GB2634928A - Engine system for gaseous fuel - Google Patents

Abstract

An internal combustion engine system comprises a gas-fuelled engine 2, a fuel supply 21 which stores pressurised gas, one or more fuel injectors 16 to deliver pressurised gas to the cylinder(s) of the engine, and a fuel rail 17 to supply pressurised gas to the fuel injector(s). The system further comprises a compressor 30 which receives gas at a first pressure from the fuel supply and delivers gas at a higher pressure to a buffer volume 34 which is connected to the fuel rail. A controller 6 controls the gas output of the compressor based at least in part on a measured gas pressure of the buffer volume. A hydraulic pump 32 may provide hydraulic fluid to drive the compressor. The controller may control the output of the compressor by varying the output of the hydraulic pump based at least in part on a measured gas pressure and temperature of the fuel supply, an engine demand signal or rotational speed and the measured gas pressure in the buffer volume. The compressor may be a doubled ended piston compressor having first and second compressor chambers 45, 46.

Description

ENGINE SYSTEM FOR GASEOUS FUEL

Background

There is an increasing need in modern technology areas to move away from fossil Theis and to replace them with renewable energy sources. This affects the transport sector significantly where conventional gasoline or diesel fuelled vehicles are being replaced by battery electric vehicles (BEV). However, current BEV technology is yet to achieve energy densities comparable with traditional fuels which compromises their range of travel. This limits the attractiveness of BEV technology particularly for heavy duty applications where the size of the battery needed to achieve range and load carrying requirements is impractical.

A known alternative is to use a conventional internal combustion engine that is fuelled by hydrogen gas Hydrogen as an energy carrier and main fuel is a promising option due to its carbon-free content, wide flammability limits and fast flame speeds. For spark-ignited internal combustion engines, utilizing hydrogen in direct injection has been proven to achieve high engine power output and efficiency combined with low emissions. Extensive research supports the feasibility of this solution, but many challenges remain to be solved to commercialise the technology on a wide scale. One challenge to using hydrogen is to deliver gas at high pressures reliably to the cylinders of the associated engine. For this purpose, a compression system may be required to decouple the engine from the main gas tank, in which storage pressure varies with the volume of gas. Such a compression system presents a further parasitic loss on the system since it needs to be driven by the engine or an electric motor, and so it is desirable to maximise the efficiency of any such compression system. It is against this background that the invention has been devised.

Summary of the Invention

The invention is defined by the features set out in the appended independent claims, with optional features set out in the dependent claims.

Further optional and advantageous features are referenced in the detailed description and the appended claims.

Brief Description of the Drawings

So that the invention may be better understood, reference will now be made by way of example only to the following drawings in which: Figure 1 is a schematic view of an internal combustion engine in which examples o the invention may be incorporated; Figure 2 is a schematic view of a compressor system associated with the ngine system; Figures 3 to 5 are flowcharts illustrating an exemplary control scheme ass opted uifh the compressor system; Figure 6 is a schematic view of a vehicle, in particular a heavy-duty vehicle such as a truck:, equipped with an example of the invention.

Detailed description he embodiments

The invention relates to the delivery of pressurised gas internal combustion engine system of a vehicle, which may be an automobile. The gas may be hydrogen gas which is known in the art as being used to fuel vehicles.

Gas-fuelled power plants usually require the delivery of gaseous fuel, hereinafter 'gas, at a narrow range of predetermined pressures. On-board storage tanks receive gas from a filling station at a certain pressure, usually about 700 bar, but also at about 350 bar. However, the pressure of gas in the tank will drop as the gas is used A suitable apparatus is therefore desirable to provide an intermediate pressure boosting and/or regulation function to ensure gas is supplied to the power plant at a steady pressure throughout the filled capacity of the main gas tank.

To put the examples of the invention into technical context, a discussion of an internal combustion engine which is fuelled with gaseous hydrogen will now he described with reference to Figure 1.

In overview, an internal combustion engine system 1 comprises an engine block 2, an air inlet system 3, a fuel delivery system 4 and an exhaust system 5. The engine system 1 further comprises a controller or 'control unit or 'ECU' 6 which is adapted to receive data input 6.1 to sense operational parameters of the engine and to provide suitable control output signals 6.2 to the engine system 1 to control its operation based on driver demands, as is conventional. The terms 'controller' and 'control unit' may be used herein synonymously.

The engine block 2 of the illustrated example comprises four combustion chambers 7, or cylinders. in an 'inline' configuration. However, it should be noted that this is for illustrative purposes only and the engine block may comprise any suitable number of combustion chambers in any suitable configuration, as would be well understood by the skilled person. Common engine configurations are single cylinder engines, twin cylinders, triples, in-line sixes or V-sixes, and V8 engines. Herein, the term 'combustion chamber' will be considered synonymous with 'cylinder'.

The air inlet system 3 comprises an air inlet 8 which feeds fresh air into a network of air pipes 9 through an air filter 10. An air mass flow sensor 11 is provided to provide data to the control unit 6 about the airflow entering the engine system 1.

The network of air pipes 9 feeds incoming air through an air compressor 12 and, subsequently, to an intercooler 13. The functionality of the air compressor 12 and the intercooler 13 are known in the art so a further discussion will not be provided. The network of pipes 9 leads from the intercooler 13 through a throttle valve 14 to an air inlet duct or 'manifold' 15. As is known, the air inlet manifold 15 directs fresh air to each of the combustion chambers 7 of the engine block 2 via separate air channels.

The fuel delivery system 4 comprises a set of one or more fuel injectors 16 (only one of which is labelled) that are arranged to inject combustible fuel, in this case hydrogen gas, into the fresh air flowing into the combustion chambers 7.

In the illustrated example, there are a plurality of fuel injectors 16, the number of which corresponds to the number of combustion chambers 7. Each of the fuel injectors 16 is arranged to inject fuel directly into a respective one of the cylinders 7.

The fuel injectors 16 are each connected to a fuel accumulator or 'common rail' or 'fuel rail' 17, which terms may be used synonymously herein. As is known, the common rail 17 provides a relatively large volume of fuel which is maintained at a predetermined, and controllable, pressure level which means that the fuel injectors 16 are connected to a source of fuel having a pressure level that is in essence static and is not affected by their operation. It should be noted, however, that the fuel pressure within the common rail 17 can be modified in use due to various requirements that are beyond the scope of this discussion.

The pressure of fuel within the common rail 17 is determined by the control unit 6 by means of a fuel pressure sensor 18. The fuel pressure sensor 18 is shown as being connected to the end of the common rail 17 which has an elongated shape, in this example. However, the shape of the common rail 17 and the relative position of the pressure sensor 18 are configurational aspects that are not central to the invention. Fuel arid air mixture in the cylinders 7 is ignited by respective spark plugs 19, in the usual manner.

The common rail 17 is supplied with fuel by a fuel supply system 20. The fuel supply system includes a pressurised fuel source or reservoir 21, a compressor system 22, a shut-off valve 23 and a gas supply line 24 which connects the shut-off valve 23 to the common rail 17. In some examples, the shut-off valve 23 may be connected directly to the common rail, although it is usual for a length of gas supply line 24 to be present so that a desired separation distance may be achieved between the engine system 1 and the fuel supply system 20. The pressurised Fuel source 21 or 'fuel tank' may suitably he configured to store hydrogen gas at an appropriate pressure level, which typically is between 350 and 700 bar when the fuel tank 21 is full; although the gas pressure in the tank 21 can drop significantly during use. The compressor system 22 is configured to maintain the gas pressure at a pressure suitable for injection, which may be between 150 bar and 300 bar in respect of direct injection systems. It should be noted that the configuration of the fuel supply system 20 is explained in more detail below.

Turning now to the exhaust system 5, combustion gases from the cylinders 7 feed into an exhaust duct or 'manifold' 25 which combines the gas out flow into a single pipe which leads to a turbine 26. As is known, the turbine 26 is connected to the air compressor 12 and, together, the turbine 26 and the air compressor 12 constitute a turbocharger of the engine system 1. Turbochargers provide a means to increase the density of the charge of air delivered to the cylinders 7 thereby providing more efficient and powerful combustion. However, their use is not essential to operation. Turbochargers are known in automotive technology so a full discussion will not be provided here for the sake of brevity.

The skilled person would appreciate that the engine system 1 that is the focus of the above discussion has been simplified for present purposes and that in practice an engine system would he more complex. However, the illustrated engine system 2 is intended to demonstrate the principal components and subsystems that are relevant to the examples of the invention.

Figure 2 illustrates an example of the compressor system 22 in more detail.

In overview the compressor system 22 comprises a gas compression device 30, a hydraulic drive system/pump 32, a heat exchanger 34, a buffer volume in the form of buffer tank 36, and an optional pressure regulator 38. The pressure regulator 38 may be advantageous in affording more accurate control over the pressure of gas supplied to the fuel rail 17 and may be controllable by the controller 6 These components are located fluidically between the storage tank 21 and the shut-off valve 23.

The gas compression device 30 takes the form of a double-ended pressure booster which works on the principle of a hydraulically driven motive piston that compresses gas in a pair of working chambers.

The hydraulic drive pump 32 is connected to the gas compression device 30 by a hydraulic circuit 33. The hydraulic drive pump 32 may be any suitable hydraulic pump but preferably should have a variable capacity. In the illustrated example, the hydraulic drive pump 32 is driven by a power take off from the engine block 2. This is a relatively simple mechanical implementation. In such a case, the hydraulic drive pump 32 may be a variable displacement pump such as a swash plate pump, the output of which is controllable by the controller 6, as will be discussed further below. Other options are possible, and preferably any such pump will be capable of pressure in the region of 300-400Bar. For example, the hydraulic drive pump may be an electrically driven pump, e.g. via a motor (not shown) driven from the vehicle's battery system. In such a case a known type of pump such as a gear pump may he suitable as its output can be controlled by varying its drive speed The hydraulic circuit 33 in the illustrated example forms a closed loop between the hydraulic pump 32, the compressor control valve 52 and back to the input of the hydraulic pump 32. An oil reservoir 35 is provided to provide the hydraulic drive pump 32 with a sufficient oil supply to run at any capacity required. In this respect, the hydraulic circuit 33 can be considered to comprise a supply line 33a to convey hydraulic fluid from the hydraulic drive pump 32 to the gas compression device 30 via the valve 52, and a return line 33b to return hydraulic fluid from the gas compression device 30 to the hydraulic drive pump 32. The oil reservoir 35 is connected in a branch line 33c connected between the supply line 33a and the return line 33b.

Other configurations are possible in order to supply the hydraulic circuit with a supply of hydraulic fluid, as would be understood by the skilled person. However, this 'dosed loop' approach is considered beneficial because it provides a greater capacity of hydraulic fluid when required and some advantage in system efficiency. Further modifications may be made to the hydraulic circuit 33 For example, a bypass line may be incorporated into the hydraulic circuit 33 to bypass the supply of hydraulic fluid away from the valve 52 when the valve 52 is not required to be driven. An implementation of.a bypass line is shown in Figure, where the input line 33a and the output line 33b are connected by virtue of the central position of the spool valve 52 so that hydraulic fluid flows through the spool valve 52 when it is in a neutral position.

As can be seen, the gas compression device 30 comprises a piston arrangement 31 including a pair of pistons 40,42 which are connected to and spaced apart by a connecting rod 44. A first compression chamber 45 is defined at the end of the first piston 40 and a second compression chamber 46 is defined at the end of the second piston 42. A respective pair of hydraulic drive chambers 48,50 are located between the pistons 40,42.

A valve arrangement 52 is operable to control the pressure of hydraulic fluid within the hydraulic drive chambers 48,50. The valve arrangement 52 in the illustrated example is a spool valve (terms may be used synonymously), the position of which is controlled by the control unit 6. The valve arrangement 52 is a three-position valve and, as such is operable to direct hydraulic ive fluid from the hydraulic drive pump 32 to either one of the hydraulic drive chambers 48,50 in dependence on the position of the valve 52. The configuration of the spool valve 52 would be understood by a skilled person so further discussion will not be provided. Other valve arrangements including one or more valves may also be used instead of a spool valve.

Supply of hydraulic drive fluid through the hydraulic circuit 33 to the first hydraulic drive chamber 48 drives the connecting rod 44 in a first direction (to the left in the drawing) therefore compressing gas in the first compression chamber 45. Similarly, hydraulic drive fluid supplied through the hydraulic circuit 33 to the second hydraulic drive chamber 50 drives the connecting rod 44 in a second direction (to the right in the drawing) therefore compressing gas in the second compression chamber 45.

In a neutral position; the valve 52 blocks the flow of hydraulic drive fluid to either of the hydraulic drive chambers 48,50 so the piston arrangement 31 remains static and so compressed gas is not pumped out of the gas compression device 30. in a neutral position, hydraulic drive fluid is permitted to circulate around the hydraulic circuit.

The compression chambers 45,46 are supplied with gas from the gas tank 21 at whatever pressure level is present. The gas pressure level and temperature may vary significantly depending on the amount of gas in the tank 21. The compression chambers 45,46 are connected to the gas tank 21 by inlet lines 53 and inlet check valves 54. Likewise, the compression chambers 45,46 are connected to an outlet line 56 by respective outlet check valves 58.

The inlet line 53 connect from the gas tank 21 to the inlet check valves 54. However, at an intermediate position, a flow control valve 55 is provided. The flow control valve 55 controls the flow of gas from the gas tank: to the inlet line 53 or, alternatively to a bypass line 57 which is connected to a position upstream of the shutoff valve 23. The flow control valve 54 is controlled by the controller 6, as will be described in more detail below.

The gas output line 56 is connected to the fuel rail via the buffer tank 36. The buffer tank 36 provides a volume of gas that can be maintained within a predetermined pressure range, as can be selected by the control unit 6. It is envisaged that the buffer tank 36 will be kept at a gas pressure level that is higher than the gas pressure level of the fuel rail 17.

The heat exchanger 34 is located fluidly downstream (or upstream) of the buffer tank 36. The role of the heat exchanger 34 is to regulate the temperature of the gas flowing to the buffer tank which may have an elevated temperature due to the compression process. The heat exchanger 34 has at least one flow passage (not shown specifically in Figure 2) for gas to flow through it which are configured to exchange thermal energy with a fluid medium flowing through separate passages, in a manner common to heat exchanger devices Not shown in Figure 2, but implied, is the presence of a counter flow of heat exchanging fluid medium from a coolant system 57 that would effect thermal energy transfer with the gas flowing through the heat exchanger 34. it should be noted that the precise form of heat exchanger is not essential to the invention, as would be understood by the skilled person.

The functionality of the heat exchanger 34 may be combined with the functionality of the buffer tank 36. Therefore, the internal volume of the heat exchanger 34 may define a sufficient volume to serve as a buffer volume that could otherwise be provided by a separate tank.

At this point it should be appreciated that the buffer tank 36 may be sized in such a way that the intermitted flow delivered by the compressor device 30 can be smoothed out before reaching the regulator 38 without causing the pressure within the buffer tank 36 to drop below an acceptable level. The acceptable level should be higher than the pressure in the fuel rail 17 so that the regulator 36 can operate normally. Generally, this means that the volume of the buffer tank 36 should be more than the volume of the fuel rail 17, for example a multiple of 2, 3, 4, or 5 times the volume of the fuel rail or up to 10 times the volume of the fuel rail. The preferred greater volumetric capacity of the buffer tank 36 compared to the fuel rail 17 should be balanced with the requirements to package the buffer tank 36 with space constraints that are typical in an automotive installation.

Beneficially, and as scribed in further pi esence of the buffer tank 36 provides a continual supply of pressurised gas to the fuel rail 17. Since the volume of the buffer tank 36 is higher than that of the fuel rail 17, the buffer tank 36 acts as an intermediary between the fuel rail 17 and the gas compression device 30 to ensure that the fuel demanded by the engine can be met by the fuel supply system 20. Moreover, the presence of the gas compression device 30 means that better use can be made of the gaseous fuel within the fuel tank. A further advantage of the system is that the gas compression device 30 and other components of the compressor system 22 can be located remotely from components of the internal combustion engine system 1. This advantage is realised by the hydraulic drive pump 32 which provides hydraulic drive fluid to the gas compression device 30 by way of the hydraulic circuit 33. In principle the hydraulic circuit 33 can be configured to be any length and so this enables the gas compressor device 30 to be separated physically from the engine block 2 and associated components. This provides a significant packaging advantage because the gas compressor device 30 does not need to be accommodated within the vehicle engine bay. This can he appreciated more fully by viewing Figure 6, which shows a heavy-duty vehicle such as a truck. In Figure 6, certain components of the engine system 1 such as the engine block 2, air inlet system 3, exhaust system 5, fuel delivery system 4 and so on, are housed within a vehicle cab area Al of vehicle 60. The vehicle cab area Al may include a suitable engine compartment 61 for housing those components. The gas compression device 30 and the fuel tank 21 are shown as being located at a region of the chassis shown as area A2 which is a different part of the vehicle. As shown here, the area A2 is an external area which is exposed to ambient conditions, in the illustrated example, although this is not essential. in other examples, the engine bay 61 may be positioned in a forward position so that a passenger cabin is located in between the engine bay and the area A2 where the gas compression device 30 is located.

The area A2 is separated from the area Al by a bulkhead 62. A safety benefit therefore is provided since the physical separation afforded by the hydraulic drive pump 32 means that the gas compression device 30 and the associated fuel tank(s) 21 can be located on a part of the vehicle at which there are fewer space constraints and where any gas leakage will be mitigated by the exposed location.

Returning to Fig 2, the control over the gas output of the compressor system 22 is achieved by controlling the position of the valve 52. The valve 52, being a spool valve, can be shifted between a central, neutral, position (in the illustrated example) which sets the piston arrangement 31 into a static position so that compressed gas does not flow out of the compressor device 30. The valve 52 is a computer-controlled valve and may be controlled by the controller 6 by appropriately configured control signals. As will be described below, the controller 6 is configured to control the gas output of the gas compression device 30 to the buffer tank 36 based at least in part of a determined gas pressure of the buffer tank 36. In this way, the gas pressure at the buffer tank 36 can he controlled to remain within a predetermined pressure range. Such a pressure range may be static or may be dynamic based on the operating conditions of the engine system 1. A benefit of this approach is that the system makes more effective use of the gas supply within the gas tank 21 because as the gas pressure within the gas tank 21 drops during use, the compressor system 22 boosts the pressure and stores gas in the buffer tank 35 so that a predictable and reliable supply is provided to the fuel rail 17.

The control over the gas output of the compressor system 22 is also benefitted by controlling the output of the hydraulic drive pump 32. As will be described below, the hydraulic drive pump is controlled in order to control the output of hydraulic oil to drive the gas compression device 30. By controlling the hydraulic oil output, a more efficient pumping action is achieved which makes the overall system more efficient.

As can be seen in Figure 2, the controller 6 is configured to receive a set of data signals and to output a set of control signals. In summary, the data signals comprise the pressure and temperature of gas in the buffer tank (P_BLIF, T_BUF), the pressure and temperature of gas in the gas tank 21 (P_INK, T_TNK), engine torque demand and engine speed (TO, SPD), whilst the control signals comprise a signa lrelating to the required position of the spool valve 52 (POS_VLV) and a signal relating to the required output of the hydraulic drive pump (PUMPS.)). Suitable sensors may be provided to provide the data signals.

Having described various hardware aspects of an illustrated example of the invention, the discussion will now turn to a control scheme, approach, or method that may be implemented by the controller 6 for operation of the engine system 1.

In Figure 3, method 100 sets out a control algorithm by which the controller 6 may control the compressor system 22 to achieve one or more of the effects mentioned above. The method 100 is intended to be implemented on a suitable execution environment defined on the controller 6 in suitable hardware/software/firmware.

The method 100 is proposed to be executed continuously in order to effect continuous control over the compressor system 22. A suitable execution period may be selected as required, for example the method may be executed one every 25ms, but it should be understood that this is just an example and other execution rates are acceptable. In principle it is desirable for the system to respond to changes in a time period in the order of 1 second, So a processing loop time of 25ms should satisfy this requirement whilst not being overly-demanding on processing resources.

The method 100 starts at step 102 at which point the controller 6 determines the pressure of gas within the gas tank 21. Optionally, the controller 6 may also determine the temperature of the gas within the gas tank 21. Suitable sensing means (not shown) may be provided at the gas tank 21 for this purpose. In principle a direct measurement of the pressure of the gas may be sufficient. However, it is also envisaged that the pressure of gas may be determined based on stored values of pressure at the beginning of an engine operation or at the beginning of a tank filling operation, and then derived from a record of gas usage by the engine system. By determining gas temperature of the gas tank 21 is may be useful to determine the quantity/mass of gas in the gas tank 21 rather than simply relying on a pressure reading. For present purposes, however, the determination of simply gas pressure will be discussed. The controller 6 receives suitable data readings P_TNK and LINK (optionally).

At step 104, the controller 6 compares the gas pressure in the gas tank 21 with a predetermined acceptable pressure threshold pressure or a pressure range. This pad of the process is provided to ensure that the gas compression system 22 is only operated when needed in order to avoid using energy unnecessarily. Vv'hen a hydrogen gas tank is filled, it is typically at a pressure of 700 bar (known as 'full pressure') or 350 bar, depending on application. The controller 6 may therefore be configured to determine whether the gas pressure has fallen below 300 bar, in the case of gas filling to 700 bar, which may indicate a significant drop in the quantity of gas in the gas tank 21.

In the ongoing discussion, it should be noted that determinations based on pressure may also be determinations based on gas quantity, which can be calculated based on the ideal gas law, gas pressure, gas temperature and known geometry of the tank.

If the gas pressure is at an acceptable level, the method 100 moves to step 106 at which it configures the flow control valve 55 into a position so as to permit gas to flow from the gas tank 21 to the main supply line 24 upstream of the shut-off valve 23. Further, the hydraulic pump 32 and the gas compression device 30 are configured to a deactivated state. In this way, the controller 6 sets the POS....VLV signal to a null or '0' state so that the piston arrangement 31 is in a static position and no gas is transferred from the gas compression device 30. Similarly, the controller 6 sets the PMP_Q signal to zero so that no hydraulic fluid is pumped to the gas compression device 30.

If the gas pressure if not at an acceptable level, then the method 100 moves to a set of actions in order to configure the operation of the gas compression device 30 and the hydraulic pump 32 at appropriate settings.

Fundamentally, the controller 6 is configured to control the operation of the gas compression device 30 so that the supply of gas to the fuel rail 17 is maintained at an appropriate level for operation of the engine system 1. Whilst the pressure of gas in the fuel tank 21 is within an acceptable range, the flow control valve 55 is configured to supply fuel directly by fuel bypass line 57 to a position upstream of the shut-off valve 23. For the purposes of this discussion, the shut-off valve 23 can he considered to be at an open position. However; it should be noted that the shut-off valve 23 may be commanded into a closed position during a shut down process of the engine system 1 or during an emergency event when it is desired to sever the supply of hydrogen to the fuel rail 17.

The buffer tank 36 provides a volume of gas that can be pressurised to an equal or greater pressure compared to the pressure of gas in the fuel rail. In this way; the buffer tank 36 can be charged with a pressure of gas that is greater than that of the gas tank 21, thereby making better use of the remaining gas in the gas tank 21 whilst still providing a reliably pressurised source of gas for the fuel rail 17. To this end, the controller 6 is configured to control the gas compression device 30 in order to maintain the pressure of gas within the buffer tank 36 within an acceptable pressure range, or at least above a minimum acceptable pressure level.

Thus, at step 108, the controller 6 measures the gas pressure, P_BUF, in the buffer tank. In the illustrated example, the controller 6 also measures the pressure of gas in the main gas tank 21, as indicated at step 110.

At step 112 the controller 6 is configured to set the valve 52 in an appropriate position or state to maintain the gas pressure in the buffer tank 36 in the required range. For this purpose, the controller 6 is configured to switch the valve state between +1 and -1 positions in order to control the reciprocating movement of the piston arrangement 31 to cause a suitable mass flow of pressurised gas form the gas compression device 30. For this purpose, the controller 6 may read from an internal data structure that sets out valve switching timings based on an error between the measured pressure of gas within the buffer tank 36 and the required pressure. Other possibilities would be conceivable by the skilled person.

At step 114, the controller 6 sets the flow output of the hydraulic drive pump 32 by outputting the PMP_C) signal. This causes the hydraulic drive pump 32 to output a suitable supply of hydraulic fluid to the gas compression device 30 through the valve 52. For this purpose, the controller 6 may read from an internal data structure that sets out valve values for the Plv1P_Q signal against gas pressureitempe.rature readings Other possibilities would be conceivable by the skilled person.

Beneficially, controlling the output of the hydraulic drive pump 32 based on the pressure of gas in the gas tank 21 provides for more efficient operation of the system as a whole. If the gas pressure is low, then the controller 6 will compensate for this by increasing the flow from the hydraulic drive pump 32 which will in turn drive the gas compression device 30 harder.

Conversely, if the as pressure in the gas tank 21 is only slightly reduced, then the flow from the hydraulic drive pump 32 can be set at a more modest level. This approach achieves a more efficient system because it provides a variable flow of hydraulic drive fluid which is tailored to the requirements of the gas compression device 30 and avoids using energy to drive the hydraulic drive pump 32 when the pressure in the gas tank 21 is at an acceptable level. A reduction is parasitic losses is therefore achieved.

Steps 110 and 114 are shown here as being executed simultaneously. However, precise simultaneous timing is not essential to the invention.

Following completion of steps 112 and 114, the method repeats at a suitable time interval as discussed above. In this way; the method applies continuous and adaptive control over the output of the hydraulic drive pump 32 and the gas compression device 30.

Another method 200 in accordance with the invention is shown in Figure 4. in method 200, method steps 102 to 112 are the same as discussed above with respect to method 100 of Figure 3 so the same steps will not be discussed again for clarity. However, the discussion will focus on the differences in the method 200 compared to method 100 and the technical significance of those differences.

In method 200, step 110 involves the controller 6 receiving data relating to the pressure of gas in the main gas tank 21, as is discussed above. However, method 200 comprises a further step (step 202) of the controller 6 receiving data relating to the operating condition of the engine system. In this specific example, the controller 6 receives an engine torque demand signal (TO) and/or and engine speed signal SPD.

Following the receipt of data relating to gas pressure (and, optionally, temperature) of gas in the gas tank 21 and also engine operating conditions (TO, SPD), the controller 6 is operable to calculate the required hydraulic fluid output from the hydraulic drive pump 32 and to control the hydraulic drive pump 32 to produce the required flow rate based on the gas pressure data and engine operating condition data (step 204). For this purpose, the controller 6 may access a multi-dimensional map which may be stored in memory and tabulates pump output PMP_Q against engine torque and/or engine speed (TO; SPD) and gas tank pressure/temperature (P_TNK, T_TNK).

The benefit of this approach is that the controller 6 is able to manage the output from the hydraulic drive pump 32 more accurately based on a broader range of operating conditions to provide a more accurate mass flow of gas from the gas compression device 30. In addition, it is believed that the method 200 will realise a reduction in activation/deactivation of the spool valve 52 which improves reliability of this component.

Another method 300 in accordance with the invention is shown in Figure 5. in method 300, method steps 102 to 112 are the same as discussed above with respect to methods 100 and of Figure 3 and Figure 4 so the same steps will not be discussed again for clarity. However, the discussion will focus on the differences in the method 300 and the technical significance of those differences.

In method 300, step 110 ivolves the controller 6 receiving data relating to the pressure of gas in the main gas tank 21, as is discussed above. However, method 300 comprises a further step (step 302) of the controller 6 receiving data relating to the operating condition of the engine system, but also data relating to the gas pressure in the buffer tank 36, P_BUF. Note that the pressure in the buffer tank 36 may also be the pressure of gas in the heat exchanger 34, where the heat exchanger 34 provides the buffer volume of the buffer tank 36.

In this specific example, therefore, the controller 6 receives an engine torque demand signal (TO) and/or and engine speed signal SPD, and also the pressure within the buffer tank 36, PBU F. Following the receipt of data relating to gas pressure (and, optionally, temperature) of gas in the gas tank 21, data relating to engine operating conditions (TO, SPD), and data relating to the buffer tank pressure, P_BUF, the controller 6 is operable to calculate the required hydraulic fluid output from the hydraulic drive pump 32 and to control the hydraulic drive pump 32 to produce the required flow rate based on the gas pressure data, engine operating condition data and buffer tank pressure data (step 304). For this purpose, the controller 6 may access a multi-dimensional map which may be stored in memory and tabulates pump output Pfv1P_Q against engine torque and/or engine speed (TO; SPD): gas tank pressure (P_TNK.) and buffer tank pressure (P_BUF) In calculating the required hydraulic fluid output from the hydraulic drive pump 32, any suitable control algorithm may be used, such as a RD (Proportional/Integral/Derivative) control structure or similar, as would be apparent to the skilled person Method 300 provides the advantages of a further refinement in efficiency in controlling the hydraulic drive pump 32 to provide a suitable flow of hydraulic fluid to drive the gas compression device to provide the required mass flow of gas to the buffer tank to set the demanded pressure level. In the methods 100, 200 and 300, it should be noted that a suitable control algorithm may be implemented to ensure that the actual/measured buffer tank pressure is controlled appropriately to minimise the error compared to the demanded buffer tank pressure, which may be in a suitable range, for example between 300 and 350 bar, depending on conditions. Suitable control algorithms would include a suitably tuned proportion-integral-differential closed loop control algorithm as would be understood by a skilled person.

As a further aspect to the above control approaches detailed in method 100, 200 and 300, it should be noted that at step 112 where the controller 6 controls the spool valve 52 to maintain the buffer tank pressure within the specified range of pressures, that the controller 6 has the authority of setting the spool valve position to one of three different positions, -1, 0, + 1, in order to drive the piston arrangement 31 left and right in the drawings. In one example, the controller 6 is configured to select a specific timing for when to control the spool valve 52 to the '0' position, thereby to stop the gas compression device 30. Specifically, when the controller 6 determines that the buffer tank pressure is within an acceptable range and so operation can be halted, in one example the controller 6 is configured to determine an equilibrium point of the gas compression device 30 and to set the spool valve 52 to the '0' position, also known as the 'off' or 'closed' position, when the gas compression device 30 is in the equilibrium point.

The equilibrium point of the gas compression device 30 is where the gas pressure in each of the compression chambers 45; 46 is substantially equal; or at least within a small range of one another, for example within 2-8%. The benefit of this approach is that whilst the piston arrangement 31 is stationary, there is no appreciable compression of the hydraulic fluid trapped within the drive chambers 48,50. This means that when the spool valve 52 is controlled to resume motion of the piston arrangement 31, the motion will resume smoothly rather than abruptly, which provides for a smoother and quieter operation of the gas compression device 30. What is more, when the engine system 1 is deactivated with the spool valve 52 in the 'off' position there is no significant pressure remaining which reduces wear on the spool valve 52.

Claims (1)

1. CLAIMS1. An internal combustion engine system (1) comprising: a gas-fuelled engine (2) co p ig one or more cylinders (7), a fuel supply (21) configured to store pressurised gas, one or more fuel injectors (16) configured to deliver pressurised gas to the cylinders of the 10 engine, a fuel rail (17) configured to supply pressurised gas to the one or more fuel injectors, a compressor device (30) configured to receive gas at a first pressure from the fuel supply (21), and to deliver gas at a higher pressure to a buffer volume (36), wherein the buffer volume (36) is connected to the fuel rail (17), further comprising a controller (6) configured to control the gas output of the compressor device (30) to the buffer volume (36) based at least in part on a measured gas pressure of the buffer volume (36) 2. The system of Claim 1, wherein the controller (6) is configured to control the output of gas from the compressor device (30) to the buffer volume (36) so that gas pressure in the buffer volume (36) remains in a predetermined pressure range.3. The system of any one of the preceding claims, further comprising a hydraulic drive pump (32) configured to provide a flow of hydraulic fluid that drives the compressor device (30).4. The system of Claim 3, wherein the controller (6) is configured to control the output of the compressor device (30) by configuring the hydraulic fluid output of the hydraulic drive pump (32).5. The system of Claim 4, wherein controller (6) is configured to control the hydraulic fluid output of the hydraulic drive pump (32) based at least in part on the measured gas pressure in the fuel supply (21).6. The system of Claim 5, wherein the controller (6) is further configured to control the hydraulic fluid output of the hydraulic drive pump (32) further based on the temperature of gas within the fuel supply (21).The system of Claims 5 or 6, wherein the controller (6) is further configured to control the hydraulic fluid output of the hydraulic drive pump (32) further based on an engine fuel demand signal.8. The system of any one of Claims 5 to 7, wherein the controller (6) is further configured to control the hydraulic fluid output of the hydraulic drive pump (32) further based on engine rotational speed.9. The system of Claim 8, wherein the controller (6) is further configured to control the hydraulic fluid output of the hydraulic drive pump (32) further based on the measured gas pressure in the buffer volume (36).10. The system of any one of the preceding claims, wherein the compressor device (3 is of the form of a doubled ended piston compressor having first and second compressor chambers (45,46).11. The system of Claim 10, wherein, when the controller (6) controls the compressor device (30) to stop gas output, the controller controls the compressor so that an equilibrium condition exists with respect to the first and second compressor chambers (45,46).12. The system of any one of the preceding claims, further comprising a heat exchanger device (34) located fluidly upstream from the fuel rail (17) and fluidly downstream from the compressor device (30).13. The system of Claim 12, wherein the buffer volume (36) forms part of he heat exchanger device (34).14. The system of Claim 12 or Claim 13, wherein the controller (46) is configured to monitor the temperature of gas within the heat exchanger, and wherein the controller is configured to stop operation of the compressor if the temperature of the gas in the heat exchanger is detected as exceeding a predetermined temperature threshold.