WO1996010688A1

WO1996010688A1 - Stratified charge engine

Info

Publication number: WO1996010688A1
Application number: PCT/GB1995/000904
Authority: WO
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Company Limited; Ford Werke Ag; Ford France S.A.; Ford Motor Company
Priority date: 1994-10-04
Filing date: 1995-04-21
Publication date: 1996-04-11
Also published as: GB9419944D0; GB2293862A

Abstract

An internal combustion engine is described comprising a cylinder having an intake valve (14), and two manifolds having branches (22, 32) that are configured to supply two gas streams to the intake valve of each cylinder. The two streams enter the cylinder separately through the valve (14) so as to produce a stratified charge within the engine cylinder. The first manifold (24) supplies a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold (34) supplies stratification gases. A non-return valve (23) is arranged in each branch (22) of the first manifold to permit gas flow in the branch only in the direction towards the intake valve (14).

Description

STRATIFIED CHARGE ENGINE

Field of the invention

The present invention relates to an internal combustion engine having a stratified charge, that is to say a charge in which the mixture in the combustion chamber is deliberated rendered inhomogeneous.

Background of the invention

Engines are known that comprise a cylinder having an intake valve, and two manifolds having branches that are configured to supply two gas streams to the intake valves of each cylinder, the first manifold supplying a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold supplying a stratification gas, which may either be recirculated exhaust gases (EGR) or air containing no fuel.

Conventional engines do not achieve charge stratification because the two streams are well mixed on entering the cylinder. In the case where EGR gases are supplied through the second manifold, during the periods that the intake valve is closed, EGR gases enter and are stored in the intake port and along the branch of the first manifold so that when the intake valve opens again, the gases first inducted contain little oxygen yet they tend to have a high fuel content picked up from the walls of the wet intake port. This can only be tolerated if the combustion chamber is designed to produce good mixing of the charge during the compression stroke, as is the case in conventional engines.

A stratified EGR charge has been produced in engines with two intake valves, by feeding the combustible charge to one intake valve and a gas stream containing the EGR gases to the other intake valve. If there is well organised symmetrical tumble motion within the combustion chamber during the compression stroke, this can result in transverse stratification, that is to say the mixture strength is inhomogeneous across the combustion chamber with the combustible mixture remaining on one side and the EGR gases on the other.

This latter proposal has two disadvantages. First, the position of the spark plug needs to be offset from the centre to lie within the combustible part of the charge. Second, a clear separation of combustible charge and EGR gases can only be achieved over a narrow range where the EGR gases and the combustible charge are of substantially the same volume.

Object of the invention

The present invention seeks to provide a stratified charge engine in which a combustible mixture and gases containing no fuel remain generally separate within the combustion chamber and in which the proportion of stratification gases can be varied over a wide range without interfering with the ignitability of the mixture.

Summary of the invention

According to the present invention, there is provided an internal combustion engine comprising a cylinder having an intake valve, and two manifolds having branches that are configured to supply two gas streams to the intake valve of each cylinder, the two streams entering the cylinder separately through the valve so as to produce a stratified charge within the engine cylinder, the first manifold supplying a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold supplying gases that contain no fuel, wherein a non-return valve is arranged in each branch of the first manifold to permit gas flow in the branch only in the direction towards the intake valve.

In the present invention, the air/fuel mixture and the stratification gases are drawn in separately through the same valve passing over different regions of the valve skirt and steps are taken to maintain the two streams of the intake charge separate within the combustion chamber during the intake and compression strokes of the engine.

The non-return valves in the branches of the first intake manifold serves first to prevent back-filling of these branches with stratification gases, thereby ensuring that no fuel is entrained from the wet intake port by the stratification gases. The non-return valves also ensure that pockets of incombustible gases are not stored in the branches of the first intake manifold. Such pockets would find their way into the combustible region of the stratified charge and thereby interfere with flame propagation within this region of the charge.

An engine having the same aim of stratifying the charge and having generally the same components as the present invention except for the non-return valves in the branches of the first intake manifold has been proposed in the Applicant's co-pending British Patent Application No. 9409400.0. In the latter patent specification, the first manifold is especially designed to act as a reservoir for gases supplied through the second manifold while the intake valve is closed so that along the length of the branches of the first manifold there is stored a column of gases that is stratified along its length and which, if drawn into the cylinder without being disturbed, gives rise to an axially stratified charge within the engine cylinder.

One of the described embodiments of the latter application ensures thorough scavenging of the closed end of the intake port during the period when the intake valve is closed, that it to say, it makes sure that the intake port is left clean of fuel-air mixture. The reverse flow of gases containing no fuel is then exploited to store within the branch of the first intake manifold a stratified charge which is subsequently drawn into the engine cylinder during the intake stroke, still as a column, to create axial stratification within the cylinder. The thorough scavenging ensures no isolated pocket of fuel-air mixture is drawn into the cylinder separate from the main mixture charge.

British Patent Application No. 9409400.0 also describes an embodiment which, during the period when the intake stroke is taking place, takes advantage of the parallel flows of fuel-air mixture from the first intake duct and gases containing no fuel from the second intake duct to create transverse stratification across the engine cylinder. The combined effect in both embodiments is one of predominately axial charge stratification along the cylinder, with stratification across the cylinder existing only as a minor feature. This works well with swirl but not as well with tumble because the axial stratification could result in a pocket of inert gases being transported by the tumble into the spark plug region and cause misfire.

A requirement of the above system is that the length of the intake ducts must be sufficient to store the column of stratified charge. Also in the case where the second intake flow of gases containing no fuel consists of EGR gases, because of the reverse flow into the first intake ducts, the intake port and the first intake duct will alternately contain fuel-air mixture and EGR gases and it is necessary to make sure there is no liquid fuel wetting the walls of the intake port otherwise this wet fuel will be entrained by the EGR gases which contains no oxygen and cannot support combustion. This restricts the application of stratified EGR to pre-mixed fuel-air mixture which is already finely atomised or fully vaporised before it is supplied to the first intake duct which should remain dry. This precludes the use of intake port fuel injection which typically deposits wet fuel on the intake port walls.

The present invention does not rely on thorough scavenging of the closed end of the intake port during the period when the intake valve is closed, but prevents the reverse flow of stratification gases into the branches of the first intake manifold by using a non-return valve in each branch. The branch cannot now be filled from its end nearer to the intake valve and does not store a column of stratified charge. Instead it remains filled with fuel-air mixture which is drawn into the cylinder as soon as the intake valve is open. This achieves stratification in the combustion chamber by parallel flows and allows more freedom in the choice of the length of the intake ducts and the quality of the fuelling system where wall wetting is permissible.

In the present invention, the non-return valves ensure that there is no axial stratification of the charge and instead the stratification is either radial or lateral depending on the port and combustion chamber design. If the intake streams are drawn in tangentially to promote organised swirl, it is possible to achieve a central core of combustible mixture surrounded by a ring of stratification gases, this being termed radial stratification. Alternatively, if the intake port or ports are designed to be symmetrical about a central plane and gases are drawn symmetrically from both sides of the central plane to produce a tumbling motion, then lateral stratification can be achieved in which a diametrical slice containing the combustible mixture is flanked by two regions containing stratification gases. In both cases, the combustible mixture in the present invention can be confined to the centre of the combustion chamber, obviating the need to reposition the spark plug from its normal central position. The branches of the second manifold leading to the intake valves are preferably of a substantial diameter as compared with the branches of the first manifold, being typically between a quarter and a half of the full flow cross section of the intake port. It may be considered that the presence of such large branches of the second manifold in the intake ports might restrict the breathing of the engine and reduce its full load capacity when the gas supply to the second manifold is shut off. However, because flow can occur in both directions along the branches of the second manifold, the second manifold will act to store gases drawn through the branches of the first manifold while the intake valves are closed and to transfer gases between branches of the first intake manifold. As a result, when the supply of stratification gases to the second manifold is shut off, both manifolds will be supplying combustible mixture to the intake valves.

It is preferred to dimension the flow cross sections of the branches of the two manifolds to be equal to one another so that under full load conditions equal quantities of air will be drawn from both manifolds for maximum performance.

The ability of the invention to vary the proportion of stratification gases over a wide range allows the effective volume of the combustible part of the charge to be varied without resorting to engine throttling. The stratification can therefore be used for load control leading to an engine with reduced air pumping losses.

In the case of stratification with EGR gases, by metering only the air supplied to the first manifold and setting the fuel quantity accordingly, it is possible to ensure that the mixture strength within the combustible part of the charge is stoichiometric, thereby permitting the use of a three-way catalyst to purify the exhaust gases. Apart from GB 9409400.0, other prior art systems have recognised the potential of achieving charge stratification by controlling the flow of two streams of gases as they are drawn into the combustion chamber during the period when the intake valve is open. However, they failed to appreciate the detrimental effect of back-filling of gases within the flow channels during the period when the intake valve is closed, and overlooked the importance of preventing such back-filling from occurring in order to maintain the purity of the two gas streams throughout the whole engine cycle. The use in the invention of a non-return valve ensures a clear definition in the purity of the respective gas streams and guarantees a high degree of charge stratification within the engine combustion chamber. Consequently it is possible to operate the engine at a significant level of overall charge dilution, resulting in substantial reductions in the engine pumping losses and exhaust NO_x emissions.

The presence of the non-return valve also ensures that backflow of residual gases into the intake port, during overlap periods when the intake and exhaust valves of a cylinder are both open simultaneously, usually termed internal EGR, is directed into the second manifold and does not mix with the fuel and air mixture drawn in through the first manifold. Thus gases from the internal as well as the external EGR are stored in the branches of the second manifold.

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 diagram of a four cylinder spark ignition internal combustion engine fitted with an intake and exhaust manifold system designed to produce a radially stratified charge,

Figure 2 schematically show in plan and sectional views the distribution of the combustible mixture within the combustion chamber in Figure 1,

Figure 3 is a view generally similar to that of Figure 1 showing an arrangement designed to produce a laterally stratified charge, and

Figure 4 comprises two views similar to those shown in Figure 2 showing the distribution of the combustible mixture within the combustion chamber of Figure 3.

Detailed description of the preferred embodiments

In Figure 1 an engine 12 has combustion chambers each with two intake valves 14 and two intake ports, a spark plug 16 and two exhaust valves 18. Shut-off valves 25 are provided in the intake ports to permit de-activation of one of the intake ports under low load conditions. Both of the intake ports are supplied with air from a first manifold that has a plenum chamber 24 and separate branches 22 leading to the individual cylinders. The plenum 24 draws in outside air through a flow meter 52 and a flow control valve 50. Fuel is introduced into the first manifold near the intake ports by injection nozzles 60, the fuel quantity being calculated in dependence upon the air flow drawn in through the first manifold only.

A second manifold having a plenum chamber 34 and separate branches 32 leading to the permanently active intake ports of the cylinders is provided to supply EGR gases under certain operating conditions. To this end the plenum chamber 34 is connected to the exhaust manifold 80 through EGR pipe 82 and a flow control valve 84 in the pipe 82 can be used to regulate the flow of EGR gases drawn into the engine. The illustrated embodiment also permits the second manifold 34 to be connected to outside air through a diverter valve 90. This valve 90 enables different compositions of the stratification gases to be used, ranging from 100% EGR to 100% air, or any desired EGR/air ratio between these two extremes.

As so far described, the engine is generally similar to that described in the previously mentioned co-pending Patent Application No. 9409400.0. In the invention however non¬ return valves 23 are provided in the branches 22 to permit gas to flow in the branches only in the forward direction, that is to say towards the combustion chambers.

The branches 32 of the second manifold are directed tangentially so that as stratification gases enter the combustion chambers, they are directed towards the outer circumference of the cylinder and the combustible fuel and air mixture is directed towards the centre of the combustion chamber. The strong swirl creates the charge distribution shown in Figure 2 wherein the shaded region 15 represents the combustible mixture and the unshaded region 17 represents the stratification gases.

As the proportion of stratification gases is varied by changing the positions of the flow control valves 84 and 50, the radial size of the combustible mixture can be varied to regulate engine load. It will be noted that if the stratification gases are EGR gases, this does not affect the stoichiometry of the overall charge as the combustion chamber is being filled with gases that contain no oxygen. Throughout the regulating range combustion remains stable as the part of the charge surrounding the spark plug is always ignitable. The non-return valves 23 serve to prevent stratification gases from flowing into the branches 22 when the intake valves are closed. This is important because it is inevitable that some fuel will be present in the intake ports and in the absence of the non-return valves 23, this fuel would be entrained within the stratification gases and the resultant charge in the combustion chamber would become mixed and poorly stratified.

It should be mentioned that long branches 22 are shown in

Figure 1 for clarity but these branches do no serve to store a stratified charge as they do in the co-pending application and in practice these branches may be short.

Under low load a stratified charge is produced in the manner described above and as the load increases the valves 25 are opened to bring both intake valves into play. At this time if stratification gases are introduced into the combustion chamber they can no longer swirl because of the symmetrical flow from the two intake valves and instead the flow will tumble within the combustion chamber leaving a single pocket of stratification gases on the side of the cylinder near the branch 32. Eventually the stratification gases are shut off completely to allow the engine to operate under full load with a full air charge.

During full load operation, the engine is operated with the valve 84 full closed. The second manifold will act to store air and to transfer air between cylinders so that the cylinders continue to receive a full air charge, albeit that some air will reach each cylinder indirectly.

Figure 3 differs from Figure 1 only in that there are branches 32, 32' of the second manifold leading to each of the valves of the engine. The geometry of the intake ports is design to be symmetrical about the diameter of the cylinder so that no swirl occurs and instead the full charge tumbles to give rise to the combustible mixture distribution shown in Figure 4 which is here referred to as lateral stratification. Because like components have been designated by the same reference numerals, it is believed that the operation of this embodiment does not call for more detailed explanation.

In both described embodiments, by linking the two flow control valves 50 and 84, it is possible to provide load control that changes the engine load by varying the proportion of the volumes of the combustible gases and the stratification gases without increasing the air pumping work done by the cylinders. It may simplify the control system further to allow the linkage connecting the control valves 50 and 84 to be moved directly by the demand pedal.

The illustrated embodiments both have multi-point fuel injection but this is acceptable as the invention does not critically depend on a dry intake manifold. It is alternatively possible to resort to a carburettor or central fuel injection directly into the plenum 24.

Claims

1. An internal combustion engine comprising a cylinder having an intake valve (14), and two manifolds (24,34) having branches (22,32) that are configured to supply two gas streams to the intake valve (14) of each cylinder, the two streams entering the cylinder separately through the valve so as to produce a stratified charge within the engine cylinder, the first manifold supplying a metered quantity of air within which the fuel to be burnt is dispersed and the second manifold supplying gases that contain no fuel, characterised in that a non-return valve (23) is arranged in each branch (22) of the first manifold (24) to permit gas flow in the branch only in the direction towards the intake valve (14) .

2. An internal combustion engine as claimed in claim 1, wherein the branches (32) of the second manifold (34) have a flow cross sectional area equal to at least one quarter of the area of the intake ports of the engine.

3. An internal combustion engine as claimed in claim 2, wherein the branches (22,32) of the first and second intake manifolds have substantially equal flow cross sections.

4. An internal combustion engine as claimed in any preceding claim, wherein the streams of gases from the branches (22,32) of the first and second manifolds are maintained separate by a physical partition until they reach the vicinity of the intake valves and are thereafter inducted in parallel into the combustion chamber without significantly mixing with one another.

5. An internal combustion engine as claimed in any preceding, wherein the flows from the branches of both manifolds enter the combustion chamber through a single valve and wherein the branches (22,32) of the first and second manifolds (24,34) are designed to give substantially parallel flows directed tangentially to the cylinder bore of the combustion chamber so as to produce swirling motion in the combustion chamber about the axis of the cylinder.

6. An internal combustion engine as claimed in any of claims 1 to 4, wherein the flows from the branches (22,32) of the two manifolds (24,34) enter the combustion chamber through two valves and wherein the branches of the first and second manifolds are designed to give substantially parallel flows directed parallel to a diameter of the cylinder bore so as to produce tumbling motion of the gases in the combustion chamber about an axis transverse to the axis of the cylinder.

7. An internal combustion engine as claimed in any preceding claim, wherein fuel is metered in dependence upon the air quantity drawn in through the first intake manifold only.

8. An internal combustion engine as claimed in Claim 7, wherein fuel is metered as a spray into a plenum chamber of the first manifold to mix with the metered air supply.

9. An internal combustion engine as claimed in Claim 7, wherein fuel is separately metered as a spray into each branch of the first intake manifold downstream of the associated non-return valve (23) .

10. An internal combustion engine as claimed in any preceding claim, wherein the gases drawn in from the second intake manifold comprise only EGR gases.

11. An internal combustion engine as claimed in Claim 10, wherein the average fuel-air ratio of the stratified intake charge is stoichiometric.

12. An internal combustion engine as claimed in any of claims 1 to 9, wherein the gases drawn in from the second intake manifold contain air.

13. An internal combustion engine as claimed in any preceding claim, wherein respective flow control valves are provided for regulating the gas supplies into the first and second manifolds, to vary the proportion of the first and second intake flows drawn into each combustion chamber in order to regulate the power output of the engine.