GB2289501A - I.c. engine with control of the number of operative cylinders - Google Patents

Abstract

The supply of air and fuel to a pair of cylinders is terminated at low load and their exhaust gases are recirculated. The volume of the manifold branch 34 downstream of the connections of the recirculation pipes 20 up to the connection 50 to the manifold branch 36 of the operative cylinders is greater than the displacement volume of one cylinder so that mixing does not take place between the recirculating and freshly produced gases. Heat is transferred to the recirculating gases and their temperature is influenced by a heat exchanger (44, Fig. 2) connected to the engine coolant circuit or a vehicle passenger compartment. <IMAGE>

Description

Internal Combustion Engine The present invention relates to a multi-cylinder spark ignition internal combustion engine having means for disabling individual cylinders, that is to say preventing individual cylinders from producing output power by interfering with the combustion cycle within the cylinder.

It is known to disable one or more cylinders of a multi-cylinder engine to improve the overall fuel economy.

Such improved economy is achieved because pumping work in the disabled cylinders is reduced. Furthermore, as the driver will tend to open the throttle to counteract the loss of power, the pumping work in the firing cylinders will also be reduced. The engine operating with a bank of cylinder disabled also produces lower hydrocarbon (HC) emissions.

The main sources of HC emissions are the crevices and the oil film in each combustion chamber which act as fuel stores. If fewer cylinders are in use, the HC emissions ar reduced in the same proportion as the number of disabled cylinders. Furthermore, the firing cylinders must work at a higher load in order to produce the same engine power output and this will also reduce their HC emissions by virtue of the improved efficiency when operating under high load conditions.Thus in addition to the improved fuel economy from the reduced pumping work, there is an advantage of lower HC emissions in the feedgas (the gas supplied from the engine to the catalytic converter) and a correspondingly lower demand on the catalytic converter to oxidise the HC emissions.

In order to disable a cylinder, it is known simply to prevent the supply of fuel to the cylinder while allowing the intake and exhaust valves to operate normally. This is not satisfactory because each disabled cylinder will transfer fresh air into the exhaust system making it impossible to maintain a stoichiometric exhaust gas composition, as is necessary for three-way catalytic conversion of the exhaust pollutants. Also the air drawn into the disabled cylinder undergoes compression followed by over-expansion because of heat loss near TDC. If the engine is not fully warmed up and if the ambient condition is cold and humid, the over-expansion may reduce the air temperature below freezing and cause ice forming on the spark plug, making it impossible to re-enable power operation of the cylinder when required.Furthermore, the exhaust gases will be excessively diluted and cooled by the pumped air to the extent that the catalytic converter may fall below its light off temperature.

It is also known to disable a cylinder by deactivating both intake and exhaust valves of the cylinder. This is the common practice and permits stoichiometric exhaust gas composition and high exhaust gas temperature to be maintained. However, this proposal is still not satisfactory because of the high complexity in the valve train design in order to achieve such valve deactivation.

Also, the same gas is permanently trapped in the disabled cylinder and this is first compressed and then undergoes repeated expansions and compressions for prolonged periods.

During these cycles, heat loss and air leakage will eventually result in an equilibrium condition in which the mean pressure in the disabled cylinders will be negative.

This causes a net inflow of lubricating oil into the combustion chamber past the piston rings. Excessive accumulation of oil may cause spark plug fouling. A further problem is that after prolonged disablement, a bank of engine cylinders may cool down excessively and give rise to problems when re-enabled.

US-A-4,304,208 and US-A-4,296,72 disclose systems in which in a six cylinder engine, banks of three cylinders at a time are disabled by shutting off their air and fuel supply and recirculating the exhaust gases of the disabled cylinders to their intake ports. These prior art systems address one particular problem that arises when disabling cylinders in this manner, namely that the recirculated gases through the disabled cylinders cool down after a period of time.Apart from the fact that the disabled cylinders would themselves become cold, which is undesirable, some of the cold gases circulated through the cylinders can find their way out of the recirculation loop and can be discharged through the exhaust pipe passing the exhaust gas oxygen sensor (EGO sensor) that is normally located upstream of the catalytic converter and used by the engine management system to maintain stoichiometry by closed loop control. This could cool the EGO sensor and cause it to lose accuracy, thereby interfering with the correct fuelling of the engine. Also these recirculated gases would contain a significant concentration of unburnt fuel trapped in the disabled cylinders, because fuel wetting the walls of the intake manifold and the intake ports is drawn into the cylinders and recirculated after the air and fuel supply has been shut off.This could cause the EGO sensor and the closed-loop control system to respond incorrectly because the measured oxygen does not correspond to the oxygen in the feedgas supplied to the catalytic converter.

To reduce these problems, these prior art references propose applying the cylinder disablement to a bank of three cylinders out of a six cylinder engine. Because of the overlapping of the intake and exhaust valve opening periods of the three cylinders in the group, most of the recirculated gases discharged from one disabled cylinder at one time will immediately be drawn back into an adjacent disabled cylinder. The net change in volume of the recirculated gases in the exhaust manifold of the disabled bank of cylinders is therefore relatively small.By specifying the volume of this exhaust manifold upstream of the EGR tapping point to be at least the same as the displacement volume of a disabled cylinder, and by further specifying a shut-off valve downstream of the EGR tapping point and upstream of the EGO sensor to isolate the disabled exhaust manifold bank from the remaining exhaust system, the problem of the recirculated gases escaping and cooling or disturbing the EGO sensor can be avoided.

This method, however, will not be effective for a bank of disabled cylinders consisting of one or two cylinders because in these cases, there is very little overlap between the intake and exhaust valve opening periods of the group of cylinders and the cyclic change in volume of the recirculated gases in the exhaust manifold bank is much larger than can be accommodated by these prior art systems.

The present invention seeks to provide an engine with cylinder disablement in which the problem of cooling of the disabled cylinders is mitigated. Furthermore, the invention aims to allow any number of cylinders to be disabled and to avoid affecting the reliability of the engine control system when reliant upon a signal from an EGO sensor in the exhaust system.

According to the present invention, there is provided a multi-cylinder spark ignition internal combustion engine having means for selectively disabling cylinders by shutting off the supply of fuel and ambient air to the intake port of each cylinder to be disabled and recirculating disabled cylinder exhaust gases to the intake port of each disabled cylinder, wherein the exhaust manifold has separate branches connected at their upstream ends to the exhaust ports of the firing and disabled cylinders, respectively, and connected to one another at their discharge ends at a point located upstream of a catalytic converter of the engine exhaust system, and wherein the gases recirculated to the intake port of a disabled cylinder are drawn from a branch of the exhaust manifold that is connected to the same cylinder or another disabled cylinder at a tapping located upstream of the discharge end of the branch, the volume of the latter branch disposed between the tapping from which the EGR gases are drawn and the discharge end of the branch being greater than the displacement volume of one engine cylinder.

The prior art offers no teaching regarding the volume of the exhaust pipe of the disabled bank of cylinders downstream of the EGR tapping point. Whereas the present invention requires this volume to exceed the full displacement volume of a disabled cylinder, the above prior art references are concerned only with the volume upstream of the tapping point.

The invention offers two advantages over the systems described in the prior art. First, the fluctuation in volume caused by the cyclic discharge and return of gases through the disabled cylinders is completely contained within this volume. If an EGO sensor is mounted downstream of this volume, no recirculated gases can escape and disturb the EGO sensor. Second, as the volume of the recirculated gases fluctuates within this branch, there is induced a corresponding flow of hot exhaust gases from the firing cylinders into and out of this branch cyclically which alternately occupies the same volume in the branch. This provides a heat transfer process whereby the pipe acts as a heat reservoir which is alternately exposed to the hot exhaust gases from the firing cylinders and the colder recirculated gases through the disabled cylinders.The transfer of heat to the recirculated gases in this manner is effective in the keeping the recirculated gases hot, and consequently the disabled cylinder will also remain hot and stable for a longer period. The hot exhaust gases from the firing cylinders entering the heat transfer volume of this pipe can only reach as far as the EGR tapping point and cannot be drawn into the disabled cylinders.

Indeed, so much heat can be transferred to the EGR gases in this way that the heat may be used to assist in engine warm up and in heating the passenger compartment of the vehicle following a cold start. To this end, a coolant heat exchanger may be provided to remove some of the heat from the recirculated gases.

The method by which the air and fuel supply are shut off to disable cylinders would depend on the fuel metering system used in the engine. For a multi-point fuel metering system which injects fuel individually into the intake port of each cylinder, the fuel injectors to the disabled cylinders are shut off at approximately the same time as the air supply are shut off and exhaust gases are recirculated to those cylinders. For a central fuel metering system which supplies a mixture of fuel and air from a common plenum to all the engine cylinders, shutting off the mixture supply to one bank of cylinders would automatically shut off both the fuel and air to disable those cylinders.

In both cases, the presence of liquid fuel which accumulates on the walls of the intake ports and intake manifold can cause a problem during the disablement of a bank of cylinders. After the metered fuel supply to those cylinders has been shut off, this liquid fuel would continue to evaporate and enter the disabled cylinders and could result in an excess of fuel in the recirculated gases. When these gases are eventually discharged when the bank of cylinders are re-enabled, high HC emissions could be released to the atmosphere because the catalytic converter will not be effective in oxidising this fuel in the absence of sufficient oxygen.

In order to mitigate this proble in a multi-point fuel injection engine, it is preferred to position the air shut off valve and the ERS connections to the intake tracts sufficiently far upstream in the intake manifold so as to trap a large volume of air in the recirculation loop of the disabled cylinders and to shut off the fuel injectors to those cylinders one or more cycles earlier than the shutting off of the air supply. The trapped air will in this case be sufficient to balance the liquid fuel that is also trapped in the recirculated gases resulting in an excess of air within those gases. This trapped fuel/air mixture may burn in the disabled cylinders and the remaining unreacted fuel and air, when they are discharged when the cylinders are re-enabled, will be fully oxidised in the catalytic converter.When cylinders are re-enabled, the fuel can also be resumed one or more cycles after resumption of the air supply to ensure that oxygen is available in the catalytic converter to complete the combustion of unburnt fuel for several cycles after the mode change.

In an engine with a central fuel injection (CFI) system, it is preferred to provide means for vaporising and atomising the metered fuel so finely that all the fuel is transported by the inducted air without being deposited on the walls of the intake tracts of the intake manifold. By ensuring a dry manifold, that is to say a manifold having substantially no liquid fuel wetting the walls of the manifold, the positions of the air shut off valve and the EGR connections to the intake tracts are no longer critical since any trapped mixture in the recirculated gases will have the correct fuel/air ratio which would either burn completely in the disabled cylinders, or when discharged through the catalytic converter, will be fully oxidised in the catalytic converter.

Considering now the operation of an EGO sensor in the exhaust system in this case, as the recirculated gases are kept hot, and as the fuel and air trapped in the disabled cylinders are in the correct fuel/air ratio, even if some of the recirculated gases should escape into the main exhaust system and flow past the EGO sensor, the sensor will not be disturbed either in temperature or in the mixture stoichiometry it detects, and the closed-loop control system will remain stable.

Because of the above, the invention makes it acceptable to tolerate some leakage or gas exchange between the recirculation volume of the disabled exhaust manifold bank and the firing exhaust manifold bank. This may occur at the exhaust manifold joint connecting the two manifold banks to the separate exhaust downpipes because of poor sealing at the joint. Indeed the EGO sensor may be located near this joint across a small opening connecting the two manifold banks without suffering any adverse effect, the small opening permitting only a negligible rate of gas exchange between the two manifold banks so that the two banks remain substantially separate from each other.

The separation of the exhaust manifold banks and downpipes also offers the advantage that when all the cylinders are firing, the manifold system has the ideal geometry of a tuned exhaust system capable of significantly improving the performance of the engine.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a schematic diagram of an engine of the invention, and Figure 2 is a side view of the engine shown in Figure 1.

The drawings show an engine 40 having a cylinder 42 that is capable of being disabled. The engine has an intake manifold with a first pair of branches 16 leading to the firing cylinders and a second pair of branches 14 leading to the cylinders capable of being disabled, the two pairs of branches being connected to a central fuel metering system comprising an air mass flow meter 6, a fuel metering unit 8 and a butterfly throttle 10. The two pairs of branches 14 and 16 are further connected to each other by means of a connection pipe containing a butterfly throttle 12.

Likewise, the exhaust manifold has a first pair of branches 26 leading to the firing cylinders and a second pair of branches 24 leading to the cylinders capable of being disabled, the two pairs of branches being connected by a further pair of downpipes 36 and 34 to an exhaust system 38 that includes a catalytic converter (not shown). An exhaust gas oxygen sensor (EGO sensor) is mounted in the exhaust system upstream of the catalytic converter at one of the positions designated 50 and 52 in the drawing. The EGO sensor forms part of a closed loop control system that varies the quantity of fuel supplied to the engine in such a manner to maintain a stoichiometric composition of the exhaust gases.

As so far described, the engine is generally known. However in the present invention, means are provided to enable 100W exhaust gas recirculation (EGR) to be applied to the individual cylinders to be disabled. To this end, an EGR pipe 20 leads from the exhaust branch 24 of cylinders 1 and 4 of the illustrated four cylinder engine to the intake ports 18 of the same cylinders. Shut off valves 22 are positioned near the junction between the EGR pipes 20 and the intake manifold branches 14.

In the invention there are separate branches 36, 34 in the exhaust manifold that lead to the exhaust ports 19 of the firing and disabled cylinders and these branches merge into one another at the point 50 located upstream of the catalyst converter, this being the discharge end of both branches.

The EGR pipe 20 is connected to the branch 24 sufficiently upstream of the discharge end 50 to allow the entire displacement volume of one cylinder of the engine to be stored in this length of the separate branch 34.

When the engine is operated normally, that is to say at high load with all cylinders firing, the valve 12 is open and the valves 22 are closed to isolate the intake manifold from the EGR pipe 20. When the cylinders 1 and 4 are to be disabled for light load operation, the fuel and air supply to these cylinders is inhibited by closing the valve 12 and EGR gases are instead supplied to the disabled cylinders by opening the shut off valves 22.

Because cylinders 1 and 4 now receive neither fuel nor air, they are disabled and no longer contribute to the output power of the engine. During this time it is preferred to continue to apply sparks to the spark plugs to keep the points clean. The disabled cylinders however do not add to the pumping work performed by the engine nor do they contribute any component to the exhaust gases that would interfere with the stoichiometry at the catalytic converter.

The exhaust and induct Ion strokes are of course not simultaneous for any given cylinder and the gases displaced during the exhaust stroke must therefore be stored temporarily within the exhaust manifold branches 24 and 34.

Because the volume of the exhaust manifold branch 34 between the EGR tapping point and the discharge end of the manifold branch 34 exceeds the displacement of one cylinder, the invention ensures that no mixing takes place between the gases from the firing and the disabled cylinders. As each cylinder discharges into the branch 34, the EGR gases will move towards the discharge end 50 but before reaching it the same or another cylinder will start drawing in the same EGR gases through the EGR pipe 20 and its intake port 18. The column of EGR gases will therefore oscillate within the branch 34 while the cylinders are disabled.

The lower end of the oscillating column of EGR gases is an interface with the exhaust gases from the firing cylinders 3 and 4. As this interface moves towards the EGR tapping point, hot exhaust gases from the firing cylinders will enter the exhaust branch 34 and heat it. When the interface moves down again towards the discharge end 50, the EGR gases occupying the branch 34 will now be heated by the heat stored in the walls of the exhaust branch 34. There is thus established a heat transfer mechanism that ensures that the recirculating EGR gases remain hot regardless of the duration over which the cylinders 1 and 4 are disabled.

The heat transferred in this manner to the recirculated EGR gases is more than sufficient to ensure that the disabled cylinder will be re-ignitable when firing in all cylinders is resumed. The surplus heat can be drawn off, as shown in Figure 2, by a heat exchanger 44 and transferred either to the coolant circuit of the engine to assist rapid warm up or to the passenger compartment of the vehicle in which the engine is installed to improve passenger comfort during cold starts.

If the engine has a wet intake manifold there is a risk of fuel being present in the recirculating gases that will be trapped without sufficient air to react with it in the catalytic converter. This can cause emission problems and if the EGO sensor should be exposed to the recirculating gases as it would be in the position designated 52, it can cause problem of instability and inaccuracy in the closed loop engine management system.

It is preferred to avoid this problem at source by not having a wet intake manifold. The problem can be reduced by resorting to multi-point fuelling but even then there is still wall wetting of the intake ports. It is therefore advantageous to use system in which the fuel is fully vaporised or atomised to very fine droplets so as to avoid the deposition of fuel on the walls of the intake manifold.

Furthermore the walls can be coated with a non-stick coating and inclined away from the intake ports so that liquid fuel does not slide down into the disabled cylinders.

If wall wetting is unavoidable, then it is preferred to shut off the fuel supply prior to shutting the air supply and to reconnect the supplies in the reverse order. This will ensure that there is always present sufficient oxygen in the recirculating gases to react with any trapped fuel in the catalytic converter.

If steps are taken to ensure the stoichiometry of the trapped recirculating charge when the cylinders are disabled, no harm can arise from exposing the EGO sensor to these gases and it is this that enables the EGO sensor to be located if desired at the position designated 52. At the point 52 there may be a small gas exchange between the recirculating gases and the exhaust gases of the firing cylinders, but this does not have undesirable consequences.

The exhaust gases from the firing cylinders will merely improve the thermal transfer while escaping recirculating gases will have no effect on the emissions nor on the engine management system. The position 52 corresponds in practice to the junction between the exhaust manifold and the exhaust system downpipe and is conventionally the optimum position for locating the EGO sensor. A small leakage between the exhaust branches at this joint is almost unavoidable but, as described above, such leakage is not detrimental and can readily be tolerated.

Because the exhaust system is of a generally conventional design, the normal advantages to be achieved by tuning the lengths of the individual pipes for engine performance can be retained and are unaffected by the EGR pipes 20 when they are shut off.

Claims (5)

1. A multi-cylinder spark ignition internal combustion engine having means for selectively disabling cylinders by shutting off the supply of fuel and ambient air to the intake port of each cylinder to be disabled and recirculating disabled cylinder exhaust gases to the intake port of each disabled cylinder, wherein the exhaust manifold has separate branches connected at their upstream ends to the exhaust ports of the firing and disabled cylinders, respectively, and connected to one another at their discharge ends at a point located upstream of a catalytic converter of the engine exhaust system, and wherein the gases recirculated to the intake port of a disabled cylinder are drawn from a branch of the exhaust manifold that is connected to the same cylinder or another disabled cylinder at a tapping located upstream of the discharge end of the branch, the volume of the latter branch disposed between the tapping from which the EGR gases are drawn and the discharge end of the branch being greater than the displacement volume of one engine cylinder.

2. An internal combustion engine as claimed in claim 1, wherein a heat exchanger is provided for cooling the exhaust gases recirculated to the intake ports of the disabled cylinders.

3. An internal combustion engine as claimed in claim 2, wherein the engine is a water cooled engine installed in a motor vehicle having a driver compartment, and wherein the heat exchanger serves to transfer heat from the recirculated gases to the coolant circuit of the engine upstream of a second heat exchanger arranged in a heater unit of the driver compartment.

4. An internal combustion engine as claimed in any preceding claim, wherein all engine cylinders are supplied with air and fuel mixture by a central fuel metering system having means for vaporising and atomising the metered fuel so finely that all the fuel is transported by the inducted air without being deposited on the walls of the intake tracts of the intake manifold.

5. An internal combustion engine, constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.