Internal combustion engine

Abstract

A hydrogen fuelled internal combustion engine comprising: engine structure comprising one or more cylinders defining an engine displacement; an intake system configured to supply a flow of intake air from an air source along an intake passage to the one or more cylinders for combusting hydrogen fuel therein; a low-pressure compressor device comprising a compressor located in the intake passage; and a high-pressure compressor device comprising a compressor located in the intake passage arranged in series with the low pressure compressor device; an exhaust system configured to exhaust a flow of exhaust gas from the one or more cylinders, along an exhaust passage to an exhaust vent, wherein at least one, of the low- pressure and high-pressure compressor devices comprises a turbine drivingly connected to the respective compressor, and wherein the or each turbine is arranged in the exhaust passage so as to be driven by the flow of exhaust gas; the engine having a specific power output of 20kW to 40kW per litre of displacement.

Description

[0001] P614603PC00

[0002] INTERNAL COMBUSTION ENGINE

[0003] FIELD

[0004] The present teachings relate to an internal combustion engine, and in particular to a hydrogen fuelled internal combustion engine. The present teachings also relate to a method of operating a hydrogen fuelled internal combustion engine and a working machine or genset incorporating a hydrogen fuelled internal combustion engine.

[0005] BACKGROUND

[0006] A transition is underway to lower or zero-carbon energy sources, such as hydrogen, to comply with environmental legislation and reduce the emission of greenhouse gases. The present inventors have recognised that for period, in certain applications, e.g. off-highway machinery such as construction and agricultural working machines, and electric generators, conventional fuel (typically diesel) internal combustion engine versions of the working machines may desirably utilise the same base machine platform as alternative fuelled machines, such as those fuelled by hydrogen. It can be costly and inefficient to manufacture multiple versions of the machines if the overall layout of the engines differs, and the engines have different space envelopes requiring different packaging into the machines when assembled.

[0007] To enhance the attraction to customers of hydrogen fuelled engines in off-highway machinery and electric generators, it is beneficial that a hydrogen fuelled engine’s power output be similar to that of a similar sized diesel fuel engine, e.g. having a similar engine displacement. However, hydrogen fuelled engines tend to require a greater volume of air in the combustion chamber per combustion cycle to achieve this. In turn this requires greater compression of the intake air supplied to the engine’s cylinders to produce the same power output as a comparable diesel fuelled engine.

[0008] It is known for some internal combustion engines to include a form of forced induction, such as a turbocharger or supercharger, to compress the flow of intake air delivered to the engine’s cylinders. T o achieve the greater compression of intake air required to produce a similar power output as a comparable diesel engine, a hydrogen engine may require a greater size and / or number of turbochargers. Therefore, it may be problematic to package such a hydrogen fuel engine within a space envelope of a machine which has been designed to receive a similar power output diesel engine.P614603PC00

[0009] Additionally, the intrinsic qualities of hydrogen as a fuel and / or the consequential changes to engine design create challenges relating to maintaining torque performance, control of emissions, transient response, efficiency, stable combustion, and / or the overall cost of the engine, which are desirably addressed to allow such engines to be commercially acceptable.

[0010] SUMMARY

[0011] The present teachings seek to overcome or at least mitigate one or more problems associated with the prior art.

[0012] The present teachings provide a hydrogen fuelled internal combustion engine according to claim 1, a method of operating a hydrogen fuelled internal combustion engine according to claim 25 and a working machine or genset according to claim 30. Optional features of the teachings are provided by the dependent claims.

[0013] An aspect of the present teachings provides a hydrogen fuelled internal combustion engine. The engine may comprise an engine structure comprising one or more cylinders defining an engine displacement; and / or an intake system configured to supply a flow of intake air from an air source along an intake passage to the one or more cylinders for combusting hydrogen fuel therein; and / or a low-pressure compressor device comprising a compressor located in the intake passage; and / or a high-pressure compressor device comprising a compressor located in the intake passage arranged in series with the low pressure compressor device; and / or an exhaust system configured to exhaust a flow of exhaust gas from the one or more cylinders, along an exhaust passage to an exhaust vent. At least one of the low-pressure and high-pressure compressor devices may comprise a turbine drivingly connected to the respective compressor. The or each turbine may be arranged in the exhaust passage so as to be driven by the flow of exhaust gas. The engine may have a specific power output of 20kW to 40kW per litre of displacement.

[0014] Advantageously, provision of the low-pressure and high-pressure compressor devices enables the intake air to be achieve a suitable compression ratio and such that the hydrogen fuelled engine produces a specific power output similar to a comparable diesel engine, which also retaining the torque and transient response characteristics required for heavy duty applications, such as in off-highway working machines.

[0015] The engine may have a maximum specific power output in the range of 25 to 30 kW / L, for example around 27kW / L.P614603PC00

[0016] The low-pressure and high-pressure compressor devices may be configured such that a maximum compression ratio of an intake air pressure downstream of the high-pressure compressor device to an inlet air pressure upstream of the low-pressure compressor device is the range of 2.5 to 4.5.

[0017] It has been found that such maximum compression ratios enable the engine to produce the maximum specific power output which is similar to comparable diesel fuel engines, whilst retaining efficiency and transient response.

[0018] The compression ratio may be in the range of 3.0 to 4.0.

[0019] The compression ratio of the low pressure compressor device may be between 2.4 and 3.2, which may occur at the full load rated speed of the engine.

[0020] This has been found to be a suitable level of contribution from the low pressure compressor device at this part of the engine map.

[0021] The compression ratio of the high pressure compressor may be between 1.6 and 2.4, which may occur at the peak torque of the engine.

[0022] This has been found to be a suitable level of contribution from the high pressure compressor device at this part of the engine map.

[0023] The low pressure and high pressure compressor devices may be configured such that the compression ratio of the intake air is at least 3.0 between 1200rpm and 1800rpm at maximum torque, optionally the compression ratio of the intake air may be at least 3.4 between 1200rpm and 1800rpm at maximum torque .

[0024] The blending of contribution from the low-pressure and high-pressure compressor devices at differing parts of the engine map allows the performance of the engine to be maintained across a wide range of engine speeds.

[0025] The high pressure compressor device may comprise a recirculation valve in the compressor thereof.

[0026] The provision of a recirculation valve enables the high pressure compressor device to respond to transient reductions in torque demand with a reduced risk of the compressor entering aP614603PC00

[0027] surge state. A recirculation valve may also assist in maintaining the efficiency of the engine, a more consistent air fuel ratio, and reducing lag when a torque demand returns.

[0028] The high pressure compressor device may comprise a variable-geometry turbine.

[0029] A variable-geometry turbocharger helps reduce turbo lag and improve turbocharger efficiency, in particular where there is high torque demand at low engine speeds.

[0030] Engines used in working machines are also often required to work cyclically, shifting rapidly from low to high load and vice-versa, in excavating operations or Y-cycle loading operations, for example. In addition to assisting with low emission and efficient steady-state operation, the provision of variable geometry vanes of the variable geometry pressure turbine and a recirculation valve also aid transient hydrogen fuelled engine operation.

[0031] The low pressure compressor device may be a fixed geometry turbocharger.

[0032] As the low pressure turbocharger is fixed geometry, its contribution to torque increases substantially in proportion to engine speed, and so at high engine speeds provides the majority of torque. By being fixed geometry the low pressure turbocharger may be lower cost, whilst overall engine performance is maintained.

[0033] The low pressure compressor device may comprise no bypass or recirculation valves.

[0034] By having no bypass or recirculation valves the low pressure compressor device may be lower cost, whilst overall engine performance is maintained.

[0035] The low pressure compressor device compressor may be solely drivable by energy in the engine exhaust gases via the turbine.

[0036] The low pressure compressor device is classed as a turbocharger and such a configuration may improve the efficiency of the engine and minimise the cost of engine manufacture.

[0037] The high pressure compressor device compressor may be solely drivable by energy in the engine exhaust gases via the turbine.

[0038] The high pressure compressor device is classed as a turbocharger and such a configuration may improve the efficiency of the engine and minimise the cost of engine manufacture.P614603PC00

[0039] The engine may further comprise a throttle (e.g. in the form of a butterfly valve or other suitable valve) in the intake system, the throttle being arranged to selectively restrict the flow of intake air.

[0040] This allows the engine to regulate the flow of intake air, particularly at low levels of torque demand where the outputs from the compression devices would otherwise to great a mass flow of air for stable combustion of the fuel required for the engine output.

[0041] The engine may further comprise a controller, the controller being configured to signal adjustment to the position of at least one of the recirculation valve, the turbine geometry and the throttle in response to changes in engine operating conditions, in particular in response to a change in a torque demand of the engine.

[0042] The controller is may be configured to signal a reduction in the effective area of the variable geometry turbine in response to an increase in torque demand at engine speeds lower than 1500rpm, optionally lower than 1300rpm.

[0043] The variable geometry arrangement of the high pressure turbocharger provides an additional contribution to torque at low engine speeds by partially closing the vanes (reducing turbine effective area) to increase the exhaust airflow velocity into the high pressure turbine and therefore increase the compression ratio of the high pressure compressor at these lower engine speeds.

[0044] The controller may be configured to signal an opening of the recirculation valve in response to a rate of reduction in torque demand above a predetermined threshold, optionally wherein the controller is configured to monitor a rate of change of demand from a throttle input device to determine a rate of reduction of torque demand, optionally wherein the controller additionally or alternatively is configured to monitor proximity of the operation of the high pressure turbocharger to a surge line of an operating map thereof in determining whether to signal an opening of the recirculation valve.

[0045] If torque demand rapidly reduces, the recirculation valve may be opened so the excess boosted air circulates back upstream of the high pressure turbocharger and the surge line of the compressor map is not reached or not materially exceeded, such that the high pressure turbocharger has a reduced risk of entering a surge state.P614603PC00

[0046] The controller may be configured to signal a reduction in the effective area of the airpath so as to restrict airflow in response to torque demand being below predetermined level.

[0047] The intake system may further comprise one or more charge air cooler devices for cooling the intake air in the intake passage. The one or more charge air cooler devices may be arranged downstream of the low-pressure compressor device, e.g. downstream of the high-pressure compressor device and / or between the low-pressure and high-pressure compressor devices.

[0048] Advantageously, such a configuration helps improve combustion stability and efficiency of the engine.

[0049] A first charge air cooler device may be positioned between the low pressure compressor device and high pressure compressor device and a second charge air cooler device may be downstream of the high pressure compressor device.

[0050] The first charge air cooler device may be an air to water cooler.

[0051] The second charge air cooler device may be an air to water cooler.

[0052] The use of air to water cooling for the interstage cooler and / or the aftercooler allows for improved heat rejection from the charge air by virtue of the greater specific heat capacity of water. For example, heat rejection across both coolers at peak torque may be approximately 40kW for the engine of the present embodiment. This allows for a lower pressure drop across the coolers compared to air to air coolers, which is beneficial for pumping efficiency of the engine. It also allows for lower air volumes of the coolers, which may improve the transient response of the engine. Surprisingly, the use of two stages of cooling has also been found to be more efficient that one stage downstream of the high pressure compressor.

[0053] The flow of water to one or both of the first and second charge air cooler devices may be configured to be adjusted in response to a temperature of the water, e.g. in the return path from the first and / or second charge air cooler devices.

[0054] High temperature intake air can affect combustion stability and efficiency so cooling the intake air in the intake passage is desirable.P614603PC00

[0055] The low-pressure and high-pressure compressor devices may be arranged in series in the exhaust passage. The turbine of the high-pressure compressor device may be upstream of the low-pressure compressor device.

[0056] Advantageously, such a configuration enables the high-pressure compressor device to extract sufficient energy from the exhaust gas to produce the required compression of the intake air.

[0057] The engine may further comprise one or more a hydrogen fuel injectors arranged to inject hydrogen fuel directly into the or each corresponding cylinder of the engine.

[0058] Another aspect of the present teachings provides a method of operating the engine of the previous aspect when comprising a controller. The method may comprise a step of the controller monitoring a torque demand of the engine and the controller signalling adjustment of one or more positions of at least one of the recirculation valve, the turbine geometry and the throttle.

[0059] This allows for optimisation of the operation of the high pressure turbocharger.

[0060] The method may further comprise a step of the controller signalling a reduction in the effective area of the variable geometry turbine in response to an increase in torque demand at engine speeds lower than 1500rpm, optionally lower than 1300rpm.

[0061] The variable geometry arrangement of the high pressure turbocharger provides an additional contribution to torque at low engine speeds by partially closing the vanes (reducing turbine effective area) to increase the exhaust airflow velocity into the high pressure turbine and therefore increase the compression ratio of the high pressure compressor at these lower engine speeds.

[0062] The method may further comprise a step of the controller signalling an opening of the recirculation valve in response to a rate of reduction in torque demand above a predetermined threshold, optionally wherein the controller monitors a rate of change of demand from a throttle input device to determine a rate of reduction of torque demand, optionally wherein the controller additionally or alternatively monitors proximity of the operation of the high pressure turbocharger to a surge line of an operating map thereof in determining whether to signal an opening of the recirculation valve.P614603PC00

[0063] If torque demand rapidly reduces, the recirculation valve may be opened so the excess boosted air circulates back upstream of the high pressure turbocharger and the surge line of the compressor map is not reached, such that the high pressure turbocharger has a reduced risk of entering a surge state.

[0064] The method may further comprise a step of the controller is signalling a reduction in the effective area of the airpath so as to restrict airflow in response to torque demand being below predetermined level.

[0065] A further aspect of the present teachings provides a working machine or genset comprising the engine of the first aspect.

[0066] According to an aspect of the present teachings there is provided a hydrogen fuelled internal combustion engine. The hydrogen fuelled internal combustion engine may comprise an engine structure defining a base, a top face and a side face, the side face extending between the base and top face, and arranged to one side of a crankshaft axis of the engine and the engine structure comprising one or more cylinders. An intake system may be configured to supply a flow of intake air from an air source along an intake passage to the one or more cylinders for combusting hydrogen fuel therein. A low-pressure compressor device comprising a compressor may be located in the intake passage; and a high-pressure compressor device comprising a compressor may be located in the intake passage arranged in series with the low pressure compressor device. Each compressor may be rotatable about a rotational centre thereof. Each compressor device may be mounted to the side face and / or the top face such that the rotational centres of the low-pressure and high-pressure compressor devices are vertically offset from each other with respect to the base.

[0067] Advantageously, provision of the low-pressure and high-pressure compressor devices enables the intake air to be sufficiently compressed such that the hydrogen fuel engine produces a specific power output similar to a comparable diesel engine. Moreover, such a mounting arrangement of the compressor devices improves their packaging on the engine so as to minimise the space envelope required to receive the engine in a machine.

[0068] The rotational centre of the low-pressure compressor device may be lower than the rotational centre of the high-pressure compressor device with respect to the base, optionally, wherein at least the low-pressure compressor device may be mounted to the side face.P614603PC00

[0069] Advantageously, such an arrangement helps to leave available space towards the top of the engine for other engine components mounted to the engine structure. Moreover, such an arrangement helps to minimise the maximum height of the engine.

[0070] The engine structure may comprise a cylinder block and a cylinder head mounted on top of the cylinder block, wherein the low-pressure compressor device may be mounted to the cylinder block and / or the cylinder head, and wherein the high-pressure compressor device may be mounted to the cylinder head and / or a cam cover mounted on top of the cylinder head, optionally wherein the high pressure compressor device may be mounted to the cylinder head via an exhaust manifold.

[0071] The rotational centre of the low-pressure compressor device may be higher than the rotational centre of the high-pressure compressor device with respect to the base; optionally, wherein at least the low-pressure compressor device may be mounted to the top face.

[0072] Such an arrangement may leave available space towards a lower part of the engine for other engine components mounted to the engine structure. Moreover, such an arrangement may enable an intake air conduit and an exhaust conduit connected to an inlet and outlet of the low-pressure compressor device respectively, to extend above the engine structure, similar to a comparable diesel engine. This enables the engine to be more easily retrofitted to a machine designed to be powered by such a diesel engine.

[0073] The engine structure may comprise a cylinder head mounted on top of the cylinder block and a cam cover mounted on top of the cylinder head, wherein the low-pressure compressor device may be mounted to the cylinder head and / or the cam cover, and wherein the high-pressure compressor device may be mounted to the cylinder block and / or the cylinder head.

[0074] The low pressure and high-pressure compressor devices may be mounted to the side face, e.g. to the cylinder block and / or a cylinder head mounted on top of the cylinder block.

[0075] Advantageously, such an arrangement helps make available space on the top face of the engine structure for other engine components to be located in.

[0076] At least one of the rotational centres of the low-pressure and high-pressure compressor devices may be higher than the cylinder block with respect to the base.

[0077] Advantageously, such an arrangement helps to make available space on the cylinder block for other engine components to be mounted to.P614603PC00

[0078] The rotational centres of the low-pressure and high-pressure compressor devices may be offset from each other along a transverse axis of the engine perpendicular to the crankshaft axis.

[0079] Advantageously, such a configuration improves access to the inlets and outlets of the compressor devices for conduits forming part of the intake and exhaust passages. Moreover, this configuration helps to reduce the streamwise length of such conduits and / or enables a simplified routing of the conduits. Further, this configuration may help improve the packaging of other engine components mounted to the engine structure.

[0080] The rotational centre of the low-pressure compressor device may have a greater transverse spacing from the crankshaft axis than the rotational centre of the high-pressure compressor device.

[0081] Advantageously, such a configuration may help improve the packaging of other engine components mounted to the engine structure.

[0082] The rotational centre of the low-pressure compressor device may be transversely spaced from the crankshaft axis less than the rotational centre of the high-pressure compressor device.

[0083] Advantageously, such a configuration helps to minimise the overall space envelope of the engine.

[0084] The rotational centres of the low-pressure and high-pressure compressor devices may be at least partially axially offset from each other along the crankshaft axis; optionally, wherein the engine structure may comprise a rear face extending between the base and top face such that the crankshaft axis intersects the rear face, the rear face may comprise a gearcase of the engine structure, wherein the rotational centre of the low-pressure compressor device may be closer to the rear face relative to the rotational centre of the high-pressure compressor.

[0085] Advantageously, such a configuration helps to reduce the streamwise length of a conduit forming part of the intake passage connecting the compressor devices, so as to minimise pressure losses in the intake air. Moreover, such a configuration helps to reduce the number of bends in such a conduit, so as to further minimise pressure losses in the intake air.

[0086] Each of the low-pressure and high-pressure compressor devices may comprise a lubricating system for lubricating one or more moving parts of the devices, wherein the lubricating system may comprise an external orifice for receiving a supply of oil therein or draining oil therefrom, and wherein the external orifices of the compressor devices may be at least partially, e.g. mostly or entirely, transversely and / or axially offset from each other.P614603PC00

[0087] Advantageously, such a configuration helps to provide space below the compressor devices for oil supply / drainage conduits extending from the compressor devices to the engine structure without or with less obstruction from the other compressor device. For drainage conduits in particular, having space below the compressor devices enables the drainage conduit to extend substantially down from the compressor device with no or minimal changes in direction to the engine structure, to allow oil to freely return to the engine structure.

[0088] Each compressor device may comprise an intake inlet in the intake passage upstream of the respective compressor, and wherein the intake inlets of the low-pressure and high-pressure compressor devices may be at least partially, e.g. mostly or entirely, vertically, transversely and / or axially offset from each other.

[0089] Advantageously, such an arrangement improves access to the intake inlets for conduits forming part of the intake passage.

[0090] The low-pressure compressor device may comprise an intake outlet in the intake passage, and the high-pressure compressor device may comprise an intake inlet in the intake passage, such that intake air may exit the low-pressure compressor device at the intake outlet and may enter the high-pressure compressor device at the intake inlet, and wherein the intake outlet and intake inlet may be at least partially, e.g. mostly or entirely, transversely and / or axially offset from each other.

[0091] Advantageously, such a configuration helps to reduce the streamwise length of a conduit forming part of the intake passage connecting the compressor devices, so as to minimise pressure loss in the intake air. Moreover, such a configuration helps to reduce the number of bends in such a conduit, so as to minimise pressure losses in the intake air.

[0092] The engine may further comprise an exhaust system configured to exhaust a flow of exhaust gas from the one or more cylinders, along an exhaust passage to an exhaust vent, wherein at least one, e.g. both, of the low-pressure and high-pressure compressor devices may comprise a turbine drivingly connected to the respective compressor, and wherein the or each turbine may be arranged in the exhaust passage so as to be driven by the flow of exhaust gas.

[0093] In such a configuration the compressor devices may be classed as a turbocharger and such a configuration may improve the efficiency of the engine.

[0094] The low-pressure and high-pressure compressor devices may be arranged in series in the exhaust passage, The turbine of the high-pressure compressor device may be upstream of the low-pressure compressor device.P614603PC00

[0095] Advantageously, such a configuration enables the high-pressure compressor device to extract sufficient energy from the exhaust gas to produce the required compression of the intake air.

[0096] The high-pressure compressor device may comprise an exhaust outlet in the exhaust passage, and the low-pressure compressor device may comprise an exhaust inlet in the exhaust passage, wherein exhaust gas may exit the high-pressure compressor device at the exhaust outlet and may enter the low-pressure compressor device at the exhaust inlet. The exhaust outlet and exhaust inlet may be vertically, axially and / or transversely offset from each other; optionally, wherein the exhaust outlet and exhaust may be substantially transversely aligned; optionally, wherein the exhaust passage may comprise an exhaust connector connecting the exhaust outlet to the exhaust inlet, and wherein the exhaust connector may be substantially straight or L-shaped.

[0097] Advantageously, such a configuration helps to reduce the streamwise length of a conduit forming part of the exhaust passage connecting the compressor devices, so as to minimise heat and pressure loss in the exhaust gas. Moreover, such a configuration helps to reduce the number of bends in such a conduit, so as to minimise pressure losses in the exhaust gas.

[0098] The exhaust passage between the low-pressure and high-pressure compressor devices may comprise a flexible portion configured to flex in response to relative movement between the low-pressure and high-pressure compressor devices; optionally, wherein the flexible portion may be expandable, e.g. comprising bellows.

[0099] Advantageously, such a configuration helps reduce the risk of damage to the exhaust passage caused by differential thermal expansion.

[0100] At least part of the exhaust passage between the low-pressure and high-pressure compressor devices may slope downwardly in a streamwise direction towards the exhaust vent for draining condensed water therealong.

[0101] Advantageously, such a configuration helps to prevent or minimise condensed water (a major combustion project of hydrogen and air) flowing from the exhaust passage into the cylinders after the engine has shutdown.

[0102] The exhaust passage may comprise an exhaust manifold mounted to the engine structure, wherein the exhaust manifold may comprise an exhaust outlet in the exhaust passage, wherein the high-pressure compressor device may comprise an exhaust inlet in the exhaust passage, wherein the exhaust inlet may be mounted directly to the exhaust outlet, such that exhaust gas exiting the exhaust outlet enters the exhaust inlet; optionally, wherein the high-P614603PC00

[0103] pressure compressor device may be mounted to a substantially upward facing surface of the exhaust manifold.

[0104] Advantageously, such a configuration helps minimise the streamwise length of the exhaust passage between the cylinder(s) and the high-pressure compressor device, which helps to minimise heat loss in the exhaust gas at the high-pressure compressor device, and thus maximise the amount of energy which can be extracted.

[0105] Each compressor device may comprise an intake inlet in the intake passage and an exhaust outlet in the exhaust passage, wherein the intake inlet and the exhaust outlet of each compressor device may be aligned with a reference axis of the device, and wherein the reference axes of the low-pressure and high-pressure compressor devices may be: substantially parallel to each other, e.g. and substantially parallel to the crankshaft axis; vertically offset from each other; transversely offset from each other; and / or may be substantially aligned with the rotational centre of the respective compressor device.

[0106] Advantageously, such a configuration improves access to the inlets and outlets of the compressor devices for conduits forming part of the intake and exhaust passages, and helps minimise the space envelope of the engine.

[0107] The low-pressure compressor device may be mounted to the engine structure via a bracket projecting from the engine structure in a generally transverse direction with respect to the crankshaft axis such that the low-pressure compressor device may be spaced from the engine structure.

[0108] Advantageously, such a configuration may improve the packaging of other components mounted to the engine structure but enabling one or more ancillary engine components to be received or routed therebetween, so as to minimise the overall space envelope of the engine, and / or improve access to components for installation or maintenance in service.

[0109] The bracket may have a first bending axis perpendicular to the crankshaft axis, and a second bending axis parallel to the crankshaft axis, wherein the bracket may be configured to be more flexible in bending about the first bending axis relative to the second bending axis; optionally, wherein the low-pressure compressor device may be secured to an end of the bracket opposite the engine structure, and wherein the bracket may be configured such that said end is more flexible in a direction parallel to the crankshaft axis relative to a direction perpendicular to the crankshaft axis.P614603PC00

[0110] Advantageously, such a configuration enables the bracket to accommodate movement of low-pressure compressor device caused by thermal expansion.

[0111] The intake system may further comprise one or more charge air cooler devices for cooling the intake air in the intake passage, wherein the one or more charge air cooler devices may be arranged downstream of the low-pressure compressor device, e.g. downstream of the high-pressure compressor device and / or between the low-pressure and high-pressure compressor devices.

[0112] Advantageously, such a configuration helps improve combustion stability and efficiency of the engine.

[0113] The engine may have a maximum specific power output in the range of 20 to 40 kW / L; optionally, in the range of 25 to 30 kW / L.

[0114] The low-pressure and high-pressure compressor devices may be configured such that a maximum compression ratio (pressure ratio) of an intake air pressure downstream of the high-pressure compressor device to an inlet air pressure upstream of the low-pressure compressor device may be the range of 2.5 to 4.5; optionally, in the range of 3.0 to 4.0.

[0115] A further aspect of the present teachings provides a working machine or genset comprising the engine of the previous aspect

[0116] BRIEF DESCRIPTION OF DRAWINGS

[0117] Embodiments will now be described by way of example only with reference to the accompanying figures, in which:

[0118] Figure 1 is a rear isometric view of an internal combustion engine according to an embodiment;

[0119] Figure 2 is a front isometric view of the engine of Figure 1 ;

[0120] Figure 3 is a schematic diagram of a layout of part of the engine of Figure 1 ;

[0121] Figure 4 is a side view of the engine of Figure 1 ;

[0122] Figure 5 is a top view of the engine of Figure 1 ;

[0123] Figure 6 is a cross-sectional view along section A-A in Figure 5;

[0124] Figure 7 is an isometric view of the engine of Figure 1 with some components omitted;P614603PC00

[0125] Figure 8 is a rear view of a turbocharger and a bracket of the engine of Figure 1 ;

[0126] Figure 9 is a schematic view of an engine according to an embodiment of the present teachings;

[0127] Figure 10 is a further schematic view of an engine according to an embodiment of the present teachings incorporating a control system;

[0128] Figure 11 is a plot illustrating the operation of turbochargers of the engine in differing operating conditions;

[0129] Figure 12 is a compressor map plot for a low pressure turbocharger of the engine of Figure 10;

[0130] Figure 13 is a compressor map plot for a high pressure turbocharger of the engine of Figure 10;

[0131] Figure 14 is a flow chart of a method of operating the low pressure and high pressure turbochargers and throttle as part of overall engine operation;

[0132] Figure 15 is a side view of a working machine according to an embodiment including an engine of the present teaching.

[0133] DETAILED DESCRIPTION

[0134] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments and the teachings. However, those skilled in the art will understand that: the present teachings may be practiced without these specific details or with known equivalents of these specific details; that the present teachings are not limited to the described embodiments; and, that the present teachings may be practiced in a variety of alternative embodiments. It will also be appreciated that well known methods, procedures, components, and systems may not have been described in detail.

[0135] References to vertical and horizontal in the present disclosure should be understood to be in relation to the engine when mounted in a machine stood on horizontal ground in a non-working condition.

[0136] Figures 1 and 2 show an internal combustion engine 100 according to an embodiment.

[0137] In the illustrated embodiment, the engine 100 is a hydrogen fuelled engine configured to be exclusively powered by hydrogen fuel. In alternative embodiments, the engine 1 may insteadP614603PC00

[0138] be configured to be powered via hydrogen and one or more different fuels; e.g. a blend of hydrogen and another gaseous fuel.

[0139] The engine 100 may be suitable for use as the prime mover in a working machine 10 (see Figure 15 which depicts a backhoe loader, but may also be a telescopic handler, a forklift truck, a wheeled loading shovel, a dumper, an excavator or a tractor, for example). Such working machines 10 are suitable for use in off-highway industries such as agriculture and construction. In these industries they are generally configured to perform tasks such as excavation, load handling, harvesting or planting crops. The engine 100 may also be utilised in a genset - a self-contained unit to provide electrical power at off-grid locations. As such the engine 100 is typically required to have certain characteristics such as a high torque output over a wide engine speed band, with peak torque occurring at a relatively low engine speed, which differ from light passenger vehicles, for example. In off-highway applications, this provides “torque backup” that enables working machines 10 to continue to carry out working operations when encountering increased loads, or resistance to a working operation - e.g. an excavator encountering a particularly solid piece of earth to be excavated.

[0140] The engine 100 includes an engine structure 102 including a crankcase 104, an oil reservoir 105 (oil pan or sump), a cylinder head 106, a cam cover 107, a cylinder block 108, and a gearcase 109.

[0141] The cylinder block 108 includes four cylinders 112 (see Figure 3, illustrating the engine 100 schematically), one of which is shown in Figure 6. The cylinder head 106 is mounted on top of the cylinder block 108 so as to cover the top of the cylinders 112. The cylinder head 106 comprises four intake runners 113 - one per cylinder 112 - which each bifurcate into two intake ports for each cylinder (not shown for clarity), supplying air to each cylinder 112. The intake runners 113 may be, at least in part be provided by an intake manifold 114. The cylinder head 106 also comprises four exhaust runners 115, one per cylinder 112, for exhausting exhaust gases from each cylinder 112 via two exhaust ports for each cylinder (not shown for clarity). Each intake port is selectively opened and closed by an intake valve (not shown), and each exhaust port selectively opened and closed by an exhaust valve (not shown).

[0142] Each cylinder 112 receives a piston 117 which is translationally movable within the cylinder 112. During operation of the engine 100, translational movement of each piston 117 is converted into rotational movement of a crankshaft 119. The crankcase 104 houses the crankshaft 119. The crankshaft 119 is aligned with a crankshaft axis X1 of the engine 100.

[0143] With reference to Figures 1, 2, 4 and 5, the engine structure 102 includes a base 102a, a top face 102b, a side face 102c, a rear face 102d, and a front face 102e. The side face 102cP614603PC00

[0144] extends between the base 102a and the top face 102b. The side face 102c is arranged to one side of the crankshaft axis X1. The rear and front faces 102d, 102e are opposite each other, and both extend between the base 102a and the top face 102b such that the crankshaft axis X1 intersects the rear and front faces 102d, 102e. In this embodiment, the rear face 102d includes the gearcase 109.

[0145] The engine 100 has a transverse axis X2 perpendicular to the crankshaft axis X1. The transverse axis X2 and the crankshaft axis X1 define a horizontal plane of the engine 100. The transverse axis X2 intersects the side face 102c of the engine structure 102.

[0146] In the present embodiment the engine 100 has a total displacement of 4.8 litres (i.e. 1.2 litres per cylinder). In engines used in off-highway applications each cylinder may typically have a displacement of between 0.75 and 1.25 litres. Such a displacement is relatively high by comparison with passenger vehicle engines, but is suited to providing the operating characteristics described above.

[0147] In the present embodiment, the engine 100 includes a hydrogen fuel delivery system by which hydrogen fuel may be directly injected into each cylinder 112 from a pressurised fuel tank 12 (see Figure 15). The rated pressure of the fuel tank 12 is typically in excess of 35MPa and the hydrogen pressure may be stepped down before being introduced into the cylinders 112. The engine 100 may be a direct injection (DI) engine in which fuel is injected directly into each cylinder 112, or a port fuel injection (PFI) engine in which fuel is injected into each intake runner 113 from which it is delivered to the corresponding cylinder 112.

[0148] In alternative embodiments (not shown), the engine 100 may have more or fewer cylinders 112, e.g. 2, 3, 6, or 8. In addition, in other embodiments the cylinders 112 may be oriented in a “V” or boxer configuration rather than inline as in the disclosed embodiment.

[0149] With reference to Figure 3, the engine 100 includes an exhaust system 206 configured to exhaust a flow of exhaust gas from the cylinders 112, along an exhaust passage 208 to an exhaust vent (122, Figure 15 - e.g. a tailpipe). Since in this embodiment the engine 100 is fuelled exclusively by hydrogen, the exhaust gas is predominantly water vapour.

[0150] With further reference to Figure 7, which shows the engine 100 with some engine components omitted, the exhaust passage 208 includes an exhaust manifold 236 mounted to the cylinder head 106. The exhaust manifold 236 includes four exhaust runners 237. Each exhaust runner 237 is connected to one of the exhaust ports 115 for exhausting exhaust gas therefrom. The exhaust runners 237 lead to an exhaust manifold outlet 238.P614603PC00

[0151] With reference to Figures 1 , 2 and 3, the engine 100 includes an intake system 110 configured to supply a flow of intake air from an air source, along an intake passage 120, to the cylinders 112 for combusting fuel therein. In the illustrated embodiment, the air source is the atmosphere. The intake system 110 supplies the intake air to each cylinder 112 via its corresponding intake port 113.

[0152] The engine 100 is configured to have a maximum specific power output (i.e. power output divided by total engine displacement) of approximately 27 kW / L, which is similar to comparable diesel fuel engines. In some embodiments, the engine 100 may have a maximum specific power output in the range of 20 to 40 kW / L, e.g. 25 to 30 kW / L.

[0153] It is known to compress the intake air supplied to cylinders of an engine to increase the engine’s maximum specific power output via some form of forced induction. However, to achieve the maximum specific power output of the engine 100, the intake air supplied to the cylinders 112 needs to be compressed to a higher pressure relative to a diesel fuel engine having the same maximum specific power output. It is difficult to achieve this high compression of the intake air via a single compressor device, such as a turbocharger, whilst maintaining acceptable engine operating characteristics. Therefore, the engine 100 of the present teachings comprises two turbochargers: a low-pressure turbocharger 200 and a high-pressure turbocharger 202. In some embodiments, the intake passage 120 may include an air filter 116 (shown in Figure 9) arranged upstream of the low-pressure turbocharger 200.

[0154] In this embodiment the compressor devices are turbochargers 200, 202 driven by engine exhaust gases. In alternative embodiments, one or both turbochargers 202, 204 may instead be replaced by a supercharger, an electric motor powered compressor (an electric supercharger), or an electrically assisted turbocharger, for example.

[0155] Each turbocharger 200, 202 includes a compressor 204a and 204b in the intake passage 120 for compressing the flow of intake air. Referring to Figures 3 and 4, the compressor 204a of the low-pressure turbocharger 200 is rotatable about a rotational axis X3 intersecting a rotational centre R1 of the compressor 204a. The compressor 204b of the high-pressure turbocharger 202 is about a rotatable about a rotational axis X4 intersecting a rotational centre R2 of the compressor 204b.

[0156] The compressors 204a, b are arranged in series in the intake passage 120 such that the flow of intake air from the air source may be first compressed by the low-pressure turbocharger 200, then may be further compressed by the high-pressure turbocharger 202, and then supplied to the cylinders 112.P614603PC00

[0157] The low-pressure turbocharger 200 may compress the intake air to increase its pressure from a first pressure P1 upstream of the low-pressure turbocharger 200 to a second pressure P2 downstream of the low-pressure turbocharger 200. The second pressure P2 is greater than the first pressure P1. The first pressure P1 is atmospheric pressure in this embodiment. The low-pressure turbocharger 200 is configured such that the maximum compression ratio of the second pressure P2 to the first pressure P1 is approximately 2.8 e.g. between 2.4 and 3.2, which occurs at the full load rated speed of the engine 100.

[0158] The high-pressure turbocharger 202 may further compress the intake air to increase its pressure from the second pressure P2 upstream of the high-pressure turbocharger 202 to a third pressure P3 downstream of the high-pressure turbocharger 202. The third pressure P3 is greater than the second pressure P2. The high-pressure turbocharger 200 is configured such that the maximum compression ratio of the third pressure P3 to the second pressure P2 is approximately 2.0 e.g. between 1.6 and 2.4, which occurs at the peak torque of the engine 100

[0159] The low-pressure and high-pressure turbochargers 200, 202 may be configured such that a maximum compression ratio of the third pressure P3 to the first pressure P1 is in the range of 2.5 to 4.5, e.g. in the range of 3.0 to 4.0. It has been found that such maximum compression ratios enable the engine 100 to produce the maximum specific power output which is similar to comparable diesel fuel engines. In some embodiments this may be assisted by inter-stage cooling as described in more detail below.

[0160] The high-pressure turbocharger 202 is smaller, and thus has a lower inertia, than the low-pressure turbocharger 200 to provide the required compression ratio of the intake air, and also being able to spin up to speed more quickly by virtue of having a lower rotational inertia.

[0161] The low-pressure turbocharger 200 includes an intake inlet 200ii in the intake passage 120 upstream of the respective compressor 204a, and an intake outlet 200io in the intake passage 120 downstream of the respective compressor 204a. The high-pressure turbocharger 202 comprises an intake inlet 202ii in the intake passage 120 upstream of the respective compressor 204b, and an intake outlet 202io in the intake passage 120 downstream of the respective compressor 204b. Intake air passes from the air source in series through the intake inlet 200ii, the compressor 204a and then the intake outlet 200io of the low-pressure turbocharger 200, and then through the intake inlet 202ii, the compressor 204b and then the intake outlet 202io of the high-pressure turbocharger 202, and then to the cylinders 112 via a conduit 203 (only part of which is shown in Figures 1 and 2).P614603PC00

[0162] The intake passage 120 includes an intake connector 263 connecting the intake outlet 200io of the low-pressure turbocharger 200 to the intake inlet 202 ii of the high-pressure turbocharger 202.

[0163] In the illustrated embodiment, the intake outlet 200io of the low-pressure turbocharger 200 faces upwardly (normal to the axis X3), so as to reduce the required streamwise length of the intake connector 263. In embodiments in which the low-pressure turbocharger 200 is higher than the high-pressure turbocharger 202, the intake outlet 200io of the low-pressure turbocharger 200 may instead face downwardly.

[0164] As the intake air is compressed by each turbocharger 200, 202, the temperature of the intake air increases. For example, the intake air may be around 120°C immediately downstream of the high-pressure turbocharger 202. Such high temperature intake air can affect combustion stability and efficiency. Therefore, in this embodiment, the intake system 110 includes a charge air cooler device 201 (illustrated schematically in Figures 3 and 5) for cooling the intake air in the intake passage 120. The charge air cooler device 201 may employ water and / or air cooling of the intake air. The charge air cooler device 201 is arranged in the intake passage 120 downstream of the high-pressure turbocharger 202. In other embodiments, the intake system 110 may additionally or alternatively include a charge air cooler device in the intake passage 120 between the low-pressure and high-pressure turbochargers 200 (i.e. downstream of the low-pressure 200 and upstream of the high-pressure 202 turbocharger).

[0165] A throttle 205 is additionally provided in the intake passage 120 downstream of the charge air cooler device 201 to regulate the flow of intake air.

[0166] Each turbocharger 200, 202 further comprises a turbine 210a, b drivingly connected to the respective compressor 204a, b via a shaft 211 such that the turbine 210a, b and respective compressor 204a, b are rotatable about the respective rotational axis X3, X4 of the turbocharger 200, 202. Each turbine 210a, b is arranged in the exhaust passage 208 such that the turbine 210a, b, and thus the corresponding compressor 204a, b, is driven by the flow of exhaust gas.

[0167] In the illustrated embodiment, the low-pressure and high-pressure turbochargers 200, 202 are arranged in series in the exhaust passage 208 such that the flow of exhaust gas from the cylinders 112 drives the turbine 210b of the high-pressure turbocharger 202, then drives the turbine 210a of the low-pressure turbocharger 200, and is then exhausted via the exhaust vent 122. In certain circumstances, this arrangement enables the high-pressure turbocharger 202 to extract more energy from the exhaust gas. The flow of exhaust gas downstream of the high-P614603PC00

[0168] pressure turbocharger 202 still has sufficient energy to drive the low-pressure turbocharger 200. The low pressure turbocharger 200 comprises a larger turbine 210a and larger compressor 204a than the high pressure turbocharger 202 to recover more of the remaining energy from the exhaust gas. However, in the present embodiment, the energy recovered from the from the exhaust gas by the respective high and low pressure turbochargers 202, 200 may be adjusted by the particular design of the turbocharger components - such as the trim of the turbine vanes - and the use of waste gates as described below. This may result in each turbocharger 200, 202 being more active at differing engine speeds and loads.

[0169] The low-pressure turbocharger 200 includes an exhaust inlet 200ei in the exhaust passage 208 upstream of the respective turbine 210a, and an exhaust outlet 200eo in the exhaust passage 208 downstream of the respective turbine 210a. The high-pressure turbocharger 202 includes an exhaust inlet 202ei in the exhaust passage 208 upstream of the respective turbine 210b, and an exhaust outlet 202eo in the exhaust passage 208 downstream of the respective turbine 210b. Exhaust gas passes from the cylinders 112 in series through the exhaust inlet 202ei, the turbine 210b and then the exhaust outlet 202eo of the high-pressure turbocharger 202, and then through the exhaust inlet 200ei, the turbine 210a and then the exhaust outlet 200eo of the low-pressure turbocharger 200, and then to the exhaust vent 122.

[0170] The exhaust inlet 202ei of the high-pressure turbocharger 202 is mounted to the exhaust manifold outlet 238, such that exhaust gas exiting the exhaust manifold outlet 238 enters said exhaust inlet 202ei. This helps minimise the streamwise length of the exhaust passage 208 between the cylinders 112 and the high-pressure turbocharger 202, which helps to minimise heat loss in the exhaust gas at the high-pressure turbocharger 202, and thus maximise the amount of energy which can be extracted. In the illustrated embodiment, the high-pressure turbocharger 202 is mounted to a substantially upward facing surface 240 of the exhaust manifold 236 so as to simplify assembly, but may not be in other embodiments. In some embodiments the exhaust inlet 202ei and the exhaust manifold outlet 238 may be manufactured as a single part.

[0171] In the illustrated embodiment, the exhaust system 206 includes two waste gates 207 for controlling the flow of exhaust gas to the turbines 210a, b of the low-pressure and high-pressure turbochargers 200, 202 (i.e. one waste gate 207 per turbocharger 200, 202). The waste gates 207 enable the speed of the turbines 210a, b to be controlled by allowing some or all of the exhaust gas to bypass the turbines 210a, b, e.g. to prevent the compressors 204a, b from generating excessively high boost pressures. In other embodiments, one or both waste gates 207 may be omitted.P614603PC00

[0172] In some embodiments, one or both of the low-pressure and high-pressure turbochargers 200, 202 may be variable-geometry turbochargers (VGTs). This helps reduce turbo lag and improve turbocharger efficiency.

[0173] In this embodiment, the low-pressure turbocharger 200 intake inlet 200ii is an axial inlet aligned with axis X3 and the intake outlet 200io is radial. Similarly, the high-pressure turbocharger 202 intake inlet 202ii is an axial inlet aligned with axis X4 and the intake outlet 202io is radial. In this embodiment, the low pressure turbocharger 200 exhaust inlet 200ei is radial and the exhaust outlet 200eo is axial and aligned with axis X3. The high pressure turbocharger 202 exhaust inlet 202ei is radial and the exhaust outlet 202eo is axial and aligned with axis X4. In other embodiments, this may not be the case.

[0174] It is common for comparable power diesel fuel engines to have, at most, a single turbocharger for compressing the intake air. Therefore, since the present engine 100 is provided with two turbochargers 200, 202, a problem arises of packaging both turbochargers 200, 202 on the engine 100 without significantly increasing the space envelope required in a machine, such as the working machine 10 or a genset, for receiving the engine 100, whilst achieving the required performance thereof.

[0175] To address this problem, each turbocharger 200, 202 is mounted to the side face 102c and / or the top face 102b of the engine structure 102 such that the rotational centres R1, R2 of the turbochargers 200, 202 are vertically offset from each other with respect to the base 102a. Advantageously, such an arrangement helps ensure that the turbochargers 200, 202 can be efficiently packaged so as to minimise the space envelope needed for the turbochargers 200, 202 as well as the portions of the intake and exhaust passages 120, 208 connecting the turbochargers 200, 202.

[0176] In the illustrated embodiment, the low-pressure and high-pressure turbochargers 200, 202 are mounted to the side face 102c. In particular, both turbochargers 200, 202 are mounted to the cylinder head 106, as will be discussed more below. In alternative embodiments, one or both of the turbochargers 200, 202 may additionally or alternatively be mounted elsewhere on the side face (e.g. the cylinder block 108), and / or to the top face 102b (e.g. the cam cover 107).

[0177] With reference to Figure 4, the rotational centre R1 of the low-pressure turbocharger 200 is lower than the rotational centre R2 of the high-pressure turbocharger 202 with respect to the base 102a. This helps to minimise the maximum height of the engine 100, and frees up space towards the top of the engine 100. In the illustrated embodiment, theentirety of the compressor 204a of the low-pressure turbocharger 200 is lower than the compressor 204b of the high-P614603PC00

[0178] pressure turbocharger 202 with respect to the base 102a. In alternative embodiments, the compressor 204a of the low-pressure turbocharger 200 may instead be partially (e.g. mostly) lower than the compressor 204b of the high-pressure turbocharger 202 with respect to the base 102a.

[0179] In the illustrated embodiment, the intake inlets 200ii, 202ii of the low-pressure and high-pressure turbochargers 200, 202 are entirely vertically offset from each other. This helps to provide clear access to the intake inlets 200ii, 202ii for conduits forming part of the intake passage 120. In alternative embodiments, the intake inlets 200ii, 202ii may be partially, e.g. mostly vertically offset from each other.

[0180] In the illustrated embodiment, the exhaust outlets 200eo, 202eo of the low-pressure and high-pressure turbochargers 200, 202 are entirely vertically offset from each other. This helps to provide clear access to the exhaust outlets 200eo, 202eo for conduits forming part of the exhaust passage 208 In alternative embodiments, the exhaust outlets 200eo, 202eo may be partially, e.g. mostly vertically offset from each other.

[0181] With reference to Figures 4 and 5, the intake inlet 200ii and the exhaust outlet 200eo of the low-pressure turbocharger 200 is aligned with a reference axis, which is aligned with the rotational axis X3 in this embodiment, but may not be in other embodiments. The intake inlet 202ii and the exhaust outlet 202eo of the high-pressure turbocharger 202 is aligned with a reference axis, which is aligned with the rotational axis X4 in this embodiment, but may not be in other embodiments. In Figure 5 the flow path of exhaust gas is indicated by hatched block arrows and the air inlet flow path is indicated by white block arrows.

[0182] In the illustrated embodiment, the rotational axes X3, X4 (and therefore reference axes) are substantially parallel to each other, but may not be in other embodiments. This helps to improve packaging of the turbochargers 200, 202.

[0183] In the illustrated embodiment, the rotational axes X3, X4 are substantially parallel to the crankshaft axis X1 , but may not be in other embodiments. This helps to improve packaging of the turbochargers 200, 202.

[0184] In the illustrated embodiment, the rotational centres R1, R2 of the low-pressure and high-pressure turbochargers 200, 202 are higher than the cylinder block 108 with respect to the base 102a. This makes space available for ancillary engine components mounted to the cylinder block 108. In alternative embodiments, one or both rotational axes R1, R2 may be at the same level as the cylinder block 108.P614603PC00

[0185] With reference to Figure 5 and to Figure 6 (which shows a cross-sectional view along section A-A in Figure 4), the rotational centres R1, R2 of the low-pressure and high-pressure turbochargers 200, 202 are transversely offset from each other along the transverse axis X2 of the engine 100. This helps to provide clear access to the inlets and outlets of the turbochargers 200, 202.

[0186] In the illustrated embodiment, the rotational centre R1 of the low-pressure turbocharger 200 is transversely spaced from the crankshaft axis X1 more than the rotational centre R2 of the high-pressure turbocharger 202. In alternative embodiments, the rotational centre R1 of the low-pressure turbocharger 200 may be transversely spaced from the crankshaft axis X1 less than, or the same as, the rotational centre R2 of the high-pressure turbocharger 202.

[0187] The intake outlet 200io of the low-pressure turbocharger 200 is transversely spaced from the crankshaft axis X1 more than the intake inlet 202ii of the high-pressure turbocharger 202, but may not be in other embodiments. This helps to reduce the length of the intake passage 120 between the turbochargers 200, 202.

[0188] With further reference to Figures 4 and 5, the rotational centres R1, R2 of the low-pressure and high-pressure turbochargers 200, 202 are axially offset from each other with respect to the crankshaft axis X1. Such an arrangement helps minimise the lengths of the intake and exhaust passages 120, 208 between the turbochargers 200, 202.

[0189] In the illustrated embodiment, the rotational centre R1 of the low-pressure turbocharger 200 is closer to the rear face 102d of the engine structure 102 relative to the rotational centre R2 of the high-pressure turbocharger 202. In alternative embodiments, the rotational centre R2 of the high-pressure turbocharger 202 may be instead be closer to the rear face 102d, or the rotational centres R1, R2 may be substantially axially equally spaced from the rear face 102d.

[0190] In the illustrated embodiment, the intake outlet 200io of the low-pressure turbocharger 200 and the intake inlet 202ii of the high-pressure turbocharger 202 are axially offset from each other along the crankshaft axis X1. This helps minimise the streamwise length of the intake passage 120 between the turbochargers 200, 202, whilst enabling the suitable routing of the conduits making up the passages 120, 208.

[0191] As best seen in Figure 4, in the illustrated embedment, the exhaust outlet 202eo of the high-pressure turbocharger 202 and the exhaust inlet 200ei of the low-pressure turbocharger 200 are axially offset from each other, but are transversely aligned (i.e. they are substantially equally transversely spaced from the crankshaft axis X1), but may be in other embodiments. Such an arrangement helps to reduce the streamwise length of the exhaust passage 208P614603PC00

[0192] between the turbochargers 200, 202. This helps to reduce heat loss in the exhaust gas at the low-pressure turbocharger 200, as well as help minimise turbo lag. The short exhaust passage 208 length also retains heat in the exhaust gas for heating any aftertreatment system downstream of the low pressure turbocharger 200 exhaust outlet 200eo, so the aftertreatment system reaches its most efficient operating temperature more rapidly.

[0193] The exhaust passage 208 includes an exhaust connector 220 connecting the exhaust outlet 202eo of the high-pressure turbocharger 202 to the exhaust inlet 200ei of the low-pressure turbocharger 200. In the illustrated embodiment the exhaust connector 220 is substantially L-shaped, but may have any suitable shape in other embodiments (e.g. substantially straight).

[0194] As previously discussed, the high-pressure turbocharger 202 is mounted to the cylinder head 106 via the exhaust manifold 236 in this embodiment. It will be appreciated that when the engine 100 is running, the exhaust manifold 236 and the high-pressure turbocharger 202 will be heated to a higher temperature relative to a bracket 230 (shown in Figure 8) and the low-pressure turbocharger 200. As such, thermal expansion of the exhaust manifold 236 and the high-pressure turbocharger 202 will impart forces on the low-pressure turbocharger 200 via the connections therebetween.

[0195] To accommodate relative movement of the turbochargers 200, 202 resulting from such thermal expansion, the exhaust connector 220 includes a flexible portion 222 configured to flex in response to such relative movement. In the illustrated embodiment, the flexible portion 222 includes bellows configured such that the flexible portion 222 is expandable. In alternative embodiments, the flexible portion 222 may have any suitable configuration.

[0196] The exhaust connector 220 includes a portion 224 which slopes downwardly in a streamwise direction towards the exhaust vent 122 for draining condensed water from the exhaust gas, and away from the cylinders 112. In some embodiments, the exhaust passage 208 may include a water trap for collecting condensed water and configured to drain the collected water therefrom.

[0197] Each of the low-pressure and high-pressure turbochargers 200, 202 includes a lubricating system for lubricating moving parts thereof, e.g. one or more bearings for the shaft 211. Each lubricating system includes a first external orifice 226 for receiving a supply of lubricant (e.g. oil) therein and a second external orifice 228 for draining lubricant therefrom. In the illustrated embodiment, the first and second external orifices 226, 228 are in a part of the respective turbocharger 200, 202 between the compressor 204a, b and the turbine 210a, b which houses a bearing for the shaft 211.P614603PC00

[0198] In the illustrated embodiment, the first and second external orifices 226, 228 of the low-pressure turbocharger 200 are vertically, axially and transversely offset from the first and second external orifices 226, 228 of the high-pressure turbocharger 202, but in other embodiments may be axially and / or transversely aligned. This enables oil drain lines illustrated by double-dashed lines 242 to extend generally straight down from the second external orifices 228 of the turbochargers 200, 202 to the engine structure 102 (e.g. the cylinder block 108 for the high pressure turbocharger 202 and gearcase 109 for the low pressure turbocharger 220) so that oil can drain freely.

[0199] The low-pressure turbocharger 200 is mounted to the engine structure 102 at a mounting position partially lower than a corresponding mounting position of the high-pressure turbocharger 202 on the engine structure 102 with respect to the base 102a. In alternative embodiments, the engine structure mounting position of the low-pressure turbocharger 200 may be at the same level as or higher than that of the high-pressure turbocharger 202.

[0200] With reference to Figure 8, the low-pressure turbocharger 200 is mounted to the cylinder head 106 via the bracket 230 projecting transversely from the cylinder head 106 with respect to the crankshaft axis X1 such that the low-pressure turbocharger 200 is spaced from the engine structure 102.

[0201] The bracket 230 includes an arc-shaped end 231 opposite the engine structure 102 for receiving a corresponding arc-shaped portion of the low-pressure turbocharger 200. The low-pressure turbocharger 200 is secured to the bracket 230 via three fasteners 233, but could be one, two or more than three fasteners 233 in other embodiments, and the portion need not be arc-shaped. The fasteners 233 are received in holes 235 adjacent the arc-shaped end 231 of the bracket 230 and corresponding holes in a flange extending from the low-pressure turbocharger 200. Providing the holes 235 adjacent the arc-shaped end 231 makes it easier to access the fasteners 233 to simplify assembly and maintenance.

[0202] The bracket 230 has a first bending axis X5 perpendicular to the crankshaft axis X1 , and a second bending axis X6 parallel to the crankshaft axis X1. In order to inhibit plastic deformation of the bracket 230 due to thermal expansion of the engine 100, the bracket 230 is configured to be more flexible in bending about the first bending axis X5 relative to the second bending axis X6. That is, for identical bending moments about the first and second bending axes X5, X6 respectively, the bracket 230 is configured to flex more about the first bending axis X5 relative to the second bending axis X6. This helps the bracket 230 to accommodate the thermal expansion without plastically deforming, which may weaken and / or damage the bracket 230. In the illustrated embodiment, the first bending axis X5 is a vertical axis, so thatP614603PC00

[0203] the arc-shaped end 231 of the bracket 230 is more flexible in a direction parallel to the crankshaft axis X1 relative to a direction perpendicular to the crankshaft axis X1.

[0204] With reference to Figures 2 and 6 in particular, the engine 100 includes a plurality of ancillary engine components mounted to the side face 102c of the engine structure 102, including a high-density power take off (HDPTO) pump 232, a conduit 234 extending from the HDPTO pump 232, and an alternator 239. In this embodiment, the HDPTO pump 232 pumps hydraulic fluid along the conduit 234 for driving hydraulic equipment on the working machine 10, such as its steering mechanism, and is separate to a main hydraulic pump used to drive actuators of the machine 10 (e.g. for moving a working implement such as an arm). The turbochargers 200 and 202, the intake passage 120 and exhaust passage 208 are required to be located with clearances to such ancillary engine components, and so that access is obtainable to fasteners for mounting such components, where applicable.

[0205] Further, different machine applications may require different ancillary components to be fitted, depending upon their particular attributes. Therefore, providing clearances to fit and access these different ancillary components may be particularly beneficial in engines 100 for use in off-highway applications, given the wide variety of components to be fitted, and space envelopes in which the engines are required to fit. In some embodiments the ancillary components may be fitted and removed to a machine whilst in service, making ease of access beneficial for service personnel.

[0206] In addition, in the illustrated embodiment, the bracket 230 spaces the low-pressure turbocharger 200 from the engine structure 102 such that the conduit 234 is interposed between the low-pressure turbocharger 200 and the engine structure 102, as shown in Figure 6. This helps to more efficiently package the engine’s externally mounted components so as to minimise the overall space envelope required for the engine 100. In other embodiments, additional or alternative ancillary engine components may be interposed between the low-pressure turbocharger 200 and the engine structure 102.

[0207] Figure 6 shows an alternative arrangement of the low-pressure turbocharger, which is indicated by reference 200’ and which is represented schematically in dashed circle. In this alternative arrangement, the rotational centre R1’ of the low-pressure turbocharger 200’ is higher than the rotational centre R2 of the high-pressure turbocharger 202 with respect to the base 102a of the engine structure 102.

[0208] In Figure 6, the low-pressure turbocharger 200’ is mounted to the top face 102b of the engine structure 102. As such, the low-pressure turbocharger 200’ is mounted to the engine structureP614603PC00

[0209] 102 ata mounting position higher than the mounting position of the high-pressure turbocharger 202 on the engine structure 102. In Figure 6, the low-pressure turbocharger 200’ is mounted to the cam cover 107. In alternative embodiments, the low-pressure turbocharger 200’ may additionally or alternatively be mounted to the side face 102c (e.g. the cam cover 107 and / or the cylinder head 106). The high-pressure turbocharger 202 may be mounted to the cylinder block 108, the cylinder head 106 and / or the cam cover 107.

[0210] In a further alternative embodiment, the low pressure turbocharger 200 may by mounted entirely or partially to the exhaust manifold 236. Mounting the low pressure turbocharger 200 on the exhaust manifold 236 in conjunction with the high pressure turbocharger 202 may reduce the amount of differential thermal expansion between the two components, which in turn may reduce the size of or obviate the need for a flexible coupling between them. In addition it may reduce the tolerance stack between the two components and also enable a pre-assembly of the turbochargers onto the exhaust manifold 236 separate from the main production line, and enable a more efficient overall assembly. This mounting position may also enable the low pressure turbocharger to be mounted in an elevated position, potentially reducing the length of the conduits connecting the two turbochargers.

[0211] In Figure 6, the rotational centre R1’ of the low-pressure turbocharger 200’ is transversely spaced from the crankshaft axis X1 less than the rotational centre R2 of the high-pressure turbocharger 202. This helps to reduce the space envelope required for the engine 100.

[0212] Figure 9 illustrates an example of a cooling system 250 incorporating charge air cooler devices in more detail. In this example two charge air cooler devices 201a, 201b are provided. The firstand second charge air cooler devices 201a, 201b are cooled by water. The cooling system 250 includes a water cooler 284, a water tank or reservoir 286 and a water pump (not shown). The cooling system 250 is configured to convey cooling water to the first and second charge air coolers 201a, 201b via cooling water circuits 288 to cool the first and second charge air coolers 201a, 201b. In this way, the cooling system 250 is capable of providing a coolant to the charge air cooler devices 201a, 201b that is separate from the coolant supplied to the internal combustion engine.

[0213] In this embodiment, the first charge air cooler device 201a is an interstage cooler located between the low pressure turbocharger 200 and the high pressure turbocharger 202 and the second charger air cooler device 201b is an aftercooler located between the high pressure turbocharger 202 and the engine structure 102.P614603PC00

[0214] In other embodiments the cooling system 250 may be shared with the cooling system (not shown) for the engine structure 102.

[0215] As indicated above, in this embodiment, the high-pressure turbocharger 202 is a variable geometry turbocharger (i.e. the high pressure turbine 210b comprises adjustable vanes to selectively restrict or de-restrict the flow of exhaust gases into the turbine 210b). The vanes may be actuated by a suitable actuator, such as an electrical or pneumatic actuator (not shown). In this embodiment, a dual axle VGT is utilised for durability in off-highway applications.

[0216] In addition, in this embodiment the high-pressure turbocharger 202 may comprise a recirculation valve (RCV) on the high pressure compressor. The RCV may be actuated by a suitable actuator, such as an electrical or pneumatic actuator.

[0217] In the embodiment depicted in Figure 9, no waste gates are provided on either the high pressure or low pressure turbines 210a, 210b, but in other embodiments such waste gates 207 may be provided in either or both turbines 210a, 210b. Further, in this embodiment, the low pressure turbocharger 200 is a fixed geometry (non-VGT) turbocharger which does not comprise an RCV or other control mechanisms. In other embodiments, this may not be the case, and the low pressure turbocharger 200 may comprise a variable geometry turbine, and RCV and / or a wastegate 207.

[0218] Referring to Figure 10, a similar layout of the engine 100 is depicted to Figure 9, but with the addition of a control system 300 incorporating a controller 302 to control operation thereof. The controller 302 may comprise: control circuitry; and / or processor circuitry; and / or at least one application specific integrated circuit (ASIC); and / or at least one field programmable gate array (FPGA); and / or single or multi-processor architectures; and / or sequential / parallel architectures; and / or at least one programmable logic controllers (PLCs); and / or at least one microprocessor; and / or at least one microcontroller; and / or a central processing unit (CPU), to perform the described methods. The controller 302 may include an associated memory or the memory may be located locally to the controller or remotely. The memory may be a nonvolatile flash memory.

[0219] The controller 302 is arranged to communicate with a plurality of sensors and actuators via a CAN bus or other suitable electronic communication link as depicted in Figure 10 by dotted lines.

[0220] In this embodiment, the controller 302 is arranged to signal actuation of the vanes 210b1 of the variable geometry high pressure turbine 210b and the recirculation valve 204b1. InP614603PC00

[0221] addition, the controller 302 may signal actuation of the throttle 205 to selectively restrict air flow into the intake manifold 114. Further, the controller 302 may signal the actuation of pumps 290 in the cooling water circuits 288 and / or operation of a fan 292 arranged to blow air over the water cooler 284. These components may additionally provide feedback on their position or speed to the controller 302.

[0222] Additionally, the controller 302 may receive signals from sensors including a mass airflow (MAF) sensor 304 upstream of the low pressure compressor 204a, a boost pressure sensor 306 downstream of the high pressure compressor 204b, temperature sensors 308 in the water cooling circuits 288 downstream of the first and second charge air coolers 201a, 201b, and engine speed via a speed sensor 310.

[0223] The controller 302 may additionally monitor other parameters commonly associated with engine 100 operation, either directly from suitable sensors, or inferred from other parameters and / or models (so-called virtual sensors), including one or more of engine load, exhaust temperature, exhaust pressure, intake temperature, speed of the low pressure and high pressure turbochargers 200, 202, throttle demand (e.g. from a throttle input device 312 such as a hand throttle lever or foot throttle pedal), ambient temperature and ambient pressure, and may additionally control other engine components such as fuel injectors, spark plugs etc. (not shown).

[0224] The controller 302 is configured to follow and air-led control strategy, that is the controller 302 determines the required air mass to achieve the desired torque output, and then meters the hydrogen fuel supplied to the cylinders 112 of the engine 100 to match the air mass in the cylinders 112 at the required air-fuel ratio.

[0225] As noted above, hydrogen fuelled engines typically require a greater air mass than comparable gasoline or diesel engines to achieve the same torque and power output. This is a consequence of seeking to minimise NOx emissions by combusting the hydrogen in a lean air-fuel mixture (a lambda in excess of 2, e.g. around or in excess of 2.4-2.5).

[0226] The temperature of exhaust gases leaving the hydrogen fuelled engine 100 is generally lower at high load and higher at low loads than a comparable diesel engine. For example, the exhaust gas temperature in the exhaust manifold 236 may reach a peak of around 500°C at maximum load and a minimum of 250°C at low loads and engine speeds, whereas these values for a diesel engine may be around 700°C and 130°C respectively. This means that on one hand the enthalpy of the exhaust gas is lower at points on the engine map where more air mass is required and less energy is available to drive the turbine 210a, 210b to provide theP614603PC00

[0227] air mass, and on the other may be too high when a lower air mass entering the cylinder(s) is required.

[0228] Figure 11 illustrates in simplified terms a torque curve of the engine of Figure 10, which closely matches that of a diesel fuelled engine of the same displacement, and in which maximum torque is relatively flat across the main working range to provide a desired performance in demanding off-highway applications as described above.

[0229] From Figure 11 it can be appreciated that the variable geometry arrangement of the high pressure turbocharger 202 provides an additional contribution to torque at low engine speeds by partially closing the vanes 210b1 (reducing turbine effective area) to increase the exhaust airflow velocity into the high pressure turbine 210b and therefore increase the boost pressure ratio of the high pressure compressor 204a at these lower engine speeds, this is represented by the additional torque above the dashed line up to around 1300rpm. This is achieved whilst retaining a high value for lambda by virtue of the additional air being forced into the engine 100 at lower engine speeds, to minimise emissions of NOx, which in hydrogen fuelled engines tends to increase as the air-fuel ratio (AFR) drops.

[0230] Conversely, at higher engine speeds (approx. 1750rpm to 2250rpm) opening the vanes 210b1 (increasing turbine effective area) provides a limited contribution to the compression ratio (boost) and consequentially limited additional torque. The declining percentage contribution at increasing engine speeds is represented generally by the dotted line of Figure 11 , although it will be appreciated that this may be adjusted to an extent by adjusting the opening of the vanes 210b1. At higher engine speeds, a greater contribution to the compression ratio and therefore torque output is provided by the low pressure turbocharger 200. As the low pressure turbocharger 200 is fixed geometry, its contribution to torque increases substantially in proportion to engine speed, and so at high engine speeds provides the majority of compression ratio and torque contribution as represented by the dot-dash line of Figure 11. This allows the two turbochargers 200, 202 to provide a combined compression ratio of 3.0 or more, e.g. 3.4 or more, between 1200rpm and 1800rpm. It will however be appreciated that at low loads and low engine speeds the compression ratio may however be at or very close to 1.

[0231] In low load conditions for the engine 100, the low pressure and high pressure turbochargers 200, 202 may be unable to limit the supply of air to the cylinders 112 to an appropriate mass, in view of their sizing and the enthalpy of the exhaust gases in these conditions. Therefore, the intake throttle 205 may be adjusted by the controller 302 to restrict air mass entering the cylinders 112 and ensure that a greater torque output than demanded is avoided, whilstP614603PC00

[0232] ensuring that the air-fuel ratio is low enough for stable combustion without misfires. A region of the operating map where the intake throttle 205 is active (partially closed) for this purpose is depicted in Figure 11. More torque than demanded is undesirable as it may make the controls of the working machine 10 too sensitive for the working operation being undertaken, or in the case of traction, wheel slip may be generated rather than forward motion in low grip conditions. However, using the throttle 205 to restrict intake air does increase pumping losses that reduce the efficiency of engine operation at low loads.

[0233] Figures 12 and 13 depict compressor maps for the low and high pressure turbochargers 200, 202 respectively. The key of Figure 12 also applies to Figure 13. Figure 12 shows that the full load line runs substantially linearly through the centre of the map, being “passive” and providing almost no boost at low speeds and full boost at full load rated speed, and further that efficiency of the turbocharger 200 is maximised at full load.

[0234] By contrast, the high pressure turbocharger 202 full load line runs across the map due to the operation of its variable geometry vanes 210b1 , meaning its efficiency reduces at higher and lower engine speeds, but a higher boost with a pressure ratio in excess of 2 is available at low engine speeds. By comparing the two maps, it can be seen that both the low pressure and high pressure turbochargers 200, 202 operate at high efficiency (most efficient pumping) where the peak engine torque is achieved at around 1500rpm. In turn, this contributes to high brake thermal efficiency (BTE) of the engine at this particular part of the engine operating map.

[0235] Engines used in working machines are also often required to work cyclically, shifting rapidly from low to high load and vice-versa, in excavating operations or Y-cycle loading operations, for example. In addition to assisting with low emission and efficient steady-state operation, the provision of variable geometry vanes 210b1 and a recirculation valve 204b1 also aid transient hydrogen fuelled engine operation.

[0236] In situations where a ramp up in torque demand is sensed, e.g. by monitoring a rate of change of demand from the throttle input device 312, the variable geometry vanes 210b1 are restricted to rapidly spin-up the compressor and raise the pressure ratio of the high pressure turbocharger 202, and therefore allow more hydrogen to be provided to the engine whilst a lean air-fuel ratio is maintained, whilst avoiding the compressor stalling (reaching the choke line). Conversely, if torque demand rapidly reduces, e.g. again by monitoring a rate of change of demand from the throttle input device 312, optionally in conjunction with determining how close to the surge line the high pressure turbocharger is operating or if the controller 302 determines the surge line is reached or is about to be reached, the recirculation valve 204b1 may be opened so the excess boosted air circulates back upstream of the high pressureP614603PC00

[0237] turbocharger 202 and the surge line of the compressor map is not crossed, or does so only to a limited degree , such that the high pressure turbocharger 202 does not enter a surge state, or only to a minimal extent.

[0238] The use of air to water cooling for the interstage cooler 201a and the aftercooler 201b allows for improved heat rejection from the charge air by virtue of the greater specific heat capacity of water, compared to air to air cooling. For example, heat rejection across both coolers 201 a, 201b at peak torque may be approximately 40kW for the engine 100 of the present embodiment. This allows for a lower pressure drop across the coolers 201a, 201b compared to air to air coolers, which is beneficial for pumping efficiency of the engine 100. It also allows for lower air volumes of the coolers 201a, 20b, which may improve the transient response of the engine 100. Surprisingly, the use of two stages of cooling has also been found to be more efficient than one stage downstream of the high pressure compressor 204b.

[0239] Due to low combustion temperatures for hydrogen compared to other fuels, it is also desirable for the intake air temperature to be low, e.g. 55°C or less at engine intake ports to avoid preignition (knock). The use of air to water cooling makes this more readily achievable, particularly at high ambient operating temperatures. For example, the engine 100 of the present embodiment is rated up to an ambient operating temperature of 46°C. This is particularly the case where the cooling system 250 is separate from the cooling system (not shown) for the engine structure 102, which typically operates at higher temperatures than those required for the cooling system 250.

[0240] The use of pumps 290 and / or one or more fans 292 operable under the influence of the temperature sensors 308 in the water cooling circuits downstream of the first and second charge air coolers 201a, 201 b enables parasitic power losses to be minimised since the pumps 290 and / or fans 292 need only operate when water temperatures above a threshold value are sensed.

[0241] A method of operating the low pressure and high pressure turbochargers 220, 202 and throttle 205 as part of overall engine operation is now described with respect to Figure 14.

[0242] With the engine 100 running, the process starts at step S400 by the controller 302 reading a value for at least current engine torque output, rate of change of engine torque output, engine speed, rate of change of engine speed, engine torque demand and rate of change of engine torque demand. These data are then utilised at step S402 for the controller 302 to determine, using a suitable look-up table or algorithm, if the current turbine vane position 210b1 is correct, or if adjustment is required. For example, if engine speed is low (e.g. less than aroundP614603PC00

[0243] 1500rpm, or less than 1300rpm) and torque demand is higher than the current level, a more restricted vane position 210b1 may be indicated to increase turbine 210b and compressor 204b speed and thus raise the pressure ratio and mass flow generated by the high pressure turbocharger 202, to allow for more fuelling at a constant air-fuel ratio, and the higher torque output to be achieved. At step S404, the controller 302 signals the adjustment of the turbine vane position 210b1 if adjustment is required and the process moves to step S406. If no adjustment is needed, the process moves straight to step S406.

[0244] At step S406, the controller 302 determines, using a suitable look-up table or algorithm, if the current compressor recirculation valve 204b1 position is correct, or if adjustment is required. For example if engine torque is high, engine speed is low or moderate (indicating that the high pressure turbocharger 202 is operating close to its surge line) and rate of change of torque demand is steeply negative (a sudden reduction) then the controller 302 may determine that the recirculation valve 204b1 should be opened to avoid the high pressure compressor 204b entering a surge state. If this is the case, the process moves to step S408 and signals the recirculation valve 204b1 to open, before moving to step S410. If no adjustment is needed, the process moves straight to step S410.

[0245] At step S410, the controller 302 determines, using a suitable look-up table or algorithm, if the current throttle 205 position is correct, or if adjustment is required. For example, if engine torque demand is lower than engine torque output, and air flow mass cannot be reduced further, e.g. by restricting airflow via the variable geometry vanes 210b1 , then the controller 302 may determine that adjustment of the throttle 205 position is required to restrict air flow and maintain stable combustion of hydrogen at a suitable air-fuel ratio. If so, at step S412, the controller 302 signals adjustment of the throttle 205 position and a cycle of the process is completed. If not, adjustment of the throttle 205 position is required at step S410, then a cycle of the process is also completed. In either case, the process returns to the start and runs repeatedly at suitable intervals whilst the engine 100 is running.

[0246] It should be noted that the turbine vane position 210b1 , recirculation valve position 204b1 and throttle 205 position may all be proportionally adjusted between fully open and fully closed or a predetermined degree of closure. The adjustment may be step-wise with a plurality of intermediate steps or may be continuous. This allows for optimisation of the operation of the high pressure turbocharger 202.

[0247] It will be appreciated that in other embodiments, the steps S402 to S412 may be re-ordered as required.P614603PC00

[0248] Off highway engines may be called up to operate at altitudes significantly above sea level. This presents a challenge to forced induction systems, as the lower density air at high altitudes requires higher compressor speeds to achieve the same mass flow as at sea level, and also creates a greater propensity for surge issues to arise and low cycle fatigue to arise where the transient operation, temperature changes etc. Thus, to provide a suitable buffer to mitigate these risks and minimise a requirement to de-rate the engine at high altitudes, the present application applies an approximate 20% margin to steady state compressor operating speeds that are determined at sea level. For example, this may allow for operation of the engine 100 up to 4000m above sea level and / or no de-rating of engine performance until 2000m above sea level.

[0249] In some embodiments, the turbocharger layout described above may be used in conjunction with exhaust gas recirculation (EGR). This may alter (reduce) the sizing of one or more of the turbochargers needed, but adds complexity and potentially makes packaging more difficult. However, it does not alter the fundamental operation of the system.

[0250] The one or more embodiments are described above by way of example only and it will be appreciated that the variations are possible without departing from the scope of protection afforded by the appended claims.

Claims

P614603PC00Claims1. A hydrogen fuelled internal combustion engine comprising:an engine structure comprising one or more cylinders defining an engine displacement; an intake system configured to supply a flow of intake air from an air source along an intake passage to the one or more cylinders for combusting hydrogen fuel therein;a low-pressure compressor device comprising a compressor located in the intake passage; anda high-pressure compressor device comprising a compressor located in the intake passage arranged in series with the low pressure compressor device;an exhaust system configured to exhaust a flow of exhaust gas from the one or more cylinders, along an exhaust passage to an exhaust vent, wherein at least one, of the low-pressure and high-pressure compressor devices comprises a turbine drivingly connected to the respective compressor, and wherein the or each turbine is arranged in the exhaust passage so as to be driven by the flow of exhaust gas;the engine having a specific power output of 20kW to 40kW per litre of displacement.

2. The engine of claim 1, having a maximum specific power output in the range of 25 to 30 kW / L, for example around 27kW / L.

3. The engine of any preceding claim, wherein the low-pressure and high-pressure compressor devices are configured such that a maximum compression ratio of an intake air pressure downstream of the high-pressure compressor device to an inlet air pressure upstream of the low-pressure compressor device is the range of 2.5 to 4.5.

4. The engine of claim 3, wherein the ratio is in the range of 3.0 to 4.0.

5. The engine of claim 3 or claim 4, wherein the compression ratio of the low pressure compressor device is between 2.4 and 3.2, which occurs at the full load rated speed of the engine.

6. The engine of any one of claims 3 to 5, wherein the compression ratio of the high pressure compressor is between 1.6 and 2.4, which occurs at the peak torque of the engine.

7. The engine of any one of claims 3 to 6, wherein the low pressure and high pressure compressor devices are configured such that the compression ratio of the intake air is at least36P614603PC003.0 between 1200rpm and 1800rpm at maximum torque, optionally the compression ratio of the intake air is at least 3.4 between 1200rpm and 1800rpm at maximum torque .

8. The engine of any preceding claim, wherein the high pressure compressor device comprises a recirculation valve in the compressor thereof.

9. The engine of any preceding claim, wherein the high pressure compressor device comprises a variable-geometry turbine.

10. The engine of any preceding claim, wherein the low pressure compressor device is a fixed geometry turbocharger.

11. The engine of any preceding claim, wherein the low pressure compressor device comprises no bypass or recirculation valves.

12. The engine of any preceding claim, wherein the low pressure compressor device compressor is solely drivable by energy in the engine exhaust gases via the turbine.

13. The engine of any preceding claim, wherein the high pressure compressor device compressor is solely drivable by energy in the engine exhaust gases via the turbine.

14. The engine of any preceding claim, further comprising a throttle in the intake system, the throttle being arranged to selectively restrict the flow of intake air.

15. The engine of any one of claims 8, 9 or 14, further comprising a controller, the controller being configured to signal adjustment to the position of at least one of the recirculation valve, the turbine geometry and the throttle in response to changes in engine operating conditions, in particular in response to a change in a torque demand of the engine.

16. The engine of claim 15, wherein the controller is configured to signal a reduction in the effective area of the variable geometry turbine in response to an increase in torque demand at engine speeds lower than 1500rpm, optionally lower than 1300rpm.

17. The engine of claim 15 or claim 16, wherein the controller is configured to signal an opening of the recirculation valve in response to a rate of reduction in torque demand above a predetermined threshold, optionally wherein the controller is configured to monitor a rate of change of demand from a throttle input device to determine a rate of reduction of torque37P614603PC00demand, optionally wherein the controller additionally or alternatively is configured to monitor proximity of the operation of the high pressure turbocharger to a surge line of an operating map thereof in determining whether to signal an opening of the recirculation valve.

18. The engine of claim 15 or claim 16, wherein the controller is configured to signal a reduction in the effective area of the airpath so as to restrict airflow in response to torque demand being below predetermined level.

19. The engine of any preceding claim, wherein the intake system further comprises one or more charge air cooler devices for cooling the intake air in the intake passage, wherein the one or more charge air cooler devices are arranged downstream of the low-pressure compressor device, e.g. downstream of the high-pressure compressor device and / or between the low-pressure and high-pressure compressor devices.

20. The engine of claim 19, comprising a first charge air cooler device between the low pressure compressor device and high pressure compressor device and a second charge air cooler device downstream of the high pressure compressor device.

21. The engine of claim 20, wherein the first charge air cooler device is an air to water cooler.

22. The engine of claim 20 or claim 21 , wherein the second charge air cooler device is an air to water cooler.

23. The engine of claim 21 or claim 22, wherein the flow of water to one or both of the first and second charge air cooler devices is configured to be adjusted in response to a temperature of the water.

24. The engine of any preceding claim, wherein the low-pressure and high-pressure compressor devices are arranged in series in the exhaust passage with the turbine of the high-pressure compressor device upstream of the low-pressure compressor device.

25. The engine of any preceding claim, further comprising one or more a hydrogen fuel injectors arranged to inject hydrogen fuel directly into the or each corresponding cylinder of the engine.P614603PC0026. A method of operating the engine of claim 15 or any claim when dependent upon claim 15, the method comprising a step of the controller monitoring a torque demand of the engine and the controller signalling adjustment of one or more positions of at least one of the recirculation valve, the turbine geometry and the throttle.

27. The method of claim 26, further comprising a step of the controller signalling a reduction in the effective area of the variable geometry turbine in response to an increase in torque demand at engine speeds lower than 1500rpm, optionally lower than 1300rpm.

28. The method of claim 26 or claim 27, further comprising a step of the controller signalling an opening of the recirculation valve in response to a rate of reduction in torque demand above a predetermined threshold, optionally wherein the controller monitors a rate of change of demand from a throttle input device to determine a rate of reduction of torque demand, optionally wherein the controller additionally or alternatively monitors proximity of the operation of the high pressure turbocharger to a surge line of an operating map thereof in determining whether to signal an opening of the recirculation valve.

29. The method of any one of claims 26 to 28, further comprising a step of the controller is signalling a reduction in the effective area of the airpath so as to restrict airflow in response to torque demand being below predetermined level.

30. A working machine or genset comprising the engine of any one of claims 1 to 24.

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