GB2473481A

GB2473481A - Method for supplying EGR in an i.c. engine

Info

Publication number: GB2473481A
Application number: GB0916023A
Authority: GB
Inventors: Thomas Tsoi Hei Ma; Hua Zhao
Original assignee: Brunel University
Current assignee: Brunel University
Priority date: 2009-09-14
Filing date: 2009-09-14
Publication date: 2011-03-16
Anticipated expiration: 2029-09-14
Also published as: GB2473481B; GB0916023D0

Abstract

Each cylinder of the engine has at least one intake valve and one exhaust valve operating on a four stroke cycle. The method comprises selectively interchanging in timing relative to the engine strokes the opening events of the intake and exhaust valves of one or more cylinders when EGR is required so that the exhaust valve is opened during a downward stroke drawing exhaust gases from the exhaust system into the cylinder and the intake valve is opened during a upward stroke discharging exhaust gases from the cylinder into the intake. The cylinder (EGR cylinder) thereby operates as a positive displacement exhaust gas charger with two gas exchange strokes and two non gas exchange strokes during the four stroke cycle delivering exhaust gases for mixing with the intake air supplied to the remaining cylinders (power cylinders). Fuel may be injected into the EGR cylinder for pre-conditioning the fuel in the hot exhaust gases in the EGR cylinder, and the EGR gases and pre-conditioned fuel may be subsequently mixed with the intake air supplied to the power cylinders of the engine for combustion within the power cylinders.

Description

METHOD FOR SUPPLYING EGR IN AN IC ENGINE

Field of the invention

The present invention relates to a method of supplying EGR in an internal combustion engine.

Background of the invention

EGR (Exhaust Gas Recirculation) is a commonly used for reducing the NOx emissions and slowing down the combustion rate of an internal combustion engine. In the case where the engine is supplied with pressurised air from a supercharger or turbocharger, it is necessary to provide the EGR at an elevated pressure higher than the pressure of the air supply to the engine in order to introduce the EGR into the intake system of the engine. This is conventionally achieved by increasing the back pressure in the engine exhaust system which has the disadvantage of increasing the pumping work and reducing the fuel efficiency of the engine.

Such a method which relies on creating a pressure difference to drive an EGR flow in the direction of the engine intake system is also unreliable because the pressures in the engine exhaust and intake systems are highly variable and the pressure difference could fluctuate or reverse very rapidly under dynamic driving conditions.

It is known that when fuel is supplied with air and hot EGR into a cylinder of the engine, the heating and dilution of the air-fuel mixture by the hot EGR gases could be sufficient under the right conditions to cause auto-ignition and combustion at a relatively low speed and low temperature resulting in near-zero NOx emissions, very low soot formation and significantly improved fuel efficiency. Such combustion is known by various names: CAl (Controlled Auto-Ignition), HCCI (Homogeneous Charge Compression Ignition), and LTC (Low Temperature Combustion) Object of the invention The present invention aims to achieve a reliable and efficient system for delivering EGR and fuel in an internal combustion engine.

Summary of the invention

According to the present invention, there is provided a method of achieving exhaust gas recirculation in a multi-cylinder four stroke internal combustion engine of which each cylinder has at least one intake valve controlling gas flow through an intake port leading from an air intake manifold and one exhaust valve controlling gas flow through an exhaust port leading to an exhaust manifold, in which method, when exhaust gas recirculation is required, the function of at least one selected cylinder is changed from a power cylinder to one acting as a positive displacement exhaust gas charger EGR cylinder by modifying the timing of the opening events of the intake and exhaust valves in such a manner that, for each EGR cylinder, the exhaust valve is opened as the volume of the combustion chamber is expanding to admit combustion gases into the combustion chamber from the exhaust manifold and the intake valve is subsequently opened while the volume of the combustion chamber is contracting to expel the previously admitted combustion gases from the working chamber into the intake manifold for mixing with the intake air of the remaining cylinders of the engine.

In order to ensure uniform distribution of EGR to all the power cylinders, it is preferred to change more than one cylinder to act as an EGR charger.

The intake and exhaust valves are herein defined as the valve connected to the intake system and the exhaust system of the engine respectively regardless of the direction of flow into or out of the engine cylinder. It is also important that the EGR cylinders and the power cylinders share the same intake system and exhaust system so that the exhaust gases discharged from the power cylinders are drawn into the EGR cylinders and the exhaust gases discharged from the EGR cylinders are mixed with the intake air supplied to the power cylinders.

In one embodiment of the invention, in order to change a power cylinder to an EGR cylinder, the timings of the opening events of the intake and exhaust valves are interchanged whereby the exhaust valve opens during what was previously the induction stroke and the intake valve opens during what was previously the exhaust stroke.

In an alternative embodiment, to change a power cylinder into an EGR cylinder, the timings of the opening events of the intake and exhaust valves are modified such that the exhaust valve opens during what was previously the expansion stroke and the intake valve opens during what was previously the compression stroke. In both of these embodiments, the EGR cylinder acts as a four stroke pump with two gas exchange strokes and two strokes in which no gas exchange takes place.

As a further possibility, in order to change a power to an EGR cylinder, the timings of the opening events of the intake and exhaust valves may be modified such that the exhaust valve opens during what were previously both the induction stroke and the expansion stroke and the intake valve opens both during what were previously both the compression stroke and the exhaust stroke. In this case, the EGR cylinder operates as a two stroke pump with gas exchange taking place in every stroke.

In the present invention, by converting a power cylinder to an EGR cylinder acting as a positive displacement exhaust gas charger, pressurised exhaust gases are transferred directly to the intake system of the engine for mixing with the intake air supplied to the power cylinders of the engine. This will take place even in the case where the engine is supplied with pressurised air at a high boost pressure exceeding the engine back pressure.

Depending on the requirement of different combustion modes, the EGR supplied to the power cylinders may be cooled or uncooled. In the present invention, in the case where the EGR cylinder is operated as a four stroke exhaust gas charger, there are non gas exchange strokes between the induction and exhaust strokes, i.e. the compression and expansion strokes during which the cylinder charge is compressed to a very small volume and to a high temperature resulting in significant heat transfer from the hot gases to the walls of the cylinder which is usually water cooled.

Thus the EGR gases delivered by the four stroke exhaust gas charger will be substantially well cooled which is suitable for reducing the NO emissions and slowing down the rate of combustion in an internal combustion engine.

In the case where the EGR cylinder is operated as a two stroke exhaust gas charger, the exhaust gases are drawn into the cylinder and immediately discharged from the cylinder without any significant heat transfer to the walls of the cylinder. Thus the EGR delivered by the two stroke exhaust gas charger will be relatively uncooled and substantially hot which is suitable for promoting auto-ignition in a CAI/HCCI engine.

The copious amount of EGR gases delivered by the EGR cylinder for mixing with the intake air supplied to the power cylinders will affect the combustion in the power cylinders in several ways. In a conventional internal combustion engine, the dilution effect of the exhaust gases will slow down the combustion rate and reduce the temperature gas peak thereby reducing the propensity of NO emissions. Also the thermal effect of the exhaust gases will heat up the air charge and evaporate any fuel that is mixed with the air. In the context of a CAl/HOd engine, in the case where the compression ratio of the engine is nominally not high enough to reach auto-ignition temperature such as in a gasoline-fuelled engine, the additional heating by hot internal EGR could bring the compressed air charge to auto-ignition temperature thereby achieving CAl. In the case where the compression ratio of the engine is nominally high enough to reach auto-ignition temperature such as in a diesel-fuelled engine, the heat release rate of the combustible charge by HCCI may be too high and this could be slowed down by the dilution effect of cooled EGR.

There may be no fuel injected into the EGR cylinder in which case the engine operates effectively as a variable displacement engine with a reduced number of power cylinders. On the other hand, the present invention may include fuel being injected into the EGR cylinder and this fuel may be carried by the EGR gases to the power cylinders for combustion or may burn within the EGR cylinder.

Thus fuel may be injected into the EGR cylinder for pre-conditioning of the fuel within the hot exhaust gases in the EGR cylinder, and the EGR gases and pre-conditioned fuel from the EGR cylinder are subsequently mixed with the intake air supplied to the power cylinders of the engine for combustion within the power cylinders. This could promote auto-ignition in the power cylinders of a CAI/HCCI engine.

The power cylinders supplied with the EGR gases and pre-conditioned fuel may operate with stoichiometric combustion or lean combustion. In the stoichiometric case, there will be no oxygen left in the exhaust gases so that any fuel injected into the EGR cylinder can be safely brought to a high temperature and high pressure without combustion during the compression and expansion strokes of the EGR cylinder when the EGR cylinder is operated in the four stroke exhaust gas charger mode.

In the lean burn case, there will be surplus oxygen in the exhaust gases. When the EGR cylinder is operated in the two stroke exhaust gas charger mode, any fuel injected into the EGR cylinder will not burn at the relatively low temperature of the exhaust gases in the EGR cylinder. On the other hand, when the EGR cylinder is operated in the four stroke exhaust gas charger mode, the high temperatures produced during the compression stroke may bring some of the fuel injected into the EGR cylinder to combustion, producing some power in the EGR cylinder and subsequently delivering the EGR together with the burnt and unburnt fuel to the power cylinders for further combustion within the power cylinders. This could promote CAI/HCCI in both the EGR cylinder and the power cylinders.

Of course, fuel is also injected directly into the power cylinder for normal combustion within the power cylinder.

The pre-conditioning of the fuel may also include fuel reforming in the exhaust gases in the EGR cylinder producing hydrogen which enhances combustion in the power cylinders.

Some of the EGR and reformed fuel containing hydrogen may also be delivered via a pipe connecting from the pressurised intake system of the engine to a particulate trap and/or catalytic converter in the exhaust system of the engine for supporting regeneration of the particulate trap and selective catalytic reduction in the catalytic converter respectively.

The above method of supplying EGR and fuel according to the present invention will be most advantageously applied with the engine cylinders grouped into pairs of EGR and power cylinders. Each EGR cylinder will have a crank angle difference of 1200 for a six cylinder engine and 180° for a four cylinder engine relative to the power cylinder so that the opening events of the intake and exhaust valves of the EGR cylinder will have a substantial overlap with at least one of the opening events of the intake and exhaust valves of the power cylinder, thus enabling the transfer of exhaust gases between the cylinders to occur by direct push-pull cooperation of the pistons during the overlapping valve opening periods of the cylinders.

Depending on the phasing of the valve opening events of the EGR cylinder in relation with the valve opening events of the power cylinder in each pair, the exhaust gas exchange between the cylinders may be selected to take place with push-pull cooperation of the pistons during the induction stroke of the power cylinder or during the exhaust stroke of the power cylinder. Either case will increase the efficiency of gas transfer as well as conserve the thermal properties of the exhaust gases during the transfer between cylinders.

Preferably, in order to assist immediate transfer of exhaust gases between the cylinders during the overlapping valve opening periods of the cylinders corresponding to the pull-pull cooperation of the pistons, the supply and discharge pipes connecting the cylinders of each pair should be close-coupled providing the shortest gas transfer path between the cylinders. Accordingly, the intake and exhaust manifolds of the engine may be modified to that effect.

In grouping the EGR cylinders and power cylinders in pairs in the multi-cylinder engine, each pair may be considered to be operating according to an eight-stroke engine cycle with the first four strokes transferring EGR gases which may include fuel via the EGR cylinder and the next four strokes drawing fresh intake air mixed with the EGR gases and fuel into the power cylinder which may include further addition of fuel producing combustion and useful work and subsequently discharging exhaust gases out of the power cylinder. In the next cycle, a proportion of the exhaust gases is drawn back into the EGR cylinder while the remainder is expelled through the engine exhaust system.

Thus there is a continuous flow of air and exhaust gases through the engine whilst a substantial recirculation of exhaust gases is taking place internally.

With the additional time and high temperatures within the EGR cylinder favourable for the pre-conditioning of the fuel and the managing the thermal properties of the EGR gases during the first four strokes, it will be possible to achieve CAI/HCCI combustion in the power cylinder over a much wider speed and load range, thus achieving a highly efficient eight-stroke CAI/HCCI engine. To that effect, the push-pull cooperation of the pistons during the overlapping induction strokes of the pair of cylinders and the close-coupling of the supply and discharge pipes connecting the cylinders at the intake side of the engine will be especially beneficial because a substantial quantity of exhaust gases discharged from the EGR cylinder will be transferred directly to the power cylinder during the middle of the eight-stroke engine cycle and the transfer delay will be minimum with less thermal loss.

In contrast, a similar push-pull effect in an alternative option of overlapping during the exhaust valve events for the pair of cylinders will have less influence because out of the exhaust gases discharged from the power cylinder at the end of the eight-stroke engine cycle only a proportion will be drawn back to the EGR cylinder at the beginning of the next cycle. Also any thermal loss within the engine exhaust system will be small because the exhaust gas temperature will be substantially the same as the mean temperature of the exhaust manifold.

In a four cylinder engine having a firing order of 1, 3, 4, 2, the ideal configuration will be to interchange the valve events of the end cylinders using them as EGR cylinders while the middle cylinders are used as power cylinders so that the intake valve events of each pair of cylinders 1, 2 and 3, 4 will be overlapping resulting in the beneficial push-pull cooperation of the pistons occurring during the middle of the eight-stroke engine cycle.

Accordingly as discussed earlier, the engine may be operated with an overall stoichiometric air/fuel ratio producing conventional combustion or CAI/HCCI combustion only in the power cylinders, i.e. one firing event during the eight-stroke engine cycle. Alternatively, the engine may be operated with an overall lean air/fuel ratio producing conventional combustion or CAI/HCCI combustion in both the EGR cylinders and power cylinders, i.e. two firing events during the eight-stroke engine cycle.

In practice, the engine will be started from cold using conventional combustion and after the engine has warmed up switched over to CAI/HCCI combustion. In the CAI/HCCI mode, the eight-stroke engine will be operated at low loads with stoichiometric mixture and a high proportion of internal EGR firing from only the power cylinders in order to increase the load factor and exhaust gas temperature of the power cylinders and thereby extend the lower range for CAl down to idle conditions. Then as the engine speed and load increase, the engine will be operated with lean mixtures and internal EGR, firing from all cylinders.

Finally and importantly, the lean burn engine can be readily boosted with air and internal EGR to extend the upper range for CAl. Because the EGR cylinders are by design highly efficient positive displacement gas chargers, they will have no problem delivering boosted EGR at any -10 -pressure together with pre-conditioned fuel to the pressurised intake system of the eight-stroke engine.

The prolonged processes and intensified thermal effects produced within the EGR cylinders during the first 4 strokes of the eight-stroke engine would also be effective with diesel fuels for fully evaporating the fuel including the heaviest fuel fractions to produce a homogeneous mixture, thus achieving true HCCI and LTC (Homogeneous Charge Compression Ignition and Low Temperature Combustion) in a diesel-fuelled engine which has eluded many researchers in the past because of the high boiling point of diesel fuel.

Another advantage of operating the EGR cylinders and power cylinders in pairs is that the average pumping work of the engine arising from low mean intake pressure and high mean exhaust pressure will be neutral because the pumping work which is negative for the power cylinders will be positive for the EGR cylinders or vice-versa balancing each other.

In implementing the method of the present invention in one or more cylinders, or in one of each pair of cylinders in a multi-cylinder engine, there are a variety of variable valve actuation systems which could be used for selectively interchanging in timing the opening events of the intake and exhaust valves of a cylinder thereby switching from a power cylinder to an EGR cylinder when EGR is required. Preferably a cam profile switching (CPS) system is used which is a well-known system found in many production engines of which the Honda VIEC is the best example. In CPS, each engine valve can be operated by one of two cams positioned adjacent to one another along a camshaft. A switchable rocker is arranged to engage selectably with one or the other cam.

The profile of the second cam may be different in timing, lift, duration and frequency from the first cam and the CPS -11 -enables the switching to either cam profile for each individual cylinder within one engine cycle.

Thus when using CPS, a first set of intake and exhaust cams may be selected for operating the cylinder as a power cylinder and a second set of intake and exhaust cams interchanged in timing relative to the first set may be selected for operating the cylinder as an EGR cylinder. The profiles determining the timing, lift and duration of the second set of cams may of course be different from the profiles of the first set of cams.

In the optional embodiments mentioned earlier, the second set of intake and exhaust cams may be phase shifted by 1800 cam angle to produce another gas charger which is one engine revolution displaced from the original gas charger. Also in the further optional case, by providing two opening events 180° cam angle apart in the second set of intake and exhaust cams, the valves will open twice in every two engine revolutions so that the EGR cylinder will operate as a two stroke exhaust gas charger when the CPS is switched to the second set of cams.

For progressive control of the flow through the exhaust gas charger, a port throttle may be provided in the engine port associated with the exhaust valve of each EGR cylinder for regulating the gas pressure drawn into the EGR cylinder in order to adjust the amount of EGR delivered by the EGR cylinder.

The engine may be boosted by a turbocharger and/or supercharger and the EGR from the EGR cylinder will have no problem joining with the intake air at boost pressure by virtue of the positive displacement gas charger provided by the EGR cylinder. The engine may also be boosted from a pressurised air storage tank onboard an air hybrid vehicle.

-12 -The engine may at any time be returned to all power cylinders by switching the EGR cylinders back to power cylinders. In this case, the present invention will become ineffective for supplying EGR to the power cylinders and any necessary EGR will have to be provided using another method such as the traditional method of increasing the engine exhaust back pressure sufficiently to force a flow of exhaust gases along an external EGR pipe against any boost pressure in the intake system of the engine.

Brief description of the drawings

The invention will now be described further by way of example with reference to the accompanying drawings in which Figure 1 is a schematic valve timing illustration comparing the opening events of the intake and exhaust valves between a power cylinder and an EGR cylinder in a four stroke internal combustion engine according to the method of the present invention, Figure 2 is a schematic valve timing illustration of an EGR cylinder operating as a two stroke exhaust gas charger, Figure 3 is a schematic drawing of a four cylinder internal combustion engine supplied with EGR from two EGR cylinders, Figure 4 is a schematic valve timing illustration of a four cylinder internal combustion engine where the cylinders are grouped into pairs of EGR and power cylinders, and Figure 5 is a schematic illustration of an eight-stroke engine cycle produced by each pair of EGR and power cylinders.

Detailed description of the preferred embodiment

Figure 1 shows a schematic valve timing illustration comparing the opening events of the intake and exhaust -13 -valves between a power cylinder and an EGR cylinder in a four stroke internal combustion engine according to the method of the present invention. The top row shows the intake and exhaust valve opening events (I and E respectively) in a power cylinder, while the middle row shows the exhaust and intake valve events (E and I respectively) in an EGR cylinder. In the present context, the terms "intake" and "exhaust" indicate whether the valves are connected to the intake system or the exhaust system and do not necessarily correspond to the direction of gas flow.

The bottom row shows the piston movement associated with the four stroke engine cycle. As is well known, the first stroke is a suction or induction stroke, which is followed by a compression stroke, then an expansion or expansion stroke, and finally a discharge or exhaust stroke.

In the power cylinder, the induction stroke draws in supply air from the engine intake system. The air is compressed to raise its temperature and pressure during the compression stroke, fuel is burned near top dead centre at the end of the compression stroke to force the piston down during the expansion stroke and in the final upwards movement of the piston the exhaust gases are discharged into the engine exhaust system.

When a power cylinder is switched to an EGR charger mode (hereinafter termed an EGR cylinder), the exhaust valve is opened during what would normally be the induction stroke to draw exhaust gases from the exhaust system of the engine into the cylinder and the intake valve is opened during what would be the exhaust stroke to discharge exhaust gases from the cylinder into the intake system of the engine. In this way the EGR cylinder operates as a four stroke positive displacement exhaust gas charger. Thus when the EGR cylinder and the power cylinders share the same intake system and exhaust system, EGR gases are delivered by the EGR cylinder -14 -for mixing with the intake air supplied to the power cylinders.

It is clear from the valve timing illustration in Figure 1 that when a cylinder is switched from a power cylinder to EGR cylinder, the opening events of the intake and exhaust valves can simply be interchanged in timing relative to the piston strokes.

As an alternative, it is possible to modify the valve timing so that the exhaust valve opens during what would, when the cylinder is acting as a power cylinder, be the expansion stroke of the cylinder and the intake valve may opened during what would be the compression stroke of the cylinder. In other words, the interchanged opening events of the intake and exhaust valves may be displaced by one engine revolution. The cylinder again operates as a positive displacement exhaust gas charger with two gas exchange strokes and two non gas exchange strokes during a four stroke engine cycle. In both cases, the EGR cylinder operates effectively as a four stroke gas charger.

As a further possibility, both the interchanged opening events and the displaced opening events of the intake and exhaust valves may be provided in the same cylinder, the EGR cylinder thereby operating as a positive displacement exhaust gas charger with two sets of two gas exchange strokes during a four stroke engine cycle, i.e. as a two stroke gas charger as shown in the second row in Figure 2.

There are a variety of variable valve actuation systems which could be used for altering a first set of valve opening events for a power cylinder to a second set of valve opening events for an EGR cylinder. Preferably a cam profile switching (CPS) system is used for providing the variable valve actuation. CPS is a well known system used in production engines of which the Honda VTEC is the best -15 -example. In CPS, each engine valve can be operated by one of two cams positioned adjacent to one another along a camshaft. A switchable rocker is arranged to engage selectably with one or the other cam. The profile of the second cam may be different in timing, lift, duration and frequency from the first cam and the CPS enables the switching to either cam profile for each individual cylinder within one engine cycle.

In the optional case mentioned earlier, the second set of intake and exhaust cams may be phase shifted by 1800 cam angle to produce another gas charger which is one engine revolution displaced from the original gas charger. Also in the further optional case, by providing two opening events 180° cam angle apart in the second set of intake and exhaust cams, the valves will open twice in every two engine revolutions so that the EGR cylinder will operate as a two stroke exhaust gas charger when the CPS is switched to the second set of cams.

Figure 3 shows a four cylinder internal combustion engine 16 equipped with a rotary air charger 10 supplying pressurised air to the engine 16 via an intake manifold 14.

Exhaust gases from the engine 16 is discharged via an exhaust manifold and exhaust pipe 20. The rotary air charger 10 may be a supercharger or a turbocharger driven mechanically or by exhaust gases respectively in the conventional manner the details of which are not shown in -16 -Figure 3 in order to avoid unnecessary complexity in the diagram. The rotary air charger 10 may also be a combined supercharger and turbocharger connected in series supplying the engine 16. In so far described, the setup of the air charge system 10, 14 for supplying pressurised air to the engine 16, and the exhaust system 20 for discharging gases from the engine 16 is conventional.

In Figure 3, cylinders 1 and 4 of the engine 16 are shown shaded which may be switched to operate as EGR cylinders from the original power cylinders by using a CPS system to interchange the timings relative to the engine strokes of the opening events of the intake and exhaust valves in each cylinder so that the exhaust valve is opened during a downward stroke of the cylinder drawing exhaust gases from the exhaust system 20 of the engine 16 into the cylinder and the intake valve is opened during a upward stroke of the cylinder discharging exhaust gases from the cylinder into the intake system 14 of the engine 16. Thus the EGR cylinders 1 and 4 operate as four stroke positive displacement exhaust gas chargers delivering pressurised exhaust gases from the engine exhaust system 20 to the engine intake system 14 for mixing with the intake air supplied to the power cylinders 2 and 3 of the engine 16.

The same CPS system may also be used to duplicate the opening events of the intake and exhaust valves in the next two strokes of the EGR cylinders 1 and 4, in which case the EGR cylinders 1 and 4 operate as two stroke exhaust gas chargers having two suction strokes and two discharge strokes in every two engine revolutions.

In Figure 3, separate throttle valves 46 are provided in the exhaust port associated with the exhaust valve of each EGR cylinder 1 and 4 for regulating the quantity of exhaust gases drawn into the cylinders 1 and 4. This effectively controls the metering of the EGR for mixing with -17 -the intake air supplied to the power cylinders 2 and 3 of the engine 16. The throttle valves 46 will be fully open when the EGR cylinders 1 and 4 are switched back to power cylinders.

The engine 16 may be boosted by a turbocharger and/or supercharger 10 and the EGR gases will have no problem joining with the pressurised air at the boost pressure of air charger 10. The engine 16 may alternatively be boosted from a pressurised air storage tank 34 onboard an air hybrid vehicle which is shown in some detail in Figure 3 though a number of components related to the air hybrid vehicle will not be described fully in the present invention.

In the air hybrid vehicle, the supercharger 10 may be driven during braking and/or all cylinders of the engine 16 may be operated as air chargers to produce pressurised air which is stored in the air storage tank 34 when the engine 16 is driven by the vehicle during deceleration. When the engine is driving the vehicle during acceleration, the stored air may be used in substitution of the supercharger for boosting the power cylinders 2 and 3 while the cylinders 1 and 4 are switched to EGR cylinders operating as four stroke exhaust gas chargers delivering boosted EGR to the cylinders 2 and 3.

In Figure 3, according to the present invention and depending on the combustion requirement of the power cylinders 2 and 3, the EGR gases may be cooled or uncooled.

In the case where the EGR cylinders 1 and 4 are operated as four stroke exhaust gas chargers, there are non gas exchange strokes between the induction and exhaust strokes, i.e. the compression and expansion strokes during which the cylinder charge is compressed to a very small volume and to a high temperature resulting in significant heat transfer from the hot gases to the walls of the cylinders 1 and 4 which are usually water cooled. Thus the EGR gases delivered by the -18 -cylinders 1 and 4 operated as four stroke exhaust gas chargers will be substantially well cooled.

In the case where the EGR cylinders 1 and 4 are operated as two stroke exhaust gas chargers, the exhaust gases are drawn into the cylinders 1 and 4 and immediately discharged from the cylinders without any significant heat transfer to the walls of the cylinders 1 and 4. Thus the EGR gases delivered by the cylinders 1 and 4 operated as two stroke exhaust gas chargers will be relatively uncooled.

The engine 16 in Figure 3 is equipped with direct fuel injection in each cylinder and this is still available for use after the cylinder is switched to EGR cylinder such as in cylinders 1 and 4. In this case any fuel injected into the EGR cylinders 1 and 4 will be pre-conditioned within the hot exhaust gases in the EGR cylinders and subsequently the EGR gases and pre-conditioned fuel will be mixed with the intake air supplied to the power cylinders 2 and 3 for combustion. This would promote auto-ignition in the power cylinders of a CAI/HCCI engine.

The power cylinders 2 and 3 supplied with the EGR gases and pre-conditioned fuel may operate with stoichiometric combustion or lean combustion. In the stoichiometric case, there will be no oxygen left in the exhaust gases so that any fuel injected into the EGR cylinders 1 and 4 can be safely brought to a high temperature and high pressure without combustion during the compression and expansion strokes of the EGR cylinders 1 and 4 when the EGR cylinders 1 and 4 are operated in the four stroke exhaust gas charger mode.

In the lean burn case, there will be surplus oxygen in the exhaust gases. When the EGR cylinders 1 and 4 are operated in the two stroke exhaust gas charger mode, any fuel injected into the EGR cylinders will not burn at the -19 -relatively low temperature of the exhaust gases in the EGR cylinders. On the other hand, when the EGR cylinder 1 and 4 are operated in the four stroke exhaust gas charger mode, the high temperatures produced during the compression stroke may bring some of the fuel injected into the EGR cylinders 1 and 4 to combustion, producing some power in the EGR cylinders and subsequently delivering the EGR gases together with the burnt and unburnt fuel to the power cylinders 2 and 3 for further combustion within the power cylinders. This could promote CAl/HOd in both the EGR cylinder 1 and 4 and the power cylinders 2 and 3.

Of course, fuel is also injected directly into the power cylinders 2 and 3 for normal combustion within the power cylinder 2 and 3.

The pre-conditioning of the fuel may also include fuel reforming in the exhaust gases in the EGR cylinders 1 and 4 producing hydrogen which enhances combustion in the power cylinders 2 and 3. Some of the EGR gases and reformed fuel containing hydrogen may also be delivered via a pipe 50 connecting from the pressurised intake system 14 of the engine 16 to a particulate trap 52 and/or catalytic converter 54 in the exhaust system 20 of the engine 16 for supporting regeneration of the particulate trap 52 and selective catalytic reduction in the catalytic converter 54 respectively. In the boosted engine 16 because the intake system 14 is at a higher pressure than the downstream pressure in the exhaust system 20, a steady flow of EGR gases and reformed fuel will flow naturally towards the trap 52 and converter 54.

The above method of supplying EGR and fuel according to the present invention will be most advantageously applied with the engine cylinders grouped into pairs of EGR and power cylinders. Each EGR cylinder will have a crank angle difference of 120° for a six cylinder engine and 180° for a -20 -four cylinder engine relative to the power cylinder so that the opening events of the intake and exhaust valves of the EGR cylinder will have a substantial overlap with at least one of the opening events of the intake and exhaust valves of the power cylinder, thus enabling the transfer of exhaust gases between the cylinders to occur by direct push-pull cooperation of the pistons during the overlapping valve opening periods of the cylinders.

Figure 4 shows a schematic valve timing illustration of a four cylinder engine where the cylinders are grouped into pairs of EGR and power cylinders. In a four cylinder engine having a firing order of 1, 3, 4, 2, the ideal configuration will be to interchange the valve events of the end cylinders using them as EGR cylinders while the middle cylinders are used as power cylinders. The interchanged valve events are shown in brackets (E) and (I) respectively. In this case, the intake valve events of each pair of cylinders 1, 2 and 4, 3 will be overlapping resulting in the beneficial push-pull cooperation of the pistons occurring during the induction strokes of the power cylinders 2 and 3 and the associated discharge strokes of the EGR cylinders 1 and 4 respectively.

Preferably, in figure 3, in order to assist immediate transfer of exhaust gases between the cylinders during the overlapping valve opening period of the cylinders corresponding to the pull-pull cooperation of the pistons, the intake manifold 14 of the engine 16 is provided with close-coupled connecting air ducts 14a between each pair of cylinders 1, 2 and 4, 3 for the shortest gas transfer path between the cylinders.

In grouping the EGR cylinders and powers cylinder in pairs in the multi-cylinder engine, each pair may be considered to be operating according to an eight-stroke engine cycle with the first four strokes transferring EGR -21 -gases which may include fuel via the EGR cylinder and the next four strokes drawing fresh intake air mixed with the EGR gases and fuel into the power cylinder which may include further addition of fuel producing combustion and useful work and subsequently discharging exhaust gases out of the power cylinder. In the next cycle, a proportion of the exhaust gases is drawn back into the EGR cylinder while the remainder is expelled through the engine exhaust system.

Thus there is a continuous flow of air and exhaust gases through the engine whilst a substantial recirculation of exhaust gases is taking place internally. This is shown in Figure 5 in a self-explanatory manner following the flow direction of the internal EGR gases.

Figure 5 also shows the expected maximum compression temperatures in the EGR cylinder and power cylinder. With the additional time and high temperatures within the EGR cylinder favourable for the pre-conditioning of the fuel during the first four strokes, it will be possible to achieve CAI/HCCI combustion in the power cylinder over a much wider speed and load range, thus achieving a highly efficient eight-stroke CAI/HCCI engine. To that effect, the push-pull cooperation of the pistons during the overlapping induction strokes of the pair of cylinders and the close-coupling of the supply and discharge pipes connecting the cylinders at the intake side of the engine will be especially beneficial because a substantial quantity of exhaust gases discharged from the EGR cylinder will be transferred directly to the power cylinder during the middle of the eight-stroke engine cycle and the transfer delay will be minimum with less thermal loss.

Accordingly as discussed earlier, the engine 16 in Figure 3 may be operated with an overall stoichiometric air/fuel ratio producing conventional combustion or CAI/HCCI combustion only in the power cylinders, i.e. one firing event during the eight-stroke engine cycle. Alternatively, -22 -the engine may be operated with an overall lean air/fuel ratio producing conventional combustion or CAI/HCCI combustion in both the EGR cylinders and power cylinders, i.e. two firing events during the eight-stroke engine cycle.

In practice, the engine 16 will be started from cold using conventional combustion and after the engine has warmed up switched over to CAI/HCCI combustion. In the CAI/HCCI mode, the eight-stroke engine will be operated at low loads with stoichiometric mixture and a high proportion of internal EGR firing from only the power cylinders in order to increase the]oad factor and exhaust gas temperature of the power cylinders and thereby extend the lower range for CAl down to idle conditions. Then as the engine speed and load increase, the engine will be operated with lean mixtures and internal EGR, firing from all cylinders.

Finally and importantly, the lean burn engine 16 can be readily boosted with air and internal EGR to extend the upper range for CAl. Because the EGR cylinders are by design highly efficient positive displacement gas chargers, they will have no problem delivering boosted EGR at any pressure together with pre-conditioned fuel to the pressurised intake system 14 of the eight-stroke engine 16.

The prolonged processes and intensified thermal effects produced within the EGR cylinder during the first 4 strokes of the eight-stroke engine 16 would also be effective with diesel fuels for fully evaporating the fuel including the heaviest fuel fractions to produce a homogeneous mixture, thus achieving true HCCI and LTC (Homogeneous Charge Compression Ignition and Low Temperature Combustion) in a diesel-fuelled engine which has eluded many researchers in the past because of the high boiling point of diesel fuel.

-23 -Another advantage of operating the EGR cylinders and powers cylinder in pairs is that the average pumping work of the engine 16 arising from low mean intake pressure and high mean exhaust pressure will be neutral because the pumping work which is negative for the power cylinders will be positive for the EGR cylinders or vice-versa balancing each other.

The engine 16 may at any time be returned to all power cylinders by switching the EGR cylinders back to power cylinders. In this case, the present invention will become ineffective for supplying EGR to the power cylinders and any necessary EGR will have to be provided using another method such as the traditional method of increasing the engine exhaust back pressure sufficiently to force a flow of exhaust gases along an external EGR pipe against any boost pressure in the intake system of the engine.

Claims

-24 -CLAIMS1. A method of achieving exhaust gas recirculation in a multi-cylinder four stroke internal combustion engine of which each cylinder has at least one intake valve controlling gas flow through an intake port leading from an air intake manifold and one exhaust valve controlling gas flow through an exhaust port leading to an exhaust manifold, in which method, when exhaust gas recirculation is required, the function of at least one selected cylinder is changed from a power cylinder to one acting as a positive disp]acement exhaust gas charger EGR cylinder by modifying the timing of the opening events of the intake and exhaust valves in such a manner that, for each EGR cylinder, the exhaust valve is opened as the volume of the combustion chamber is expanding to admit combustion gases into the combustion chamber from the exhaust manifold and the intake valve is subsequently opened while the volume of the combustion chamber is contracting to expel the previously admitted combustion gases from the working chamber into the intake manifold for mixing with the intake air of the remaining cylinders of the engine.
2. A method as claimed in claim 1, wherein to change a power cylinder to an EGR cylinder the timings of the opening events of the intake and exhaust valves are interchanged whereby the exhaust valve opens during what was previously the induction stroke and the intake valve opens during what was previously the exhaust stroke.
3. A method as claimed in claim 1, wherein to change a power cylinder to an EGR cylinder the timings of the opening events of the intake and exhaust valves are modified such that the exhaust valve opens during what was previously the expansion stroke and the intake valve opens during what was previously the compression stroke.

-25 -
4. A method as claimed in claim 1, wherein to change a power cylinder to an EGR cylinder the timings of the opening events of the intake and exhaust valves are modified such that the exhaust valve opens during what were previously both the induction stroke and the expansion stroke and the intake valve opens both during what were previously both the exhaust stroke and the compression stroke.
5. A method as claimed in any preceding claim, wherein fuel is injected into the EGR cylinder for pre-conditioning of the fuel within the hot exhaust gases in the EGR cylinder without combustion, and the EGR gases and pre-conditioned fuel from the EGR cylinder are subsequently mixed with the intake air supplied to the power cylinders of the engine for combustion within the power cylinders.
6. A method as claimed in any one of claims 1 to 4, wherein fuel is injected into the EGR cylinder for pre-conditioning of the fuel including combustion within the hot exhaust gases in the EGR cylinder, and the EGR gases together with the burnt and unburnt fuel from the EGR cylinder are subsequently mixed with the intake air supplied to the power cylinders of the engine for combustion within the power cylinders.
7. A method as claimed in any one of claims 1 to 4, wherein fuel is injected into the EGR cylinder for reforming of the fuel within the hot exhaust gases in the EGR cylinder, and the EGR gases and reformed fuel containing hydrogen from the EGR cylinder are subsequently mixed with the intake air supplied to the power cylinders of the engine for enhancing the combustion within the power cylinders.
8. A method as claimed in any preceding claim, wherein fuel is injected directly into each power cylinder for combustion within the power cylinder.

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9. A method as claimed in any preceding claim, wherein each EGR cylinder is paired with a power cylinder such that the opening events of the intake and exhaust valves of the EGR cylinder have a substantial overlap with at least one of the opening events of the intake and exhaust valves of the power cylinder, thus enabling the transfer of exhaust gases between the cylinders to occur in direct push-pull cooperation of the pistons during the overlapping valve opening periods of the cylinders.
10. A method as claimed in claims 9, wherein the supply and discharge pipes connecting the cylinders of each pair are close-coupled for the shortest gas transfer path between the cylinders during the overlapping valve opening period of the cylinders.
11. A method as claimed in claim 9 or 10, wherein the pair of cylinders is operated with an overall stoichiometric air/fuel ratio producing output power from the power cylinder.
12. A method as claimed in claim 9 or 10, wherein the pair of cylinders is operated with an overall lean air/fuel ratio producing output power from both the EGR cylinder and power cylinder.
13. A method as claimed in claim 7, wherein some of the EGR gases and reformed fuel containing hydrogen from the EGR cylinder are delivered via a pipe connecting from the pressurised intake system of the engine to a particulate trap and/or catalytic converter in the exhaust system of the engine for supporting regeneration of the particulate trap and selective catalytic reduction in the catalytic converter respectively.

-27 -
14. A method as claimed in any preceding claim, wherein the engine is boosted with pressurised air from a turbocharger and/or supercharger.
15. A method as claimed any one of claims 1 to 13, wherein the engine is boosted with pressurised air from a boost air storage tank onboard an air hybrid vehicle.
16. A method as claimed in any preceding claim, wherein a cam profile switching system is provided for modifying the opening events of the intake and exhaust valves of the selected EGR cylinders.
17. A method as claimed in any preceding claim, wherein a port throttle is provided in the engine port associated with the exhaust valve of each EGR cylinder for regulating the gas pressure drawn into the EGR cylinder in order to adjust the amount of EGR delivered by the EGR cylinder.