GB2642282A

GB2642282A - Identification of engine oil performance

Info

Publication number: GB2642282A
Application number: GB2409381.7A
Authority: GB
Inventors: Bellingham Sarah; Blundell Guy; John Struwe Frank Iii
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2024-06-28
Filing date: 2024-06-28
Publication date: 2026-01-07
Also published as: WO2026006507A1; GB202409381D0

Abstract

A method of identifying local engine oil performance after operation of a combustion engine and a characterisation of local engine oil performance are provided. The engine oil is configured to lubricate the combustion engine. The method comprises operating the engine during an operation period 110. Data indicative of engine operation during the operation period 120 is obtained. The data is used as an input to an oil evolution function 130 to provide a generated engine oil evolution 140. Local engine oil performance is identified by comparing the generated engine oil evolution to a reference characterisation of engine oil evolution. The reference characterisation comprises an association of each of a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine and/or an association of each of a plurality of reference differences between a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine.

Description

[0001] Identification of engine oil performance

[0002] Field of the disclosure

[0003] The disclosure relates to the field of engine oil.

[0004] Background

[0005] It is known to use engine oil to lubricate combustion engines, protecting and prolonging the life of engine components. The engine oil may optionally perform additional functions, particularly in an event that additives are added to the engine oil. Functions of the engine oil may include cleaning (using detergents added to the engine oil), cooling, inhibiting corrosion, neutralising acids from combustion, and so on. The engine oil performance for both lubrication and any other functions evolves over time. In many instances, as it evolves the engine oil performance is acceptable for successful engine operation. However, if the engine oil is utilised for too long, the performance of the engine oil may deteriorate, possibly to the point that damage to the engine may occur. To overcome this, it is known to periodically change the engine oil. The time interval between engine oil changes may be called the service interval or oil change interval. It is also possible that the engine oil performance evolution is such that undesirable engine oil performance characteristics may develop at shorter time scales than the engine oil change interval.

[0006] Oil sample analysis may be used to characterise some changes to the oil throughout the service period. Conventionally, the engine oil is analysed using analytical chemistry techniques. These techniques may include spectroscopy, such as Fourier-transform infrared (FTIR) spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES) or Raman spectroscopy. The techniques may include Gas Chromatography, Gel permeation chromatography (GPC), or other performance characterisations such as viscometry. For example, in the case of FTIR spectroscopy the resulting spectrum is integrated over broad wavelength ranges to calculate values indicative of oxidation, nitration and sulfation of the engine oil. These values provide some indication of broad changes to the engine oil.

[0007] Summary of the disclosure

[0008] Against this background, there is provided: a method of identifying local engine oil performance after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: a. operating the engine during an operation period; b. obtaining data indicative of engine operation during the operation period; c. using the data as an input to an engine oil evolution function to provide a generated engine oil evolution of the engine oil; and d. identifying local engine oil performance by comparing the generated engine oil evolution to a reference characterisation of engine oil evolution; wherein the reference characterisation comprises an association of each of a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine and/or an association of each of a plurality of reference differences between a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine.

[0009] In this way, an evolution of the engine oil may be predicted using the engine oil evolution function, based on data from the engine and without taking samples of the engine oil. The generated engine oil evolution may be a predicted evolution of bulk engine oil. Local engine oil evolutions that make up the bulk engine oil evolution may be identified. Local operational conditions to which some or all of the bulk oil has been exposed may be identified. A change or changes to a spectrum of engine oil may provide a fingerprint for a particular operational condition, and each of the reference spectra and/or reference differences may provide a fingerprint that is indicative of a particular operational condition. The presence (or absence) of the fingerprint for a particular operational condition in the generated spectra may be indicative that the bulk engine oil has been exposed to (or has not been exposed to) that particular operational condition. The operational conditions associated with the reference spectra and/or reference differences may each be a local operational condition of a component, sub-system or system of the engine. Identifying local engine oil evolutions may provide information as to the health or performance of the engine oil and/or information as to the health or performance of a component, sub-system or system of the engine.

[0010] There is also provided: a characterisation of local engine oil performance after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine. The characterisation comprises a generated engine oil evolution, wherein the generated engine oil evolution is obtained by using data indicative of engine operation during the operation period as an input to an engine oil evolution function to provide the generated engine oil evolution of the engine. The characterisation further comprises an identification of local engine oil performance based on a comparison of the generated engine oil evolution to a reference characterisation of engine oil evolution. The reference characterisation comprises an association of each of a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine and/or an association of each of a plurality of reference differences between a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine.

[0011] Brief description of the drawings

[0012] A specific embodiment of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a flowchart illustrating a method of identifying engine oil performance according to an embodiment of the present disclosure.

[0013] Figure 2 shows examples of changes to peaks of a spectrum.

[0014] Figure 3 shows examples of changes to troughs of a spectrum.

[0015] Figure 4 shows a flowchart illustrating a method of obtaining a reference characterisation of the evolution of engine oil, according to an embodiment of the present disclosure.

[0016] Figure 5 shows an example of a spectrum used in a method obtaining a reference characterisation of the evolution of engine oil according to an embodiment of the present disclosure.

[0017] Figure 6 shows a schematic illustration of examples of spectra indicative of engine undergoing a "normal" evolution pathway. 4 -

[0018] Figure 7 shows a schematic illustration of examples of spectra indicative of engine undergoing an "abnormal" evolution pathway.

[0019] Figure 8 shows a schematic illustration of examples of spectra indicative of engine undergoing a "normal" evolution pathway.

[0020] Figure 9 shows a schematic illustration of examples of spectra indicative of engine undergoing an "abnormal' evolution pathway.

[0021] Detailed description

[0022] During operation of a combustion engine, engine oil may lubricate the combustion engine.

[0023] The engine oil may be configured to lubricate one or more components, sub-systems or systems of the combustion engine. The engine oil may be configured to travel to the components, systems or subsystems via a lubrication system. The engine oil may move through the various parts of the lubrication system or combustion engine at different mass flow rates. Furthermore, the engine oil may perform additional functions, particularly in an event that additives are added to the engine oil. Functions of the engine oil may include cleaning (using detergents added to the engine oil), cooling, inhibiting corrosion, neutralising acids from combustion, and so on. As used herein, "engine oil" may refer to both a base engine oil (or base stock) and any additives, if present. A local residence time of the engine oil in a particular component, system or subsystem may be the average time a particular volume of engine oil spends at or in a particular component, system or subsystem.

[0024] In certain examples, an oil pump may pressurise the engine oil, such that the engine oil passes through a lubrication system to the components, sub-systems and systems of the combustion engine. In an example, the oil pump may pressurise the engine oil such that the engine oil passes into an inlet comprising a suction tube into an oil pan or sump. From the oil pan, the pressurised oil may flow to an oil cooler and then to oil filters, or straight to oil filters. The engine oil may then be routed to a main distribution volume of the engine. The main distribution volume may be referred to as an oil gallery or galleries. The engine

[0025] -

[0026] oil may pass from the gallery to other components, systems or subsystems of the combustion engine.

[0027] A portion of the engine oil may pass from the galleries to directly fed components, systems or subsystems via passages. The passages may be configured to physically connect the directly fed components, systems or subsystems to the main gallery or galleries. The passages may, for example, comprise tubes or pipes. The combustion engine may comprise a plurality of passages connecting the galleries to components, systems or subsystems. For example, one or more passages may connect the galleries to bearings. A bearing may result in a restriction of flow to the lubrication system. The mass flow rate of the engine oil through the components, systems or subsystems may vary. The local residence time of engine oil in a directly fed component, system or subsystem may be the volume of the component, system or subsystem divided by the mass flow rate.

[0028] A portion of the engine oil may pass from the galleries to indirectly fed components, systems or subsystems, based on directed flow from the main oil gallery. Indirectly fed components, systems or subsystems may not be physically connected to the main gallery or galleries via passages. Possible examples of indirectly fed components, systems or subsystems may be a cylinder bore, a piston cooling gallery, valve guides and valve seats.

[0029] For example, an amount of engine oil may reside in aerosol form inside the combustion engine. Some of this engine oil in aerosol form may pass to indirectly fed components, systems or subsystems. In another example, piston cooling jets may be fed from the main gallery and provide engine oil to indirectly fed components, systems or subsystems. Due to a mass flow of engine oil in the combustion engine, for example through and out of directly fed components, systems or subsystems, there may be engine oil in motion in the combustion engine, some of which may be passed to the indirectly fed components, systems or subsystems. As an example, engine oil may be used to cool a piston, wherein the piston is directly fed. The piston cooling gallery may be a roughly toroidal shaped volume in the piston, with an inlet and an outlet. A piston cooling jet may provide engine oil to the piston. In certain examples, the end of the jet (comprising, for example, a nozzle) may not be connected to the piston. The jet outlet may not provide perfect columnar flow. Engine oil exiting the jet may "broom" and spread out as it exits the jet. Any engine oil exiting the jet that does not enter the piston may be incident on the bottom of the piston or may land on walls of a cylinder bore, so lubricating the cylinder wall. The cylinder wall is, in 6 -this example, indirectly fed. The cylinder wall may be additionally or alternatively lubricated by engine oil that is expelled from bearings in the vicinity of the cylinder wall.

[0030] The engine lubrication system provides engine oil to several locations within the engine.

[0031] These locations may comprise specific component(s), sub-systems or systems that receive engine oil and introduce the engine oil into areas where the engine oil is to be used. The engine oil may exist within component interfaces or within an engine system or sub-system for a certain length of time. For example, the engine oil may be introduced to component tribological interfaces and/or the engine oil may be introduced to systems to be cooled by the engine oil. The engine oil may be introduced to a system or component to do mechanical work (such as hydraulic lash adjusters). The local engine oil comprises a particular quantity of oil in an area where the engine oil is in use. For example, the local engine oil may comprise a particular quantity of oil in the tribological interfaces (between surfaces), or in surfaces to be cooled, or in an area where the engine oil is doing mechanical work of or for a specific component(s), sub-system and / or system. Examples of local engine oil include engine oil in a main crankshaft bearing, engine oil on the cylinder bore, the engine oil film between the cylinder bore and piston ring or piston skirt, or the engine oil in the piston cooling gallery.

[0032] Examples of engines that may use engine oil for lubrication and, optionally, other functions include an engine in a vehicle, an engine in a work machine or work equipment, a marine propulsion engine, a locomotive engine, an engine used in a stationary or mobile or towable configuration for electrical power generation, an engine used in a stationary or mobile or towable configuration to support petroleum industry functions, an engine used on a vehicle or marine vessel for auxiliary power, an industrial machine, or other engine applications During operation of the engine, the engine oil is exposed to various conditions within the engine. The engine oil evolves over time, such that changes occur in the engine oil. The engine oil performance evolves over time. These changes may include one or more of physical changes, chemical changes and contamination. Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Chemical changes might include chemical reactions in the engine oil, and/or chemical species being formed or consumed.

[0033] Contamination increasing or decreasing, or deposits forming in the engine might result in 7 -changes to the engine oil. As noted, the term "engine oil" may refer to both the base engine oil (or base stock) and any additives, if present. The changes to the engine oil may comprise changes to the engine oil itself and/or changes to the additives, if additives are present. The engine oil performance evolves over time. The engine oil performance may be the performance of the engine oil in relation to one or more of its functions, including but not limited to lubrication.

[0034] Engine oil passes through an engine, such that engine oil passes through or near to various engine subsystems or resides in various engine subsystems. An engine system, subsystem or component may expose engine oil that is in proximity to said system, subsystem or component to certain local conditions that result in chemical reactions of the engine oil, contaminants to appear (as a result of wear materials contaminating the engine oil, for example), or other changes to the engine oil to occur. In other words, the engine oil evolves in a certain manner due to being exposed to certain local conditions within the engine. An engine system, subsystem or component may expose engine oil to certain local conditions in an event that the engine oil passes through or by the system, subsystem or component, or in an event that the engine oil resides in the system, subsystem or component.

[0035] Engine oil may evolve when exposed to consistent local conditions. In other words, a change to engine oil does not necessarily represent a change in local conditions to which the engine oil is exposed. However, evolution of the engine oil may depend on the local conditions to which the engine oil is exposed, so a change to the engine oil may represent a change in local conditions to which the engine oil is exposed. A change in performance of an engine system, subsystem or component may alter the local conditions to which the engine oil passing through or residing in the engine system, subsystem or component is exposed. Since evolution of the engine oil may depend on the local conditions to which the engine oil is exposed, changes to the performance of an engine system, subsystem or component may affect the evolution of engine oil passing through or residing in the engine system, subsystem or component. Certain changes to engine oil may represent changes in performance of the engine system, subsystem or component. Other changes to engine oil may not represent changes in performance of the engine system, subsystem or component. 8 -

[0036] The engine oil may pass through different engine subsystems at different mass flow rates, such that the engine oil is in proximity to different engine subsystems for different lengths of time. Furthermore, the engine oil may follow different routes through the engine. A plurality of routes may be followed by the engine oil, with the engine oil splitting such that a given time the engine oil follows a plurality of different routes through the engine. For example, a quantity of the engine oil may follow a first route through the engine, while a sub-quantity of engine oil may diverge from this route. The sub-quantity of engine-oil may return to the first route or may mix with the rest of the engine oil in the oil pan or sump.

[0037] Certain engine systems, subsystems or components may rely on the state or performance of the engine oil that is passing through or by said system, subsystem or component. For example, certain engine systems, subsystems or components may rely on rheological performance of the engine oil (wherein the engine oil forms films of oil to separate adjacent surfaces) or tribochemical performance of the engine oil (wherein the engine oil forms protective films on a surface, and performs other functions), and/or on other functions of the engine oil such as cleaning, cooling, inhibiting corrosion, and neutralising acids from combustion. The performance of an engine system, sub-system or component may be affected by the performance of the engine oil. Evolution of engine oil in a certain engine system, subsystem or component may affect other engine system(s), subsystem(s) or component(s), which rely on engine oil performance. In other words, oil evolution that is caused or driven by interaction of the engine oil with a first engine component, sub-system or system may alter the bulk oil chemistry of the engine oil, especially for abnormal oil evolution. The new bulk oil chemistry may result in changes of performance of the engine oil that impact the interaction of the engine oil with a second engine component, sub-system or system and that therefore impact the performance of the engine oil in the second engine component, sub-system or system. For example, the new bulk oil chemistry of the engine oil may result in changes in performance of the engine oil that results in abnormal performance of the second engine component, sub-system or system, such as increased wear or damage, to the second engine component, sub-system or system.

[0038] Changes to the performance of the engine oil may affect the health of engine components, subsystems or systems that are reliant on the performance of the engine oil. The change to the health of the engine components, subsystems or systems may, in turn, change the conditions to which the engine oil is exposed in those components, subsystems or systems. The engine oil will continue to evolve based on those changed conditions, which may further affect the engine components, subsystems, or systems, and so on. Changes to the performance of the engine oil due to exposure to certain conditions in a particular engine component, subsystem or system may affect the health of that engine component, subsystem or system and/or the health of a different engine component, subsystem or system.

[0039] Health of an engine or engine components, subsystems or systems may be reflected by the performance or operation of the engine. Good engine health may be reflected by good or acceptable overall engine performance; reliable engine performance or a lack of reliability issues; and a lack of abnormal wear of engine components, wherein abnormal wear may comprise accelerated wear or a high magnitude of wear. Poor engine health may be reflected by fair to unacceptable overall engine performance or operational. For example, the engine may be functional but with performance that is below expected performance. The engine components may be suffering from abnormal wear. A system or component of the engine may be undergoing initial or intermediate phases of a component or system failure or performance degradation.

[0040] As described above, an engine system, subsystem or component may expose engine oil that is in proximity to said system, subsystem or component to certain local conditions that result in chemical reactions of the engine oil, contaminants to form, or other changes to the engine oil to occur. In other words, the engine oil evolves in a certain manner due to being exposed to certain local conditions within the engine. Specific evolution pathways of the engine oil can be associated with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. Engine oil may be considered to be in proximity to an engine subsystem when the engine oil is exposed to certain conditions by that engine subsystem. For example, the engine oil may pass through, by or near to said subsystem, or may reside near to the subsystem such that the engine oil is exposed to certain engine conditions as a result of its proximity to the subsystem. The evolution pathways may comprise changes to the engine oil, including one or more of chemical changes, physical changes and contamination.

[0041] Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Chemical changes might include chemical reactions in the engine oil, and/or chemical species being formed or consumed. Contamination increasing or decreasing, or deposits forming in the engine might result in changes to the engine oil. Changes to the engine oil may be evident in an -10 -analysis of chemistry of the engine oil, such as a spectral analysis of the engine oil. As an example, a spectral analysis of engine oil might provide information relating to the chemical species within the engine oil or to the chemical bonds that are present or that have formed or have broken within the components of the engine oil. The changes that are shown in a chemical analysis of the engine oil may include one or more of physical changes, chemical changes and contamination. An evolution pathway of engine oil including one or more of physical changes, chemical changes and contamination may therefore be interrogated via chemical analysis.

[0042] The characterisation of the evolution of the engine oil may include "normal" and "abnormal" evolutions of the engine oil. The normal and abnormal evolution of engine oil may be specific to the specific engine components, subsystems or systems in which the oil resides. The normal and abnormal evolution of engine oil may differ for engine oil residing in different engine architectures. The normal and abnormal evolution of engine oil may be indicative of normal and abnormal health, respectively, of engine components or subsystems. In other words, particular changes to the engine oil may correlate to particular changes to the engine. Abnormal evolution of engine oil may indicate damage or issues with engine components or subsystems, before the damage is observed at an engine level. The normal and abnormal evolution of engine oil may be indicative of normal and abnormal operation, respectively, of engine components or subsystems. Normal engine operation may comprise expected levels of one or more of overall engine performance; oil consumption; oil physical properties; chemistry of the engine oil; contamination of engine oil; and magnitude and rate of component wear. In this context, "expected" may mean within limits, between which the performance of an engine, component, sub-system or system may be considered acceptable. For example, the engine, component, sub-system or system may be considered to meet customer requirements of engine, machine or application performance or lifespan. Abnormal engine operation may comprise performance outside the expected limits.

[0043] Changes to the engine oil are caused by conditions or states to which the engine oil has been exposed and that have caused chemical reactions to occur, contaminants to form, or other changes to the engine oil to occur. The conditions or states to which the engine oil is exposed are defined by the engine operation. Changes to the engine oil may be indicative of changes to the engine itself, including but not limited to changes to the physical state of the engine components or systems (such as wear) or changes to the engine operating conditions (such as speed, load, etc.). The conditions to which engine oil is exposed may vary between engine subsystems and components, so engine oil in or near to a particular engine subsystem or component is exposed to a local condition. This causes local engine oil evolution.

[0044] However, larger quantities of the engine oil circulate through the engine than is found at a given moment in a local area. The engine oil may return to an oil pan or sump and may be considered as a substantially homogeneous mixture within the oil pan, wherein the engine oil in the oil pan is a mixture of the engine oil that has passed through the various engine components, sub-systems and systems. The engine oil in the oil pan or sump may be referred to as bulk engine oil. The bulk oil may be approximately homogeneous, but is not static while in the engine. The engine oil is transported from the sump via the oil pump and distributed into the lubrication system continuously and repeatedly while the engine is in operation. The engine oil in the lubrication system is distributed to specific locations within the engine. After passing through a local component, sub-system or system in the engine, the engine oil flows into the sump. In the sump, the engine oil flowing from the local area is mixed to re-establish a new bulk oil.

[0045] Bulk engine oil may further describe engine oil that is added to the engine lubrication system at fill, and the engine oil within the engine lubrication system before the engine is operated. Once the engine begins to be operated, the bulk engine oil is made up of engine oil that has passed through local components, sub-systems or systems of the engine. At a given moment, engine oil in different local components, sub-systems or systems of the engine may be exposed to different local conditions, such that the engine oil in different local components, sub-systems or systems of the engine undergoes different local engine oil evolutions. The bulk engine oil is a mixture of engine oil that has passed through local components, sub-systems or systems of the engine, and therefore the bulk engine oil has a bulk evolution that is a consequence of various local engine oil evolutions. Since the engine oil may follow different routes through the engine, and may have different mass flow rates through different engine subsystems, the bulk engine oil evolution is a complex combination of the various local engine oil evolutions.

[0046] Bulk oil evolution describes the changes over time of the bulk engine oil. These changes may include one or more of physical changes, chemical changes and contamination. As the bulk engine oil is made up of engine oil received from local areas, the bulk oil evolution -12 -is affected by the evolution of engine oil within local areas of the engine. Local oil evolution may comprise changes over time to the local engine oil, while the engine oil is within a component interface or within a system or sub-system. The changes may include one or more of physical changes, chemical changes and contamination. The local oil evolution within a component interface or within a system or sub-system may be affected by the local residence time for that component interface or system or sub-system. The local residence time is an average length of time that a particular quantity of oil remains in a particular local area, such as a component interface or system or sub-system. The local area may, for example, be within lubrication surfaces of a system or sub-system or within a system or sub-system. The particular quantity of oil may be in motion within that local area during the local residence time. In certain embodiments, the local residence time may be an average length of time that a droplet of oil remains in a particular local area, or an average length of time that a cubic millimeter of oil remains in a particular local area. The local residence time may be defined for any component interface, sub-system or system. For example, a local residence time may apply to an oil pan or sump, an oil passage of a cylinder block, a bearing, or other system or subsystem. In a certain example, a local residence time for a bearing may be the average length of time that a particular quantity of oil remains in a clearance volume of a bearing. A clearance volume of a bearing comprises a volume between the bearing and a component, wherein the volume is configured to be filled with engine oil. The component may, for example, be a journal (shaft) or other component. The engine oil between the bearing and the component may be a film of engine oil, such that the bearing and component are separated by the film of engine oil. In a particular example, the bearing may have a hollow cylindrical shape such that an inner diameter of the bearing may contact engine oil or a component. Said component may be cylindrical and pass through the bearing such that an outer diameter of the component may contact engine oil or the inner diameter of the bearing, wherein the clearance volume has a shape of a thin-walled cylinder between the bearing inner diameter and the component outer diameter. The local residence time for said bearing may be the average time taken for the particular quantity of oil to enter and exit the clearance volume of a bearing.

[0047] The local residence time may differ between different components, systems or subsystems. The local residence times may differ between directly and indirectly fed components, systems or subsystems. For example, the local residence time of engine oil on the cylinder wall may be significantly longer than the local residence time in the piston cooling gallery.

[0048] The local residence time in the piston cooling gallery may be longer than the local -13 -residence time of main or rod bearings. In certain examples, the local residence time of engine oil on the cylinder wall may be in the order of minutes. In certain examples, the local residence time of engine oil on the piston cooling gallery may be in the order of tenths of a second. In certain examples, the local residence time of engine oil in the main or rod bearings may be in the order of hundredths of a second. However, these values are examples only. The local residence times may vary from these examples.

[0049] The local residence time of engine oil in an engine component, subsystem or system may depend on a mass flow rate of engine oil through the engine component, subsystem or system. As discussed above, specific evolution pathways of the engine oil can be associated with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. The rate of evolution of engine oil as a result of proximity to or operating conditions within specific engine components, subsystems or systems may be affected by mass flow rate of engine oil though the engine components, subsystems or systems. For example, engine components, subsystems or systems that have a higher mass flow rate of engine oil may cause a higher rate of evolution of engine oil than engine components, subsystems or systems that have a lower mass flow rate of engine oil. A higher mass flow rate through a particular engine component, subsystem or system may result in a higher rate of evolution of engine oil than a lower mass flow rate through that engine component, subsystem or system. However, other examples are possible. For example, an engine component, subsystem or system that has a relatively low mass flow rate of engine oil may cause a relatively high rate of evolution of engine oil.

[0050] A method of identifying local engine oil performance is provided. A user of a vehicle or equipment comprising an engine operates the engine to perform work. During operation of the engine, the engine oil evolves. In this context, operating an engine to perform work may mean any operation of an engine that drives equipment, a machine or a vehicle. The engine may be used for propulsion or in a stationary configuration. Using an engine to drive equipment may mean using an engine in an end use or in a customer application configuration. The end use may be any application or function for which an engine is designed or used. A customer may be a customer of the engine manufacturer. A customer application configuration may be used at a site or location of said customer. An application configuration may refer to any equipment, machine or vehicle driven by the engine. The end use of the engine or a customer application configuration of the engine may include any of an engine in a vehicle, an engine in a work machine or work equipment, a marine -14 -propulsion engine, a locomotive engine, an engine used in a stationary or mobile or towable configuration for electrical power generation, an engine used in a stationary or mobile or towable configuration to support petroleum industry functions, an engine used on a vehicle or marine vessel for auxiliary power, an industrial machine, or other engine applications. In other words, equipment driven by the engine may include any of a vehicle, a work machine or work equipment, a marine vessel, a locomotive vehicle, an electrical power generator, an auxiliary power provider on a vehicle or marine vessel, an industrial machine, or any other engine application.

[0051] Data indicative of engine operation is obtained from the engine. The data is used as an input to an engine oil evolution function. An output of the engine oil evolution function may be used to provide a generated engine oil evolution that may be used to identify local engine oil performance, using a reference characterisation of engine oil evolution. The engine oil evolution function may comprise any function or model configured to receive data indicative of engine operation as an input and provide an output. The output may be used to provide a generated engine oil evolution, such as one or more generated spectra or one or more variances between two or more generated spectra. For example, the engine oil evolution function may comprise any of a function, an analytical model, a numerical model, a computer program or model, an algorithm, or other type of function or model. An analytical model may comprise a mathematical model, physics-based model, quantitative model or computational model.

[0052] As noted above, specific evolution pathways of the engine oil can be associated with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. The reference characterisation may be used to interpret the determined engine oil evolution, associating the determined engine oil evolution with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. This interpretation can provide information relating to local engine oil performance. The interpretation depends on several factors, and the same change to a spectrum might mean something different depending on other metrics. As an example, a particular change to a spectrum might mean one thing for relatively new engine oil, and something different for relatively old engine oil. The local engine oil performance may be used in a variety of ways, to aid subsequent operation of the engine.

[0053] -15 -The local engine oil performance may, for example, be used to determine when to replace the engine oil. This may allow the interval between replacing the engine oil to be extended safely, by more accurately predicting when the engine oil might need to be replaced. When the engine oil needs to be replaced may depend on multiple factors. In general, the engine oil does not need to be replaced as long as the engine oil performance is such that the engine oil safely protects the engine in all conditions. In an event that the engine oil performance diminishes such that the engine oil cannot safely protect the engine in all conditions, the engine oil may need to be replaced. However, the extent to which the performance can diminish before the engine oil needs to be replaced may depend on several factors, including the specifics of the engine and the operation of the engine. For example, a particular engine may be able to withstand a lower performing engine oil than another engine, or a mode of operation of an engine may affect the importance of engine oil performance. In another example, the local engine oil performance may be used to determine whether any local issues resulting from this oil evolution and performance are likely to occur, such that the engine oil can be replaced before the issues arise.

[0054] Additionally, or alternatively, the local engine oil performance might allow the engine to be operated appropriately based on the performance of the engine oil. For example, in an event that the engine oil is "high performing" (in other words is performing well and safely protecting the engine), an engine might be safely operated in any operating condition. The engine may be operated intensively or at high loads, in the knowledge that the engine oil is able to effectively protect the engine. On the other hand, in an event that the engine oil evolves such that local engine oil performance is diminished, the engine oil may not be able to protect the engine in all operating conditions. The engine might be limited to operate only in conditions at which the engine oil can still protect the engine. The local engine oil performance may be used to change an engine calibration depending on the health of the oil, for example to allow operators to run the engine hard when the local areas are protected by high performing oil, or to run the engine more carefully if the oil evolves to be lower performing such that the engine oil cannot protect the engine as effectively.

[0055] With reference to Figure 1, a method of identifying local engine oil performance after operation of a combustion engine according to an embodiment of this disclosure is illustrated. The engine oil is configured to lubricate the combustion engine. The method comprises operating the engine during an operation period at step 110. At step 120, data indicative of engine operation is obtained during the operation period. The data may be -16 -obtained at any time during the operation period. The data may be obtained once, or more than once. The method further comprises using the data at step 130 as an input to an engine oil evolution function, such that a generated engine oil evolution is obtained. At step 140, the generated engine oil evolution is compared to a reference characterisation of engine oil evolution, and local engine oil performance is identified. The reference characterisation may comprise a plurality of reference spectra and/or a plurality of reference variances between reference spectra. Each reference spectrum and/or each reference difference may be associated with one or more of a plurality of operational conditions of the combustion engine.

[0056] Operating the engine at step 110 may comprise operating the engine to perform work according to the definition provided above. For example, operating the engine may comprise any of operating the engine to provide power to a vehicle, machine, equipment or tool; operating the engine as a traction engine; operating the engine as a generator; or operating the engine for some other purpose.

[0057] The operation period of the combustion engine may comprise any time period during which the combustion engine is operated, either continuously or over discrete sub-intervals.

[0058] During the operation period, the engine may be operated continuously over the operation period or the engine may be operated over discrete sub-intervals, wherein the engine is not operated between the sub-intervals. The operation period of the combustion engine may comprise any time period during which the combustion engine is operated, either continuously or over discrete sub-intervals. The operation period may comprise one or more sub-intervals during which the combustion engine is operated. The operation period may comprise one or more sub-intervals during which the combustion engine is in standby, turned off, or in storage.

[0059] The data may be obtained from an engine control module (or engine control unit), or from any sensor or controller associated with the engine. The data may be obtained at intervals, or continuously. In certain embodiments, the data may be obtained at constant intervals. In other embodiments, the data may be obtained at varying intervals. For example, the intervals may reduce in length over time so that data is obtained more frequently. Data may be obtained more frequently in circumstances where the risk of engine oil performance degrading is considered to be higher. For example, the interval might be longest -17 -immediately after the engine oil has been changed, and may then reduce over time so that as the engine oil becomes older data is obtained more frequently.

[0060] At step 130, the data is used as an input to an engine oil evolution function to obtain a generated engine oil evolution. The generated engine oil evolution may be indicative of how properties of the engine oil change over time, wherein the changes to the properties of the engine oil may be indicative of one or more of physical changes to the engine oil, chemical changes to the engine oil and contamination of the engine oil.

[0061] In certain embodiments, the output of the engine oil evolution function may comprise one or more generated spectra of the engine oil. Generated spectra may provide predicted properties of the engine oil at a particular time point. The generated spectra may, for example, indicate elemental composition of the engine oil, or provide information regarding the bonds of the molecules within the engine oil. The engine oil evolution function may be used to obtain more than one generated spectrum, wherein each generated spectrum is a predicted spectrum of the engine oil at a particular time point. These generated spectra may be used to obtain a generated engine oil evolution. The generated spectra may be in graphical form or may be provided as generated spectral data.

[0062] The generated engine oil evolution may be characterised by the generated spectra as a function of time. The generated engine oil evolution may be characterised by predicted variances between spectra of the engine oil at different time points, wherein the predicted variances are obtained by comparing two or more generated spectra. The generated engine oil evolution may be characterised by a combination of generated spectra as a function of time and predicted variances between generated spectra of the engine oil at different time points.

[0063] In certain embodiments, the output of the engine oil evolution function may comprise predicted variances between spectra of the engine oil at different time points, without outputting the spectra themselves. The generated engine oil evolution may be characterised by the predicted variances between spectra of the engine oil at different time points.

[0064] -18 -The generated engine oil evolution may be characterised by predicted variances between spectra of the engine oil at different time points, wherein the predicted variances are obtained by comparing two or more generated spectra to obtain one or more differences between generated spectra. The one or more differences determined between the generated spectra may comprise one or more of peak (and/or trough) height, peak (and/or trough) location and peak (and/or trough) shape differences. The one or more differences may comprise differences to one peak (or trough) or to multiple peaks (and/or trough). The one or more differences between the generated spectra are indicative of changes to the engine oil. The engine oil may undergo changes as a result of being exposed to conditions in the engine that force chemical reactions to occur, or contaminants to form, or other changes to the engine oil to occur. The changes to the engine oil may depend on where in the engine the engine oil is located, since different systems or components of the engine the engine may expose the engine oil to different conditions. The changes to the engine oil may depend on the length of time for which the engine oil is exposed to the engine conditions.

[0065] The generated spectra may be indicative of bulk oil evolution. As described above, the bulk engine oil is made up of engine oil that has passed through local components, sub-systems or systems of the engine. The bulk evolution of the bulk engine oil is, therefore, a result of a combination of a plurality of local oil evolutions. Analysis of a volume of bulk oil may be used to identify local evolution of the engine oil in a particular component, sub-system or system. The features of the generated spectra may be used to isolate local oil evolution and local conditions to which the oil has been exposed. The engine may change over time, in which case the conditions to which the engine oil is exposed will change and the local oil evolution for a particular subsystem may change. The local engine oil evolution may be used to identify changes to the conditions to which the engine oil has been exposed, and therefore to identify changes to the engine. Changes to the engine may include changes to a component, sub-system or system. Changes to a component, sub-system or system may include physical changes, such as wear, and/or changes to performance of a component, sub-system, system or engine.

[0066] In an event that all the components and subsystems of the engine are performing as intended and there are no issues with the engine, the engine oil evolves in a certain way.

[0067] This evolution may be referred to as a normal evolution pathway. In an event that there is -19 -an issue with any part of the engine, or a component or subsystem of the engine is not performing as expected, the engine oil may evolve in a different way. This evolution may be referred to as an abnormal evolution pathway.

[0068] The difference(s) between the generated spectra are indicative of the conditions to which the engine oil has been exposed within the engine. These conditions may be conditions experienced by the engine oil within components or systems of the engine or in proximity to components or systems of the engine, while those engine components or systems are undergoing certain engine conditions. The conditions in the engine may be influenced by various parameters. Examples of parameters that are influenced by engine operational conditions and that affect the evolution of engine oil may include temperature, pressure, engine oil volume, engine oil flow rate, final combustion products, interim combustion products, local engine oil chemical state, air or other gas concentrations, shear, friction, or other property of the engine. Combustion products may include nitrogen oxides, blowby gases, intermediary gases, soot, or other products. For example, the contact area between a gas and the engine oil may affect the engine oil, for example if the gas is in bubbles or a sheet. Changes in these parameters may be indicative of changes to certain engine components or to the operation of certain engine components. These parameters may influence the bulk engine oil. The parameters may influence the local engine oil in a component, sub-system or system during the local residence time in that component, sub-system or system. Different components, sub-systems and systems can all be differently exposed to different combinations of these parameters.

[0069] The generated spectra may each comprise a plurality of features. There may be one difference between the generated spectra that may comprise a change to one feature. For example, one peak might change in height. There may be multiple differences between the generated spectra, comprising changes to multiple features. The combination of differences may indicate a particular change to the engine oil and its performance. For example, a change to a first feature may indicate one of two changes to the engine oil, depending on a change to a second feature. In an example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak decreases in height, but the first peak increasing in height may indicate a second change to the oil in an event that the second peak increases in height. In another example, there may be one difference between the spectra, comprising a change to a first feature, wherein the change to the engine oil indicated by the difference depends on another unchanged feature of the -20 -spectrum. For example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak is lower than the first peak, but may indicate a second change to the oil in an event that the second peak is not lower than the first peak.

[0070] The features of the generated spectra may comprise peaks and troughs. The difference(s) between the generated spectra may include one or more of the following: one or more peaks shifting with respect to the y axis; one or more peaks shifting with respect to the x axis; one or more peaks scaling with respect to the y axis; one or more peaks scaling with respect to the x axis; one or more troughs shifting with respect to the y axis; one or more troughs shifting with respect to the x axis; one or more troughs scaling with respect to the y axis; one or more troughs scaling with respect to the x axis. The differences may comprise a combination of more than one of these differences. The difference(s) between generated spectra may include one or more of a change to peak height ratios; a change to the number of peaks in a specific wavenumber range; a change to integrals of the generated spectrum within a specific wavenumber range; and roughness over a specific wavenumber range. A change in peak height may comprise a change in the absolute value of absorbency of the peak (i.e. measured from the origin), or a change in height as measured from the base of the peak.

[0071] Figures 2 and 3 to indicates some simple examples of possible differences between generated spectra. These examples are merely illustrative, and show simplified peaks (Figure 2) and troughs (Figure 3). For each graph, the solid line indicates a part of a primary generated spectrum, and the dashed and dotted lines indicate parts of possible secondary generated spectra, wherein the primary generated spectrum and secondary generated spectrum would be generated spectra of engine oil at different time points.

[0072] Figure 2A shows a peak shifting with respect to the y-axis. The dashed line 212 shows a positive translation of the peak 211 of the primary generated spectrum with respect to the y-axis (i.e. translated to higher y values), and the dotted line 213 shows a negative translation of the peak 211 of the primary generated spectrum with respect to the y-axis (i.e. translated to lower y values). Figure 2B shows a peak shifting with respect to the x-axis. The dashed line 222 shows a positive translation of the peak 221 of the primary generated spectrum with respect to the x-axis (i.e. translated to higher x values), and the dotted line 223 shows a negative translation of the peak 221 of the primary generated spectrum with respect to the x-axis (i.e. translated to lower x values). Figure 2C shows a -21 -peak scaling with respect to the y-axis. The dashed line 232 shows the peak 231 of the primary generated spectrum stretched parallel to the y-axis. The dotted line 233 shows the peak 231 of the primary generated spectrum compressed parallel to the y-axis. Figure 2D shows a peak scaling with respect to the x-axis. The dashed line 242 shows the peak 241 of the primary generated spectrum stretched parallel to the x-axis. The dotted line 243 shows the peak 241 of the primary generated spectrum compressed parallel to the x-axis.

[0073] Figure 3A shows a trough shifting with respect to the y-axis. The dashed line 312 shows a positive translation of the trough 311 of the primary generated spectrum with respect to the y-axis (i.e. translated to higher y values), and the dotted line 313 shows a negative translation of the trough 311 of the primary generated spectrum with respect to the y-axis (i.e. translated to lower y values). Figure 3B shows a trough shifting with respect to the x-axis. The dashed line 322 shows a positive translation of the trough 321 of the primary generated spectrum with respect to the x-axis (i.e. translated to higher x values), and the dotted line 323 shows a negative translation of the trough 321 of the primary generated spectrum with respect to the x-axis (i.e. translated to lower x values). Figure 3C shows a trough scaling with respect to the y-axis. The dashed line 332 shows the trough 331 of the primary generated spectrum stretched parallel to the y-axis. The dotted line 333 shows the trough 331 of the primary generated spectrum compressed parallel to the y-axis. Figure 3D shows a trough scaling with respect to the x-axis. The dashed line 342 shows the trough 341 of the primary generated spectrum stretched parallel to the x-axis. The dotted line 343 shows the trough 341 of the primary generated spectrum compressed parallel to the x-axis.

[0074] The primary and secondary generated spectra may comprise multiple peaks and troughs.

[0075] As discussed above, the difference between the secondary and primary generated spectra may comprise one change to one peak or trough, such as one of those changes described in relation to Figures 2 and 3. The difference between the secondary and primary generated spectra may comprise multiple changes to multiple peaks or troughs, such as a combination of those changes described in relation to Figures 2 and 3. A given change to a given feature of the primary generated spectrum may be associated with different evolution pathways of the engine oil, depending on the change to another feature(s) of the generated spectrum, or the lack of change to another feature(s) of the generated spectrum, or some other characteristic of other feature(s) in the generated spectrum. For example, a given change to a given feature of the primary generated spectrum may be associated with different evolution pathways of the engine oil depending on whether or not another feature -22 -has changed. In another example, a given change to a given feature of the primary generated spectrum may be associated with different evolution pathways of the engine oil depending on how another feature has changed. In another example, a given change to a given feature of the primary generated spectrum may be associated with different evolution pathways of the engine oil depending on characteristics of another feature relative to a changed feature (such as relative peak/trough height, or relative peak/trough position, etc.).

[0076] In an event that obtaining the generated engine oil evolution requires the comparison of generated spectra at different time points, the comparison of the generated spectra to identify changes between them may be carried out by any suitable method. More than one method of analysis may be combined. Analysis of the generated spectra aims to identify the degree of variation between the generated spectra associated with engine oil at different time points. Changes to the features of the generated spectra can indicate both how the engine oil evolves, and how the engine components are performing. The changes to the features of the generated spectra can be indicative of performance of any component, subsystem or system that interacts with the engine oil. In certain embodiments, trends may be identified in the changes to the generated spectra.

[0077] Data that is output by the function may be compared using simple mathematical comparisons, such as finding numerical differences between the raw data. Given the complex nature of the generated spectra, other techniques may be used to identify the changes to the generated spectra. These may include data science techniques, such as dimension reduction techniques or binary additive operations, or simplifying the generated spectra by experiments. Data science is an interdisciplinary method, that may use one or more of statistics, algorithms, scientific methods, numerical methods of analysis, domain knowledge, and other methods to analyse complex data sets. The application of data science techniques to chemical information may also be referred to as chemometrics. Data science may be used to identify trends in the changes in features of the generated spectra. Trends of changes to a particular feature of the generated spectra, either in isolation or in combination with trends of changes of other features of the spectra, allow a better description of the evolution of the engine oil. The degree of variation between the generated spectra can be analysed.

[0078] Techniques for identifying changes to the generated spectra may be applied to a matrix or matrices of generated spectrum data. The matrix or matrices may comprise data for two or -23 -more generated spectra, such that analysis of the data can be used to identify changes in the generated spectra. A simple mathematical assessment or assessments may be applied to a matrix of the spectra data, or more advanced techniques may be used to identify changes between generated spectra. Such advanced techniques may include data science techniques.

[0079] Data science techniques may be used to identify how variations are occurring and to isolate variations to specific variables. In this case, data science may be used to analyse generated spectra of engine oil in order to establish how the engine oil evolved. There are many data science techniques that can be used to identify variations in the generated spectra. As a non-limiting example, a matrix containing the data can be simplified via dimension reduction. It may then be identified where in the generated spectrum a change is seen. Once these variations have been identified, they may be interlinked to identify changes in features that are affected by other features or by changes to other features.

[0080] In a specific example, a set of spectroscopy data can be presented as a matrix (rows and columns). The set of data may comprise data for more than one generated spectrum. Either the rows or the columns may be described as features or dimensions. The rows (or columns) may be individual 'dimensions" such that a data set with numerous rows (or columns) is known as multidimensional data. Examples of a data science technique that can be applied to multidimensional generated spectra data sets are dimensional reduction techniques, used to reduce the number of rows (or columns) of the data set and thereby increase interpretability of data while maintaining the trends and patterns within the data. In other words, the data set is simplified without losing the patterns in the data that need to be extracted and analysed, and particular variations in the data may be isolated. The dimension reduction techniques may be applied directly to the generated spectral data, or to processed generated spectral data.

[0081] In certain examples, each set of data in a matrix of generated spectroscopy data may be compared, and a new matrix may be determined containing numerical indications of differences between each data set and/or variations within each data set. Dimension reduction techniques or other techniques may be used to isolate specific variations. In a specific example, a matrix of generated spectral data may comprise a plurality of sets of data. Each set of data may provide modelled generated spectral data for engine oil at a particular time point, in the form of signal magnitudes and wavenumbers. A new matrix -24 -containing numerical indications of differences between and/or within each data set may be obtained, and analysed to determine the degree of variation between and/or within the data sets. Specific variations may be isolated, for example using dimension reduction techniques or otherwise.

[0082] An example of a common data science dimensional reduction technique is Principal Component Analysis (RCA). RCA linearly transforms a data set into a new coordinate system. In the new coordinate system, variations in the data can be described with fewer dimensions than in the original data. In use, RCA is carried out using a matrix of generated spectral data comprising a plurality of sets of data, wherein each set of data provides generated spectral data for engine oil in the form of signal magnitudes and wavenumbers. For each wavenumber, the mean of signal magnitudes across the sets of data may be obtained. The data sets may be normalised with respect to that mean value. A covariance matrix may then be obtained, and analysed to determine the degree of variation between and/or within the data sets.

[0083] RCA can be used to characterise primary variation between the generated spectra. For example, changes in the shapes of the curves may be characterised, including slopes or gradients, and magnitude and position of peaks and valleys. Alternatively or additionally, the variations between the generated spectra may be characterised by other techniques.

[0084] Irrespective of the technique used to obtain the characterisation, the characterisation may provide an easily recognisable visual representation of differences between generated spectra, which may be more easily interpreted than a visual assessment of the raw data. Patterns between engine oil at different times can be established by comparing principle components of the characterisation or differences between the generated spectra data sets.

[0085] At step 140, the generated engine oil evolution is compared to a reference characterisation of engine oil evolution, and local engine oil performance is identified.

[0086] In certain embodiments, the generated engine oil evolution may be characterised by generated spectra of the engine oil at different time points. The reference characterisation may comprise a plurality of reference spectra.

[0087] -25 -In certain embodiments, the generated engine evolution may be characterised by predicted variances between the generated spectra of the engine oil at the different time points. These predicted variances may be output directly from the engine oil evolution function, or may be obtained by processing or analysing the output of the engine oil evolution function.

[0088] The reference characterisation may comprise a plurality of reference spectra and/or a plurality of reference differences variances between reference spectra. Each reference spectrum and/or each reference difference may be associated with one or more of a plurality of operational conditions of the combustion engine. The reference variances may each comprise changes to one or more features of a spectrum, which are indicative of changes to engine oil chemistry or changes to the rheological characteristics of the engine oil.

[0089] The reference characterisation may comprise a plurality of reference spectra and/or a plurality of reference differences variances between reference spectra. Each reference spectrum and/or each reference difference may be associated with one or more of a plurality of operational conditions of the combustion engine. The reference variances may each comprise changes to one or more features of a spectrum, which are indicative of changes to engine oil chemistry or changes to the rheological characteristics of the engine oil.

[0090] Step 140 may be carried out after (and separately to) step 130 of obtaining a generated engine oil evolution. Alternatively, the reference characterisation may be embedded within the engine oil evolution function such that the output of the engine oil evolution function comprises a comparison of the generated engine oil evolution to the reference characterisation. For example, in an event that the engine oil evolution function outputs a generated spectrum, the engine oil evolution function may further associate a reference spectrum of the reference characterisation with the generated spectrum and may compare the generated spectrum and the reference spectrum, such that the comparison of step 140 is carried out as part of step 130.

[0091] In an event that comparing the generated engine oil evolution to the reference characterisation comprises comparing generated spectra to reference spectra of the reference characterisation, the comparison of the generated engine oil evolution to the reference characterisation may comprise identifying the reference spectrum that is closest to the generated spectrum. In an event that comparing the generated engine oil evolution -26 -to the reference characterisation comprises comparing generated variances between measured spectra to reference variances of the reference characterisation, the comparison of the generated engine oil evolution to the reference characterisation may comprise identifying one or more reference variance that is closest to the one or more differences between the generated spectra. The reference variances may comprise individual reference variances (for example, changes to one feature of a spectrum) or a combination of reference variances (for example, changes to more than one feature of a spectrum). Where more than one difference has been identified between generated spectra, more than one reference variance may be identified as being closest to the more than one difference between generated spectra.

[0092] Identifying the reference spectrum or reference variance that is closest to the generated spectrum or generated variances, respectively, may be carried out by any appropriate method. More than one method of analysis may be combined. For example, the reference spectrum or reference variance that is closest to the generated spectrum or generated variances may be identified by visual inspection. The spectra and/or variances may be visually inspected in graphical form, or the raw data may be inspected. In another example, the reference spectrum or reference variance that is closest to the generated spectrum or generated variances may be identified via simple mathematical comparisons, such as by subtracting the reference spectrum or reference variance from the generated spectrum or generated variances, respectively (or vice versa) or by finding numerical differences between the reference spectrum or reference variance and the generated spectrum or generated variances, respectively. In another example, the reference spectrum or reference variance that is closest to the generated spectrum or generated variances may be identified using other mathematical methods, such as using data science techniques.

[0093] The reference spectrum or reference variance that is closest to the generated spectrum or generated variances may be the reference spectrum or reference variance that has fewest differences to the generated spectrum or generated variances, or that has the smallest differences to the generated spectrum or generated variances, or that has the least significant differences to the generated spectrum or generated variances.

[0094] The reference characterisation may associate other data with the reference spectra and/or reference variances, wherein the data is indicative of properties of the engine oil or conditions to which the engine oil was exposed. For example, the data may comprise one -27 -or more of the number of hours for which the engine oil has been used, or a mass flow rate of the engine oil, or meta data associated with the engine. Identifying the reference spectrum or reference variance that is closest to the generated spectrum or generated variances may further comprise using other data or information associated with the reference spectra and/or reference variances. Data associated of the reference spectrum or reference variance may be compared with similar data associated with the generated spectrum or generated variances to help identify the reference spectrum or reference variance that is closest to the generated spectrum or generated variances. In a specific example, a similar mass flow rate associated with a given reference spectrum and a given generated spectrum may mean that it is more likely that the reference spectrum is a close match to the generated spectrum than if the mass flow rates differed significantly.

[0095] The reference characterisation may comprise reference spectra and/or reference variances for more than one evolution path, wherein for each evolution path the reference characterisation comprises a plurality of reference spectra and/or a plurality of reference differences variances between reference spectra. Each reference spectrum and/or each reference difference may be associated with one or more of a plurality of operational conditions of the combustion engine. Identifying the reference spectrum or reference variance that is closest to the generated spectrum or generated variances, respectively, may entail comparing the generated spectrum or generated variance to all the reference spectra or reference variances, irrespective of evolution path. Otherwise, identifying the reference spectrum or reference variance that is closest to the generated spectrum or generated variances, respectively, may entail comparing the generated spectrum or generated variance to the reference spectra or reference variances only for a particular evolution path. The evolution path may be selected based on a previous comparison between a generated spectrum or generated variance and the reference characterisation for that engine (for example, a previous iteration of a method as described herein, wherein data from the engine is obtained earlier in the engine oil evolution), or based on other data such as data indicative of properties of the engine oil or conditions to which the engine oil was exposed (for example, one or more of the number of hours for which the engine oil has been used, or a mass flow rate of the engine oil, or meta data associated with the engine).

[0096] Once the reference spectrum or reference variance that is closest to the generated spectrum or generated variances has been identified, the reference spectrum or reference variance is compared to the generated spectrum or generated variances. Each reference -28 -spectrum or reference variance is associated with one or more of a plurality of operational conditions of the combustion engine. The reference characterisation may comprise reference spectra and/or reference variances that are indicative of normal and/or abnormal engine oil evolutions.

[0097] An exact or close match between a reference spectrum and a generated spectrum or between a reference variance and a generated variance is indicative that the generated engine oil evolution is the same or similar to the evolution path of the reference spectrum or reference variance. This may be indicative that the engine oil has been exposed to the same operational condition(s) as is associated with the reference spectrum or reference variance.

[0098] A less close match between a reference spectrum and a generated spectrum or between a reference variance and a generated variances may be indicative that the generated engine oil evolution is different to the evolution path of the reference spectrum or reference variance. This may indicate that the wrong reference spectrum or reference variance has been compared to the generated spectrum or generated variance, or that the generated spectrum or generated variance has been compared to the wrong evolution path of the reference characterisation, or that the generated engine oil evolution is not an evolution path contained within the reference characterisation. The generated spectrum or generated variance being compared to the wrong evolution path of the reference characterisation may, in certain scenarios, indicate that the engine oil has changed evolution path, for example as a result of a fault or issue that has occurred or may occur in future in the combustion engine. For example, in an event that the reference characterisation contains only reference spectra and/or reference variances that are indicative of normal engine oil evolutions then a less close match between a reference spectrum and a generated spectrum or between a reference variance and a generated variance may be indicative that the engine oil has evolved in an abnormal way. This may, for example, be indicative of a fault with the combustion engine that has happened or that may occur in future.

[0099] In an event that a less close match is identified between a reference spectrum and a generated spectrum or between a reference variance and a generated variances, the differences between the reference spectrum or reference variance and the generated spectrum or generated variance may be used to provide information about the evolution of the engine oil. For example, the differences may be indicative of deviation of the generated -29 -engine oil evolution from an evolution path, and may be used to provide information as to potential reasons for the deviation. In an event that the less close match is as a result of the generated spectrum or generated variance being compared to the wrong evolution path of the reference characterisation, a new comparison may be carried out to a different evolution path, either for this generated spectrum or generated variance, or for the next generated spectrum or generated variance (output from the next iteration of the method). The change to the evolution path may provide information as to the evolution of the engine oil and/or to the performance of the combustion engine. Similarly, in an event that the less close match is as a result of the generated spectrum or generated variance being compared to the wrong reference spectrum or reference variance, a new comparison may be carried out either for this generated spectrum or generated variance, or for the next generated spectrum or generated variance (output from the next iteration of the method).

[0100] The comparison between the generated spectra or generated variances and the reference characterisation may be used to identify engine oil performance and changes in engine oil performance over time.

[0101] The reference spectra or variances may link changes to the engine oil with changes to parameters or variables of the engine, and/or to particular local operating conditions of the engine. In certain examples, the reference variances may link changes to the engine oil with changes to parameters or variables of the engine and further interpretation may be carried out using knowledge of the engine operation (such as mass flow rates of the oil around the engine, and operating conditions of the engine) to identify a location within the engine at which a change has occurred that resulted in the change to the parameters or variables.

[0102] The particular change to the engine oil may be indicative of a change of a local performance attribute of the engine oil, which may be improving or worsening. A change to the performance of the engine oil may result in a change to the health of one or more engine components, sub-systems or systems that rely on the performance of the engine oil.

[0103] A change to the health of an engine component, sub-system or systems may, in turn, expose the engine oil to different conditions and may further change the local engine oil evolution. In other words, a change to the performance of the engine oil may be indicative of a prior change to the engine and/or may be predictive of a future change to the engine.

[0104] -30 -In this way, the changes of the generated spectra can be interpreted to provide the evolution of the engine oil. The evolution of the engine oil may be further interpreted, using the reference characterisation, to provide information relating to the performance of the engine oil, the performance of the engine or engine component, sub-system or system, and other attributes of the engine oil and engine.

[0105] The comparison of a generated spectrum or the generated variance to the identified closest reference spectrum or variance of engine oil evolution may be carried out by any appropriate method. The comparison may be carried out via visual inspection. Alternatively, or additionally, the comparison may be carried out using mathematical comparison. For example, peak locations or the changes in peak locations in the generated spectra may be identified and compared to the reference characterisation. As another example, where more than one reference variance has been identified, simultaneous equations may be solved to evaluate the combination of reference differences of the reference characterisation that result in the evolution of engine oil that has been determined from the more than one differences of the generated variance. Other mathematical methods may be used as appropriate. Assumptions may be made to assist the comparisons. For example, the assumptions may include mass flow rates of engine oil in difference locations in the engine, or operating conditions that result in the reference differences of the reference characterisation.

[0106] The comparison of the generated spectra or variances to a reference characterisation of engine oil evolution may allow various conclusions to be drawn. In certain embodiments, the methods herein may be carried out to monitor the evolution of the engine oil and to determine any changes. The methods may further comprise interpreting those changes to the evolution of the engine oil. In other embodiments, the methods herein may be carried out in reaction to a change in performance of an engine, in order to determine the cause of that change in performance. For example, the evolution of the engine oil may be compared to the reference characterisation in order to diagnose the cause of a failure mode by determining the local area of the engine in which something has changed and/or what parameter of the engine has changed.

[0107] In certain embodiments, the comparison may allow the evolution of the engine oil to be determined to be "normal' or "abnormal". In an event that the evolution of the engine oil is determined to be abnormal, the method may further comprise identifying a cause or likely -31 -consequence of the abnormal evolution. For example, the reference characterisation may be used to link the evolution of the engine oil to a particular change to engine performance that has already occurred and has resulted in a change to the engine oil. In certain examples, this may be done by using the reference characterisation to identify a change in the evolution of the engine oil that is "abnormal", and then using the reference characterisation to identify what variable or parameter of the engine has changed. Using knowledge of the engine operation (such as mass flow rates of the oil around the engine, and operating conditions of the engine), the location (e.g. component, sub-system or system) within the engine that has changed can then be determined.

[0108] Additionally, or alternatively, the reference characterisation may be used to link the evolution of the engine oil to potential consequences of an evolution of engine oil, such as potential damage to an engine subsystem that relies on engine oil performance. Monitoring of the engine can then be targeted based on the evolution of the engine oil. For example, if it is known that a particular change to the evolution of the engine oil may cause deposit formation, deposit formation can be monitored so that the engine oil can be changed before there are any negative consequences. In other examples, a different particular change to the evolution of the engine oil may cause an increase in wear rate (or surface damage) to a cam lobe to cam follower interface at the surfaces of the cam lobe and/or cam follower (wherein the cam follower is either a roller-follower or a 'flat tappet' follower). Another particular change to the evolution of the engine oil may cause damage to a piston ring to cylinder bore inner diameter surfaces. Another particular change might cause damage to a bearing inner diameter (also known as a running surface). Other changes might have other effects for which it would be useful to introduce targeted monitoring, if it is known that those effects are likely.

[0109] This means that in an event that the evolution of the engine oil comprises a change that is not expected (i.e. is not part of the "normal" evolution"), the reference characterisation may be used to both link that change to the cause of the change and to link that change to a potential future change to the engine or engine oil. For example, a change in the evolution of the engine oil may be identified as meaning that a change has occurred at a first local area of the engine and that a potential future change may occur at a second local area of the engine.

[0110] -32 -The performance of the engine oil may be indicative of issues within the engine that have begun to occur, or that will occur. The plurality of reference differences of the reference characterisation may correspond to changes to the engine oil, which may in turn correspond to changes to engine health. The changes to engine health may comprise changes to the health of the engine as a whole, or changes to systems or components of the engine.

[0111] The performance of the engine oil may affect performance of the engine. The performance of the engine as a whole or of systems or components of the engine may be affected. For example, a particular engine subsystem may rely on the oil rheological or tribochemical performance. Changes to the engine oil may result in unwanted attributes such as deposit formation, increased wear of engine component surfaces, surface damage such as scuff, smearing, galling, spalling or seizure, or other unwanted attributes. These in turn affect the engine performance.

[0112] The method may further comprise using the local engine oil performance to determine current and future local engine health. The local engine health may comprise performance/health of components, sub-systems or systems of the engine. The local engine health may comprise issues that are likely to occur in systems or components of the engine. The likely issues may be identified based on the performance of the engine oil. The likely issues may be a result of the performance of the engine oil, or may be a result of other factors.

[0113] The generated engine oil evolution is obtained using an engine oil evolution function, with data from the engine as inputs. As discussed above, the outputs of the generated engine oil evolution may comprise generated spectra (in numerical or graphical form) and/or generated variances between spectra of the engine oil at different time points, or the outputs may be further processed to obtain generated variances between spectra of the engine oil at different time points. The engine oil evolution function may be developed by any appropriate method. Several functions will be described in more detail by way of example, but it will be understood that other functions may be used. The examples of methods of obtaining the engine oil evolution function described below may be used alone or in combination.

[0114] -33 -In a first example, the engine oil evolution function may be generated using experimental equipment. The experimental equipment may comprise bench-top laboratory equipment such as a bench-top engine oil reactor. The experimental equipment may be used to replicate the conditions of an individual subsystem. For example, conditions in a top ring turn around zone may be replicated. Changes to the engine oil, including physical changes and chemical changes, may be evaluated. The changes to the engine oil may, for example, be evaluated using spectral analysis of the engine oil. This may be repeated, using the experimental equipment to replicate conditions of a plurality of different individual subsystems. The changes to the engine oil may be associated with each set of conditions, such that the results may be used to generate the engine oil evolution function, wherein engine conditions are associated with changes to the engine oil such that data from an engine may be used as an input to the engine oil evolution function. In certain examples, the engine oil evolution function may then be validated and/or expanded using data from an engine. Data from the engine may include spectral analysis of engine oil samples from an engine, and/or data that is indicative of properties of the engine oil or conditions to which the engine oil was exposed in the engine (such as the number of hours for which the engine oil has been used, or a mass flow rate of the engine oil, or meta data associated with the engine). The data may be associated with changes to the engine oil and/or to engine conditions replicated by the experimental equipment.

[0115] In a second example, the engine oil evolution function may be based on engine operating data and measured spectra of engine oil samples. An engine may be operated, either in a test scenario or to drive equipment (wherein driving equipment refers to using the engine to drive any equipment, machine or vehicle in an end use application or in a customer application configuration). As a result of operating the engine, the engine oil evolves.

[0116] Engine operating data may be recorded using sensors on the engine, or obtained from the Engine Control Module (ECM). Data obtained from the ECM may comprise calculated parameters or data based on sensor data. Examples of engine operating data may include, but not be limited to, engine speed, engine fuelling amount, engine load, fluid temperatures (such as air temperature, jacket water temperature, engine oil temperature), engine oil pressure, ambient temperature, engine oil type, engine type, and so on. Engine operating data may be recorded as a function of time, or as a histogram with respect to a parameter. Samples of the engine oil may be obtained at sample time points during the operation of the engine (where a sample taken at time = zero is new engine oil that has not been used in engine operation, and a sample taken at time t has been in an engine that has been -34 -operated for time 0. The samples of the engine oil may be taken when the combustion engine is in operation or when the combustion engine is not in operation. In an event that the samples are taken when the combustion engine is in operation, the sample may be taken downstream of the oil pump such that the oil is under pressure and can flow out of the engine into a sampling container when a valve is opened. The samples may be taken automatically on the engine during operation of the combustion engine. The samples may be taken during a service of the combustion engine, for example by a service engineer. The samples may be taken by an operator, for example before or after the operator operates the combustion engine.

[0117] Each sample may be analysed using a spectrometer to obtain a measured spectrum for each sample time point. The measured spectra for each sample time may be associated with the engine operating data for the sample time point. The engine operating data for a sample time point may comprise engine operating data at the sample time point or during a time period preceding the sample time point. At time zero (new engine oil in the engine), certain types of engine operating data that requires operation of the engine may not be available, but certain engine operating data such as engine oil type and engine type may still be associated with the measured spectra at time zero. Meta-data may also be associated with the measured spectra at the sample time points.

[0118] An engine oil evolution function may be obtained from the engine operating data and the measured spectra using analytics, algorithms, or other mathematical techniques. For example, a regression model may be used to fit the data, or other data science techniques may be used. The resulting engine oil evolution function may take engine operating data as inputs, and output generated spectra (or generated variances) for a particular time point.

[0119] The generated spectra comprise fitted data, and are not the measured spectra. The techniques used to create the engine oil evolution function may further reduce the number of input parameters needed, from the initial number of parameters (data types or channels) used to create the engine oil evolution function. The engine oil evolution function may be obtained from engine operating data and measured spectra obtained from a plurality of engines. The engine oil evolution function may be verified by further operating engines and obtaining further engine operating data and further measured spectra of engine oil samples. The further engine operating data may be used as inputs to the engine oil evolution function, and the outputs of the engine oil evolution function compared with the further measured spectra of engine oil samples.

[0120] -35 -In a third example, the engine oil evolution function may be based on one or more models that describe engine operation and/or engine oil evolution. The one or more model may comprise any function or model. For example, the one or more model may comprise any of a function, an analytical model, a numerical model, a computer program or model, an algorithm, or other type of function or model. An analytical model may comprise a mathematical model, physics-based model, quantitative model or computational model. In an example, the engine oil evolution function for an engine may be based on two separate models: an engine operation model and a model of predicted engine oil evolution.

[0121] The engine operation model may provide a modelled engine operation. Local conditions in the engine may be monitored using sensors. Data obtained from the sensors on the engine may be used as inputs to the model of engine operation to model the local conditions within the engine. The data inputs may include engine speed and fuelling information, and/or other inputs such as manifold pressures, temperatures, or other parameters. The model of predicted engine oil evolution may provide an expected evolution path of the engine oil over its life cycle for particular engine conditions.

[0122] The modelled engine operation may be used in combination with the expected evolution path to determine at what point in the expected evolution path the engine oil in the engine currently is, providing the generated engine oil evolution that is compared to the reference characterisation. For example, a numerical fit may be carried out between the modelled engine operation and the modelled evolution path to determine the extent to which the engine oil has evolved along the expected evolution path. This may provide a generated spectrum of the engine oil at a particular point in time for particular engine conditions, and/or a generated variance between spectra of the engine oil at particular points in time for particular engine conditions. The generated spectrum and/or generated variance may depend on the length of time that the engine oil has been in the engine and on the engine conditions to which the engine oil was exposed during that time. The generated spectra and/or generated variances may be compared to the reference characterisation.

[0123] Furthermore, the future engine oil evolution may be predicted, since the extent to which the engine oil has evolved so far (i.e. the point at which the engine oil has reached along the predicted evolution path) has been determined.

[0124] -36 -In certain embodiments, the engine operation model may provide information regarding the current state of the engine oil. The current state of the engine oil may be a local engine oil state. The engine operation model may use data from one or more engine sensors as inputs, in order to provide the current state of the engine oil. The engine operation model may be weighted to emphasise engine parameters of particular significance, such as fuel or engine speed. The weighting may be carried out by any method. For example, dimension reduction may be used. In certain embodiments, the outputs of the engine operation model may include a data set that is a time series. The time series data set may be arranged into a histogram, with data in a certain time range put into a certain histogram bin. The size (time range) of the histogram bins may vary. Smaller histogram bins can be used to provide more data where the risk to degradation of engine oil performance is greater. For example, the size of the histogram bins might reduce at later times in the time series, since the evolution of the engine oil may be at a later stage and therefore the risk of a decrease in performance may increase. Alternatively, the size of the histogram bin might depend on the engine operating conditions, such that the histogram bin is smaller for riskier engine operating conditions.

[0125] In certain embodiments, the model of predicted engine oil evolution may be based on a database of engine oil data. For example, the data may be based on spectra of engine oil.

[0126] In a specific example, the spectra may be Fourier-transform infrared (FTIR) spectra. The predicted evolution path may be obtained from the database, for example using data science techniques. In certain embodiments, dimension reduction techniques may be used to obtain the predicted evolution path. The accuracy of the predicted evolution path may be weighted to variables or areas related directly to engine oil performance. For example, in certain implementations the concentration of phosphorus and calcium might be important.

[0127] In other implementations, the weighted variables might differ. The predicted evolution path may identify how the engine oil is expected to evolve, depending on the current or previous parameters of the engine oil. For example, the engine oil may be expected to evolve in different ways from the same set condition, depending on the composition of the engine oil.

[0128] A relationship between the output of the engine operation model and the model of predicted engine oil evolution may be obtained using prior data. In certain embodiments, the relationship may be obtained using a neural network or other modelling techniques. The relationship may be used to determine the extent to which the engine oil has evolved -37 - (i.e. the point reached on the predicted evolution path). The relationship may be used to predict the future evolution of the engine oil.

[0129] Determining the extent to which the engine oil has evolved allows the generated engine oil evolution to be compared to the reference characterisation. This comparison allows interpretation of the evolution of the engine oil, as described above.

[0130] In certain embodiments, the point reached on the predicted evolution path can be used to extract the generated engine oil evolution around that point, to compare variances in spectra to the reference characterisation. The predicted evolution path may comprise spectra, such that determining the point reached on the predicted evolution path allows a particular spectrum to be identified. Variances between that spectrum and previous (or later) spectra in the predicted evolution path can then be compared to the reference characterisation. The predicted evolution path may additionally or alternatively comprise variances between spectra, such that determining the point reached on the predicted evolution path allows variances between spectra to be identified directly and compared to the reference characterisation. In other embodiments, the engine oil evolution model may be run at more than one time point, wherein at each time point the engine oil evolution model determines a point reached on the predicted evolution path. The different points reached on the predicted evolution path can then be analysed to determine one or more differences between the points reached on the predicted evolution path, wherein the one or more differences can be compared to the reference characterisation. For example, predicted evolution path may comprise spectra, such that determining the point reached on the predicted evolution path allows a particular spectrum to be identified. Each point reached on the predicted evolution path corresponds to a spectrum, so one or more differences between those spectra can be identified and compared to the reference characterisation.

[0131] In another example, the engine oil evolution model may be developed using machine learning algorithms.

[0132] The methods described herein require a reference characterisation of evolution of the engine oil. A method of characterising evolution of engine oil to obtain a reference characterisation will now be described.

[0133] -38 -With reference to Figure 4, at step 410 the method comprises using a first spectrometer to analyse a primary sample of a first volume of the engine oil and provide a primary spectrum 411 of the first volume of engine oil. The primary sample may be a sample of bulk engine oil. The method further comprises exposing the first volume of engine oil to a first operational condition (or conditions) at step 420. At step 430 the first spectrometer is then used to analyse a secondary sample of the first volume of engine oil after the volume of engine oil has been exposed to the first operational condition, and provide a secondary spectrum 431 of the first volume of engine oil. The primary spectrum 411 and the secondary spectrum 431 of the first volume of engine oil are compared at step 440, to determine one or more differences between the primary spectrum and the secondary spectrum wherein the one or more differences are indicative of a change in the first volume of engine oil. As discussed above, a change in the first volume of engine oil may include one or more of a physical change, chemical change or contamination. Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Chemical changes might include chemical reactions in the engine oil, and/or chemical species being formed or consumed. Contamination increasing or decreasing, or deposits forming in the engine might result in changes to the engine oil. The method further comprises associating the one or more differences with the first operational condition at step 450.

[0134] The first volume of engine oil may comprise any quantity of engine oil that is exposed to the first operational condition. The first volume may be of constant or varying quantity. The first volume of engine oil may be exposed to the first operational condition for any length of time. For example, the first operational condition may include an oil volume of engine oil residing in or passing through an engine, or a component, subsystem or system of an engine for a particular length of time. The first volume of engine oil may be exposed to the first operational condition in a combustion engine, or the first volume of engine oil may be exposed to a first experimental condition corresponding to the first operational condition or conditions of the combustion engine.

[0135] The first volume of engine oil may be exposed to the first operational condition or conditions of the engine operation between the sample times at which the primary and secondary samples are taken from the first volume of the engine oil. For example, a first volume of engine oil may comprise the engine oil that is in an engine. A primary sample of engine oil taken from an engine at a first time, and analysed to provide a primary spectrum.

[0136] -39 -The engine oil remaining in the engine may then be exposed to operational conditions while the engine is in use. After a period of use of the engine, a secondary sample of engine oil may be taken from an engine at a second time, and analysed to provide a secondary spectrum.

[0137] The primary and secondary samples may be taken from the first volume of engine oil. The primary sample may be taken from the first volume of engine oil before the first volume of engine oil has been exposed to the first operational condition, and the secondary sample may be taken from the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. The primary sample may be analysed prior to exposing the first volume of the engine oil to the first operational condition and the secondary sample may be analysed after exposing the first volume of the engine oil to the first operational condition, or both the primary and secondary samples may be analysed after the exposing the first volume of the engine oil to the first operational condition.

[0138] Otherwise, the primary and secondary samples may remain in the first volume of engine oil, and may be analysed by the first spectrometer while remaining in the first volume of engine oil. The primary sample may be analysed before the first volume of engine oil has been exposed to the first operational condition, and the secondary sample may analysed after the first volume of engine oil has been exposed to the first operational condition.

[0139] The first operational condition of step 420 may be achieved by any method that can be used to implement the first operational condition(s) or to replicate the first operational condition(s), or to achieve a first experimental condition(s) corresponding to the first operational condition(s).

[0140] In an example, the first experimental condition may be achieved using an experimental rig configured to replicate conditions found within one or more subsystems, systems or components of an engine. As used herein, an experimental rig may be any experimental equipment or laboratory equipment, such as bench-top test equipment.

[0141] In another example, the first operational condition may be achieved using an engine. The first volume of engine oil may be distributed within the lubrication system of the engine. The primary sample may comprise a sample of bulk engine oil. The samples of bulk engine oil may be removed from the engine after the engine oil has passed through an oil pump but before the engine oil has passed through the oil filter. At this point the engine oil comprises -40 -bulk oil and is under pressure, so will flow into a collection container. A sample of bulk engine oil may also be removed from the sump or oil pan, which may require use of a vacuum or other extraction device.

[0142] Where the first operational condition is achieved using an engine, the engine may or may not be used to drive equipment.

[0143] In an example where the first operational condition is achieved using an engine or cell that is not used to drive equipment, the first operational condition may be achieved using a test engine or test cell to implement first experimental condition(s) corresponding to the first operational conditions(s), for example using a dynamometer. The test engine or cell does not drive equipment. In this context, driving equipment refers to using the engine to drive any equipment, machine or vehicle in an end use application or in a customer application configuration. Driving equipment does not, in this example, refer to: a dynamometer; any test device used to either measure torque or speed of the engine; or any test device used to provide simulated loading of the engine.

[0144] In an example where the first operational condition is achieved using an engine that is used to drive equipment, the first operational condition may be achieved by using an engine to drive equipment such that the first operational condition(s) is/are implemented.

[0145] Alternatively, the first operational condition may be identified by using an engine to drive equipment and using data relating to engine use to identify the first operational condition(s). The primary sample comprises a bulk engine oil sample. In this context, equipment may refer to any equipment, machine or vehicle driven by the engine. The engine may be used for propulsion or in a stationary configuration. Using an engine to drive equipment may mean using an engine in an end use or in a customer application configuration. The end use may be any application or function for which an engine is designed or used. A customer may be a customer of the engine manufacturer. A customer application configuration may be used at a site or location of said customer. An application configuration may refer to any equipment, machine or vehicle driven by the engine. The end use of the engine or a customer application configuration of the engine may include any of an engine in a vehicle, an engine in a work machine or work equipment, a marine propulsion engine, a locomotive engine, an engine used in a stationary or mobile or towable configuration for electrical power generation, an engine used in a stationary or mobile or towable configuration to support petroleum industry functions, an engine used on -41 -a vehicle or marine vessel for auxiliary power, an industrial machine, or other engine applications. In other words, equipment driven by the engine may include any of a vehicle, a work machine or work equipment, a marine vessel, a locomotive vehicle, an electrical power generator, an auxiliary power provider on a vehicle or marine vessel, an industrial machine, or any other engine application.

[0146] These examples of methods for obtaining the operational condition will be discussed in more detail below.

[0147] At step 440, the primary spectrum 411 and the secondary spectrum 431 of the first volume of engine oil are compared. The one or more differences determined between the primary spectrum 411 and the secondary spectrum 431 of the first volume of engine oil may comprise one or more of peak height, peak location and peak shape differences. The one or more differences may comprise differences to one peak or to multiple peaks.

[0148] In certain embodiments, the first volume of engine oil may be homogenous such that the composition, chemical properties and physical properties of the primary and secondary samples are substantially the same as those of the first volume of engine oil. In other embodiments, the first volume of engine oil may be inhomogeneous. The primary and secondary samples may be indicative of properties of the first volume of engine oil.

[0149] As an example of a homogeneous first volume, the first volume of engine oil may be a bulk oil volume, and so the primary and secondary spectra are indicative of bulk oil evolution. As described above, the bulk engine oil is made up of engine oil that has passed through local components, sub-systems or systems of the engine. The bulk evolution of the bulk engine oil is, therefore, a result of a combination of a plurality of local oil evolutions. Analysis of a volume of bulk oil may be used to identify local evolution of the engine oil in a particular component, sub-system or system. The features of the primary and secondary spectra may be used to isolate local oil evolution and local conditions to which the oil has been exposed. The engine may change over time, in which case the conditions to which the engine oil is exposed will change and the local oil evolution for a particular subsystem may change. The local engine oil evolution may be used to identify changes to the conditions to which the engine oil has been exposed, and therefore to identify changes to the engine. Changes to the engine may include changes to a component, sub-system or system. Changes to a component, sub-system or system may include physical changes, such as wear, and/or changes to performance of a component, sub-system, system or engine.

[0150] -42 -The difference(s) between the primary spectrum and the secondary spectrum are indicative of the conditions to which the engine oil has been exposed. The difference(s) between the spectra are associated with the first operational condition (step 450). This may entail associating the secondary spectrum with the first operational condition, and/or associating variances in spectra features (based on difference(s) between the primary spectrum and the secondary spectrum) with the first operational condition. Associating a spectrum or variance with an operational condition may comprise recording the operational condition against the spectrum or variance, or labelling the spectrum and/or variance such that it is linked to the operational condition, or other method of associating a spectrum or variance with an operational condition.

[0151] Other information may be associated with the first operational condition (or with the secondary spectrum or variances in spectra features). Examples include but are not limited to: a mass flow rate of the engine oil; a length of time for which the engine oil has been exposed to the first operational conditions; a length of time for which the engine oil has been exposed to any engine conditions; data relating to engine operation (such as a percentage duty cycle, or other data); information relating to engine hardware; engine age; or other information.

[0152] The first operational condition(s) may correspond to conditions that would be experienced by the engine oil within components or systems of the engine or in proximity to components or systems of the engine, while those engine components or systems are undergoing certain engine conditions. In an event that the first volume of engine oil is exposed to a first experimental condition corresponding to the first operational condition(s), the first experimental condition may aim to isolate operational condition(s) for particular components or systems of the engine, or may aim to correspond to operational condition(s) for multiple components or systems of the engine. Similarly, the first experimental condition may aim to isolate a particular engine condition or may aim to correspond to more complex engine conditions. For example, the first operational condition(s) may correspond to engine oil within a particular component or system of the engine, or to engine oil in proximity to or within multiple components or systems of the engine, or to engine oil within the engine as a whole. The first operational condition(s) may correspond to the engine undergoing one particular engine condition, such as a steady state engine operation. The first operational condition(s) may correspond to the engine undergoing multiple engine conditions. The first -43 -operational condition may isolate one or more of the multiple engine conditions. For any given engine condition, the engine oil may experience multiple operational conditions.

[0153] The first operational condition(s) may correspond to conditions experienced under "normal" operation of a combustion engine. Alternatively, the first operational condition(s) may correspond to conditions experienced under "abnormal" combustion operation of an engine, such as in an event that an engine component, system or sub-system is experiencing a failure mode or has suffered a failure mode. A failure mode may be an error, a defect, or any operational condition in which an engine component, subsystem or system is not performing as expected. A particular failure mode may be indicated by a unique signature of changes of the engine oil, that can be identified in the secondary spectrum. However, a change of the engine oil does not necessarily represent a change in performance of a component, subsystem or system of the engine.

[0154] Examples of parameters that are influenced by engine operational conditions and that affect the evolution of engine oil may include temperature, pressure, engine oil volume, engine oil flow rate, final combustion products, interim combustion products, local engine oil chemical state, air or other gas concentrations, shear, friction, or other property of the engine. Combustion products may include nitrogen oxides, blowby gases, intermediary gases, soot, or other products. For example, the contact area between a gas and the engine oil may affect the engine oil, for example if the gas is in bubbles or a sheet. Changes in these parameters may be indicative of changes to certain engine components or to the operation of certain engine components. These parameters may influence the bulk engine oil. The parameters may influence the local engine oil in a component, sub-system or system during the local residence time in that component, sub-system or system.

[0155] Different components, sub-systems and systems can all be differently exposed to different combinations of these parameters.

[0156] As described above, exposing the first volume of engine oil to a first operational condition or to a first experimental condition corresponding to the first operational condition may comprise using experimental equipment, a test engine or test cell, or use of an engine to drive equipment. The first volume of engine oil may comprise all of or a portion of the engine oil within the experimental equipment, test engine, or engine. The primary sample may comprise a portion of the first volume of engine oil that is extracted from the first volume of engine oil prior to exposing the first volume of engine oil to the first operational -44 -condition. The secondary sample may comprise a portion of the first volume of engine oil that is extracted from the first volume of engine oil after exposing the first volume of engine oil to the first operational condition. In other words, the analysis of the primary and secondary samples may occur separately to the first volume of engine oil, such that the analysis occurs separately to the equipment or engine used to expose the first volume to the first operational condition. Or, the primary and secondary samples may remain as part of the first volume of engine oil during analysis. The analysis of the primary and secondary samples may occur in situ in the equipment or engine used to expose the volume to the first operational condition.

[0157] The analysis of the primary sample may occur prior to exposing the first volume to the first operational condition. Alternatively, the analysis of the primary sample may occur after exposing the first volume to the first operational condition. The analysis of the primary and secondary samples may occur at any time and in any order.

[0158] In an event that the analysis of the primary and secondary samples occurs separately to the equipment or engine used to expose the first volume to the first operational condition, the analysis may be performed on-site. Here, "on-site" refers to a location of the equipment or engine used to expose the first volume to the first operational condition, such as a laboratory, test site, work site or other location. Alternatively, the analysis may be performed off-site. The primary and secondary samples may be extracted by any means from the equipment or engine used to expose the volume to the first operational condition.

[0159] In an event that the analysis of the primary and secondary samples occurs in situ in the equipment or engine used to expose the volume to the first operational condition, the primary and secondary samples may remain part of the first volume and the analysis of the primary and secondary samples may be performed while the primary and secondary samples remain in the equipment or engine used to expose the volume to the first operational condition. An analysis device may be permanently fitted to the equipment or engine used to expose the volume to the first operational condition, or may be a removable device that may be removably fitted to the equipment or engine used to expose the volume to the first operational condition. In an example, an analysis device may be fitted to an engine oil circuit at a point at which the engine oil is pressurised, such that the engine oil flows through the analysis device. The engine oil may return to the engine oil circuit from the analysis device. The engine oil flowing through the analysis device may be analysed, -45 -for example via spectroscopy. The engine oil may be diluted in the analysis device, for example using a solvent. In a particular, non-limiting, example, the analysis device may comprise a microfluidic channel into which the engine oil flows, wherein the engine oil is analysed while in the microfluidic channel.

[0160] Exposing the first volume of engine oil to a first experimental condition corresponding to the first operational condition(s) may comprise using experimental equipment to replicate conditions found within one or more specific component, sub-system or system of the combustion engine. The experimental equipment may therefore be used to age or react the first volume of the engine oil as though it was being used only in that specific component(s), sub-system(s) or system(s) of the combustion engine under the first operational condition. Effectively, the first experimental condition may isolate one or more component, sub-system or system of the combustion engine. Alternatively, the experimental equipment may be used to replicate conditions found within multiple components, sub-systems or systems, or to replicate conditions found within the engine as a whole.

[0161] The experimental equipment may be configured to control one or more of the temperature, pressure, engine oil volume, engine oil flow rate, intermediate and final products of combustion, engine oil evolution, air or other gas concentrations, shear, friction, or other property of the engine. Gas flows may be controlled, and reaction rates may be measured. In certain embodiments, the experimental equipment may comprise a pressure burette, which may be used to measure reaction rates. The experimental equipment may comprise one or more of a stirrer, a condenser and a high pressure, high temperature beaker. The high pressure, high temperature beaker may be used to apply temperatures and pressures at and above room temperature and atmospheric pressure. Different gases may flow through the system, and the flows of those gases can be controlled. The experimental equipment may comprise components configured to shear the engine oil thereby introducing frictional forces into the engine oil, such as a high frequency reciprocating rig or a ball on disc tribometer. In certain embodiments, the experimental equipment may comprise more than one experimental rig, wherein the engine oil is put into the more than one experimental rig sequentially. For example, the engine oil may be first exposed to experimental conditions of high temperature and limited shear using a first experimental setup, and second exposed to lower temperature and high shear using second -46 -experimental setup. This approach may be used to replicate operational conditions for engine oil having proximity to more than one component or system of the engine.

[0162] In certain embodiments, use of an experimental rig to expose the engine oil to experimental conditions may comprise holding the oil under a number of conditions. The experimental rig may be used to individually change a plurality variables. For example, a volume of engine oil may be held under a plurality of values of a first variable. A sample of the volume of engine oil may be analysed after exposure to each of the plurality of values of the first variable. The sample is a bulk oil sample, indicative of the volume of engine oil within the experimental rig. The volume of engine oil may be exposed to the plurality of values of first variable sequentially, to analyse the cumulative effect of changing the first variable. Otherwise, a new volume of engine oil may be exposed to each of the plurality of values of first variable This may be repeated for one or more other variables. Analysis of a sample of engine oil from the volume of engine oil may allow trends to be identified and linked to changes in a particular variable. The particular variable that controls a certain reaction in or change to the volume of engine oil may be isolated. Using knowledge of parameters and variables within an engine, the analysis may be interpreted to identify engine conditions that result in certain reactions of the engine oil or change to the engine oil.

[0163] For example, certain systems or locations within the engine may be highly sensitive to combustion conditions. To replicate this, the engine oil may be exposed to high temperatures and multiple combustion products. In other systems or locations within the engine, the engine oil may be exposed to lower temperatures and higher shear rates.

[0164] The engine oil may undergo reactions while under operational conditions. These reactions may be driven by different energies, depending on the reaction. For some reactions, the activation energy may be provided by temperature and/or pressure. For other reactions, shear may reduce the activation energy required.

[0165] As described above, the first operational condition may be achieved using a test engine or using an engine to drive equipment. The first operational condition may include the time interval over which the engine oil is within the engine may be set. For example, samples may be taken of the engine oil at specific time intervals. The first operational condition(s) may correspond to engine oil within the engine as a whole, wherein the engine undergoes one or more particular engine conditions. The first operational condition may depend on the -47 -time period over which the engine oil is in the engine under the particular engine conditions. Data from the equipment driven by the engine or from the test engine may provide information as to the operational conditions to which the engine oil was exposed over the time period. The engine may be run in a particular way to aim to achieve particular operational condition. Or, the engine may be run without aiming for a particular operational condition, wherein data from the engine or equipment provides information as to the actual operational conditions.

[0166] As described with reference to the spectra of the samples above, the primary and secondary spectra may each comprise a plurality of features. There may be one difference between the primary and the secondary spectra that may comprise a change to one feature. For example, one peak might change in height. There may be multiple differences between the primary and secondary spectra, comprising changes to multiple features. The combination of differences may indicate a particular change to the engine oil and its performance. For example, a change to a first feature may indicate one of two changes to the engine oil, depending on a change to a second feature. In an example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak decreases in height, but the first peak increasing in height may indicate a second change to the oil in an event that the second peak increases in height. In another example, there may be one difference between the primary and secondary spectrum, comprising a change to a first feature, wherein the change to the engine oil indicated by the difference depends on another unchanged feature of the spectrum. For example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak is lower than the first peak, but may indicate a second change to the oil in an event that the second peak is not lower than the first peak.

[0167] The features of the primary and secondary spectra may comprise peaks and troughs. The difference(s) between the primary spectrum and the secondary spectrum may include one or more of the following: one or more peaks shifting with respect to the y axis; one or more peaks shifting with respect to the x axis; one or more peaks scaling with respect to the y axis; one or more peaks scaling with respect to the x axis; one or more troughs shifting with respect to the y axis; one or more troughs shifting with respect to the x axis; one or more troughs scaling with respect to the y axis; one or more troughs scaling with respect to the x axis. The differences may comprise a combination of more than one of these differences.

[0168] The difference(s) between the primary spectrum and the secondary spectrum may include -48 -one or more of a change to peak height ratios; a change to the number of peaks in a specific wavenumber range; a change to integrals of the spectrum within a specific wavenumber range; and roughness over a specific wavenumber range. A change in peak height may comprise a change in the absolute value of absorbency of the peak (i.e. measured from the origin), or a change in height as measured from the base of the peak.

[0169] Several differences may be associated with an operational condition.

[0170] The output of the methods described herein may comprise a characterisation of engine oil, comprising a plurality of associations of differences between spectra with a plurality of operational conditions. A given operational condition may be associated with one difference between spectra, or with a particular combination of differences between spectra. In use, it may be possible to identify one or more changes in an engine oil volume (by analysing a sample of the volume of engine oil using spectrometry), and use the characterisation to look up the associated operational condition.

[0171] The first spectrometer may comprise a Fourier-transform infrared (FTIR) spectrometer, allowing analysis of the bonds of the molecules within the oil. Analysis of individual peaks allows analysis of the same bond within different molecules. For example, a double bond between carbon and oxygen may be found in two different molecules, and the changes of that bond in the two molecules may have different implications for the evolution of the oil.

[0172] FTIR spectrometry is a well-known technique. In brief, an FTIR spectrum indicates the absorption of light at various wavelengths. Light at different frequencies is incident on the sample, and absorption or transmittance is measured for each frequency. This is achieved by using a Michelson interferometer to block or transmit certain wavelengths of light from a light source by altering a mirror position, such that the frequency of light incident on the sample varies as a function of time. The absorbance or transmittance may be measured against each frequency as a function of time. A Fourier transform may be used to convert the displacement of the mirror into the wavenumber of the light, providing a spectrum of absorption against wavenumber.

[0173] As described, an interpretation of a change to a particular peak or trough of a spectrum may change depending on other features of the spectrum. With reference to Figure 5, an example of a section of FTIR spectra is illustrated. Absorbance is plotted against wavenumber, and each line represents a sample taken at a different time. This is an example only. Alternatively, transmittance (wherein absorbance = 2 - -49 -log(transmittance)) may be plotted against wavenumber, or the absorbance or transmittance may be plotted against wavelength or frequency. In this example, each sample was taken approximately 50 hours apart, but this is merely illustrative. The spectrum of Figure 5 is cropped with respect to the y-axis, and shows only a selected range of wavenumbers. The FIR spectra used may have a different range of wavenumbers, and may show an absorbance axis from 0 to 1. To provide examples of changes that may occur to the spectrum, four peaks are indicated (510, 520, 530, 540). In an event that peak 510 grows (i.e. absorbance increases) over time, it may be an indication that the engine oil is degrading. However, in an event that peak 520 has low absorbance before peak 510 grows, the growth of peak 510 may not indicate that the engine oil is degrading; the performance of the engine oil may improve or remain the same. The growth of peak 510 may also indicate other changes to an engine component. Changes to peak 530 may indicate changes to a characteristic that is improving, but the interpretation of changes to peak 530 may depend on the height of peak 520. Peak 540 may relate to another characteristic of the engine oil. If peak 540 decreases, this may indicate either an improvement to or a worsening of that characteristic of the engine oil, depending on other peaks of the spectrum. Examples of changes to engine characteristics that may be inferred by scrutiny of the spectra include changes to anti-wear performance of an engine component, growth of deposits on an engine component, changes to the turbo bearing, rheological changes, soot induced wear and other changes.

[0174] With reference to Figures 6 and 7, further examples of spectra indicative of engine oil evolution are illustrated. Figures 6 and 7 show schematics that are representative of sections of FTIR spectra, but that do not show real data. Figure 6 illustrates a section of an FTIR spectra for engine oil undergoing a "normal" evolution pathway. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. A peak at a particular wavenumber is indicated by arrow 610 and a trough at a particular wavenumber is indicated by arrow 620. The peak 610 increases in height over time. The trough 620 increases in depth over time. Figure 7 shows a section of an FTIR spectra for engine oil undergoing an "abnormal" evolution pathway, for the same wavenumber range as shown in Figure 6. The spectra shown in Figure 7 are for engine oil undergoing a particular failure mode. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. Arrow 710 indicates a peak at the same wavenumber as peak 610 of Figure 6. The peak 710 has a greater increase in height over time than peak 610. Arrow 720 indicates a peak at the same wavenumber as trough 620 of Figure 6.

[0175] -50 -Rather than the peak 610 of normal evolution, the abnormal evolution shows a small peak 710. The combination of these two changes may be a signature of a particular failure mode. One of those changes without the other may be a signature of a different failure mode.

[0176] With reference to Figures 8 and 9, further examples of spectra indicative of engine oil evolution are illustrated. Figures 8 and 9 show schematics that are representative of sections of FTIR spectra, but that do not show real data. Figure 8 illustrates a section of an FTIR spectra for engine oil undergoing a "normal" evolution pathway. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. A peak at a particular wavenumber is indicated by arrow 810 and a trough at a particular wavenumber is indicated by arrow 820. The peak 810 increases in height over time. The trough 820 increases in depth over time. Figure 9 shows a section of an FTIR spectra for engine oil undergoing an "abnormal" evolution pathway, for the same wavenumber range as shown in Figure 8. The spectra shown in Figure 9 are for engine oil undergoing a particular failure mode. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. Arrow 910 indicates a peak at the same wavenumber as peak 810 of Figure 8, but that decreases in height over time and eventually becomes a trough. Arrow 920 indicates a feature at the same wavenumber as trough 820 of Figure 8.

[0177] The trough 920 decreases in depth with time, and becomes a peak increasing in height over time. The combination of these two changes may be a signature of a particular failure mode. One of those changes without the other may be a signature of a different failure mode.

[0178] Figures 6 to 9 each show spectra for engine oil at intervals of 100 hours. However, other sampling intervals may be used. The sampling interval may be shorter or longer than 100 hours, and may be any length. The sampling intervals may be consistent, or may vary in length. The sampling interval may be defined by a time period or by reference to an engine oil change interval. For example, samples of engine oil may be obtained at certain points in an engine oil change interval. In specific, non-limiting examples, an engine oil change interval may be used as a sampling interval. In certain examples, the engine oil change interval may be between 250 and 1,000 hours, or an engine oil change interval may be less than or equal to 4,000 hours, or an engine oil change interval may be greater than or equal to 10 hours. Other engine oil change intervals may be used. Samples of engine oil may be taken at certain points in an engine oil change interval other than (instead of or in addition -51 -to) at the end of an engine oil change interval. In another specific example, one sample of engine oil may be obtained in the middle of an engine oil change interval, and one sample of engine oil may be obtained at the end of an engine oil change interval (when the engine oil is changed). Or, the samples of engine oil may be obtained more frequently during the engine oil change interval. The sampling interval may depend on one or more of: the type of engine; how frequently an operator is able to or wishes to pause operation of the engine; whether the operator suspects an issue; and so on. For example, a shorter sampling interval may be required for a diesel engine than for a spark ignited industrial natural gas engine. In another example, if an operator suspects an issue with the engine or engine oil, the sampling interval may be in the region of 10 hours. Other sampling intervals may be used.

[0179] The spectra illustrated in Figures 6 to 9 show each feature (peak or trough, in this case) having a linear change with respect to time. In other words, the change in height of a peak or change in depth of a trough is shown as being linear with time. In fact, the changes may be non-linear with respect to time, for normal or abnormal evolutions. A charge may be linear with respect to time for a certain length of time, and then may develop to be nonlinear with respect to time. For example, a change may be linear with respect to time in a normal evolution, but if a failure mode develops the change may become non-linear with respect to time. In other examples, a change may be non-linear with respect to time in a normal evolution. Furthermore, the spectra illustrated in Figures 6 to 9 show peaks and troughs changing in height or depth over time. As discussed above, other changes to features of the spectra may occur.

[0180] The changes to the features of the spectra can indicate both how the engine oil evolves, and how the engine components are performing. For example, a rate of consumption (i.e. rate of decrease in peak height, where peak height is measured from the base to the top of the peak) of a particular peak may be affected by NO gas exposure. Inspection of the rate of consumption of that peak may indicate a change to the combustion within the engine. In another example, the rate of change of the whole spectrum may be affected by temperature. Inspection of this effect may indicate how the turbo is performing. In another example, a change in a ratio of certain peaks may indicate a local high temperature and may be indicative of piston top ring conditions. In another example, the quantity of soot may affect the skew of the trace. The changes to the features of the spectra can be -52 -indicative of performance of any component, subsystem or system that interacts with the engine oil.

[0181] The method may further comprise analysing further samples of further volumes of engine oil exposed to different operational conditions, with a view to associating a plurality of different operational conditions with differences between spectra of the engine oil. The different operational conditions may correspond to conditions experienced in an engine operating under "normal" and/or "abnormal" conditions.

[0182] The method may further comprise analysing further samples of further volumes of the engine oil. The volumes of engine oil may be exposed to further operational conditions. In an event that an experimental rig is used, further experimental conditions may correspond to different operational conditions of the same part of the combustion engine, and/or operational conditions of a different part of the engine. In an event that a test engine is used or an engine is used to drive equipment, the further operational conditions may correspond to a different time interval with the same engine conditions, and/or different engine conditions.

[0183] The method may further comprise using the first spectrometer to analyse a second volume of the engine oil. To do so, a primary sample of the second volume of engine oil may be analysed, providing the primary spectrum of the second volume of engine oil. The second volume of engine oil may be exposed to a second operational condition of a second part of the combustion engine. The first spectrometer may then be used to analyse a secondary sample of the second volume of engine oil and provide a secondary spectrum of the second volume of engine oil. The method may further comprise comparing the primary spectrum and the secondary spectrum of the second volume of engine oil to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the second operational condition.

[0184] The first volume and the second volume may be the same engine oil, wherein the engine oil is sequentially exposed to the first operational condition and then to the second operational condition. The first operational condition may differ simply in time periods, such that the first volume is exposed to a particular condition or conditions for a first time period, and the second volume is exposed to the same condition or conditions for a second time -53 -period. The first and second time periods may be adjacent time periods, or may have a time period between them, or may overlap. In certain embodiments, the second volume may be exposed to the first operational condition prior to being exposed to the second operational condition, such that the primary sample of the second volume has been exposed to the first operational condition but not to the second operational condition. For example, the primary sample of the first volume may be taken at time to. The secondary sample of the first volume may be taken at time ti, later than time to. The primary sample of the second volume may be taken at time t2, which may be the earlier than, the same as or later than t1 (the primary sample of the second volume may the same as or different from the secondary sample of the first volume). The secondary sample of the second volume may be taken at time t3, which is later than time t2. Between to and t3, the conditions to which the engine oil is exposed may remain the same or may be varied. For example, a test engine may be run with a certain quantity of engine oil. The first volume of engine oil may refer to that quantity of engine oil exposed to a first operational condition within the test engine over a first time period. The second volume of engine oil may refer to that same quantity of engine oil exposed to a second operational condition within the same test engine over a second time period. The second time period may immediately follow the first time period, may occur some time after the first time period ends, or may overlap with the first time period.

[0185] The first volume of engine oil and second volume of engine oil may be exposed to the first and second operational conditions, respectively, using the same equipment or engine. However, the engine oil may be changed between using the equipment or engine to expose engine oil to the first and second operational conditions, such that the first and second volumes of engine oil are different engine oil. The second volume of engine oil has not, therefore, been exposed to the first operational condition prior to being exposed to the second operational condition.

[0186] The first volume of engine oil and the second volume of engine oil may be exposed to the first and second operational conditions, respectively, using different equipment or engines.

[0187] The method may further comprise exposing further volumes of engine oil to further operational conditions, each of further components, sub-systems or systems of the engine.

[0188] For each volume of engine oil exposed to an operational condition, a primary spectrum may be obtained prior to exposing the volume of engine oil to the operational condition and -54 -a secondary spectrum may be obtained after exposing the volume of engine oil to the operational condition. The further volumes be the same as or different to each other and/or to the first and second volumes. The further operational conditions may be applied using the same as or different equipment or engines to each other and/or the first and second operational conditions.

[0189] In addition to or instead of the second volume of engine oil, the method may comprise using the first spectrometer to analyse a third volume of the engine oil and provide a primary spectrum of the third volume of engine oil. To do so, a primary sample of the third volume of engine oil may be analysed, providing the primary spectrum of the third volume of engine oil. The method may comprise exposing the third volume of engine oil to a third operational condition, of the same component, sub-system or system of the combustion engine as the first operational condition. The first spectrometer may then be used to analyse a secondary sample of the third volume of engine oil and provide a secondary spectrum of the third volume of engine oil. The primary spectrum and the secondary spectrum of the third volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the third operational condition.

[0190] In addition to or instead of the second and/or third volumes of engine oil, the method may comprise using the first spectrometer to analyse a primary sample of a fourth volume of the engine oil and provide a primary spectrum of the fourth volume of engine oil. The method may comprise exposing the fourth volume of engine oil to a fourth operational condition.

[0191] The fourth operational condition may correspond to different engine condition(s). The first spectrometer may then be used to analyse a secondary sample of the fourth volume of engine oil and provide a secondary spectrum of the fourth volume of engine oil. The primary spectrum and the secondary spectrum of the fourth volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the fourth operational condition.

[0192] In addition to or instead of the second and/or third and/or fourth volumes of engine oil, the method may comprise using the first spectrometer to analyse a primary sample of a fifth -55 -volume of the engine oil and provide a primary spectrum of the fifth volume of engine oil. The method may comprise exposing the fifth volume of engine oil to a fifth operational condition. The fifth operational condition may correspond to the same engine condition(s) as another operational condition, over a different time interval. The first spectrometer may then be used to analyse a secondary sample of the fifth volume of engine oil and provide a secondary spectrum of the fifth volume of engine oil. The primary spectrum and the secondary spectrum of the fifth volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the fifth operational condition.

[0193] The method may comprise exposing any number of volumes of engine oil of engine oil to any number of operational conditions, and analysing the volumes of engine oil before and after exposing the volumes of engine oil to each operational condition. Any of the volumes of engine oil may be analysed by one or more of the methods described herein.

[0194] In certain embodiments, the same volume of engine oil may be exposed to multiple operational conditions in succession, with the volume of engine oil being analysed between each operational condition. The operational conditions may have a cumulative effect on the engine oil. For example, the second volume of engine oil may comprise the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. In an event that a given volume of engine oil is exposed to successive operational conditions, the secondary sample taken after exposing the volume of engine oil to a particular operational condition may also be used as the primary sample of the volume of engine oil taken prior to exposing the volume of engine oil to the next operational condition. Or, in an event that a given volume of engine oil is exposed to successive operational conditions, the primary sample of the volume of engine oil taken prior to exposing the volume of engine oil to a particular operational condition may be different from the secondary sample taken after exposing the volume of engine oil to the previous operational condition.

[0195] In other embodiments, different volumes of engine oil may each be exposed to one or more different operational conditions. For example, a one volume of engine oil may be exposed to a first operational condition followed by a second operational condition, while another volume of engine oil may be exposed to only a third operational condition.

[0196] -56 -In certain embodiments, one or more volumes of engine oil may each be exposed to multiple operational conditions in succession, and one or more volumes of engine oil may be exposed to different operational conditions.

[0197] Any number of volumes of engine oil may be exposed to any number of operational conditions, either separately or in succession.

[0198] The method may further comprise using a second spectrometer to analyse a different property of the engine oil to the first spectrometer. For example, the first spectrometer may analyse bonds, and the second spectrometer may carry out elemental analysis.

[0199] The method may further comprise, before exposing the first volume of engine oil to the first operational condition, using a second spectrometer to analyse a tertiary sample of the first volume of the engine oil and provide a tertiary spectrum of the first volume of engine oil.

[0200] After exposing the first volume of engine oil to the first operational condition, the method may comprise using the second spectrometer to analyse a quaternary sample of the first volume of engine oil and provide a quaternary spectrum of the first volume of engine oil. The tertiary spectrum and the quaternary spectrum of the first volume of engine oil may then be compared to determine one or more differences between the tertiary spectrum and the quaternary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the first operational condition. The tertiary sample may be the same as or separate from the primary sample of the first volume of engine oil. The quaternary sample may be the same as or separate from the secondary sample of the first volume of engine oil.

[0201] In any of the examples provided above, a second spectrometer may be used to analyse a tertiary sample of the volume of the engine oil and provide a tertiary spectrum of the volume of engine oil. After exposing the volume of engine oil to the particular operational condition, the method may comprise using the second spectrometer to analyse a quaternary sample of the volume of engine oil and provide a quaternary spectrum of the volume of engine oil. The tertiary spectrum and the quaternary spectrum of the volume of engine oil may then be compared to determine one or more differences between the tertiary spectrum and the quaternary spectrum, wherein the one or more differences are -57 -indicative of a change in the engine oil. The one or more differences may be associated with the operational condition.

[0202] Any of the methods described above with relation to the first volume and the first operational condition may be applied to other volumes and other operational conditions.

[0203] In certain embodiments, the second spectrometer may comprise an inductively coupled plasma atomic emission spectrometer (ICP-AES, also referred to as an inductively coupled plasma optical emission spectrometer ICP-OES).

[0204] The spectra obtained in any of the methods above are compared to identify changes between them. This may be carried out by any suitable method. More than one method of analysis may be combined. Analysis of the spectra aims to identify the degree of variation between the spectra associated with different oil volumes of engine oil. Changes to the features of the spectra can indicate both how the engine oil evolves, and how the engine components are performing. The changes to the features of the spectra can be indicative of performance of any component, subsystem or system that interacts with the engine oil. In certain embodiments, trends may be identified in the changes to the spectra.

[0205] The spectra may be inspected visually to identify changes. The spectra may be visually inspected in graphical form, or the raw data may be inspected. The raw data may be compared using simple mathematical comparisons, such as finding numerical differences between the raw data. Given the complex nature of the spectra, other techniques may be used to identify the changes to the spectra. These techniques may include binary additive operations, simplifying the spectra through a combination of experiments and mathematical functions, or data science tools such as dimension reduction techniques. Data science is an interdisciplinary subject, that may use one or more of statistics, algorithms, scientific methods, numerical methods of analysis, domain knowledge, and other methods to analyse complex data sets. The application of data science techniques to chemical information may also be referred to as chemometrics. Data science may be used to identify trends in the changes in features of the spectra. Trends of changes to a particular feature of the spectra, either in isolation or in combination with trends of changes of other features of the spectra, allow a better description of the evolution of the engine oil and the associated operational conditions. The degree of variation between the spectra can be analysed, with the variation then being characterised as being due to a condition or -58 -conditions of an individual engine component, subsystem or system. This information may be recorded, and may be used as a reference characterisation for engine oil, for example when analysing engine oil of an engine in use.

[0206] Techniques for identifying changes to the spectra may be applied to a matrix or matrices of spectrum data. The matrix or matrices may comprise data for two or more spectra, such that analysis of the data can be used to identify changes in the spectra. A simple mathematical assessment or assessments may be applied to a matrix of the spectra data, or more advanced techniques may be used to identify changes between spectra.

[0207] Data science may be used to analyse spectra of volumes of engine oil in order to establish how the engine oil evolved. There are many data science techniques that can be used to identify variations in the spectra. As a non-limiting example, a matrix containing the data can be simplified via dimension reduction. It may then be identified where in the spectrum a change is seen, and that variation may be linked to a property of the engine, for example.

[0208] Once these variations have been identified, they may be interlinked to identify changes in features that are affected by other features or by changes to other features. Knowledge of the engine and its components may also be used to assist in interpreting the changes, such as to link particular engine conditions to particular reactions of the engine oil or changes to the engine oil.

[0209] In a specific example, a set of spectroscopy data can be presented as a matrix (rows and columns). The set of data may comprise data for more than one spectrum. Either the rows or the columns may be described as features or dimensions. The rows (or columns) may be individual 'dimensions" such that a data set with numerous rows (or columns) is known as multidimensional data. Examples of a data science technique that can be applied to multidimensional spectra data sets are dimensional reduction techniques, used to reduce the number of rows (or columns) of the data set and thereby increase interpretability of data while maintaining the trends and patterns within the data. In other words, the data set is simplified without losing the patterns in the data that need to be extracted and analysed, and particular variations in the data may be isolated. The dimension reduction techniques may be applied directly to the spectral data, or to processed spectral data.

[0210] In certain examples, each set of data in a matrix of spectroscopy data may be compared, and a new matrix may be determined containing numerical indications of differences -59 -between each data set and/or variations within each data set. Dimension reduction techniques or other techniques may be used to isolate specific variations. In a specific example, a matrix of spectrometer data may comprise a plurality of sets of data. Each set of data may provide spectrometry data for an engine oil sample, in the form of signal magnitudes and wavenumbers. A new matrix containing numerical indications of differences between and/or within each data set may be obtained, and analysed to determine the degree of variation between and/or within the data sets. Specific variations may be isolated, for example using dimension reduction techniques or otherwise.

[0211] An example of a common data science dimensional reduction technique is Principal Component Analysis (PCA). PCA linearly transforms a data set into a new coordinate system. In the new coordinate system, variations in the data can be described with fewer dimensions than in the original data. In use, PCA is carried out using a matrix of spectrometer data comprising a plurality of sets of data, wherein each set of data provides spectrometry data for an engine oil sample in the form of signal magnitudes and wavenumbers. For each wavenumber, the mean of signal magnitudes across the sets of data may be obtained. The data sets may be normalised with respect to that mean value. A covariance matrix may then be obtained, and analysed to determine the degree of variation between and/or within the data sets.

[0212] PCA can be used to characterise primary variation between the spectra. For example, changes in the shapes of the curves may be characterised, including slopes or gradients, and magnitude and position of peaks and valleys. Alternatively or additionally, the variations between the spectra may be characterised by other techniques. Irrespective of the technique used to obtain the characterisation, the characterisation may provide an easily recognisable visual representation of differences between spectra, which may be more easily interpreted than a visual assessment of the raw data. Patterns between samples of oils can be established by comparing principle components of the characterisation or differences between the spectra data sets. This may yield an understanding of how the engine oil evolved within the spectra data sets. The primary variation between the engine oil samples due to the evolution can be isolated using this technique. Any variation can then be characterised as being due to a condition or conditions of an individual engine component, subsystem or system. This can be used to determine changes in engine component, subsystem or system health, and to predict subsequent engine oil performance in that engine component, subsystem or system.

[0213] -60 -Associating the one or more differences between spectra with an operational condition may be carried out by any suitable method. More than one method may be combined.

[0214] For normal engine oil evolution, one or more volumes of engine oil may be exposed to one or more operational conditions of the combustion engine, and the one or more volumes of engine oil may be analysed as described above. This may be repeated for a plurality of operational conditions to build up a reference characterisation of normal engine oil evolution comprising a plurality of reference spectra or reference variations between spectra that are indicative of normal engine oil evolution. More than one volume of engine oil may be exposed to the same operational condition to validate the normal engine oil evolution. The reference characterisation for a normal engine oil evolution may be associated with certain engine types, engine meta data, or other information. For example, a normal engine oil evolution may differ between different engines or between different operation modes of a particular engine. As an example, consistently operating an engine at a low engine load may result in a different normal engine oil evolution than consistently operating the same engine at a high engine load.

[0215] For abnormal engine oil evolution, one or more volumes of engine oil may be exposed to one or more operational conditions of the combustion engine, wherein the one or more operational conditions correspond to a failure mode or other condition resulting in abnormal engine oil evolution. The one or more volumes of engine oil may be analysed as described above. This may be repeated for a plurality of operational conditions to build up a reference characterisation of abnormal engine oil evolution comprising a plurality of reference spectra or reference variations between spectra that are indicative of abnormal engine oil evolution.

[0216] More than one volume of engine oil may be exposed to the same operational condition to validate the abnormal engine oil evolution. The reference characterisation for a normal engine oil evolution may be associated with certain engine types, engine meta data, or other information.

[0217] The resulting spectra for volumes of engine oil exposed to operational conditions resulting in abnormal engine oil evolution may, in certain scenarios, be associated with operational conditions by analysis of the spectra to determine which chemical bonds have changed, and using knowledge of certain operating conditions that would result in these changes.

[0218] For example, it may be known that a certain failure mode of the combustion engine causes -61 -a certain reaction in the engine oil that results in a particular change to the chemical bonds of a component of the engine oil. In other scenarios, a reference characterisation for abnormal engine oil may be produced by observing or replicating failure modes (or other conditions that lead to abnormal engine oil evolution) and establishing correlation and causality between a particular failure mode or condition and particular variances between spectra. For example, a volume of engine oil may be exposed to one or more operational conditions corresponding to a particular failure mode or other condition that leads to abnormal engine oil evolution, either by deliberately implementing or replicating the operational conditions or by observing the operational conditions. The volume of engine oil may be analysed as above. The resulting spectra may be compared to a reference characterisation for normal engine oil evolution, and/or to any other reference characterisation for abnormal engine oil evolution. Differences may be identified between the spectra and the reference characterisation for normal engine oil evolution (and/or any other reference characterisation for abnormal engine oil evolution) such that particular variances in the spectra may be associated with the particular failure mode or other condition that leads to abnormal engine oil evolution. This may be repeated, exposing other volumes of engine oil to the same operational condition, to validate an association between particular variances and the particular failure mode or other condition that leads to abnormal engine oil evolution.

[0219] By way of example, several specific conditions or failure modes of a combustion engine that may result in an abnormal engine oil evolution will now be described. These are examples only, and other conditions or failure modes may result in abnormal engine oil evolution.

[0220] As a first example, abnormal wear of a cylinder bore top ring turn around (TRTA) zone may lead to an abnormal engine oil evolution. A cylinder bore wall may comprise grooves on an inner surface. The grooves may, for example, be arranged in a cross-hatching pattern. The grooves may be configured to accommodate a flow of engine oil to lubricate an interface between the cylinder bore and a piston and/or between the cylinder bore and piston rings.

[0221] For example, the interface between the cylinder bore and the top ring of the piston. In an event that abnormal wear of the cylinder bore occurs, the grooves may be worn away such that there is a loss of lubrication to the piston ring(s). In an event that lubrication to the top ring of the piston is lost, the top ring may rub against the cylinder bore, causing damage to -62 -the top ring and affecting a sealing ability of the top ring. This change in performance may, in turn, affect the evolution of the engine oil.

[0222] As a second example, carbon deposits at a piston top land may result in an abnormal engine oil evolution. There may typically be a clearance between a surface of the piston top land and a cylinder bore. This clearance may allow for thermal expansion of the piston. Certain conditions (such as engine oil entering the combustion chamber, certain combustion conditions, or piston temperatures exceeding a threshold) may lead to carbon deposits building up on the top land of the piston. As the piston changes direction, the piston may rock around a piston pin, which may bring the top land of the piston closer to the cylinder bore. Carbon deposits on the top land may reduce the clearance between the top land and the cylinder bore, such that in an event that the top land of the piston is brought closer to the cylinder bore due to the piston changing direction, there may be contact between the carbon deposits on the top land and the cylinder bore. The carbon deposits may then abrade the cylinder bore along the bore. The cylinder bore may have grooves configured to accommodate a flow of engine oil to lubricate an interface between the cylinder bore and the piston. Abrasion of the cylinder bore along the bore may wear away the grooves such that there is a loss in lubrication between the cylinder bore and the top land of the piston. This change in performance may, in turn, affect the evolution of the engine oil.

[0223] Other examples of failure modes of a combustion engine may include abnormal wear of a camshaft lobe to follower interface; piston ring groove deposits; piston ring sticking, leading to scuff; abnormal wear of a piston ring face; abnormal wear or scuff of a piston second land; abnormal wear or scuff of a piston skirt; scuff or bore marking of a piston ring liner; abnormal wear or scuff or seizure of a piston pin joint; connecting rod small end bushing chemical leaching; abnormal wear, scuff or corrosion of a connecting rod crank end bearing; abnormal wear, scuff or corrosion of a main bearing; abnormal wear, scuff or corrosion of a cam bushing; abnormal wear of valve to seat; valve sticking in guide; abnormal wear, scuff or corrosion of a rocker arm bushing; scuff of a pushrod to rocker arm spherical joint; or other failure modes.

[0224] A characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, may be obtained by any of the methods described herein.

[0225] -63 -The characterisation comprises one or more differences between a primary spectrum and a secondary spectrum of a first volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation further comprises an association of the one or more differences to a first operational condition of the combustion engine. The primary spectrum is obtained by using a first spectrometer to analyse a primary sample of the first volume of engine oil and the secondary spectrum is obtained by using the first spectrometer to analyse a secondary sample of the first volume of engine oil. Between obtaining the primary spectrum and the secondary spectrum, the first volume of engine oil is exposed to a first operational condition. In other words, the primary sample is indicative of the first volume of engine oil prior to being exposed to the first operational condition, and the secondary sample is indicative of the first volume of engine oil after being exposed to the first operational condition.

[0226] The characterisation may further comprise one or more differences between a primary spectrum and a secondary spectrum of a second volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation may comprise an association of the one or more differences to a second operational condition of the combustion engine. The primary spectrum may be obtained by using a first spectrometer to analyse a primary sample of the second volume of engine oil and the secondary spectrum may be obtained by using the first spectrometer to analyse a secondary sample of the second volume of engine oil. Between obtaining the primary spectrum of the second volume of engine oil and the secondary spectrum of the second volume of engine oil, the first volume of engine oil may be exposed to a second operational condition. In other words, the primary sample may be indicative of the second volume of engine oil prior to being exposed to the second operational condition, and the secondary sample may be indicative of the second volume of engine oil after being exposed to the second operational condition.

[0227] The second volume of engine oil may be the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. Otherwise, the second volume of engine oil may be separate to the first volume of engine oil.

[0228] The second operational condition may correspond to any of those detailed above. The characterisation may comprise differences between a primary spectrum and a secondary -64 -spectrum of multiple volumes of engine oil exposed to multiple operational conditions, as described in any of the methods above.

[0229] The characterisation may further comprise one or more differences between a tertiary spectrum and a quaternary spectrum of the first volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation may further comprise an association of the one or more differences to a first operational condition of a first part of the combustion engine. The tertiary spectrum and the quaternary spectrum may be obtained by using a second spectrometer to analyse a tertiary sample and a quaternary sample of the first volume of engine oil, respectively. Between obtaining the tertiary spectrum and the quaternary spectrum, the first volume of engine oil may be exposed to a first operational condition.

[0230] The characterisation may comprise differences between primary and secondary spectra of primary and secondary samples, respectively, of multiple volumes of engine oil, wherein each volume of engine oil was exposed to an operational condition between taking the primary sample and the secondary sample.

[0231] The characterisation may further comprise any processed data provided by any of the methods herein, and/or any trends in the data identified by any of the methods herein, and/or any association of differences in spectra of engine oil samples to operational conditions of a combustion engine provided by any of the methods herein.

Claims

1. -65 -CLAIMS: 1. A method of identifying local engine oil performance after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: a. operating the engine during an operation period; b. obtaining data indicative of engine operation during the operation period; c. using the data as an input to an engine oil evolution function to provide a generated engine oil evolution of the engine oil; and d. identifying local engine oil performance by comparing the generated engine oil evolution to a reference characterisation of engine oil evolution; wherein the reference characterisation comprises an association of each of a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine and/or an association of each of a plurality of reference differences between a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine.The method of claim 1, wherein the plurality of operational conditions are each an operational condition of a component or sub-system or system of the combustion engine.

2. The method of claim 1 or 2, wherein the generated engine oil evolution comprises a bulk engine oil evolution.

3. The method of any preceding claim further comprising using the local engine oil performance to determine local engine health.

4. The method of any preceding claim wherein the method further comprises using the local engine oil performance to determine a time by which the engine oil should be replaced.

5. The method of any preceding claim wherein the data is obtained from an engine control module. 2. 3. 4. 5. 6.

6. -66 - 7. The method of any preceding claim wherein step d) comprises: comparing generated spectra of the generated engine oil evolution to reference spectra of the reference characterisation; and/or comparing generated variances between spectra of the generated engine oil evolution to reference variances of the reference characterisation.

7. The method of any preceding claim, wherein at step d) comparing the generated engine oil evolution to a reference characterisation of engine oil evolution is achieved using one or more of: visual inspection; mathematical comparison; binary additive operations; simplification of spectra; and data science tools.

8. The method of any preceding claim, wherein the engine oil evolution function is obtained using experimental equipment to expose engine oil to one or more operational conditions of a combustion engine and by evaluating changes to the engine oil.

9. The method of any preceding claim, wherein the engine oil evolution function is obtained using engine operating data of a combustion engine and measured spectra of engine oil samples from the combustion engine.

10. The method of any preceding claim, wherein the engine oil evolution function is obtained using one or more models that describe engine operation of a combustion engine and/or engine oil evolution.

11. The method of any preceding claim wherein the reference characterisation comprises: one or more differences between a primary reference spectrum and a secondary reference spectrum of a first reference volume of the engine oil, wherein the one or more differences are indicative of a change in the first volume of engine oil; and 8. 9. 10. 11. 12.

12. -67 -an association of the one or more differences to a first operational condition of a first part of the combustion engine; wherein: the primary reference spectrum is obtained by using a first spectrometer to analyse a primary reference sample of the first reference volume of engine oil, wherein the primary reference sample is indicative of the first reference volume of engine oil prior to being exposed to the first operational condition; and the secondary reference spectrum is obtained by using the first spectrometer to analyse a secondary reference sample of the first reference volume of engine oil, wherein the secondary reference sample is indicative of the first reference volume of engine oil after being exposed to the first operational condition.

13. The method of claim 12 wherein the reference characterisation further comprises: one or more differences between a primary reference spectrum and a secondary reference spectrum of a second reference volume of the engine oil, wherein the one or more differences are indicative of a change in the second reference volume of engine oil; and an association of the one or more differences to a second operational condition of the combustion engine; wherein: the primary reference spectrum is obtained by using a first spectrometer to analyse a primary reference sample of the second reference volume of engine oil, wherein the primary reference sample is indicative of the second reference volume of engine oil prior to being exposed to the second operational condition; and the secondary reference spectrum is obtained by using the first spectrometer to analyse a secondary reference sample of the second reference volume of engine oil, wherein the secondary reference sample is indicative of the second reference volume of engine oil after being exposed to the second operational condition.

14. The method of claim 13 wherein the second operational condition corresponds to a different component, sub-system or system of the engine than the first operational condition.

15. -68 - 15. The method of claim 13 wherein the second operational condition corresponds to the same component, sub-system or system of the engine as the first operational condition.

16. The method of any of claims 13 to 15 wherein the second operational condition corresponds to an engine condition over a different time period than the first operational condition.

17. The method of any of claims 12 to 16, wherein the reference characterisation further comprises: one or more differences between a tertiary reference spectrum and a quaternary reference spectrum of the first reference volume of the engine oil, wherein the one or more differences are indicative of a change in the first reference volume of engine oil; and an association of the one or more differences to a first operational condition of a first part of the combustion engine; wherein: the tertiary reference spectrum is obtained by using a second spectrometer to analyse a tertiary reference sample of the first reference volume of engine oil, wherein the tertiary reference sample is indicative of the first reference volume of engine oil prior to being exposed to the second operational condition; and the quaternary reference spectrum is obtained by using the second spectrometer to analyse a quaternary reference sample of the first reference volume of engine oil, wherein the quaternary reference sample is indicative of the first reference volume of engine oil after being exposed to the second operational condition.

18. A characterisation of local engine oil performance after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the characterisation comprising: a generated engine oil evolution, wherein the generated engine oil evolution is obtained by using data indicative of engine operation during the operation period as an input to an engine oil evolution function to provide the generated engine oil evolution of the engine; and -69 -an identification of local engine oil performance based on a comparison of the generated engine oil evolution to a reference characterisation of engine oil evolution; wherein the reference characterisation comprises an association of each of a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine and/or an association of each of a plurality of reference differences between a plurality of reference spectra to one of a plurality of operational conditions of the combustion engine.

19. The characterisation of claim 18, wherein the plurality of operational conditions are each an operational condition of a component or sub-system or system of the combustion engine.

20. The characterisation of claim 18 or 19, wherein the generated engine oil evolution comprises a bulk engine oil evolution.