Background of the invention Hybrid powertrain that uses compressed air to help power a vehicle could dramatically improve the fuel economy, particularly in cities and urban areas where the traffic conditions involve a lot of stops and starts. In such conditions, a large amount of fuel is needed to accelerate the vehicle, and much of this is converted to heat in brake friction during deceleration. Capturing, storing and reusing this braking energy to give additional power can therefore improve fuel efficiency, and this can be achieved by using the momentum of the vehicle during coasting and deceleration to drive an air compressor, and the compressed air can be stored and later used to propel the vehicle during cruising and acceleration.

The air hybrid vehicle described above would have the most economic impact if the engine itself is used as the compressor and expander, transmitting power through the pistons and the crankshaft of the engine thus braking and propelling the vehicle using the existing drive train of the vehicle. This eliminates the need of a separate air compressor and separate air motor thus reducing weight and complexity. SAE Paper 2003-01-0038 proposes one such air hybrid engine, but there are design limitations and practical complexities in the proposed concept which requires adapting and altering the function of the existing

engine valves for air hybrid operation. The present invention aims to avoid such adaptations.

Summary of the invention According to a first aspect the present invention, there is provided a regenerative air hybrid engine for a road transport vehicle, the engine being an internal combustion cycle engine and having at least one cylinder operating at selected times including coasting and deceleration as an air compressor inducting ambient air and compressing the air into a compressed air storage tank, while the intake and exhaust valves of the cylinder continue to operate normally during all the gas exchange and work phases of the engine according to the operating internal combustion cycle in the absence of fuel, characterised in that timed transfer of compressed air from the engine cylinder into the compressed air storage tank for storage in the tank is performed in two steps, using a buffer chamber having a predetermined volume and connected to the engine cylinder and to the compressed air storage tank by respective timed first and second shut-off valves synchronised with the gas exchange and work phases of the engine, wherein during the first step, the buffer chamber is connected to the engine cylinder during at least the later part of the compression phase of the engine by opening and closing the first shut-off valve at one set of predetermined times during the engine cycle while the second shut-off valve remains closed, and during the second step, the buffer chamber is connected to the compressed air storage tank during another part of the gas exchange and work phases other than the compression phase of the engine by opening and closing the second shut-off valve at another set of predetermined times while the first shut-off valve remains closed.

According to a second aspect of the present invention, there is provided the said regenerative air hybrid engine with the said cylinder operating at other selected times including cruising and acceleration as an air expander using high pressure air from the said compressed air storage tank, while the intake and exhaust valves of the cylinder continue to operate normally during all the gas exchange and work phases of the engine according to the operating internal combustion cycle in the absence of fuel, characterised in that timed admission of high pressure air from the compressed air storage tank into the engine cylinder for expansion in the cylinder is performed in two steps, using the said buffer chamber and the said timed first and second shut-off valves synchronised with the gas exchange and work phases of the engine, wherein during the first step, the buffer chamber is connected to the compressed air storage tank during one part of the gas exchange and work phases other than the expansion phase of the engine by opening and closing the second shut-off valve at one set of predetermined times during the engine cycle while the first shut-off valve remains closed, and during the second step, the buffer chamber is connected to the engine cylinder during a substantial part of the expansion phase of the engine by opening and closing the first shut-off valve at another set of predetermined times while the second shut-off valve remains closed.

In the above invention, the term'internal combustion cycle'is herein defined as the generic air power cycle experienced by the working fluid according to a prescribed sequence of gas exchange and work phases characterising a thermodynamic cycle which has a nominal internal combustion phase but which may or may not include actual combustion during that phase depending on the presence or absence of fuel in the working fluid. In the invention, by means of the buffer chamber and shut-off valves, the engine is used at selected times as an air compressor during the intake and

compression phases of the internal combustion cycle in the absence of fuel, separated by the expansion and exhaust phases which continue to operate normally through the cycle, and at other selected times the engine is used as an air expander during the expansion and exhaust phases of the internal combustion cycle in the absence of fuel, adding power to the cycle which continues to operate in full with inducted ambient air and completes the cycle substantially reversibly for the ambient air. Thus the invention does not rely on adapting or altering the function of the existing engine valves for regenerative air hybrid operation and is applicable to any engine operating according to a nominal internal combustion cycle as defined above.

The invention may be applied to a variety of combustion engines including reciprocating, rotary, poppet-valve, sleeve-valve, piston-port-valve, four-stroke-cycle, two- stroke-cycle, petrol, diesel, gaseous-fuel, spark-ignition, compression-ignition etc, each with individual cylinder fuel shut-off means to deny fuel to those cylinders operating in the regenerative compressor or expander mode.

The invention may also be applied to compressed air powered engines operating according to a nominal internal combustion cycle defined above but at all times without fuel. Finally, the invention may be applied to a non- powered engine with no thermodynamic energy supplied to the engine but still operating according to a nominal internal combustion cycle as defined above. This will be the case where a bank of cylinders in a multi-cylinder engine is at times motored substantially reversibly according to a nominal internal combustion cycle in the absence of fuel, and at other times operated in the regenerative mode either as a compressor or as an expander according to the present invention.

Going back to the case where the engine is a fuel burning engine, to ensure no fuel will enter the compressed air. storage tank, care should be taken to purge the engine cylinder of any residual fuel by motoring the cylinder for several cycles with no fuel before connecting with the buffer chamber.

In the case of a spark ignition engine controlled by a main throttle, an air bypass throttle bypassing the main throttle may preferably be provided in the intake system of the engine so that during coasting and deceleration with the fuel shut off and the main throttle closed, the bypass throttle may be opened to allow a full charge of air to be inducted into the cylinders working as compressors. In the case of an engine not controlled by a main throttle, for example, a diesel compression ignition engine or a direct injection spark ignition engine, there will be no need of a bypass throttle.

In the first aspect of the invention, the timed discharge of compressed air from the engine cylinder into the compressed air storage tank performed in two steps has significant advantage in that the compression work, or braking load experienced by the engine, is substantially constant and therefore predictable to the driver, and not dependent on the state of charge in the compressed air storage tank. This is because each compression phase of the piston works against a constant end volume which includes the volume of the buffer chamber isolated from the compressed air storage tank by the second shut-off valve and enables a substantially constant maximum compression pressure to be generated within the engine cylinder during the course of the compression phase irrespective of the state of charge in the compressed air storage tank. In contrast, had the engine cylinder been connected directly to the compressed air storage tank through a conventional compressor non-return valve, a low state of charge in the

tank would result in early opening of the non-return valve into the tank and a low compression pressure generated within the engine cylinder resulting in little braking work.

At the other end of the range, a fully charged tank will keep the non-return valve closed, not accepting any more compressed air and this air will re-expand in the engine cylinder balancing out the compression work. Only the middle range of charge in the tank (5-15 bar air pressure) would result in sufficient compression work and provide a useful braking load to the engine. This makes such a compression braking system rather unpredictable to the driver and at times ineffective, and this is obviate by the present invention.

In the second aspect of the invention, the timed admission of high pressure air into the engine cylinder performed in two steps has advantage in that it relaxes the actuation design specification of the high pressure air admission valve into the engine cylinder, the valve being the same as the buffer chamber first shut-off valve which stays open for a relatively long period of time during a substantial part of the expansion phase of the engine cycle.

This is to be contrasted with the high pressure air admission valve in a conventional air expander connected directly to a stock compressed air supply, where the valve must be opened and closed very quickly within a very short period of time while the piston is still near its top dead centre (TDC) in order to limit the high pressure air entering the cylinder for the air to expand with sufficient expansion ratio after the admission valve is closed. Such very short opening period of the admission valve poses severe problems to the design of the actuation system of the valve, and this is obviated in the present invention.

In the present invention, the timed actuation of the first and second shut-off valves may be performed using a variety of actuating means, including mechanical, electrical

hydraulic and pneumatic actuators. The shut-off valves may be shuttle valves, rotary valve or poppet valves.

In the first aspect of the invention, the first shut- off valve may be timed to remain open longer than the compression phase of the internal combustion engine cycle, and the second shut-off valve timed to remain open shorter than the remaining gas exchange and work phases. Preferably, the first shut-off valve is timed to remain open during most of the intake and compression phases of the internal combustion engine cycle, and the second shut-off valve timed to remain open during most of the expansion and exhaust phases, thus resetting the initial pressure and providing substantially equal time periods for the air connection of the buffer chamber alternately with the engine cylinder and with the compressed air storage tank. On the other hand, the first shut-off valve may be timed to open after the intake period of each engine cycle, in which case, the initial pressure in the cylinder will rise progressively through consecutive cycles resulting in higher and higher compression pressures forced into the buffer chamber.

In the second aspect of the invention, the first shut- off valve may be timed to remain open longer than the expansion phase of the internal combustion engine cycle, and the second shut-off valve timed to remain open shorter than the remaining gas exchange and work phases. Preferably, the first shut-off valve is timed to remain open during most of the expansion and exhaust phases of the internal combustion engine cycle, and the second shut-off valve timed to remain open during most of the intake and compression phases, thus providing substantially equal time periods for the air connection of the buffer chamber alternately with the engine cylinder and with the compressed air storage tank.

Preferably, the volume of the buffer chamber isolated by the second shut-off valve from the compressed air storage

tank is sufficiently small for it to be included with the clearance volume of the engine cylinder when operating in the compressor mode such that the resulting compression ratio is at least 5: 1. In this respect, the invention will be more effective when applied to a diesel engine of high compression ratio of approximately 18: 1 so that when the buffer chamber volume is included with the clearance volume, the resulting compression ratio is still relatively high at approximately 9: 1.

Also preferably, the volume of the buffer chamber isolated by the second shut-off valve from the compressed air storage tank is sufficiently small for it to be included with the clearance volume of the engine cylinder when operating in the expander mode, such that high pressure air expanding from the buffer chamber directly into the cylinder during the expansion phase achieves a substantially constant expansion ratio relative to the volume of the buffer chamber sufficiently to bring the expanded air pressure to substantially ambient pressure at the end of the expansion phase.

The above buffer chamber with its first and second shut-off valves may be designed as a compact unit with the two shut-off valves combined into a multi-channel valve actuated by a two-position actuator. This makes it relatively easy to adapt an engine cylinder to operate as a regenerative compressor and expander by connecting the compact unit to a convenient access hole in the cylinder, while the intake and exhaust valves of the cylinder continue to operate normally during all the gas exchange and work phases of the engine according to the internal combustion cycle.

Preferably, in a multi-cylinder air hybrid engine of the present invention, the cylinders are divided into two banks with one bank operating continuously as firing

cylinders and the other bank operating as regenerative compressor or expander cylinders at most times continuously except when switched back to become firing cylinders during high engine load conditions. In this case, the second bank will be motored substantially reversibly during normal operation through the said nominal internal combustion cycle, except when operated in the regenerative mode either as a compressor or as an expander according to the present invention.

This arrangement will be sufficient to provide adequate power to propel the vehicle in most urban driving conditions using one bank of firing cylinders supplemented with regenerative power using the other bank of compressor/ expander cylinders, the two banks working in parallel proportioning power according to need controlled by a central processing unit. Of course, the engine may be switched off when the vehicle stops, and restart again just before take-off using the stored compressed air and the expander cylinders to crank the engine.

The above multi-cylinder air hybrid engine has similar attributes to those of a variable displacement engine by disabling a bank of cylinders from firing, thus putting the other bank of firing cylinders on a higher load factor and higher efficiency. Moreover, it saves cost and complexity by eliminating the system hardware necessary for disabling the intake and exhaust valves of the non-firing cylinders, and at the same time putting those cylinders to good use for regenerative compression braking and expansion driving instead of just letting them stay dormant, thereby improving even more the vehicle fuel economy.

In the case where an exhaust catalytic converter is provided for aftertreatment of the exhaust gases from the firing cylinders, it may be necessary to provide separate exhaust manifold and catalyst systems to each bank of

cylinders in order to avoid air dilution of the catalyst.

Alternatively two catalytic converters may be arranged in series, with the exhaust from the firing bank of cylinders connected upstream of the first catalyst and the exhaust from the regenerative bank of cylinders connected downstream of the first catalyst and upstream of the second catalyst, so that the first catalyst treats the exhaust gases from the firing bank of cylinders and the second catalyst treats the exhaust gases from the regenerative bank when the latter is switched to become firing cylinders. In this way, the second catalyst will be kept hot by the exhaust gases from the firing bank of cylinders.

The air hybrid engine of the present invention has many advantages over an electric hybrid system by eliminating the electric generator, motor and battery components which are additional to the engine. This reduces cost, complexity and weight while providing similar function and benefits.

Another important advantage arises in the case where secondary air is required in the engine exhaust system for rapid light-off of the catalytic converter during cold start. By making sure a sufficient quantity of compressed air is retained in the compressed air storage tank at the end of a trip, this air will be available to supply the necessary volume of secondary air to the catalytic converter at the start of the following trip. This again saves cost, complexity and weight by eliminating the need for a separate electrically driven air pump while providing similar function and benefits.

The air hybrid vehicle of the present invention could replace a battery electric vehicle, with added advantages of lower cost, lighter weight, quicker recharge and longer range, by arranging for an additional larger capacity higher pressure stock air cylinder to be connected via a pressure regulator to the compressed air storage tank. The stock air

cylinder supplies additional compressed air to the compressed air storage tank in order to maintain a minimum air pressure in the tank such that the expander cylinder may be powered by air from the stock air cylinder as well as by air from the compressor cylinder. This retains the regenerative capability of the engine by maintaining an optimum air pressure (approx. 10 bar) in the compressed air storage tank while the stock air cylinder may be refilled with compressed air (up to 300 bar) when the vehicle is taken to a refilling station, thus enabling the vehicle to be used in regulated zero emission areas as a rechargeable and regenerative compressed air driven vehicle without burning fuel.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic view of a regenerative air hybrid engine operating at selected times as an air compressor or air expander with timed connection of high pressure air according to the present invention, Figures 2a and 2b are schematic views of two alternative embodiments of the air connection valve assembly for use in substitution in Figure 1, Figure 3 is a schematic view of a multi-cylinder air hybrid engine divided into banks of firing and regenerating cylinders, the latter cylinders fitted with the air connection valves of Figure 2a, and Figure 4 with inset 4a is a schematic view of an air powered air hybrid engine operating at selected times as an air compressor or air expander with additional compressed air supplied from a stock air cylinder.

Detailed description of the preferred embodiment Figure 1 shows a schematic view of a cylinder 10 of a reciprocating internal combustion four-stroke cycle engine with a piston 12 reciprocating in a bore 14. Exhaust and intake valves 16, 18 connecting the cylinder 10 with exhaust and intake ports 20, 22 are shown in their closed positions during the compression and expansion strokes of the piston 12. A main throttle 24 in the intake port 22 controls the air flow to the engine while a bypass throttle 25 provides additional air when desired. The engine also includes a fuel system and an ignition system which are not shown in Figure 1.

A compressed air storage tank 30 stores high pressure air charged into it by an air compressor driven by the momentum of the vehicle during deceleration. This compressed air is later used in an air expander to propel the vehicle during acceleration. In the invention, the cylinder 10 is used at selected times as the air compressor or air expander in the absence of fuel, and at other times as a firing cylinder in the presence of fuel according to the normal internal combustion engine cycle. The cylinder 10 with its piston 12 is connected to the crankshaft of the engine which in turn is connected to the drive train of the vehicle.

In Figure 1, the cylinder 10 is operated at selected times including coasting and deceleration as an air compressor inducting ambient air and compressing the air into a compressed air storage tank 30, while the intake and exhaust valves 16,18 continue to operate normally during all the gas exchange and work strokes of the engine according to the operating internal combustion cycle in the absence of fuel, characterised in that timed transfer of compressed air from the engine cylinder 10 into the compressed air storage tank 30 for storage in the tank 30 is

performed in two steps, using a buffer chamber 32 having a predetermined volume and connected to the engine cylinder 10 and to the compressed air storage tank 30 by respective timed first and second shut-off valves 26, 28 synchronised with the gas exchange and work strokes of the engine, wherein during the first step, the buffer chamber 32 is connected to the engine cylinder 10 during at least part of the compression stroke of the engine by opening and closing the first shut-off valve 26 at one set of predetermined times during the engine cycle while the second shut-off valve 28 remains closed, and during the second step, the buffer chamber 32 is connected to the compressed air storage tank 30 during another part of the gas exchange and work strokes other than the compression stroke of the engine by opening and closing the second shut-off valve 28 at another set of predetermined times while the first shut-off valve 26 remains closed.

In Figure 1, the cylinder 10 is operated at other selected times including cruising and acceleration as an air expander using high pressure air from the compressed air storage tank 30, while the intake and exhaust valves 16,18 continue to operate normally during all the gas exchange and work strokes of the engine according to the operating internal combustion cycle in the absence of fuel, characterised in that timed admission of high pressure air from the compressed air storage tank 30 into the engine cylinder 10 for expansion in the cylinder 10 is performed in two steps, using the said buffer chamber 32 and the said timed first and second shut-off valves 26,28 synchronised with the gas exchange and work strokes of the engine, wherein during the first step, the buffer chamber 32 is connected to the compressed air storage tank 30 during one part of the gas exchange and work strokes other than the expansion stroke of the engine by opening and closing the second shut-off valve 28 at one set of predetermined times during the engine cycle while the first shut-off valve 26

remains closed, and during the second step, the buffer chamber 32 is connected to the engine cylinder 10 during a substantial part of the expansion stroke of the engine by opening and closing the first shut-off valve 26 at another set of predetermined times while the second shut-off valve 28 remains closed.

In the above invention, the term'internal combustion cycle'is defined as the generic air power cycle experienced by the working fluid according to a prescribed sequence of gas exchange and work phases (or strokes) characterising a thermodynamic cycle which has a nominal internal combustion phase but which may or may not include actual combustion during that phase depending on the presence or absence of fuel in the working fluid. In the invention, by means of the buffer chamber 32 and shut-off valves 26,28, the engine is used at selected times as an air compressor during the intake and compression strokes of the internal combustion cycle in the absence of fuel, separated by the expansion and exhaust strokes which continue to operate normally through the cycle, and at other selected times the engine is used as an air expander during the expansion and exhaust strokes of the internal combustion cycle in the absence of fuel, adding power to the cycle which continues to operate in full with inducted ambient air and completes the cycle substantially reversibly for the ambient air. Thus the invention does not rely on adapting or altering the function of the existing engine valves for regenerative air hybrid operation and is applicable to any engine operating according to a nominal internal combustion cycle as defined above.

The invention may be applied to a variety of engines including reciprocating, rotary, poppet-valve, sleeve-valve, piston-port-valve, four-stroke-cycle, two-stroke-cycle, petrol, diesel, gaseous-fuel, spark-ignition, compression- ignition etc each with individual cylinder fuel shut-off means to deny fuel to those cylinders operating in the

regenerative compressor or expander mode. To ensure no fuel will enter the compressed air storage tank 30, care should be taken to purge the engine cylinder 10 of any residual fuel by motoring the cylinder for several cycles with no fuel before connecting with the buffer chamber 32.

Figure 1 shows a conventional spark ignition engine controlled by a main throttle 24. An air bypass throttle 25 bypassing the main throttle 24 is provided in the intake system of the engine so that during coasting and deceleration with the fuel shut off and the main throttle 24 closed, the bypass throttle 25 may be opened to allow a full charge of air to be inducted into the cylinders working as compressors. Of course, in the case of an engine not controlled by a main throttle, for example, a diesel compression ignition engine or a direct injection spark ignition engine, there will be no need of a bypass throttle.

When operating as a compressor, the timed discharge of compressed air from the engine cylinder 10 into the compressed air storage tank 30 performed in two steps has significant advantage in that the compression work, or braking load experienced by the engine, is substantially constant and therefore predictable to the driver, and not dependent on the state of charge in the compressed air storage tank 30. This is because each compression phase of the piston 12 works against a constant end volume which includes the volume of the buffer chamber 32 isolated from the compressed air storage tank 30 by the second shut-off valve 28 and enables a substantially constant maximum compression pressure to be generated within the engine cylinder 10 during the course of the compression phase irrespective of the state of charge in the compressed air storage tank 30. In contrast, had the engine cylinder 10 been connected directly to the compressed air storage tank 30 through a conventional compressor non-return valve, a low state of charge in the tank would result in early opening of

the non-return valve into the tank and a low compression pressure generated within the engine cylinder resulting in little braking work. At the other end of the range, a fully charged tank will keep the non-return valve closed, not accepting any more compressed air and this air will re- expand in the engine cylinder balancing out the compression work. Only the middle range of charge in the tank (5-15 bar air pressure) would result in sufficient compression work and provide a useful braking load to the engine. This makes such a compression braking system rather unpredictable to the driver and at times ineffective, and this is obviate in the present invention shown in Figure 1.

When operating as an expander, the timed admission of high pressure air into the engine cylinder 10 performed in two steps has advantage in that it relaxes the actuation design specification of the high pressure air admission valve 26 into the engine cylinder, the valve being the same as the buffer chamber first shut-off valve 26 which stays open for a relatively long period of time during at least the expansion stroke of the engine cycle. This is to be contrasted with the high pressure air admission valve in a conventional air expander connected directly to a stock compressed air supply, where the valve must be opened and closed very quickly within a very short period of time while the piston is still near its top dead centre (TDC) in order to limit the high pressure air entering the cylinder for the air to expand with sufficient expansion ratio after the admission valve is closed. Such very short opening period of the admission valve poses severe problems to the design of the actuation system of the valve, and this is obviated in the present invention shown in Figure 1.

In Figure 1, the timed actuation of the first and second shut-off valves 26,28 may be performed by a variety of actuating means, including mechanical, electrical,

hydraulic and pneumatic actuators (not shown). The shut-off valves may be shuttle valves, rotary valve or poppet valves.

When operating as a compressor, the first shut-off valve 26 may be timed to remain open longer than the compression stroke of the internal combustion engine cycle, and the second shut-off valve 28 timed to remain open shorter than the remaining gas exchange and work strokes.

Preferably, the first shut-off valve 26 is timed to remain open during most of the intake and compression strokes of the internal combustion engine cycle, and the second shut- off valve 28 timed to remain open during most of the expansion and exhaust strokes, thus resetting the initial pressure and providing substantially equal time periods for the air connection of the buffer chamber alternately with the engine cylinder and with the compressed air storage tank. On the other hand, the first shut-off valve 26 may be timed to open after the intake period of each engine cycle, in which case, the initial pressure in the cylinder 10 will rise progressively through consecutive cycles resulting in higher and higher compression pressures forced into the buffer chamber 32.

When operating as an expander, the first shut-off valve 26 may be timed to remain open longer than the expansion stroke of the internal combustion engine cycle, and the second shut-off valve 28 timed to remain open shorter than the remaining gas exchange and work strokes. Preferably, the first shut-off valve 26 is timed to remain open during most of the expansion and exhaust strokes of the internal combustion engine cycle, and the second shut-off valve 28 timed to remain open during most of the intake and compression strokes, thus providing substantially equal time periods for the air connection of the buffer chamber 32 alternately with the engine cylinder 10 and with the compressed air storage tank 30.

Preferably, the volume of the buffer chamber 32 isolated by the second shut-off valve 28 from the compressed air storage tank 30 is sufficiently small for it to be included with the clearance volume of the engine cylinder 10 when operating in the compressor mode such that the resulting compression ratio is at least 5: 1. In this respect, the invention will be more effective when applied to a diesel engine having a high compression ratio of approximately 18: 1 so that when the buffer chamber volume 32 is included with the clearance volume of the engine cylinder 10, the resulting compression ratio is still relatively high at approximately 9: 1.

Also preferably, the volume of the buffer chamber 32 isolated by the second shut-off valve 28 from the compressed air storage tank 30 is sufficiently small for it to be included with the clearance volume of the engine cylinder 10 when operating in the expander mode, such that high pressure air expanding from the buffer chamber 32 directly into the cylinder 10 during the expansion stroke achieves a substantially constant expansion ratio relative to the volume of the buffer chamber 32 sufficiently to bring the expanded air pressure to substantially ambient pressure at the end of the expansion stroke.

Figure 2a shows an embodiment of the air connection valve assembly in which the buffer chamber 32a and its first and second shut-off valves 26a, 28a are designed as a compact unit 50a with the two shut-off valves combined into a multi-channel rotary valve 26a/28a actuated by a two- position actuator. This makes it relatively easy to adapt an engine cylinder to operate as a regenerative compressor and expander by connecting the compact unit 50a to a convenient access hole in the cylinder, while the intake and exhaust valves of the cylinder continue to operate normally during all the gas exchange and work strokes of the engine according to the internal combustion cycle.

Figure 2b shows an alternative embodiment of the air connection valve assembly in which the buffer chamber 32b and its first and second shut-off valves 26bF 28b are designed as a compact unit 50b with the two shut-off valves combined into a multi-channel shuttle valve 26b/28b actuated by a two-position actuator. In this case, the shuttle valve has a conical seat at its end and may be spring-loaded to seal off the dead volume in the access hole to the cylinder 10, thus removing the disadvantage of introducing an additional crevice volume in the combustion chamber when the cylinder is operating as a firing cylinder.

Figure 3 shows a multi-cylinder air hybrid engine of the present invention in which the cylinders are divided into two banks with one bank 110 operating as firing cylinders and the other bank 120 (fitted with the multi- channel valve units 50a of Figure 2a) operating as regenerative compressor or expander cylinders except when switched back to become firing cylinders during high engine load conditions. In this case, the bank 120 will be at times motored substantially reversibly during normal operation through the nominal internal combustion cycle in the absence of fuel, and at other times operated in the regenerative mode either as a compressor or as an expander according to the present invention.

This arrangement will be sufficient to provide adequate power to propel the vehicle in most urban driving conditions using one bank of firing cylinders 110 supplemented with regenerative power using the other bank 120 of compressor/ expander cylinders, the two banks working in parallel proportioning power according to need controlled by a central processing unit 200. Of course, the engine may be switched off when the vehicle stops, and restart again just before take-off using the stored compressed air and the expander cylinders 120 to crank the engine.

In the case where an exhaust catalytic converter is provided for aftertreatment of the exhaust gases from the firing cylinders, it may be necessary to provide separate exhaust manifold and catalyst systems (not shown) to each bank of cylinders in order to avoid air dilution of the catalyst. Alternatively two catalytic converters 130,140 may be arranged in series as shown in Figure 3, with the exhaust from the firing bank of cylinders 110 connected upstream of the first catalyst 130 and the exhaust from the regenerative bank of cylinders 120 connected downstream of the first catalyst 130 and upstream of the second catalyst 140, so that the first catalyst 130 treats the exhaust gases from the firing bank of cylinders 110 and the second catalyst 140 treats the exhaust gases from the regenerative bank 120 when the latter is switched to become firing cylinders. In this way, the second catalyst 140 will be kept hot by the exhaust gases from the firing bank of cylinders 110.

The air hybrid engine of the present invention has many advantages over an electric hybrid system by eliminating the electric generator, motor and battery components which are additional to the engine. This reduces cost, complexity and weight while providing similar function and benefits.

Another important advantage arises in the case where secondary air is required in the engine exhaust system for rapid light-off of the catalytic converter 130 during cold start. By making sure a sufficient quantity of compressed air is retained in the compressed air storage tank 30 at the end of a trip, this air will be available to supply at the start of the following trip the necessary volume of secondary air through a valve 150 (controlled by the central processing unit 200) along a pipe 160 to the catalytic converter 130 in order to achieve rapid light-off of the catalytic converter 130. This again saves cost, complexity and weight by eliminating the need for a separate

electrically driven secondary air pump while providing similar function and benefits.

The air hybrid vehicle of the present invention could replace a battery electric vehicle, with added advantages of lower cost, lighter weight, quicker recharge and longer range by arranging (as shown in Figure 4) an additional larger capacity higher pressure stock air cylinder 60 to be connected via a pressure regulator 62 to the compressed air storage tank 30. The stock air cylinder 60 supplies additional compressed air to the compressed air storage tank 30 in order to maintain a minimum air pressure in the tank 30 such that the expander cylinder 10 may be powered by air from the stock air cylinder 60 as well as by air from the compressor cylinder 10. This retains the regenerative capability of the engine by maintaining an optimum air pressure. (for example 10 bar) in the compressed air storage tank 30 while the stock air cylinder 60 may be refilled with compressed air (up to 300 bar) when the vehicle is taken to a refilling station, thus enabling the vehicle to be used in regulated zero emission areas as a rechargeable and regenerative compressed air driven vehicle.

Figure 4 also shows a schematic view of a compressed air powered air hybrid engine operating according to a four- stroke internal combustion cycle and at all times without fuel. In Figure 4, the cylinder 70 has a combined exhaust and intake port 72 controlled by a combined exhaust and intake valve 76 with an opening duration spanning the whole period of the exhaust and intake strokes of the internal combustion engine cycle. Provided that the cylinder 70 is operated in the absence of fuel, this simplified valve arrangement with shared intake and exhaust flows is just as effective for operating the cylinder 70 as a regenerative compressor and expander as the cylinder 10 of Figure 1.

This constitutes a low cost air powered air hybrid engine for a purpose-built rechargeable and regenerative compressed

air driven vehicle suitable for inner city transport replacing battery electric cars and fuel burning cars.

The engine in Figure 4 may be substituted by another compressed air powered reciprocating engine with side ports operating according to a two-stroke internal combustion cycle and at all times without fuel (as shown in inset 4a).

Because of the absence of fuel, a tuned combined intake and exhaust side port 78 with shared intake and exhaust flows will be sufficient connected to atmosphere, crankcase air charging could be deleted, making a very low cost rechargeable and regenerative air powered engine with only a small number of moving parts. Also because the engine will never get hot, the. crankcase may be sealed with permanent lubrication for low maintenance.

Alternatively, the engine in Figure 4 may be substituted by yet another compressed air powered rotary Wankel engine (not shown) with side ports operating according to a four-stroke internal combustion cycle and at all times without fuel.

Thus, the invention may be applied to a road transport vehicle powered by a fuel burning engine or a compressed air powered engine. It may also be applied to a pedal powered vehicle such as a bicycle where a regenerative air hybrid engine is provided which is at times motored substantially reversibly during normal operation according to a nominal internal combustion cycle, and at other times operated in the regenerative mode either as a compressor or as an expander.