GB2306194A

GB2306194A - Stratified charge engine

Info

Publication number: GB2306194A
Application number: GB9521141A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1995-10-14
Filing date: 1995-10-14
Publication date: 1997-04-30
Also published as: GB9521141D0

Abstract

Two manifolds 24, 34 have branches 22 and 32, 32' that are configured to supply four gas streams to each cylinder which enter the cylinder separately through the valves 14 so as to produce a tumble flow stratified charge within the cylinder. The manifold 24 supplies a metered quantity of air within which the fuel to be burnt is dispersed and the manifold 34 supplies stratification gases. The supply throttles 50, 54 to the manifolds 24, 34 are ganged to operate in unison in order to maintain the ratio of volumes of fuel and air mixture and stratification gases constant.

Description

STRATIFIED CHARGE ENGINE Field of the invention The present invention relates to a stratified charge internal combustion engine comprising at least one cylinder having two intake valves, and two manifolds having branches supplying first and second gas streams to both intake valves of each cylinder, the two streams entering the cylinder separately through different regions of the valves so as to produce a stratified charge in the form of a vertical sandwich within the engine cylinder, the first manifold supplying a stream comprising a metered quantity of air within which fuel to be burnt is dispersed and the second manifold supplying a stream comprising gases that contain no fuel, the first stream being arranged to pass over the intake valves in regions adjacent to one another between the two valves and the second gas stream being split into two partial streams and being arranged to pass over the intake valves in two regions one on either side of the first stream and substantially parallel with the first stream. Such an engine will hereinafter be referred to as "an engine of the type described initially".

Background of the invention Engines are known that comprise a cylinder having at least one 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 recirculated exhaust gases (EGR).

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

However, in a stratified charge engine, this storage of EGR gases in the branches of the first manifold must be avoided as such stratification would result in poor combustion quality.

In an engine of the type described initially, it is important to control the contents and the velocities of the two streams entering the combustion chamber such that there is minimum mixing between them during the intake and compression strokes in order to conserve the stratification up to the time of ignition. In order to achieve this, two conditions must be met. First, the content of the two streams must be kept separate for as long as possible and second the relative velocity of the two streams when they contact each other within the combustion chamber must be kept as close to zero as possible.To meet these conditions, it is necessary to avoid balancing flows between the two manifolds at those intake ports which are not undergoing an intake stroke, and to control the velocities in the two streams at a fixed ratio of 1:1 at the intake port undergoing an intake stroke in order to conserve the sandwich stratification.

Object of the invention The present invention seeks to provide a robust and low cost system which maintains the above conditions over a range of engine speed and load conditions where a stratified charge is to be achieved.

Summary of the invention According to the present invention, there is provided an engine of the type set out initially, wherein the supply throttles to the two intake manifolds are ganged to one another to operate in unison and the area ratio of the first and second supply throttles is in all throttle positions substantially equal to the ratio of the effective flow cross-sectional areas of the first and second gas streams as they pass over the intake valves.

In the invention, the volume flow ratio in the supply throttles is set equal to the ratio of the effective flow cross-sectional areas of the first and the two second streams at the intake port end. For example, if the exit streams area ratio is 5:6, then the supply volume flow ratio should be kept at 5:6. By selecting the dimensions of the supply throttles to have the same size ratio of 5:6 and ganging them together to operate in unison, the desired volume flow ratio is kept constant over a range of throttle positions. The velocity ratio of the first and the two second streams, which can be expressed as the ratio of the volume flow to the flow cross-sectional area, will automatically remain constant at 1:1.

To ensure that the cross-sectional areas of the throttles should remain constant over a wide range of positions, it is important that the throttles should be of similar geometry to one another. For example, they may both be butterfly throttles with butterfly areas in the desired ratio.

A velocity ratio of 1:1 is the ideal value for sandwich stratification where the relative velocity between the first and second streams should be kept as close to zero as possible in order to conserve the stratification during the intake and compression strokes. A velocity ratio of 1:1 is also the unique condition at which the pressures in the two plenum chambers will be equal thereby avoiding balancing flows. When these conditions are met by design according to the present invention, the stratified charge engine will operate reliably with good sandwich stratification.

In the invention, the volume ratio of the two streams forming the vertically layered stratified charge is determined primarily by the design of the partitioned area ratio at the intake ports and correspondingly by the same size ratio of the ganged supply throttles. This ratio in turn sets the degree of charge stratification which should remain the same over the range of engine speed and load operating conditions where a stratified charge is to be achieved. For a high degree of charge stratification, that is to say a small volume of combustible charge sandwiched between two large volumes of stratification gas (air or EGR), the partitioning of the intake port may be designed such that the area of the second stream exceeds the area of the first stream. For lower degrees of charge stratification, the area of the second stream may be equal to or smaller than the area of the first stream.

A shut-off valve may be included in series with the supply throttle of the second stream. When this shut-off valve is open, conditions are met for equal pressures in the plenum chambers and sandwich stratification is achieved in the combustion chamber. When this shut-off valve is closed, large balancing flow are created between the plenum chambers and the charge entering the combustion chamber becomes homogeneous. These two modes may be selected for the lower load range and the higher load range respectively.

In the invention, for a given fixed area ratio for the two streams at the intake port, the supply throttles of the engine are deliberately constrained relative to one another to hold the velocity ratio into the combustion chamber constant at 1:1. Because of the otherwise flexible design of the system with separate manifolds and separate throttles, it would be possible to select a wide range of velocity ratios other than 1:1 if the two supply throttles were unganged and operated independently of one another with no regard to the area ratio at the intake port. However such velocity ratios other than 1:1 would create balancing flows between the branches and would not conserve the sandwich stratification through the intake and compression strokes.

To verify that the velocity ratio of the two streams in the intake port is 1:1, a differential pressure gauge may be provided to measure the pressure difference between the plenum chambers of the two manifolds supplying the two streams. This should read zero when the conditions of the invention are met.

The branches of the second manifold leading to the intake valves are preferably of a substantial cross-section as compared with the branches of the first manifold, being typically 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, under conditions when the pressures in the two manifolds are unequal, 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.

The stratification gases may either be air, EGR gases or a mixture of the two. In the case of stratification with EGR gases alone, by metering the air supplied only 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.

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 sandwich stratified charge, and Figure 2 schematically shows horizontal and vertical sections through a combustion chamber to show the distribution of fuel and air mixture and stratification gas within the combustion chamber produced by the intake system of Figure 1.

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. 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 chamber 24 draws outside air through a flow meter 52 and a supply throttle 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, 32', leading to the intake ports of the cylinders is provided to supply stratification gases under certain operating conditions. To this end, the plenum chamber 34 is connected to the exhaust manifold 80 through an EGR pipe 82 and a supply throttle 54 is 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.

The branches 32, 32 of the second manifold are directed symmetrically about the diameter of the cylinder and supply into the combustion chamber two stratification gas streams, one on each side of a fuel and air mixture stream supplied by the branch 22 of the first manifold. The intake charge tumbles within the combustion chamber to give rise to the combustible mixture distribution shown in Figure 2 which is here referred to as sandwich stratification. In Figure 2 the shaded region 15 represents the combustible mixture which is sandwiched between the unshaded regions 17, 17' representing the stratification gases.

In Figure 1 the volume flow ratio of the supply throttles 50, 54 is set equal to the ratio of the effective flow cross-sectional areas of the first and second streams at the intake port end. By selecting the dimensions of the supply throttles 50, 54 to have the same size ratio as the volume flow ratio and by ganging the throttles together to operate in unison, the volume flow ratio is kept constant over a range of throttle positions. The velocity ratio of the first and second streams, which can be expressed as the ratio of the volume flow to the flow cross-sectional area at the intake port, will automatically remain constant at 1:1 over the range of throttle positions.

A velocity ratio of 1:1 is the ideal value for sandwich stratification where the relative velocity between the first and second streams are kept as closed to zero as possible in order the conserve the stratification during the intake and compression strokes. A velocity ratio of 1:1 is also the unique condition at which the pressures in the two plenum chambers 24, 34 are equal thereby avoiding balancing flows between the branches 22 and 32, 32'. The absence of balancing flows ensures that the contents of the first and second streams are kept separate and distinct at all times that the throttles 50 and 54 are used to regulate the respective gas supplies to both manifolds.

In Figure 1, the degree of initial stratification, that is to say the volume ratio of the first and second streams entering the combustion chamber, is kept constant at substantially 1:1 for a range of engine speed and load conditions. This represents a dilution ratio of 50% in the intake charge and a significant reduction in the pumping work through reduced throttling which is beneficial for reducing fuel consumption and exhaust emissions.

Figure 1 also shows a shut-off valve 56 positioned in series with the supply throttle 54 and downstream of the same.

When this shut-off valve 56 is open, then the engine operates with a stratified charge as described above.

However, for homogeneous charge operation, for example at full load, the shut-off valve 56 is closed in order to isolate the plenum 34 from the ambient and from the EGR pipe 82. Under this condition, there will be a large pressure difference between the plenums 24 and 34 causing a large balancing flow along the branches 22 and 32, 32'. The second manifold will act to store air and fuel mixture and to transfer the mixture between cylinders so that the cylinders will receive a combustible mixture from both manifolds, albeit that some of the mixture will reach each cylinder indirectly.

A convenient method for calibrating the size ratio of the ganged throttles 50, 54 against the area ratio of the first and second streams at the intake port is to make use of a differential pressure gauge 70 connected between the two plenum chambers 24, 34 and to verify that a zero differential pressure is achieved.

The illustrated embodiment has multi-point fuel injection.

However as the invention does not critically depend on a dry intake manifold, it is alternatively possible to use a carburettor or a central fuel injection system supplying fuel directly into the plenum 24.

Claims

1. A stratified charge internal combustion engine comprising at least one cylinder having two intake valves (14), and two manifolds (24,34) having branches supplying first (22) and second (32, 32') gas streams to both intake valves (14) of each cylinder, the two streams entering the cylinder separately through different regions of the valves (14) so as to produce a stratified charge in the form of a vertical sandwich within the engine cylinder, the first manifold (24) supplying a stream comprising a metered quantity of air within which fuel to be burnt is dispersed and the second manifold (34) supplying a stream comprising gases that contain no fuel, the first stream (22) being arranged to pass over the intake valves in regions adjacent to one another between the two valves and the second gas stream being split into two partial streams (32, 32') and being arranged to pass over the intake valves in two regions one on either side of the first stream (22) and substantially parallel with the first stream (22), wherein the supply throttles (50, 54) to the two intake manifolds (24, 34) are ganged to one another to operate in unison and the area ratio of the first and second supply throttles (50,54) is, in all throttle positions, substantially equal to the ratio of the effective flow cross-sectional areas of the first (22) and second (32, 32') streams as they pass over the intake valves (14).

2. An internal combustion engine as claimed in claim 1, wherein the branches (22 and 32, 32') of the first and second intake manifolds at the intake ports have substantially equal flow cross sections.

3. An internal combustion engine as claimed in Claim 1 or Claim 2, wherein the streams of gases from the branches (22 and 32, 32') of the first and second manifolds are maintained separate by physical partitions 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.

4. An internal combustion engine as claimed in any preceding, wherein the flows from the branches of both manifolds enter the combustion chamber through common intake valves and wherein the branches (22 and 32, 32') of the first and second manifolds (24,34) are designed to give substantially parallel flows across the cylinder bore of the combustion chamber so as to produce tumbling motion in the combustion chamber about an axis perpendicular to the axis of the cylinder.

5. 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 (24) only.

6. An internal combustion engine as claimed in Claim 5, wherein fuel is metered into the plenum chamber (24) of the first manifold to mix with the metered air supply.

7. An internal combustion engine as claimed in Claim 5, wherein fuel is separately metered into each branch of the first intake manifold.

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

9. An internal combustion engine as claimed in Claim 8, wherein the average fuel-air ratio of the stratified intake charge is stoichiometric.

10. An internal combustion engine as claimed in any of claims 1 to 7, wherein the gases drawn in from the second intake manifold contain air.

11. An internal combustion engine as claimed in 10, wherein a diverter valve (90) is located upstream of the supply throttle (54) leading to the second manifold (34), the diverter valve (90) supplying air to the second manifold (34) in a first position, and supplying EGR gases to the second manifold (34) in a second position.

12. An internal combustion engine as claimed in any preceding claim, wherein a shut-off valve (56) is positioned in series with the supply throttle (54) leading to the second manifold (34) serving when closed to cut off the supply of stratification gases.

13. An internal combustion engine as claimed in any preceding claim, wherein a differential pressure gauge (70) is provided to measure the pressure difference between the plenum chambers (24, 34) of the two intake manifolds supplying the two gas streams and wherein the volume ratio of the flows supplied to the cylinder by the two intake manifolds is set to maintain the pressure difference to zero.

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