The present invention concerns a hydrogen fuelled internal combustion engine.

There is widespread recognition of the global need to move away from use of internal combustion ("1C") engines fuelled by hydrocarbons due to the damage they cause to the environment, especially through emission of carbon compounds and other pollutants into the atmosphere. Atmospheric carbon is of course regarded as a major causative factor in global warming. Much attention is focussed at present on electric power as a substitute for 1C engines in vehicles, but current electric vehicles suffer from numerous disadvantages including their limited range, their relatively high cost of manufacture, the time taken to re-charge batteries, and the environmental disadvantages associated with the manufacture and disposal of large volumes of electric batteries.

Hydrogen is potentially a very good fuel for internal combustion engines. This fact has long been recognised: hydrogen was for example used by Isaac De Rivaz in the first decade of the nineteenth century to fuel what some regard as the very first internal combustion engine. Hydrogen has a very high rate of diffusion, allowing almost 100% mixing with air on entry to a combustion chamber. It can be ignited by a low energy spark. Under the right conditions it is able to combust in a very uniform manner at about the same flame speed as petroleum (in US English, "gasoline"). The sole products of combustion of hydrogen with oxygen (neglecting any other reactions taking place) are energy and water. From the environmental perspective, it is also hugely significant that hydrogen fuel contains no carbon and so its combustion does not in itself contribute to carbon loading of the atmosphere. So a hydrogen engine has the potential to be very environmentally friendly, especially if the hydrogen itself is obtained using a renewable source of energy.

There are however technical obstacles to the widespread adoption of hydrogen as a fuel for 1C engines. One of these concerns emission of nitrogen oxides ("NO _x "), an important class of atmospheric pollutants. Nitrous oxide (N2O) is believed to be an important greenhouse gas, contributing to global warming, and to have an important deleterious effect on the ozone layer. Other nitrogen oxides contribute for example to the formation of smog and acid rain. Emission of NO _x by vehicles is subject to official regulation in many countries.

The high temperatures created in an 1C engine's combustion chamber can cause reaction of atmospheric nitrogen and oxygen, creating NO _x .

In a hydrocarbon fuelled 1C engine, NO _x is chemically reduced by reaction with carbon monoxide (CO), a product of the combustion process, yielding nitrogen gas (and in some reactions water and carbon dioxide). This reduction of NO _x is for example key to the operation of catalytic exhaust gas converters. But in a hydrogen fuelled engine the main combustion process does not involve any carbon compound. Hence no CO is produced upon combustion to mitigate NO _x release. As a consequence a hydrogen fuelled engine, operated at an air-fuel ratio comparable to that of a petroleum or diesel fuelled engine (close to a stoichiometric ratio, l= 1) can emit unacceptably high levels of NO _x .

The automobile industry has for some time focussed much development on downsizing of hydrocarbon-fuelled 1C engines, using forced induction and other technologies to obtain high power capacity from engines of relatively modest swept volume. A reduction in engine size may for example result in reduced parasitic losses and hence improved efficiency.

Modern engines often use high pressure direct fuel injection, with the timing and conditions of fuel injection being varied according to engine load and other factors, in an effort to maximise efficiency and reduce emissions. Directly injected engines often use swirl chambers and like features in the combustion chamber to localise the air-fuel mixture.

The present inventor has recognised that modern hydrocarbon-fuelled 1C engines are in various respects poorly suited to conversion for use with hydrogen fuel, and that a different approach to engine design is needed to provide an effective hydrogen fuelled engine.

A specific example of the present invention will now be described, by way of example only, with reference to the accompanying drawing, Figure 1, which is a section through part of an engine constructed in accordance with the present invention.

The drawing shows part of a hydrogen fuelled, four stroke internal combustion engine 8 whose intake port 10 leads to combustion chamber 12 via intake valve 14. The combustion chamber 12 is formed inside cylinder 16 by piston 18. The piston 18 drives a crank (not shown) through connecting rod 20. Exhaust gas leaves the combustion chamber 12 via exhaust valve 22 and exhaust port 24.

The inventor has recognised that a hydrogen engine may advantageously be designed with a relatively modest maximum output power in proportion to its swept volume. This underlying design philosophy runs counter to the trend of recent decades for downsizing of conventional hydrocarbon-fuelled 1C engines. Further, the general assumption in engine design is that larger engines are used to provide increased power capacity. Hence conventional engines which are large - in terms of swept volume - generally have component parts (camshaft, connecting rods, shaft bearings and so on) designed to handle the large stresses associated with generation of high power. In short, larger engines (in terms of swept volume) are typically expected to be powerful but also heavier. By reversing this design thinking, and designing a hydrogen fuelled engine to have a modest power capacity in proportion to its swept volume, many advantageous developments are made possible. The present engine is configured to run leaner than a conventional hydrocarbon fuelled engine. Air to fuel ratio is conventionally expressed using l values, where a stoichiometric air/fuel mix has a l value of 1, a richer mixture has l less than one and a lean mixture has a l value greater than one. The present engine is expected to run with l values between approximately 2.2 and 40. The fuel/air mix is of course adjusted according to instantaneous factors including engine power demand.

By running a hydrogen engine with a lean mixture, NO _x formation can be reduced. In fact operating at a l value of 2.2 or higher it is possible to produce almost zero NO _x . Hydrogen has a very high rate of diffusion, so that it mixes very quickly with the air. This, combined with its excellent combustion properties, make it possible to run a hydrogen fuelled engine at very lean l values, compared with an engine fuelled on hydrocarbons.

Of course if one takes the volume of air in the combustion chamber to be fixed (as a function of the engine swept volume and the air intake pressure) then a leaner air/fuel ratio implies that less fuel is combusted and so less power is produced. This is one reason why the present engine has a modest power capacity in proportion to its swept volume.

The engine 8 is configured to operate with low BMEP (Brake Mean Effective Pressures) of up to a maximum of 1100 kPa (11 Bar).

Compared with a conventionally designed engine, swept volume of the engine 8 must be approximately 1.5 to 2 times larger for a given maximum power output.

This does not however mean to say that the present engine need necessarily be heavy or excessively bulky in relation to its power capacity. The approach of designing an engine with a relatively low power in proportion to swept volume clearly implies that the cylinders themselves will be large in relation to output power. But other engine components can be designed commensurately with the output power and BMEP. Hence the cylinder block (item 26 in the drawing) is of relatively lightweight construction in proportion to the swept volume. The engine 8 uses a cylinder block 26 comprising aluminium with steel cylinder liners, which is in itself a conventional combination of materials, but the block's wall thicknesses can be designed with reference to the modest BMEP. The components making up the engine's crank mechanism including the crankshaft, its bearings, the connecting rods and gudgeon pins (in US English, wrist pins) are likewise all of modest strength and mass, compared with a conventional engine of corresponding swept volume. Because the power of the engine 8 is modest in proportion to its swept volume, the heat generated by combustion is correspondingly modest and the capacity of the engine cooling system can be smaller than would be expected for a conventional hydrocarbon fuelled engine, which assists in minimising weight and bulk of the engine itself and also reduces the power required for coolant circulation.

The piston 18 has - in comparison with the piston of a conventional hydrocarbon fuelled engine, and commensurately with its modest BM EP, a reduced piston ring set having modest ring tension. The piston ring set may comprise a single sealing ring in addition to an engine oil scraper ring.

Running the hydrogen fuelled engine 8 at high l values results, as noted already, in an exhaust containing very low levels of undesirable emissions, its main exhaust output comprising hot air and the water vapour that results from hydrogen combustion. The present engine 8 is configured to operate at high l values and without exhaust gas recirculation in a lower power operating range. In a higher power operating range the air fuel mixture is richer (lower l) and exhaust gas recirculation is used to reduce NO* production.

In the present embodiment the engine's exhaust system has no catalytic converter. It has no provision for chemical conversion of exhaust components. In comparison to a conventional, modern hydrocarbon fuelled engine, this results in a considerable saving in weight and bulk of the power plant as a whole.

The resultant engine 8 is similar in overall weight and packaging requirements to a modern diesel engine of corresponding power capacity.

The engine 8 uses forced induction to provide boost pressure of up to 300 kPa (3Bar) gauge pressure. More specifically, the present embodiment uses a turbocharger (not shown) which is designed for high boost at low engine power. An intercooler is provided to cool intake air downstream of the turbocharger and upstream of the intake port 10. This takes the form of a charge air cooler, in the present embodiment.

The engine 8 uses spark ignition achieved using a spark plug in the combustion chamber 12. The spark plug is omitted from Figure 1 for the sake of representational simplicity. Unlike a conventional petrol fuelled spark ignition engine, the engine 8 has no throttle in the air intake path. Engine load (engine power) is controlled by control of the quantity of hydrogen injected. The hydrogen fuelled engine 8 is capable of operating effectively at high air fuel ratio (high l) so that even when idling or operating at very low load (so that the quantity of fuel injected is small) it is not necessary to throttle intake air. Stratified charge systems are not used in the present embodiment.

The combustion chamber 12 and the intake port 10 are "open". Not only is there no throttle, but the intake port and combustion chamber are configured to minimise swirl and tumble in the engine intake. The intake port on a two valve per cylinder engine 10 is aligned along a radius of the cylinder 16, rather than being offset from a radius to promote swirl, or on a 4 valve per cylinder engine the port is angled steeply to minimise the effect of tumble. The engine intake port 10 is an open passage without internal features. It has no swirl features. The piston crown is in the form of an open bowl 28 in the illustrated embodiment. It could alternatively be flat or substantially flat. The piston crown lips are radiussed. Whereas a conventional petrol or diesel engine is designed to create swirl and tumble, the resultant air motion serving to promote fuel/air mixing and flame propagation, the engine 8 provides less swirl and tumble and less turbulence. It has been found that excessive air movement in a hydrogen engine can cause breakdown of the flame and consequent misfiring. Because of hydrogen's very high rate of diffusion, an open intake and combustion chamber provide more than adequate fuel/air mixing. The open intake port also provides a flow path for passage of air into the combustion chamber with minimal restriction (i.e. minimal drag to impede the flow).

The engine 8 may be direct fuelled or port fuelled. That is, fuel injectors may supply hydrogen fuel into the air intake port or directly into the combustion chamber 12. Conventional solenoid type injectors may be used. The present embodiment uses direct injection, alleviating risk of pre-ignition in the intake port.

The engine may be configured to operate according to the Miller cycle. The engine configuration may be such that the intake valve remains open through an early phase of the engine's compression stroke. This can result in expulsion of gas from the cylinder during the early phase of the compression stroke. The result is that the expansion ratio of the engine is greater than the compression ratio. Miller cycle engines can be advantageous in operating with reduced NO* emissions.

Hydrocarbon fuelled engines commonly operate with "valve overlap", which is to say that during the exhaust stroke, whilst the exhaust valve 22 is open and as the piston 18 approaches top dead centre, the intake valve 14 is opened a certain period before the exhaust valve 22 is closed. Thus there is a period during which both valves 14, 22 are open. This assists in exhaustion of combustion products from the combustion chamber 16. However because of the ease of ignition and high diffusion rate of hydrogen, an extended valve overlap period can create a risk of hydrogen combustion in the air intake (backflash) which is deleterious to performance. The engine 8 is thus designed to have little or no valve overlap. Overlap may be less than 4 degrees of camshaft angle, which is to say that the camshaft rotates through no more than 4 degrees whilst both valves are open. Overlap may be less than 2 degrees of camshaft angle. The present invention imposes no particular limitation on the mechanical means used to control the intake and exhaust valves 14, 22 but in the conventional case where they are cam controlled, the cam profiles are configured to provide this low (or no) valve overlap characteristic.