Internal combustion engine, vehicle and a method of operating them

The invention relates to an internal combustion engine comprising at least a cylinder including at least an inlet valve which can be operated between an open and a closed position, an air inlet system including at least an inlet duct mounted to the cylinder, such that it communicates with the cylinder when the inlet valve is opened, thus allowing air to flow from the inlet duct into the cylinder, an air control valve which can be operated between an open and a closed position and which is located in the inlet duct upstream of the inlet valve, a pre-chamber which is substantially defined by the space between the air control valve, the inner wall of the inlet duct and the inlet valve, and an engine management system which controls at least the air control valve.

Conventional four-stroke spark-ignition engines have a throttle valve in the air inlet system in order to control the airflow to the engine. The opening position of the throttle valve is related to the power output of the engine. The more air is allowed to flow to the engine the more fuel can be burned, generating a higher power output. When the engine runs at low load the throttle valve has such a position that the flow- through area is narrow. This situation is disadvantageous for engine efficiency as the vacuum introduced by the flow restriction introduces energy losses, because the piston has to work against the vacuum to draw air in the cylinder which effect is known as pumping losses. This is one of the main reasons of increased fuel consumption of spark-ignition engines at decreasing engine load. Therefore, some engine manufacturers develop spark- ignition engines comprising variable inlet valve timing mechanisms and eliminate the throttle valve (BMW s Valvetronic® or camless valve actuation) .

When the inlet valve timing is variable and the throttle valve is eliminated the inlet can be closed during the inlet stroke, for example, before the piston arrives at bottom dead centre. In that case air flows without restriction of a throttle

valve into the cylinder and filling is stopped when the inlet valve is closed. In this way the period in which the inlet valve is open is directly related to the engine load. A disadvantage of such a variable valve timing system is that the mechanism is complex and expensive. Therefore, simpler and cheaper methods are desired.

A type of internal combustion engine such as defined in the preamble is known in the art and is described in the European patent application EP 1 236 875. The engine shown in this document comprises an air control valve in the inlet duct, which air control valve is in its open position during a part of the period in which the inlet valve of the cylinder is open. In that situation air may flow from the inlet duct into the cylinder without throttling. The air control valve can be switched to its closed position at a predetermined moment during the inlet stroke when the inlet valve is still open. In that situation the piston is still moving downwards and as a consequence the air trapped in the cylinder and in the inlet duct downstream of the air control valve is expanded. At the beginning of the compres- sion stroke the inlet valve will be closed such as in conventional engines and the air in the cylinder will be compressed. The expansion work which is performed by the engine during the period that the air control valve is closed is neutralized by the compression work that the expanded air performs on the piston during the first part of the compression stroke. The net effect is that under part load conditions the cylinder is filled by air under atmospheric pressure until the moment that the control valve is closed, and thus total pumping work is reduced. In fact, the air control valve is a replacement of a variable inlet valve timing. However, as the air control valves described in EP 1 236 875 are placed in the inlet duct of each cylinder the valves have to operate at high speed and under all conditions. This makes that the requirements of the air control valves are very high and that there is no real advantage com- pared to camless valve actuation.

It is an object of the present invention to provide an internal combustion engine with improved part-load efficiency.

To obtain this object, the internal combustion engine comprises an air injection device which is connected to the pre- chamber for metering air to the pre-chamber.

The advantage of providing air to the pre-chamber is that it can be filled with air from an external air supply system at a pressure higher than the pressure in the inlet system upstream of the air control valve. In this way it is possible to not only eliminate the negative pumping losses but instead replace it by a positive pumping loop. The air injection device comprises an air transfer line connected to the pre-chamber and communicating with an air source, and a metering valve located in the air transfer line adapted to control the air flow from the air source to the pre- chamber through the air transfer line, which metering valve is under control by the engine management system. This configuration is relatively simple to provide air to the pre-chamber. Controlling the metering valve by the engine management system has the advantage that the valve can be operated flexibly. Metering air to the pre-chamber is preferably under part load conditions and up to intermediate engine speed; this means that the specifications for the metering valves are very common to already existing LPG or CNG injectors.

In a preferred embodiment the air source comprises a compressed air tank, which can be filled with air by an air com- pression device. When compressed air is stored in the tank it is possible to provide air to the pre-chamber, independent from other engine conditions.

The air compression device may be drivable by a driven shaft of the engine, preferably via a transmission adapted to vary the operating speed of the compression device with respect to that of the internal combustion engine. It is beneficial that the air compression device is drivable by the engine, because it is not necessary to apply a separate driving means. This also saves space, which is relevant if the internal combustion engine is mounted to a vehicle. A transmission is desired to provide the opportunity to operate the compression device at a high operating speed with respect to the actual engine speed. Thus, at a decreasing engine speed, for example during deceleration of a vehicle, a high air compression power can be maintained. These

features mean that a positive pumping loop can be achieved by re-using brake energy by means of energy regeneration of a compression device which fills the compressed air tank during braking . The air source alternatively comprises a high-pressure part in the air inlet system of the internal combustion engine. The high-pressure part may be located in the air inlet system downstream of an air filter or downstream of an inlet air compressor in the case of a supercharged engine, for example. If the air pressure in the high-pressure part is sufficiently high to enable engine running, it is advantageous to use the high- pressure part as an air source since it is a simple configuration. It is also possible to use the compressed air of a turbocharged engine at part load conditions to create a positive pumping loop. Where in normal situations the turbopressure is blocked by the throttle valve to create a vacuum downstream of the throttle valve this pressure can be used to create the desired high pre-chamber pressure and is thus not wasted.

A preferred embodiment is a combination of the com- pressed air tank and the high-pressure part in the air inlet system. In this embodiment the air injection device comprises a controllable selection valve, which has at least two positions so as to connect the compressed air tank or the high-pressure part of the air inlet system with the air transfer line. The ad- vantage of the combination is that air in the compressed air tank can be saved when sufficient air can be provided by the high-pressure part of the air inlet system.

The compressed air tank can also be filled by the air compression device at low engine speed and part load operation when the compressed air tank pressure has to be kept at a certain minimum level and air from the tank is used to create a positive pumping loop. In this case the engine runs at a higher load than demanded so as to provide energy for air compression. Generally, engine efficiency increases with increasing load. This means that compressed air is generated at a relatively high engine efficiency. Nevertheless, compression of air requires energy, but this loss may be smaller than the benefit of engine operation with a positive pumping loop.

In an advantageous embodiment of the engine at least the inlet valve is operated by a camless valve operating system. Such a system enables to open and close the inlet valve at any desired timing. It appears that this feature provides particular benefits in terms of engine efficiency to the engine according to the invention. When the pre-chamber is filled with pressurized combustion air, the inlet valve can be opened for a short period, hence _. enabling the pressurized air to flow into the cylinder and applying a positive force on the piston. After the inlet valve is closed the trapped air in the cylinder will be expanded instead of the air contained in the pre-chamber and the cylinder together such as in an embodiment with a conventional cam-driven inlet valve having fixed timing.

The invention also provides an internal combustion en- gine including at least a compressor for providing compressed air to the engine, said compressor comprising an inlet and an outlet and a bypass between the inlet and the outlet, which bypass can be opened and closed by a bypass valve allowing air to flow between the inlet and the outlet through the bypass when the bypass valve is open. This configuration is particularly beneficial to turbocharged engines as compressor work is directly related to turbine work of the turbocharger . This means that under certain conditions the compressor may provide too high air pressure. As a consequence the throttle valve will be further closed to reduce the airflow. Under these conditions the compressor compresses air against a back pressure in the air inlet system, which is in fact useless energy consumption. In such a situation it is efficient to apply a bypass to the compressor according to the invention. Another advantage of the bypass is that it can be opened during a switch from high to low engine load so that the turbocharger maintains a higher rotation speed than when compressing air against back pressure. This improves turbocharger response during a subsequent load increase, reducing the known turbo lag effect. The invention further provides a vehicle having an internal combustion engine as described above.

In a preferred embodiment of the vehicle, driving means of the air compression device are connected or connectable to rotating wheels of the vehicle via a transmission, such that the

air compression device is drivable by rotating wheels of the vehicle when this is running. These features enable the compression device to compress air to fill the compressed air tank during deceleration of the vehicle. Besides, the vehicle has more braking power when the engine is decoupled. It is also possible to use the engine itself as a compressor during a dec- celeration of the vehicle.

The invention also provides a method of operating the internal combustion engine, wherein air is metered to the pre- chamber at least when the air control valve is in the closed position and at least when the inlet valve is closed. When both valves are closed and air is metered to the pre-chamber up to above atmospheric pressure there is no pumping work to be achieved by the pistons and immediately after the inlet valve opens the air pressure in the pre-chamber will exert a positive force on the piston which results in improved efficiency.

Preferably, the air control valve is maintained in the closed position during at least part-load running of the engine. This means that under these operating conditions combustion air to the cylinder is only supplied via the air injection device. As under these conditions no air flows to the cylinder via the throttle valve and the air control valve, pumping losses are eliminated. These settings at part-load may be maintained up to a relatively high part-load level if the air pressure in the pre-chamber, before the inlet valve opens, is brought at a relatively high level so as to supply an amount of air to the cylinder corresponding to that high part-load level. As it is intended to provide the possibility to increase the pre-chamber pressure above atmospheric pressure, for example 4 bar, the air control valve can be maintained at a closed position up to a relatively high part-load condition, before it has to be opened in order to enable more air to flow into the cylinder.

Alternatively, in an engine embodiment of which the cylinder includes at least an outlet valve for releasing exhaust gases from the cylinder which outlet valve is operated between an open and a closed position, the inlet valve can be opened after the outlet valve is closed. This method creates a negative valve overlap, which may be beneficial in terms of engine effi-

ciency and exhaust gas emissions since it may generate internal EGR (Exhaust Gas Recirculation) .

Preferably, the cylinder is provided with a piston which moves within the cylinder between top dead center and bot- torn dead center, wherein the outlet valve is closed before the piston reaches top dead center and the inlet valve is opened after the piston has left top dead center. In this case exhaust gases are kept in the cylinder after the outlet valve is closed. These are compressed by the piston between the time of closing the outlet valve and top dead center. After the piston has left top dead center the exhaust gases are expanded. As this method provides the opportunity to fill the pre-chamber with pressurized air the inlet valve can already be opened at an early stage after top dead center without backflow of exhaust gases from the cylinder to the pre-chamber. This results, for example, in the possibility of achieving relatively high EGR rates at part-load running.

The invention further provides a method of operating a vehicle, wherein the air compression device is driven by rotat- ing wheels of the vehicle during vehicle decelerations. This method serves to fill the compressed air tank by using energy which otherwise would have been converted to heat generation in vehicle brakes.

The invention further provides a method of converting braking energy of a vehicle to a positive pumping loop during the inlet stroke of an internal combustion engine mounted to the vehicle, wherein the vehicle comprises a compressed air tank and an air compression device which is driven by the vehicle during at least a part of vehicle decelerations to supply air to the compressed air tank, and the engine comprises at least a cylinder and an air injection device which is operated to transfer combustion air from the compressed air tank to the cylinder. The features of this method enable to supply air of high pressure to the cylinder such that the air exerts a force on the piston dur- ing the inlet stroke when the piston moves from top dead centre to bottom dead centre, whereas the energy for air compression originates from vehicle braking. Thus, braking energy is regenerated to engine work. This method is typically advantageous during part-load operation of the engine.

This method may be applied on an engine of which the cylinder may include at least an inlet valve and which cylinder communicates with a pre-chamber when the inlet valve is open, wherein the combustion air is transferred to the pre-chamber substantially when the inlet valve is closed before it is transferred to the cylinder when the inlet valve is open. This provides the opportunity to prepare a certain amount of combustion air at relatively high pressure in the pre-chamber corresponding to the demanded engine power such that when the inlet valve opens the combustion air flows into the cylinder and exerts a positive force on the piston.

The pre-chamber may communicate with the cylinder only, when the inlet valve is open. This means that only the prepared quantity of combustion air in the pre-chamber can at least partly enter into the cylinder, which part is related to the demanded engine power. In other words, the cylinder and the pre- chamber form one closed space when the inlet valve is open. Therefore, in the period within which the inlet valve is open the pressurized air in the pre-chamber flows first into the cyl- inder and is expanded during the inlet stroke, for example down to below 0.5 bar. The expansion is typically for part-load conditions since only the amount of air required for the corresponding demanded power is transferred to the pre-chamber. In the case of full-load the pre-chamber would also communicate with the inlet system upstream of the air control valve. It may be clear that according to this method the air which is pressurized during a deceleration is converted to a positive pumping loop during part-load conditions.

The invention also provides a method of operating an internal combustion engine such as for driving a vehicle, the engine comprising an air inlet system and a turbocharger for compressing inlet air, wherein additional air is injected into the air inlet system particularly during a sudden load change such as an acceleration of the vehicle, which additional air is provided by an air source which comprises a compressed air tank filled by an air compression device that is preferably activated and driven by the vehicle during decelerations . For spark- ignition engines this method enables to achieve a temporary power output increase, which is sometimes desired during accel-

erations. Another advantage of this method is that it can be applied to compression ignition engines, as well, so as to reduce the known turbo lag effect.

These and other aspects and advantages of the invention will be apparent from the following description with reference to the drawings.

Fig. 1 is a schematic perspective cutaway view of the internal combustion engine according to the invention. Fig. 2a - 2f are schematic sectional views of an engine cylinder and a portion of the air injection device at different crankshaft positions during the outlet, inlet and compression stroke of the engine according to the invention.

Fig. 3a - 3b are schematic graphs of the four-stroke proc- ess at part load for a conventional engine (a) and the internal combustion engine according to the invention (b) .

Fig. 4 is a schematic perspective cutaway view of a bypass of a turbocharger according to the invention.

Fig. 5a - 5b are graphs of results of simulation computa- tions with different embodiments of an internal combustion engine according to the invention, illustrating the result related to an embodiment with a cam-driven inlet valve (5a) and one with a camless inlet valve system (5b).

Fig. 6 is a graph of a result of a simulation computation with an embodiment of an internal combustion engine according to the invention wherein the engine is operated with a negative valve overlap, illustrating the result of different engine parameters as a function of crank angle.

An embodiment of the internal combustion engine according to the invention is illustrated in Fig. 1. In the embodiment shown the internal combustion engine is a four-stroke spark- ignition engine having an air inlet system 1. The engine has four cylinders 2. Each of the cylinders 2 has an inlet valve 3 and an outlet valve 4, and a spark plug 5 to ignite a combusti- ble mixture trapped in the cylinder 2 above a piston 6, which piston is movable up and down in the cylinder 2. The piston 6 is connected to a crankshaft 7 via a connecting rod 8. The inlet valve 3 and outlet valve 4 are driven by the crankshaft 7 via a transmission (not shown) . The opening and closing of both valves

3, 4 are dependent on the rotational position of the crankshaft 7.

Each of the cylinders 2 is connected to an inlet duct 9 which communicates with the cylinder 2 when the inlet valve 3 is open. In that case air may flow from the inlet duct 9 into the cylinder 2 along the opened inlet valve 3.

In each of the inlet ducts 9 an air control valve 10 is mounted. This can be operated between an open and a closed position. The space between the air control valve 10, the inlet valve 3 and the inner wall of the inlet duct 9 is defined as a pre-chamber 11. When the air control valve 10 is in its closed position and the inlet valve 3 is closed as well, the pre- chamber 11 forms a closed space. In practice the pre-chamber 11 may have a volume of an order of magnitude of 0.3 dm ³ . In the embodiment of Fig. 1 a throttle valve 12 is located in the air inlet system 1 upstream of the inlet ducts 9. The throttle valve 12 can be operated between an open and a closed position which is controlled by an engine management system 13. Under normal engine operating conditions the air control valves 10 are fully opened and the throttle valve 12 is operated to control the air flow to the cylinders 2. Opening the throttle valve 12 means that more air is allowed to flow into the cylinders 2. Closing the throttle valve 12 results in a smaller airflow to the cylinders 2, such that less fuel can be burnt and the power output of the engine is reduced. Each of the cylinders 2 is provided with a fuel injector (not shown) which is located upstream of the inlet valve 3 or in the cylinder 2. The fuel injection is controlled by the engine management system 13 and the amount of metered fuel is related to the amount of air trapped in the cylinder 2.

In the embodiment shown in Fig. 1 the engine comprises a turbocharger 14. The turbocharger 14 comprises a rotatable compressor 15 and turbine 16 which have a common shaft. The turbine is driven by exhaust gas from the cylinders 2, which flows from the outlet valves 4 of the cylinders 2 to the inlet of the turbine 16 through a duct (not shown) . As a consequence the compressor 15 is driven by the turbine 16 via the common shaft and compresses air which flows from the environment through an air filter 17 to the inlet of the compressor 15.

In the embodiment of Fig. 1 the internal combustion engine is provided with an air injection device 18. The engine is further provided with an air compression device 19, which is shown as a roots blower type compressor, but may be another type of air compression device 19. The inlet air of the air compression device 19 is preferably taken from the air inlet system 1 of the engine downstream of the air filter 17. This has the advantage that no separate air filter is necessary. The outlet of the air compression device 19 is connected to a compressed air tank 20 such that it fills the compressed air tank 20 by compressed air when the air compression device 19 is driven.

Upstream of the air compression device 19 a closing device 21 is located to avoid backflow of air from the compressed air tank 20 into the air inlet system 1 of the engine. This closing device 21 may comprise an electrically controlled shut-off valve, a one-way valve or the like.

The air compression device 19 may be driven by the crankshaft 7 of the engine via a transmission (not shown) . It could be possible that in this embodiment the air compression device 19 is also used as a mechanical supercharger for the engine by replacing the turbocharger 14. If the air compression device delivers more compressed air than needed for the actual engine power output, the extra air can be used to fill the compressed air tank 20. As in this case the air compression device 19 is coupled to the crankshaft 7 via a transmission it provides the opportunity to drive the air compression device 19 during engine motoring; this is the situation in which the engine is driven such as by a vehicle during deceleration in the case that the engine is still mechanically connected with the rotating wheels. Then the compressed air tank 20 can be refilled during decelerations, for example. The advantage of filling the compressed air tank during decelerations is that kinetic energy of the vehicle is converted to compressed air, whereas the compressed air is used to create the positive pumping loop resulting in a more ef- ficient engine operating condition. This is particularly beneficial under low-load conditions as it is generally known, that in conventional spark-ignition engines with a throttle valve the pumping losses increase with decreasing engine load. Furthermore, the vehicle has extra brake power and reduced brake

wear. In a preferred embodiment the transmission is controllable such that the ratio between engine speed and operation speed of the air compression device 19 can be varied. It is, for example, desired that during engine motoring the air compression device 19 is maintained at a high operation speed to deliver compressed air as much as possible.

It is also possible that the air compression device 19 is connectable to the crankshaft 7 and is only operated when desired, comparable to an air conditioning system on a vehicle, for example. In this case the air compression device 19 is only operated during a deceleration for refilling the compressed air tank 20 and is not used as a supercharger for compression of inlet air to the cylinders 2.

In another embodiment the air compression device 19 may be mounted to the vehicle without being connected to the engine (this embodiment is not shown) . The air compression device 19 may be connected to the wheels of the vehicle and being activated during decelerations.

The compressed air tank 20 may be filled to a pressure of about 2-4 bar or higher. A moderate maximum air pressure of 2-4 bar has the advantage that a relatively simple and lightweight tank may be applied. The tank volume is a compromise between the mass and volume of the tank 20 and the air required for the engine to reduce fuel consumption under practical conditions. The tank volume may be 100 to 200 liter, for example. If the embodiment of Fig. 1 is applied in a vehicle the tank 20 may have an alternative shape, which for example fits in a cavity of the vehicle chassis or hollow parts of the chassis may form the compressed air tank. In the embodiment of Fig. 1 there are two different air sources that can be connected to the air injection device 18: the compressed air tank 20 and a high-pressure part 22 of the engine air inlet system 1 downstream the compressor 15 of the turbocharger 14. In the case of a naturally aspirated engine the high-pressure part of the air inlet system is formed by a duct downstream the air filter (not shown) . For both naturally aspirated engines and supercharged engines the air pressure is about atmospheric pressure or higher in the high-pressure part during engine running.

An alternative air source may be atmospheric ambient air. If providing this to the pre-chamber 11 a separate air filter is desired so as to feed clean air to the engine.

The air injection device 18 in the embodiment of Fig. 1 comprises two air transfer lines 23a, b and two metering valves 24a, b connected to each of the inlet ducts 9. The metering valves 24a, b are mounted to the pre-chamber 11 and can be operated by the engine management system 13. The metering valves 24a, b are time-base controlled valves which are preferably open within the period in which the inlet valve 3 is closed. When the metering valves 24a, b are opened air from one of the air sources, the compressed air tank 20 or the high-pressure part 22 of the engine air inlet system 1, is transferred through the air transfer lines 23a, b to the pre-chamber 11 if the pressure in the pre-chamber 11 is lower than in the air source where the air transfer lines 23a and/or 23b are communicating with.

In the embodiment of Fig. 1 the air transfer line 23b communicates with the high-pressure part 22 of the engine air inlet system 1 when the throttle valve 12 is opened. The air transfer line 23a may communicate with the high-pressure part 22 of the engine air inlet system 1 or the compressed air tank 20, depending on the position of a selection valve 25 which may receive a signal from the engine management system 13. During a filling cycle of the pre-chamber 11 it may be first provided with air from the high-pressure part 22 via the air transfer line 23b, possibly also via the air transfer line 23a, and later on with air from the compressed air tank 20. This saves the compressed air in the tank 20. If the air is also provided via the air transfer line 23a and directly followed by air from the com- pressed air tank 20 the selection valve 25 must be a quickly switching valve. In an alternative embodiment (not shown) the selection valve 25 may be eliminated so that the air transfer line 23a only communicates with the compressed air tank 20, whereas the air transfer line 23b only communicates with the high-pressure part 22 of the engine air inlet system 1.

If the air source comprises atmospheric ambient air (not shown) the pre-chamber 11 may be first provided with ambient air up to ambient pressure during a filling cycle. After that the

pre-chamber 11 can be filled further by air from the compressed air tank 20. This method saves compressed air in the tank 20.

Further alternative embodiments are obtained when the air transfer lines 23b are eliminated, while the air transfer lines 23a communicate with the compressed air tank only (without selection valve 25), or with the high-pressure part 22 only (without selection valve 25) , or with one of both by means of the selection valve 25 such as shown in Fig. 1.

Controlling each metering valve 24a, b by the engine manage- ment system 13 provides the opportunity to vary the amount of air injection over the different cylinders 2. This may be desired when the fuel delivery of the different fuel injectors differs or when cylinder filling between the cylinders 2 is not equal . An alternative embodiment may be that the pre-chamber 11 has a variable volume (not shown) . This gives an additional control parameter, such as allowing a higher degree of filling with an enlarged pre-chamber 11 whereas the air pressure is the same. The operation of the internal combustion engine and the air injection device 18 such as shown in the embodiment of Fig. 1, but without air transfer lines 23b and metering valves 24b, is explained below by examples of different engine operating conditions when the engine and air injection device are applied in a vehicle . When the engine is idling and the pressure in the compressed air tank 20 is still low, for example atmospheric pressure, the air may be fed from the high-pressure part 22 of the engine air inlet system 1, where the air pressure is also about atmospheric pressure at idle running. In this case the air control valves 10 are closed, the selection valve 25 is switched such that the high-pressure part 22 communicates with the air transfer line 23a, and the metering valve 24a is operated by the engine management system 13 to meter the amount of air which is required for combustion of fuel needed for idle running of the engine.

When the vehicle is accelerated and the compressed air tank 20 is not filled yet the situation does not change from idling, except that the metering valve 24a has a longer opening period

to increase the amount of air required for achieving a higher power output.

When further increasing the engine power the required amount of air may be such that it is not possible to meter it all during the period when the inlet valve 3 is closed. In that case the control valves 10 can be opened slightly to assist in filling the pre-chamber 11 or in an extreme case the engine may be switched to normal operation (still assuming that the compressed air tank is not filled) . This means that the air control valves 10 will be fully opened, the operation of the metering valves 24a will be stopped and the air flow to the cylinders 2 will be controlled by the throttle valve 12. In an alternative embodiment in which the throttle valve 12 is eliminated, the air control valves 10 may be operated between their fully open and closed positions to control the air flow.

When the engine is idling or it runs at part-load and the pressure in the compressed air tank 20 is higher than the air pressure in the high-pressure part 22 of the air inlet system 1 air from the compressed air tank 20 may be provided to the pre- chamber 11 by the metering valve 24a. In that case the selection valve 25 is switched to an opposite position in which the compressed air tank 20 communicates with the air transfer lines 23 and the air control valves 10 are closed. Operating the metering valve 24a in this situation means that air from the compressed air tank 20 is provided to the pre-chamber 11.

The air feeding process by the air injection device 18 and consequences for engine performance will be explained with reference to Fig. 2a-2f. These drawings illustrate the different steps of the air inlet flow process of a four-stroke spark- ignition engine under part-load conditions. The pressure values shown in the drawings are added to illustrate the pressure trace in the cylinder 2 and the pre-chamber over the subsequent steps. In practice the values may be different, because they strongly depend on demanded engine power. Fig. 2a: Air is injected by the metering valve 24a into the closed pre-chamber 11, whereas the outlet valve 4 is open and the piston 6 is moving upwards to press the residual exhaust gas originating from the last combustion stroke out of the cylinder 2. The inlet valve 3 is

closed. The air pressure upstream of the air control valve 10 is 1 bar (atmospheric pressure) . Due to the pressure difference over the opened metering valve 24a air flows into the pre-chamber 11. Fig. 2b: The piston 6 is in top dead centre, the metering valve 24a is already closed and the pressure in the pre- chamber 11 is 2.5 bar, the inlet valve 3 starts to open and the outlet valve 4 is closed. A pressure of 2.5 bar means that the engine runs at part-load. If a higher engine load is demanded the pressure in the pre-chamber

11 may be increased up to 4 bar, for example, which can be achieved by a longer opening period of the metering valve 24a. Fig. 2c: The piston 6 is just below top dead centre and air flows into the cylinder 2 via the opened inlet valve 3 as a consequence of the pressure difference between the pre-chamber 11 and the cylinder 2. Fig. 2d: The piston 6 is halfway between top dead centre and bottom dead centre and the pressure in the pre-chamber 11 and the cylinder 2 is more or less balanced; in reality the piston 6 is moving downward creating more volume hence generating continuous air flow from the pre-chamber 11 into the cylinder 2. The pressure values shown in Fig. 2d only serve to illustrate that the pressure in both the pre-chamber 11 and the cylinder 2 are more or less similar and will be reduced simultaneously.

Fig. 2e: The piston 6 has arrived in bottom dead centre, the inlet valve 3 is just closed and the pressure in the cylinder 2 and the pre-chamber 11 has finally reached

0.6 bar. This pressure value depends on the initial pressure in the pre-chamber 11 before the inlet valve 3 is opened. If the pre-chamber 11 was filled with ambient air of 1 bar instead of 2.5 bar the final pressure could be 0.25 bar for example. The final pressure also depends on the volume ratio between the pre-chamber 11 and the cylinder 2. Fig. 2f: During the compression stroke the air trapped in the cylinder 2 is compressed to 10 bar, for example. In the

meantime the pre-chamber 11 can be refilled as the inlet valve 3 is closed.

It may be clear from Fig. 2a-2f that the air control valve 10 is maintained at a closed position during the entire operat- ing cycle of the engine.

In Fig. 3 the gas exchange process is schematically illustrated by pressure - volume graphs (pressure and volume are referred to conditions in the cylinder) . Fig. 3a represents a part-load condition of a conventional four-stroke engine pro- vided with a throttle valve 12. After combustion and opening of the outlet valve the cylinder pressure decreases down to about ambient pressure in bottom dead centre. During the exhaust stroke the cylinder pressure more or less remains equal. During the inlet stroke the cylinder pressure is below ambient pres- sure. It is well-known in the art that this conventional gas exchange process has a negative work cycle, such as shown by the arrows in anti-clockwise direction in Fig. 3a.

Fig. 3b shows an example of the four-cycle process according to the invention. The letters a - f refer to the process steps described above and illustrated in Fig. 2. The exhaust stroke represented by the line from a to b is like the conventional process. When the inlet valve 3 opens near b the cylinder pressure rises due to air flowing from the pre-chamber 11 into the cylinder 2. During the inlet stroke when the piston 6 trav- els from top dead centre towards bottom dead centre (c-d-e in Fig. 3b) the cylinder pressure decreases down to below ambient pressure. Near e the inlet valve 3 will be closed. The first part of the compression stroke (from e towards f in Fig. 3b) the cylinder pressure will be more or less similar as during the last part of the inlet stroke. Comparing Fig. 3a and 3b turns out that the gas exchange process has changed from a negative cycle to a positive cycle, which is the consequence of the relatively high air pressure in the pre-chamber 11 which forces the piston 6 downwards when the air flows into the cylinder 2. An additional advantage of the positive pumping loop is that during expansion of the air in the pre-chamber 11 and the cylinder 2 the air temperature decreases. This provides the opportunity to operate the engine under a higher compression ratio in this situation which has a positive effect on engine effi-

ciency. This means that the invention has an additional advantage for engines comprising variable compression ratio.

As the inlet valve 3 is closed during about 1.5 engine revolutions a relatively long time period is available for air injection. Therefore, the air metering valves 24a, b may be compact. If a single metering valve 24a or 24b for each pre-chamber 11 appears to be insufficient it is possible to apply more than one. It is possible to use standard automotive gas injectors, such as for LPG injection, as metering valves 24a, b. In the case of applying disc valves, these may have an outer diameter of about 20 mm and a valve lift of 3 mm, for example.

If the engine is provided with both a throttle valve 12 and air control valves 10 the engine management system 13 will be programmed such that a shift from the air injection mode to the normal engine operation mode will occur smoothly. In this case it is preferred that the air control valves 10 are opened quickly and that the throttle valve 12 position is directed to the position that is required to the corresponding demanded engine power. Such a shift in operation mode happens, for example, when a higher engine power is demanded than can be provided by one of the air sources.

Fig. 4 shows an embodiment of a part of the air inlet system 1 which is provided with a turbocharger 14. The compressor 15 of the turbocharger 14 has a bypass 26 which can be opened and closed by a bypass valve 27. The advantage of this configuration is that in certain cases the bypass valve can be opened such that no pressure is built-up by the compressor 15 or a maximum pressure level is built-up. This situation is typically desired when the turbine runs at a speed at which a too high pressure is build-up. It is efficient to open the bypass valve so that less pressure is built-up. When applying the bypass in turbocharged engines the waste gate used in conventional turbo- charged engines may be eliminated. Another situation occurs when a sudden load change from high load to low load is effected: the turbocharger speed will decrease because of less power input from the exhaust gas, and also because of air compression work by the compressor 15. The latter effect will be eliminated by opening the bypass valve 27 reducing the back pressure on the compressor 15. As a consequence, at the next load step the tur-

bocharger will have a higher starting speed hence improving response time and reducing the known turbo lag effect.

The turbo lag effect is also known for turbocharged compression ignition engines. This effect typically generates the well-known black plume due to temporary air shortage in the cylinders after a load step. The air injection device 18 according to the invention can be effectively applied for this type of engine. If compressed air is provided to the tank by a compression device which is driven by rotating wheels of a vehicle during decelerations and the compressed air is added to the inlet system during accelerations the turbo lag effect can be reduced. Furthermore, as more air is available a higher temporary power output can be achieved.

In an alternative embodiment of the internal combustion en- gine according to the invention the inlet valve 3 is driven by a camless valve operating system (not shown) . Such a system enables to open and close the inlet valve 3 at any desired timing. Although it is known that such a system is relatively expensive over a cam-driven valve operating system, it offers particular benefits in respect of the engine according to the invention. This can be clarified by Fig. 5a and b.

Fig. 5a shows a result of a simulation computed for an engine which is similar to the embodiment such as shown in Fig. 1 and described above. The inlet valve 3 is driven by a camshaft in this case. In Fig 5a it can be seen that from the start of the exhaust stroke (a) until the moment of opening the inlet valve (near b) the cylinder pressure is more or less stable. After the inlet valve 3 has opened the cylinder pressure rises as a consequence of the higher pressure in the pre-chamber 11 as well as of closing the outlet valve 4. A certain period of time after passing top dead center the cylinder pressure decreases. During the inlet stroke (roughly from c to e) the air in the pre-chamber 11 as well as in the cylinder 2 is expanded. Near bottom dead center the inlet valve 3 is closed (e) and the pis- ton 6 starts to compress the air in the cylinder 2. It can be seen in Fig. 5a that the pressure trace during the first part of the compression stroke (e to f) significantly differs from that of the expansion during the inlet stroke.

Fig. 5b shows a result of a simulation computation with an embodiment of the engine having a camless valve operating system selected for the computations. The engine load is similar to that of the embodiment of which the computation results are shown in Fig. 5a. In the present case the cylinder pressure also increases after opening the inlet valve 3 (b) . However, the inlet valve 3 is closed short after top dead center (near c) , such that the cylinder pressure stays at a relatively high level compared to Fig. 5a. It can be seen that a big part of the ex- pansion pressure trace during the inlet stroke (roughly from c to e) is nearly similar to that of the first part of the compression stroke (e to f) . This appears to result in a significant improvement of engine efficiency with respect to the embodiment having a cam-driven inlet valve 3. It should be noted that in the case of Fig. 5a the air in the pre-chamber 11 as well as in the cylinder 2 is expanded between c and e, whereas in Fig. 5b only the air trapped in the cylinder is expanded after the inlet valve 3 is closed.

The advantage is not achieved with an engine which has a camless valve operation system only (without a pre-chamber 11 and an air injection device 18), because the positive pumping loop, such as indicated by a "+" in Fig. 5b is not achieved when the pressure upstream of the inlet valve 3 is equal to ambient pressure. In still another alternative embodiment of the internal combustion engine according to the invention the engine is provided with a negative valve overlap. In this case inlet valve 3 is opened after the outlet valve 4 is closed, whereas in conventional engines the inlet valve 3 already opens when the outlet valve 4 is not closed yet. Also in this case the valves 3, 4 may be driven by a camless valve operating system. It appears that an embodiment of an engine having a negative valve overlap has particular benefits in the case of pressurized air injection in the pre-chamber 11.

Negative valve overlap can be applied to create internal EGR (Exhaust Gas Recirculation) , which may be beneficial in terms of engine efficiency and exhaust gas emissions. The effect of negative valve overlap on the alternative embodiment of the engine according to the invention is described with reference to Fig. β. This Fig. shows cylinder pressure (Pcyl), pre-chamber

pressure (Ppc), inlet valve 3 lift, outlet valve 4 lift and air injection (AI) as a function of crank angle. The outlet valve 4 is closed (OC) before the piston 6 reaches top dead center. This means that a part of the exhaust gases that are still in the cylinder 2 stay in the cylinder 2. As the inlet valve 3 is still closed the cylinder pressure (Pcyl) starts to rise and reaches a maximum at top dead center. A certain period after top dead center the inlet valve 3 is opened (10) and the cylinder pressure (Pcyl) has already decreased due to expansion of the exhaust gases as a consequence of the displacement of the piston 6 in the direction of bottom dead center. In an engine which is operated with a negative valve overlap and which has atmospheric pressure (Pamb) as maximum pressure in the inlet system the exhaust gases will partly flow back into the inlet system when the cylinder pressure (Pcyl) is still above atmospheric pressure (Pamb). In the present alternative embodiment of the engine pressurized air is injected into the pre-chamber 11 before the inlet valve 3 is opened. As a consequence the inlet valve 3 can be opened earlier. Fig. 6 shows that the air injection system 18 injects air into the pre-chamber during the period indicated by AI. It can be seen that the pressure (Ppc) in the pre-chamber 11 rises to a certain level which is higher than the pressure in the cylinder 2 when the inlet valve 3 opens (10) . As a consequence, the ex- haust gases in the cylinder 2 do not flow back into the pre- chamber 11, but the pressurized air flows from the pre-chamber 11 into the cylinder 2. This means that relatively high EGR rates can be achieved with internal EGR.

From the foregoing it will be clear that the invention pro- vides an internal combustion engine which reduces the pumping losses which typical adversely affects the fuel consumption of conventional spark-ignition engines which are provided with a throttle valve 12. Due to the air injection device 18 according to the invention air can be metered to the cylinders without generating pumping losses. It also appears that the engine according to the invention provides particular benefits when the inlet valve is operated by a camless valve operating system and/or when the engine is operated with a negative valve overlap.

A vehicle which is provided with the internal combustion engine, the air injection device 18 and the compressed air tank 20 which is filled during decelerations according to the invention has an extra advantage: braking energy, which is normally destroyed, is converted to compressed air which is used to operate the engine at higher efficiency under part-load conditions. The advantage will be greater for vehicles which are used in dynamic traffic situations with a lot of accelerations and decelerations . The invention is not restricted to the above-described embodiments as shown in the drawings, which can be varied in several ways without departing from the scope of the claims. For example, the engine may have more or less cylinders, or the engine may be of a V-type. Each cylinder may have more than one inlet and outlet valve and more than one inlet duct per cylinder. It is also possible, for example, that the pre-chamber 11 has more than one metering valve 24a, b for air injection. Furthermore, the throttle valve 12 may be eliminated whereas the air control valves 10 may be operated to control the airflow to the engine during the normal engine operating mode as well as during the air injection mode. The air injection device can be applied for spark-ignition engines with direct as well as indirect fuel injection. It is also possible to connect the air injection device directly to the cylinder such that combustion air at elevated pressure is directly injected into the cylinder. In the case of a camless valve operating system or valve deactivation system the inlet valve could even be maintained closed when combustion air is injected directly into the cylinder.