WO2019021022A1

WO2019021022A1 - Enhanced combustion engine

Info

Publication number: WO2019021022A1
Application number: PCT/GB2018/052142
Authority: WO
Inventors: Gary MCMAHON
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-07-27
Filing date: 2018-07-27
Publication date: 2019-01-31
Anticipated expiration: 2020-01-27
Also published as: GB201712111D0; GB2564906A

Abstract

A dual-fuel combustion engine system (100) comprises an air intake system (107), an engine (111) having a combustion chamber (112), an exhaust system (114), a primary fuel source (110) and a secondary fuel source (118). A quantity of secondary fuel supplied to the combustion chamber (112) is less than 10% of the a quantity of primary fuel supplied to the combustion chamber (112), so as to cause cracking of the primary fuel, thereby causing more complete combustion of the primary fuel. A secondary fuel controller (117) is configured to use feedback control of the quantity of secondary fuel delivered to the combustion chamber (112) in order to minimise particulate emissions measured by a sensor system (115) provided within the exhaust system (114).

Description

ENHANCED COMBUSTION ENGINE

The present invention relates to improving efficiency in combustion engines, and in particular in combustion engines which utilise two different fuels. The invention provides a method of operating a combustion engine, a combustion engine system, a kit for retrofitting a combustion engine and a secondary fuel controller. A fuel system is described, in particular an addition-type secondary-fuel system, which works with but may not directly alter a current fuel system for a combustion engine.

The compression-ignition style engine market is vast and comprises a range of vehicle and plant sizes, including light and heavy goods vehicles, rail, maritime and aerospace transport sectors. For transport of goods, the cost of fuel and impact on the environment is significant and it is desirable to reduce both of these by more efficiently and completely utilising any fuel required.

Combustion engines provide energy to systems by combusting hydrocarbon fuels in the presence of oxygen. The primary liquid fuels used in most combustion engines are expensive, long-chain hydrocarbons, such as diesel.

The most efficient fuel combustion occurs, particularly in the context of diesel engines, when there is a homogeneous air/fuel mix, which produces carbon dioxide (C0₂) and water (H₂0) as the only waste products. However, due to the length of the hydrocarbons, complete combustion often does not take place. This incomplete combustion leads to exhaust gases containing unburnt fuel, which reduces the fuel efficiency of the engine, as well as other harmful waste products, such as carbon monoxide (CO) and nitrogen oxides (NOx).

It is known to provide combustion engines for vehicles which operate using a mixture of two or more different fuels, which are known as dual fuel engines. In dual fuel engines, the more expensive "primary fuel" will typically be a long-chain hydrocarbon, such as diesel or petrol, which is then supplemented by a cheaper "secondary fuel", such as natural gas (CNG), liquefied petroleum gas (LPG), or hydrogen.

Duel fuel vehicles will typically utilise as much of the secondary fuel as possible. This minimises costs, because these secondary fuels are typically significantly cheaper for a given calorific content than the primary fuel, and also minimises emissions, because short-chain secondary fuels typically combust more completely than the longer-chain primary fuel. There are two main types of known dual fuel systems.

In some systems, the quantity of primary fuel introduced into the engine is unchanged from a conventional engine, and the secondary fuel is introduced as additional fuel. This is known as an "addition" system. The principle involved is that the introduction of the secondary fuel increases the power/torque of the engine for a given supply of primary fuel, and that consequential adaptations made either by the original engine control system and/or by the operator results in a net fuel cost saving. Some systems of this type may also employ some crude forms of control to reduce the primary fuelling, typically by changing the inputs from sensors or modifying torque or speed control inputs.

In an addition system, the amount of secondary fuel that can be introduced is limited by the ability of the engine to combust the secondary fuel, such as due to a lack of oxygen, commonly known as "oxygen depletion", or due to the secondary fuel increasing the combustion temperature within the engine the primary fuel. Operating with excess secondary fuel leads to poor fuel consumption and high emissions due to incomplete combustion and the pass through of un-burnt fuel products, which exit the exhaust. The fuel cost saving from an addition system is therefore not guaranteed and can in some circumstances be negative as well as positive. Furthermore, addition systems often cause over powering of the engine whereby it operates outside of its normal operational conditions. This will have negative implications in terms of the manufacturer's warranty, insurance approvals, safety certification and potential engine life.

The second type of dual fuel system works by introducing a secondary fuel in combination with a reduced quantity of the primary fuel. These are commonly known as "substitution" systems. In such systems, both the primary and secondary fuels are directly controlled with the goal that when both fuels are combusted simultaneously they generate approximately the same power/torque as the original engine when operating only on the primary fuel.

Such substitution systems typically provide a more consistent improvement in terms of fuel cost savings and also avoid over powering the engine, but are more complex and expensive to retrofit into existing vehicles than addition systems because the entire engine control system must be replaced or overridden.

In most dual fuel vehicles, the quantity of secondary fuel supplied is typically substantially greater than 25% of the total fuel delivered to the engine. For example, a typical primary/secondary fuel ratio used in high-speed dual fuel engines is around 40%/60% and for low speed engines it is possible to reach a primary/secondary fuel ratio as low as 10%/90%.

More recently, it has been proposed in WO 2013/061094 to utilise a secondary fuel to cause hydrocarbon cracking of the primary fuel within an engine to improve combustion and performance. Hydrocarbon cracking is the process of breaking a long-chain hydrocarbon into two or more shorter ones. As discussed above, short chain hydrocarbons are easier to ignite resulting in more complete combustion. The consequence of this is that short-chain hydrocarbons produce fewer emissions and provide more energy for a given mass than long-chain hydrocarbons.

In WO 2013/061094, the secondary fuel comprises a shorter chain hydrocarbon in the form of a gaseous fuel. This fuel is injected with air into the engine, and the application describes that this secondary fuel cracks as it is compressed within the engine to form free radicals. The application describes that when the primary fuel is then injected into the engine, these free radicals act to crack the longer-chain hydrocarbon of the primary fuel into shorter chains, thereby allowing more efficient and complete fuel combustion. In other words, for a given amount of primary fuel, a greater energy output is achieved. Since a typical engine control unit (ECU) is configured to adjust the quantity of primary fuel in order to provide a desired energy output, the engine's ECU will automatically adjust to provide less primary fuel to achieve the same energy output, thereby providing greatly improved efficiency.

In contrast to existing dual fuel vehicles, the application suggests that only a small amount of secondary fuel is required to achieve this effect, typically less than 15% of the total quantity of fuel. In WO 2013/061094, the amount of secondary fuel injected is determined according to a fuel mapping profile. This profile specifies the quantity of secondary fuel to be injected as a percentage of the quantity of primary fuel being supplied for a number of different engine states, the main states being: 'idle', 'cruise', 'normal' and 'other'.

The system of WO 2013/061094 has been shown to provide substantial improvements in fuel efficiency and performance.

Viewed from a first aspect, the present invention provides a combustion engine system comprising: a combustion chamber; an exhaust system; a source of a primary fuel; a source of a secondary fuel; a secondary fuel controller configured to control supply of the secondary fuel to the combustion chamber; and a sensor system provided at the exhaust system for measuring emissions of the

combustionchamber, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber, and the sensor system being communicatively connected to the secondary fuel controller, wherein the secondary fuel controller is configured to use feedback control during operation of the combustion engine system to control the quantity of the secondary fuel delivered to the combustion chamber, the feedback control continuously varying the quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the emissions measured by the sensor system.

The above system seeks to operate based on the same cracking principle set out in WO 2013/061094. However, the predictive mapping profiles described therein have been found to restrict deployment of the system because a new profile is required for each model of vehicle, which is expensive and time consuming to generate. Furthermore, whilst a particular fuel mapping profile may work well in test situations, it may perform sub-optimally when deployed in a specific vehicle on the road. Further, it has been found that the generation of predictive mapping profiles limits the application to the automotive sector, whereas a self-mapping system as proposed applies to all transport sectors.

In accordance with the above described system, the secondary fuel controller uses feedback control in order to continuously optimise the cracking process. It has been found that the cracking process has an optimum rate of supply of the secondary fuel that achieves a maximum degree of combustion of the primary fuel (corresponding to maximum primary fuel efficiency and minimum emissions). By monitoring emissions that correspond to the degree of combustion of the primary fuel, and continuously adjusting the flow rate of the secondary fuel to maximise the degree of combustion of the primary fuel, the system permits wide scale deployment across a large number of transport vehicles and ensures that the engine always operates at maximum efficiency.

Preferably, the secondary fuel controller is configured to deliver a quantity, preferably a flow rate, of secondary fuel to the combustion chamber which is less than about 25% of a quantity, preferably a flow rate, of primary fuel delivered to the combustion chamber, more preferably less than about 15% of the quantity of primary fuel delivered to the combustion chamber, and more preferably less than about 10% of the quantity of primary fuel delivered to the combustion chamber.

These quantities are well below what would be typically seen in dual fuel engines which seek to combust the secondary fuel instead of the primary fuel in order to achieve costs savings due to the relative price differences between the fuels. However, levels within this range are still presently understood to be sufficient to cause cracking of the primary and secondary fuels and to achieve improved combustion of the primary fuel.

In one embodiment, the secondary fuel controller is configured to deliver a quantity, preferably a flow rate, of secondary fuel to the combustion chamber which is at least about 1 % of the quantity, preferably a flow rate, of primary fuel delivered to the combustion chamber, and may be between about 1 % and about 8% of the quantity of primary fuel delivered to the combustion chamber. Optimally, a quantity of secondary fuel less than about 5%, but greater than about 2%, of the quantity of primary fuel will be delivered to the combustion chamber.

It will be appreciated that, in reality, primary fuel is not supplied to an engine at a consistent flow rate, but rather as a series of short bursts as the primary fuel is injected into the combustion chamber. Accordingly, the "flow rate" of the primary fuel referred to above may, for example, be understood as an average flow rate of the primary fuel measured over a period of one or more engine cycles, or an equivalent flow rate calculated as a quantity of fuel injected into the combustion chamber per engine cycle divided by a duration of the engine cycle. Similar considerations may apply also to the flow rate of the secondary fuel, where the secondary fuel is not supplied at a continuous flow rate.

The secondary fuel controller may be separate from an engine control unit for controlling a quantity of the primary fuel delivered to the combustion chamber. For example, the secondary fuel controller may be configured to permit retrofit into an existing vehicle. Alternatively, the secondary fuel controller may be incorporated into an engine control unit that also controls a quantity of the primary fuel delivered to the combustion chamber, for example in the case of an OEM system.

The system may comprise a primary fuel injector for supplying the primary fuel to the combustion chamber. The primary fuel injector may supply the primary fuel directly into the combustion chamber.

The primary fuel may be a hydrocarbon fuel, and is preferably a longer- chain hydrocarbon fuel than the secondary fuel. The primary fuel may comprise diesel. However, other primary fuels may include petrol. The source of primary fuel may comprise a primary fuel tank.

The secondary fuel may be a hydrogen-based fuel, and may be a

hydrocarbon fuel. The secondary fuel may be a shorter-chain hydrocarbon fuel than the primary fuel. The secondary fuel may comprise a gaseous hydrocarbon fuel, such as liquefied petroleum gas (LPG). However, other secondary fuels may comprise any one or more of liquefied natural gas, compressed natural gas, methane, propane, butane, hydrogen or oxyhydrogen (Brown's gas).

The source of secondary fuel may comprise a secondary fuel tank. The secondary fuel tank may store the secondary fuel as a gas or as a liquid. The secondary fuel tank may be a liquid-feed tank or a vapour-feed tank. When the secondary fuel exits the secondary fuel tank, it may pass through a pressure regulator.

The secondary fuel may be delivered to intake air of the combustion engine system. The secondary fuel may be mixed with the intake air upstream of the combustion chamber.

The engine system may comprise a turbocharger. The secondary fuel may be delivered to the intake air of the combustion engine system upstream or downstream of a compressor of the turbocharger.

The sensor system comprises a particulate matter sensor. The particulate matter sensor may comprise an optical sensor adapted to measure particulate matter of the emissions. The particulate matter may comprise unburned primary fuel. The particulate matter sensor may comprise an optical emitter and an optical receiver. The optical receiver may be configured to detect light emitted by the optical receiver having passed through the emissions. The particulate matter sensor may be configured to measure opacity of the emissions or a degree of diffusion caused by the emissions. The optical emitter may be configured to emit visible light or non-visible light, such as infrared or ultraviolet light. The optical emitter may be configured to emit a laser beam.

The secondary fuel controller may be configured to maximise the degree of combustion of the primary fuel by minimising particulate matter in the emissions.

The sensor system may comprise more than one sensor. The sensor system may comprise any one or more of a C0₂ sensor, a flow rate sensor, a NOx sensor and an 0₂ sensor. Many such sensors are commonly built into modern engines and may be employed by the secondary fuel controller. The secondary fuel controller may be configured to monitor a quantity of the primary fuel delivered to the combustion chamber. The secondary fuel controller may be configured to control a flow rate of the secondary fuel delivered to the combustion chamber as a proportion of a flow rate of the primary fuel delivered to the combustion chamber. The secondary fuel controller may be in communication with a primary fuel sensor or with a primary fuel controller.

Viewed from a second aspect, the present invention provides a method of operating a combustion engine, the method comprising: supplying a primary fuel and a secondary fuel to the combustion chamber of the combustion engine;

measuring emissions from the combustion engine, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber; and continuously varying a quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the emissions measured by the sensor system.

The quantity of secondary fuel supplied to the combustion chamber may remain less than about 25% of the quantity of primary fuel delivered to the combustion chamber, and preferably less than about 15% of the quantity of primary fuel delivered to the combustion chamber, and most preferably less than about 10% of the quantity of primary fuel delivered to the combustion chamber.

The quantity of secondary fuel supplied to the combustion chamber may remain above at least about 1 % of the quantity of primary fuel delivered to the combustion chamber, and may be between about 1 % and about 8% of the quantity of primary fuel delivered to the combustion chamber.

The secondary fuel is supplied to the combustion chamber via the intake air of the engine system.

Measuring the emissions from the combustion chamber may comprise measuring particulate matter exhausted from the combustion chamber.

Viewed from yet another aspect, the present invention provides a fuel controller for controlling a supply of a secondary fuel into a combustion chamber of a combustion engine, the controller being configured to receive measurements from a sensor system measuring emissions of the combustion engine, the emissions being indicative of a degree of combustion of a primary fuel in the combustion chamber, and the controller being configured to use feedback control during operation of the combustion engine system to control supply of the secondary fuel to the combustion chamber, the feedback control continuously varying a quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the emissions measured by the sensor system.

The sensor system may comprise a particulate matter sensor, and the controller may be configured to maximise the degree of combustion of the primary fuel by minimising particulate matter in the emissions.

The controller is configured to deliver a quantity of secondary fuel to the combustion chamber which is less than 25% of the quantity of primary fuel delivered to the combustion chamber, preferably less than 15%, and more preferably less than 10%.

The fuel controller may incorporate any one or more of the features of the secondary fuel controller of the first aspect.

Viewed from another aspect, the present invention provides a kit for retrofitting a combustion engine designed to combust a primary fuel, the kit comprising: a fuel controller as described above; a tank for holding a secondary fuel; and a sensor system for measuring emissions from the combustion engine or an interconnect for connection to a sensor system for measuring emissions from the combustion engine, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber.

The sensor system may comprise a particulate matter sensor. The particulate matter sensor may comprise an optical sensor adapted to measure unburned primary fuel in the emissions. The particulate matter sensor may incorporate any one or more of the features of the particulate matter sensor of the first aspect

Viewed from another aspect, the present invention provides a method comprising retrofitting a vehicle to use a secondary fuel by installing a kit according as described above.

Certain preferred embodiments of the present invention are described below, by way of example only and with reference to the accompanying drawings, in which:

Figures 1 is a schematic diagram illustrating a combustion engine system in accordance with an embodiment of the present invention;

Figure 2 is a block diagram showing the components of the combustion engine system of Figure 1 ; Figure 3 is a graph showing how the degree of primary fuel combustion varies with the rate of secondary fuel supplied to the combustion chamber of the combustion engine system of Figure 1 ;

Figure 4 shows an exhaust particulate sensor for use with the combustion engine system of Figure 1 ;

Figure 5 shows schematically a secondary fuel kit that can be retrofitted to an existing engine;

Figure 6a shows a side view of a vehicle before a retrofit of the secondary fuel kit of Figure 5;

Figure 6b shows a side view of a vehicle after a retrofit of the secondary fuel kit of Figure 5; and

Figure 6c shows an opposite side view to Figure 6b of a vehicle after a retrofit of the secondary fuel kit.

As is well known, a conventional internal combustion engine comprises a piston that reciprocates within a cylinder and a crank mechanism for converting the reciprocating movement of the piston into a rotational output. The operation and efficiency of an internal combustion engine depends on a great number of factors, including the type and mixture of fuel used, the compression ratio, the dimensions of the piston / cylinder, the valve timing, the ignition timing, the temperature and distribution of temperature within the combustion chamber.

One of the main factors that determines the overall efficiency of the engine is the manner in which the fuel is burned, which is typified by the speed and completeness of the combustion process and/or the homogeneity of the air-fuel mixture.

Relatively complex hydrocarbon fuels such as diesel have a molecular structure which is long and relatively slow to combust, which prevents some of the hydrocarbons from fully burning. Moreover, these long-chain hydrocarbons have a tendency to coalesce, preventing efficient mixing with air or oxygen during the combustion process.

Also, diesel burnt in an enclosed chamber that is externally cooled will tend to ignite first in the centre of the chamber and the ignition will then spread outwards towards the edges of the chamber. If the spread of this flame front is incomplete or inefficient then smoke and particulate matter will result which will be emitted during the exhaust phase of the engine. To maximise the efficiency of an engine it is important to burn as much of the primary fuel as possible during the power stroke. However, the chemistry and thermodynamics of combustion place practical limits on the maximum percentage of the fuel that can actually be combusted during the power stroke, which generally leads to an amount of un-combusted fuel remaining in the cylinder after the power stroke.

Typically, a conventional, heavy-duty diesel engine combusts only up to about 80% of the fuel present in the cylinder during the power stroke. Embodiments of the invention aim to increase this percentage, i.e. closer to 100%, by enhancing the combustion process. The principles of the invention are equally applicable to other types of combustion engine, such as rotary or turbine combustion engines.

Figures 1 and 2 illustrate an exemplary embodiment of a combustion engine system 100 for a vehicle. The combustion engine system 100 includes an air intake system 107, a combustion engine 11 1 having at least one combustion chamber 112, and an exhaust system 1 14.

The combustion engine system 100 has a supply of primary fuel in the form of diesel, stored in a primary fuel tank 110, and a supply of secondary fuel in the form of a gaseous hydrocarbon fuel, such as liquefied petroleum gas (LPG), stored in a secondary fuel tank 118.

Broadly, the primary fuel is delivered from the primary fuel tank 1 10 to the combustion chamber 1 12. The combustion chamber 112 also receives a mixture of air and the secondary fuel from the air intake system 107. The air is compressed in the combustion chamber 112, which causes the temperature within the combustion chamber 1 12 to rise and this causes ignition of both the primary and secondary fuels. The resulting exhaust gas is removed from the combustion chamber 12 by the exhaust system 114.

As used herein, a "gaseous hydrocarbon fuel" refers to a hydrocarbon fuel that is gaseous at the temperatures and pressures within the combustion chamber 112. As is known in the art, gaseous hydrocarbon fuels may be short-chain hydrocarbons, such as propane or butane, while primary fuels, such as diesel for example, may be long-chained hydrocarbons.

The secondary fuel is preferably a hydrogen-based fuel. The secondary fuel preferably cracks when compressed to generate free radicals, as will be explained in greater detail later. A preferred secondary fuel is liquefied petroleum gas.

However, exemplary secondary fuels may include any of liquefied natural gas, compressed natural gas, methane, propane, butane, hydrogen or oxyhydrogen (Brown's gas).

The secondary fuel tank 1 18 may store the gaseous hydrocarbon fuel under pressure either as a liquid or as a gas. The secondary fuel tank 118 may be a liquid-feed tank, meaning that the secondary fuel is drawn in liquid form from the bottom of the tank, or alternatively, may be a vapour-feed tank, meaning that the secondary fuel is drawn in gaseous form from the top of the tank. Additionally, the secondary fuel tank 1 18 may be capable of operating in both a liquid-feed and vapour-feed modes of operation. Preferably, the secondary fuel tank 118 is a vapour-feed tank.

The engine system 100 comprises a turbocharger 101. A compressor 106 of the turbocharger 101 is located in the air intake system 107 upstream of the combustion chamber 1 12. The compressor 106 comprises a rotating impellor that sucks air in, via air-intake 102, to enhance the combustion process.

Prior to boosting by the compressor 106, the air from the air intake 102 passes through an air filter 105 (not shown in Figure 1) designed to block entry of contaminants into the engine 1 11. One (or more) secondary fuel injector 128 is provided on a gas supply line 124 downstream of the air filter 105 and upstream of the compressor 106. The secondary fuel injector 128 receives fuel from the secondary fuel tank 1 18, and injects it into the air stream to be supplied to the combustion chamber 1 12. A pressure regulator (reducer) 119 may be located between the secondary fuel tank 1 18 and injector 128. The pressure regulator may reduce the pressure of the secondary fuel to a pressure at or slightly above the pressure of the air at the secondary fuel injector 128.

A gas controller 117 (i.e. a secondary fuel controller) is provided for controlling the quantity of secondary fuel supplied to the filtered air upstream of the compressor 106. In this embodiment, by controlling the secondary fuel injector 128, the gas controller 117 can control the amount of secondary fuel that is injected into the filtered air prior to the compressor 106, and consequently the amount of secondary fuel that is injected into the combustion chamber 1 12. The operation of the gas controller 117 will be discussed in greater detail later.

When the secondary fuel is injected into the filtered air, the secondary fuel and air mix together. Further mixing occurs in the compressor 106, creating an even distribution of the air and secondary fuel within the mixture. The mixture may be passed through an optional intercooler 108, located between the compressor 106 and the combustion chamber 1 12. The mixture of air and secondary fuel is then passed, for example via intake valves or gas injection solenoids, into the

combustion chamber 1 12.

Whilst the above embodiment injects the secondary fuel upstream of the compressor 106 of the turbocharger 101 , in an alternative configuration, the secondary fuel injector 128 may instead be located downstream of the compressor 106 of the turbocharger 101. In yet another embodiment, the secondary fuel may be directly injected into the combustion chamber 1 12. In any case, air is

incorporated with the secondary fuel and forms a mixture before and/or within the combustion chamber 1 12.

Within the combustion chamber 1 12, the air and secondary fuel mixture undergoes compression. It is presently understood that the compression of the air and secondary fuel mixture causes the short-chained gaseous secondary fuel to split into even shorter-chained molecules (or even atoms), known as radicals or free radicals.

Primary fuel from the primary fuel tank 110, for example diesel, is delivered to the combustion chamber 112. The primary fuel is injected directly into the combustion chamber 1 12 by one (or more) primary fuel injector 127. The primary fuel injector 127 is controlled by a primary fuel controller of an engine control unit (ECU) 104. The primary fuel controller controls only the primary fuel supply and related apparatus. In this embodiment, the ECU 104 does not control the secondary fuel supply or its related apparatus, which is instead controlled by the gas controller 1 17 as described above.

The ECU 104 transmits control instructions to the primary fuel injectors 127 of the engine 1 11 along a data transmission line 125 to the primary fuel injectors 127 to adjust the flow accordingly. The data transmission line 125 may use any form of transmission, including wire, optical fibre or other physical connections, but may also be wireless such as radio transmission for example. The flow rate of the primary fuel can be adjusted, i.e. regulated, by the ECU 104 by adjusting a duty cycle of the primary fuel injector 127 for each combustion chamber 1 12 of the engine 11 1.

Once the primary fuel has been injected into the combustion chamber 112, it is presently understood that the radicals produced from the secondary fuel, which are present in the combustion chamber 112, bind with the longer-chained primary fuel (diesel) hydrocarbons, causing the primary fuel long chain hydrocarbons to split into shorter molecules that are more easily combusted.

It is presently understood that this cracking of the primary fuel by the free radicals from the secondary fuel increases the efficiency of the combustion of the primary fuel within the combustion chamber 1 12, resulting in the observed significantly improved fuel efficiency and reduced emissions.

The combustion products leave the combustion chamber 112 via the exhaust system 1 14, which expands the combustion products via a turbine 116 of the turbocharger 101. The turbine 116 of the turbocharger 101 is coupled to drive the compressor 106 of the turbocharger 101. The expanded combustion products are then exhausted from the combustion engine system 100 at 103.

The primary fuel controller of the ECU 104 is preferably a substantially conventional primary fuel controller. The ECU 104 controls how much of the primary fuel is supplied based on an accelerator pedal position and sensor readings from the engine system 100.

A primary fuel sensor 126 may be provided to sense the pressure and/or flow rate of the primary fuel as it leaves the primary fuel tank 110 to be injected into the combustion chamber 112. This pressure and/or flow rate data is transmitted along a data transmission line 130 to the ECU 104, which can calculate from the pressure data and/or read from the flow rate data a flow rate of the primary fuel.

The data transmission line may use any form of transmission, including wire, optical fibre or other physical connections, but may also be wireless such as radio transmission for example.

An exhaust sensor system 1 15 may be provided in the exhaust system 1 14 for monitoring properties of the combustion products from the combustion chamber 112. The exhaust sensor system 1 15 is located on the exhaust manifold, prior to a diesel particulate filter and/or a catalytic reduction system (not shown). The exhaust sensor system 115 should ideally also be fitted prior to any exhaust gas recirculation valve.

Data measured by this sensor system 1 15 may be transmitted via a transmission line 132 to the ECU 104. As described above, the transmission line 132 may be tangible, e.g. a wire, or intangible, e.g. wireless. The ECU 104 may control the quantity of primary fuel to be injected into the combustion chamber 1 12 based on the monitored properties of the exhaust, as is known in the art. Torque and/or rotational speed sensors 120 may be provided on a drive shaft of the engine 1 11 to monitor the output power of the engine 1 11 , in order for the ECU 104 to control the quantity of primary fuel to be injected into the

combustion chamber 1 12. Data measured by these sensors may be transmitted via a transmission line 121 to the ECU 104. The transmission line may be tangible, e.g. a wire, or intangible, e.g. wireless. This data is used by the ECU to adjust the short-term and long-term fuel trims of the engine 1 11 , which are stored in the ECU 104. The short-term and long-term fuel trims provide the mapping for how the duty cycle of the primary fuel injectors 127 needs to be regulated to adjust the flow of primary fuel to the combustion chamber 112 at any particular time.

By measuring the combustion products in the exhaust via sensor system 115, particularly an oxygen level of the exhaust, the engine fuel trims can be adjusted to provide optimised quantities of primary fuel to be injected into the combustion chamber 1 12.

The skilled person will appreciate that trimming the primary fuel profile is different to the feedback control of the secondary fuel as in the embodiments described below.

In accordance with an embodiment of the present invention, the gas controller 1 17 uses feedback control in order to optimise the combustion process, and is presently understood to optimise the cracking process described above.

Figure 3 illustrates how the degree of combustion of the primary fuel varies with the rate of supply of the secondary fuel (for a fixed rate of supply of the primary fuel).

Initially, as more secondary fuel is injected into the combustion chamber 112, an increased degree of combustion of the primary fuel occurs, which is presently believed to be caused by a greater number of free radicals being generated during the compression stage due to the increased quantity of secondary fuel. However, above a certain level of supply of the secondary fuel, it has been found that the degree of combustion of the primary fuel begins to decrease as the quantity of secondary fuel continues to increase. Thus, as can be seen, the process has an optimum rate of supply of the secondary fuel, illustrated by line 136, that achieves a maximum degree of combustion of the primary fuel.

It is desirable to operate the engine system 100 with this optimum level of secondary fuel supply because doing so will maximise fuel efficiency and minimise engine emissions. The gas controller 1 17 does this by estimating a degree of combustion of the primary fuel based on real-time monitored properties of the engine 11 1 , and continuously adjusting the flow rate of the secondary fuel in order to maximise the degree of combustion of the primary fuel.

In the present embodiment, the gas controller 117 is connected to the exhaust sensor system 115 via a transmission line 134. As described above, the transmission line 134 may be tangible, e.g. a wire, or intangible, e.g. wireless. The gas controller 1 17 monitors one or more properties of the exhaust gases from the combustion chamber 1 12 in order to estimate a degree of combustion of the primary fuel.

The exhaust sensor system 1 15 may comprise a first sensor 140 in the form of a particulate sensor, as shown in Figure 4, which measures the emissions of the engine 11 1. In particular, the particulate sensor 140 measures a level of particulate matter (or soot) in the emissions of the engine 111. In the case of a diesel engine 11 1 , this particulate matter is substantially composed of unburned diesel and so provides a strong indication of the degree of combustion of the diesel in the combustion chamber 1 12. The particulate sensor 140 outputs a signal corresponding to the level of measured particulates in the emissions to the gas controller 1 17 via transmission line 134.

In the exemplary embodiment, the particulate sensor 140 comprises at least an optical emitter 137 and an optical receiver 139. The optical emitter 137 emits a beam of light through an exhaust stream 138. The light may be visible light or non- visible light, such as ultra violet or infrared light. In one embodiment, the beam of light is a laser beam. The optical receiver 139 is configured to receive light from the optical emitter 137 that has passed through the exhaust stream 138.

In the Figure 4 embodiment, the optical receiver 139 measures the opacity of the exhaust gas 138, i.e. the degree of occlusion of the light beam by the particulates in the exhaust gas 138. However, in alternative embodiments, the optical receiver 139 may be positioned so as to measure a degree of diffusion caused by the exhaust gas 138. That is to say, the optical receiver 139 is not positioned within the path of the light beam, but is instead positioned so as to receive light reflected and diffused by the particles in the exhaust gas 138.

The gas controller 1 17 receives the signal from the emissions sensor 140 and responsively varies the quantity of the secondary fuel delivered to the combustion chamber 1 12. This will cause a change in the degree of combustion of the primary fuel and consequently a change in the emissions of the engine 1 11 , which causes a change in the output of the particulate sensor 140. In other words, there is a closed feedback loop involving emissions and the amount of secondary fuel delivered to the combustion chamber.

The gas controller 1 17 is configured to continuously vary the quantity of secondary fuel delivered to the combustion chamber 112 in order to achieve the maximum degree of combustion of the primary fuel, i.e. a minimum level of particulates in the exhaust emissions.

For example, if the gas controller 1 17 previously increased the quantity of the secondary fuel supplied to the combustion chamber 1 12 and then detects a subsequent increase in particulate emissions from the engine 1 11 , then the gas controller can determine that the engine 11 1 is operating above the optimum level 136 of secondary fuel and that the gas controller 117 should reduce the quantity of secondary fuel being supplied to the combustion chamber 112.

By way of example only, a rudimentary feedback control that could be implemented by the gas controller 117 would comprise continuously increasing or decreasing the quantity of secondary fuel supplied to the combustion chamber 1 12 by a fixed increment at fixed intervals, based on a change in the exhaust emissions detected response to the previous increment. If no change is detected, then the flow rate of the secondary fuel would still be incremented, either up or down at the designer's preference, to ensure continuous feedback control.

It will be appreciated that the above is merely a simplified example, and that in practice the feedback control performed by the gas controller 117 will be much more complex.

In the above example, the gas controller 117 estimates the degree of combustion of the primary fuel based on unburned primary fuel emissions in the engine exhaust, detected using the exhaust particulate sensor 140. Whilst this type of sensor is presently considered to be the most direct measurement of the degree of combustion of the primary fuel, those skilled in the art will appreciate that the degree of combustion of the primary fuel may be estimated based on the output of other sensors also.

For example, in one alternative embodiment, a C0₂ sensor may detect a percentage of C0₂ present in the emissions, which is a product of the combustion. This data may be coupled with data from an exhaust flow rate sensor and data relating to the quantity of primary and secondary fuel injected into the combustion chamber 1 12 to the estimate the degree of combustion of the primary fuel. ln another alternative embodiment, a NOx sensor may detect a percentage of NOx present in the emissions, which is another indicator of incomplete combustion of the primary fuel. However, factors other than the degree of combustion of the primary fuel will also affect NOx emissions. In particular, low engine temperature and insufficient oxygen levels may also cause NOx levels to reduce, which could occur if excess secondary fuel is added, even when low degrees of primary fuel combustion are being achieved. Accordingly, NOx emission data should be combined with engine exhaust temperature sensors and 0₂ sensors.

It will be appreciated that the gas controller 117 may of course combine one or more of the sensor arrangements discussed above to provide a more accurate estimation of the degree of combustion of the primary fuel.

Whilst the gas controller 1 17 can operate purely based on feedback from the emissions sensors 115, the gas controller 1 17 may advantageously incorporate an element of predictive control. For example, the gas controller 1 17 may be connected to the ECU 104 via a transmission line 135. As described above, the transmission line 135 may be tangible, e.g. a wire, or intangible, e.g. wireless. The ECU 104 may transmit engine operating data to the gas controller 117 to allow it to predictively adjust the flow rate of the secondary fuel.

For example, the gas controller 1 17 may receive data indicating a flow rate of the primary fuel supplied to the combustion chamber 112 in real time. This may be transmitted from the ECU 104, or the gas controller 117 could alternatively connect to the primary fuel sensor 126 or use a separate primary fuel sensor. For example, the volume of the primary fuel used in the engine 112 can be determined by measuring a pressure in the primary fuel rail and/or an opening time of the primary fuel injector 127 used to introduce the primary fuel into the engine 11 1. The pressure in the primary fuel rail can be determined using data obtained from a data bus connection such as the CAN bus.

It has been found that the optimum level 136 of secondary fuel varies, during at least some engine operating conditions, in a manner approximately proportional to the flow rate of the primary fuel. Thus, if the gas controller 1 17 knows the flow rate of the primary fuel, the gas controller 1 17 could adjust the flow rate of the second fuel as a proportion of the flow rate of the primary fuel. In the example above, the increments could then be increments to this proportion. Such control may permit the gas controller 1 17 to respond more rapidly in the event of significant fluctuations in the quantity of primary fuel delivered to the combustion chamber 1 12, for example due to acceleration, deceleration or changes in driving conditions such as an inclined road surface.

The gas controller 1 17 may receive data from the ECU 104 indicative of an operating state of the engine 1 11. Exemplary engine states may include, for example, 'idle', 'cruise', 'normal', 'braking' and 'other'. The gas controller 117 may change the type of feedback control used when the engine is operating in different engine states. For example, if the engine 11 1 is in an idle or braking state, then the gas controller 1 17 may set the flow rate of secondary gas to a substantially constant and pre-determined level known to correspond to the quantity of primary fuel required when the engine 1 11 is operating at minimum power.

As discussed above, it is presently understood that only a small amount of secondary fuel is required to cause the cracking effect. This is because the primary purpose of the secondary fuel is to cause cracking of the diesel fuel to promote combustion. It is presently understood that the cracking process encourages a chemical chain reaction to take place throughout the chamber 12 which results in a more homogeneous fuel/air mix.

Typically, the maximum proportion of the flow rate of the secondary fuel relative to the flow rate of the primary fuel delivered to the combustion chamber 112 is less than about 15%, and often less than about 10%. In practice, it has been found that the optimum level 136 of the flow rate of the secondary fuel is usually between about 1 % and about 8% of the flow rate of the primary fuel, and is often about 4% of the quantity of primary fuel for typical diesel goods vehicles (e.g.

between about 3% to about 6%).

The above percentages refer to the volumetric flow rate of the secondary fuel which needs to be injected as a relative percentage of the volumetric flow rate of primary fuel to be injected. For example, if the primary fuel had a flow rate of 50 litres per hour, and the corresponding quantity of secondary fuel to be supplied were 3.8%, the quantity of the secondary fuel that would be injected is 1.9 litres per hour (3.8% of 50 litres per hour).

Thus, in one embodiment, the volume of secondary fuel required is calculated based on the amount of primary fuel supplied to the engine 1 11 and the measured emissions from the engine 1 11. The amount of secondary fuel required is translated into the required opening times for the secondary fuel injector 128, which controls the delivery of the secondary fuel. The opening times of the secondary fuel injector 128 can be adjusted according to the pressure and temperature of secondary fuel.

It will be appreciated that, although the above system functions primarily using feedback, the gas controller 1 17 will include at least one initial value for supply of the secondary fuel. This may include a flow rate, or a fraction of a flow rate of the primary fuel, at which the secondary fuel is to be initially supplied, and from which the feedback control will then optimise the flow rate of the secondary fuel to achieve the optimum level 136. The initial value(s) may be a single value of a secondary fuel flow rate, or may be a more complex profile such as used in WO 2013/061094.

The gas controller 1 17 includes a processor for performing the calculations and a memory. The algorithms which control the operation of the gas controller 117 are stored in the memory and can be implemented in software, firmware or a combination of both.

The controller is an ECU but it is separate from the main ECU 104 for the engine system 100. The main ECU 104 can therefore be the existing and unmodified ECU 104 for the engine 1 11. Also, the particulate sensor 140 could be an existing sensor present in the vehicle. Indeed, most of the components are standard and unmodified engine components. A kit 200 in the form of the gas controller 1 17, a secondary fuel tank 118 and a secondary fuel supply system can easily be retrofitted to a conventional engine.

Figure 5 shows a kit 200 that can be retrofitted to an existing engine according to an embodiment of the present invention.

The kit comprises a gas controller 1 17 that controls the secondary fuel injectors 128 (via a control line 238) for supplying a quantity of the secondary fuel from the secondary fuel tank 1 18 to the existing internal combustion supply line. Thus, this kit 200 is intended to be bolted onto an existing internal combustion engine 11 1. The kit can be retrofitted to a conventional internal combustion engine 11 1 with no modifications to the ECU 104, the combustion chamber 112 or control of the connections supplying it.

The kit 200 may comprise an exhaust sensor system 115 for connection to an exhaust system 1 14 of the existing engine. However, many diesel engines already include suitable sensor systems 1 15 for estimation of a degree of combustion of the primary fuel, and the kit 200 may therefore be able to interface with these existing sensor systems. For example, the kit 200 may include an interconnect for connection to an existing emissions sensor system 1 15 of the vehicle. The kit 200 has further optional elements located between the gas tank 118 and the injectors 128. For example, a first pressure and temperature sensor 220, two electrical gas valves 224 and 230 able to perform as a solenoid or electrical shut-off, a vaporiser 226 capable of performing a gas regulation function and attached to a temperature sensor 228, a second pressure sensor 232 and a manual gas valve for providing a mechanical shut-off function. The secondary fuel tank 118 may be equipped with a float 240. Electrical and mechanical valves may be installed for safety reasons to enable the gas supply to be shut off in the event of a fault mode or otherwise.

Figures 6a-6c show examples of a vehicle before and after a retrofitting operation. More specifically, Figure 6a shows a side view of a vehicle before a retrofit of the gas controller system 200. In Figure 6a is shown a side view in which air tanks and a battery pack are fitted to a left hand side of the undercarriage of a truck. In Figure 6b, after the retrofit, the air tanks have been moved inside the chassis of the truck and have been replaced by a gas tank 1 18 (i.e. secondary fuel supply). In Figure 6c, after the retrofit, and when viewed from the opposite side, the vehicle shows gas flowing from the gas tank 1 18 through a gas solenoid

(performing a similar function to the gas valves 224 and 230 in Figure 5) and a gas regulator (performing a similar function to the gas vaporiser 226 of Figure 5).

Whilst a retrofit system 200 is described above, it will be appreciated that the described engine system 100 may alternatively be incorporated in the vehicle as a complete unit, for example at the time of manufacture. Consequently, the functionality of the gas controller 117 may be incorporated into the existing ECU 104 without affective the effectiveness of the system.

The above described system provides significant advantages in terms of primary fuel efficiency and improved emissions. Testing has found that the dual- fuel system provides fuel savings of greater than 25%. Regarding emissions, carbon particulate was reduced by up to 95%, NOx was reduced by 80% and C0₂/CO emissions were reduced by 30%.

Furthermore, the above described system is easily deployed across a wide range of vehicles. Each model of engine may operate differently, depending on the variety of factors discussed previously. Thus, using a non-feedback gas controller would require extensive testing for that particular model to determine the correct mapping profile (or of course a mapping profile for a different engine could be applied, which would achieve non-optimal performance). By instead using feedback, the present system avoids the need for a unique mapping profile to be used and allows the system to be deployed for any vehicle, and particularly for less common vehicles that might not otherwise have commercially justified the work required to generate a mapping profile. For example, large vehicles such as trains and ships may also benefit from this invention, but it is not practical for them to be tested under a wide range of operating conditions using a rolling track to generate a fuel mapping profile, as might be done for a road vehicle.

It will be appreciated that the described technique is particularly applicable to diesel engines. However, it may be employed also in petrol engines. In such engines, the turbocharger 101 and the intercooler 108 will typically not be present.

Whilst exemplary embodiments of the invention have been described, it will be appreciated that various modifications and improvements can be made to the above without departing from the scope of the invention as defined by the appended claims.

The following clauses set out features of the invention which may not presently be claimed, but which may form the basis for future amendment.

1. A combustion engine comprising:

a combustion chamber;

an exhaust system;

a primary fuel source;

a secondary fuel source;

a controller configured to deliver a quantity of secondary fuel to the combustion chamber; and

a first sensor provided at the exhaust system for measuring emissions of the combustion engine and communicatively connected to the controller,

wherein the controller is configured to vary the quantity of secondary fuel delivered to the combustion chamber based on the emissions measured by the first sensor.

2. A combustion engine in accordance with clause 1 , wherein the controller is configured to vary the quantity of secondary fuel delivered to the combustion chamber to reduce pollutants in the emissions.

3. A combustion engine in accordance with clause 1 or 2, wherein the controller is configured to vary the quantity of secondary fuel between zero and a maximum proportion of the total fuel delivered to the combustion chamber. 4. A combustion engine in accordance with any preceding clause, wherein the maximum proportion is less than 25%.

5. A combustion engine in accordance with any preceding clause, wherein the maximum proportion is less than 15%.

6. A combustion engine in accordance with any preceding clause, wherein the controller comprises an ECU which may be separate from the main ECU for the engine.

7. A combustion engine in accordance with any preceding clause, wherein the primary fuel comprises diesel or petrol.

8. A combustion engine in accordance with any preceding clause, wherein the secondary fuel comprises liquefied petroleum gas (LPG).

9. A combustion engine in accordance with any preceding clause, wherein the secondary fuel comprises liquefied natural gas, compressed natural gas, methane, hydrogen or oxyhydrogen (Brown's gas).

10. A combustion engine in accordance with any preceding clause, wherein the secondary fuel is delivered to the intake air of the engine.

11. A combustion engine in accordance with any preceding clause, wherein the first sensor comprises an oxygen sensor.

12. A combustion engine in accordance with clause 1 1 , wherein the first sensor comprises a wide band Lambda sensor.

13. A combustion engine in accordance with any of clauses 1 to 10, wherein the first sensor comprises a laser or light sensor adapted to measure the opacity of the emissions.

14. A combustion engine in accordance with any of clauses 1 to 10, wherein the first sensor comprises a carbon dioxide sensor.

15. A combustion engine in accordance with any of clauses 1 to 10, wherein the first sensor comprises a particulate sensor.

16. A combustion engine in accordance with any of clauses 1 to 10, wherein the first sensor comprises a NOx sensor.

17. A method of enhancing combustion within an engine having a combustion chamber, an exhaust system and a primary fuel source, the method comprising: providing a secondary fuel source;

providing a controller configured to deliver a quantity of secondary fuel to the combustion chamber; and providing a first sensor at the exhaust system for measuring emissions of the combustion engine and communicatively connecting the first sensor to the controller,

wherein the method includes configuring the controller to vary the quantity of secondary fuel delivered to the combustion chamber based on the emissions measured by the first sensor.

18. A method in accordance with clause 17, including configuring the controller to vary the quantity of secondary fuel delivered to the combustion chamber to reduce pollutants in the emissions.

19. A method in accordance with clause 17 or 18, including varying the quantity of secondary fuel between zero and a maximum proportion of the total fuel delivered to the combustion chamber.

20. A method in accordance with clause 19, wherein the maximum proportion is less than 25%.

21. A method in accordance with clause 19, wherein the maximum proportion is less than 15%.

22. A method in accordance with any of clauses 17 to 21 , including providing a controller which is separate from the main ECU for the engine.

23. A method in accordance with any of clauses 17 to 22, including delivering the secondary fuel to the intake air of the engine.

24. A method in accordance with any of clauses 17 to 23, including measuring the opacity of the emissions.

25. A sensor adapted to measure the emissions from a combustion engine, wherein the sensor is adapted to measure the opacity of the emissions.

26. A sensor in accordance with clause 25, wherein the sensor comprises a laser or light sensor.

27. A sensor in accordance with clause 25 or 26, wherein the sensor is provided at the exhaust system of the engine.

Claims

1. A combustion engine system comprising:

a combustion chamber;

an exhaust system;

a source of a primary fuel;

a source of a secondary fuel;

a secondary fuel controller configured to control supply of the secondary fuel to the combustion chamber; and

a sensor system provided at the exhaust system for measuring emissions of the combustion chamber, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber, and the sensor system being communicatively connected to the secondary fuel controller,

wherein the secondary fuel controller is configured to use feedback control during operation of the combustion engine system to control supply of the secondary fuel to the combustion chamber, the feedback control continuously varying the quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the emissions measured by the sensor system.

2. A combustion engine system in accordance with claim 1 , wherein the sensor system comprises a particulate matter sensor.

3. A combustion engine system in accordance with claim 2, wherein the particulate matter sensor comprises an optical sensor configured to measure unburned primary fuel in the emissions.

4. A combustion engine system in accordance with any preceding claim, wherein the primary fuel comprises diesel.

5. A combustion engine system in accordance with any preceding claim, wherein the secondary fuel comprises liquefied petroleum gas (LPG).

6. A combustion engine system in accordance with any preceding claim, wherein the secondary fuel controller is configured to deliver a quantity of secondary fuel to the combustion chamber which is less than 25% of the quantity of primary fuel delivered to the combustion chamber.

7. A combustion engine system in accordance with any preceding claim, wherein the secondary fuel controller is separate from an engine control unit for controlling supply of the primary fuel to the combustion chamber.

8. A combustion engine system in accordance with any preceding claim, wherein the secondary fuel is mixed with intake air of the combustion engine system upstream of the combustion chamber.

9. A combustion engine system in accordance with any preceding claim, wherein the secondary fuel controller is configured to maximise the degree of combustion of the primary fuel by minimising particulate matter in the emissions.

10. A method of operating a combustion engine, the method comprising:

supplying a primary fuel and a secondary fuel to the combustion chamber of the combustion engine;

measuring emissions from the combustion engine, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber; and

continuously varying a quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the measured emissions.

11. A method in accordance with claim 10, wherein measuring emissions from the combustion chamber comprises measuring particulate matter exhausted from the combustion chamber.

12. A method in accordance with claim 10 or 1 1 , wherein a quantity of secondary fuel supplied to the combustion chamber remains less than 25% of a quantity of primary fuel delivered to the combustion chamber.

13. A method in accordance with any of claims 10 to 12, wherein the secondary fuel is supplied to the combustion chamber via intake air of the engine system.

14. A fuel controller for controlling a supply of a secondary fuel into a

combustion chamber of a combustion engine, the controller being configured to receive measurements from a sensor system measuring emissions of the combustion engine, the emissions being indicative of a degree of combustion of a primary fuel in the combustion chamber, and the controller being configured to use feedback control during operation of the combustion engine system to control supply of the secondary fuel to the combustion chamber, the feedback control continuously varying a quantity of the secondary fuel delivered to the combustion chamber in order to maximise the degree of combustion of the primary fuel based on the emissions measured by the sensor system.

15. A fuel controller in accordance with claim 14, wherein the sensor system comprises a particulate matter sensor, and wherein the controller is configured to maximise the degree of combustion of the primary fuel by minimising particulate matter in the emissions.

16. A fuel controller in accordance with claim 14 or 15, wherein the controller is configured to deliver a quantity of secondary fuel to the combustion chamber which is less than 25% of a quantity of primary fuel delivered to the combustion chamber.

17. A kit for retrofitting a combustion engine designed to combust a primary fuel, the kit comprising:

a fuel controller according to any of claims 14 to 16;

a tank for holding a secondary fuel; and

a sensor system for measuring emissions from the combustion engine or an interconnect for connection to a sensor system for measuring emissions from the combustion engine, the emissions being indicative of a degree of combustion of the primary fuel in the combustion chamber.

18. A combustion engine system in accordance with claim 17, wherein the sensor system comprises a particulate matter sensor.

19. A combustion engine system in accordance with claim 18, wherein the particulate matter sensor comprises an optical sensor adapted to measure unburned primary fuel in the emissions.

20. A method comprising retrofitting a vehicle to use a secondary fuel by installing a kit according to any of claims 17 to 19.