LOAD CONTROL SYSTEM FOR A STRATIFIED CHARGE ENGINE

Abstract: A method is described for varying the engine load while maintaining optimum charge stratification in a stratified charge engine. In the invention, at least under low and part load conditions, the intake streams (22, 32) supplying the stratified charge are throttled (50, 60) in proportion to one another so as to vary the charge density uniformly across the entire combustible charge without significantly varying the relative volumes of the separate streams nor their relative velocities as they are drawn into the combustion chamber, the relative velocities being such as to minimise mixing within the combustion chamber between the intake streams during the intake and compression periods of the engine.

Description: Load Control System for a Stratified Charge Engine Field of the invention The present invention relates to a system for load control of an engine running with a stratified charge. Background of the invention Multi-cylinder spark ignition stratified charge internal combustion engines have previously been proposed in which the intake charge is drawn into each combustion chamber through at least one intake valve supplied in parallel by at least two flow streams, one being a core stream and the other at least one dilution stream, each stream passing through a respective flow channel in the intake port that is connected to a separate intake manifold regulated by a respective throttle valve, charge stratification being achieved by adding fuel to the core stream, and controlling the relative volumes and flow directions of the core and dilution streams as they enter the engine combustion chamber such that the fuel contained in the core stream is concentrated near the spark plug. Such an engine will hereinafter be termed a stratified charge engine of the type described. Because of inevitable mixing within the charge during the engine intake and compression periods, the core and dilution streams will become more and more homogeneous, that is to say, less and less stratified, with time, and the effectiveness of the initial separate entry of the two streams is useful only to the degree of charge stratification that may remain at the time of ignition. The higher the degree of stratifica ion at ignition, the higher the overall dilution limit that can be tolerated by the engine, and consequently the higher the fuel efficiency and the lower the NO x emissions that the engine may produce. A problem can arise in that varying the strength and the volume of the core mixture is usually the method for varying the power output or load of a stratified charge engine. Because these also affect charge stratification, varying the engine load using these parameters may compromise the quality of stratification at ignition and reduce the overall dilution limit below the full capability of the engine. Object of the invention The present invention seeks to provide a method of load control which does not significantly affect the quality of the charge stratification. Summary of the invention According to a first aspect of the present invention, there is provided a method for varying the engine load while maintaining optimum charge stratification in a stratified charge engine of the type herein described, the method comprising the steps: • supplying to the combustion chamber a first regulated mass of air that passes through both intake manifolds, the proportions of the regulated air mass passing along the two manifolds being in a fixed ratio to one another, • supplying fuel in an amount which, when the engine is warm, is stoichiometrically related to the first regulated air mass flowing through both manifolds, and • supplying a second regulated mass of gases into the dilution stream, the second regulated mass being in a fixed ratio to the first regulated mass, the ratios being such as to minimise mixing within the combustion chamber between the core stream and the dilution stream during the intake and compression periods of the engine. In the invention, under all operating conditions other than high load when some of the efficiency achieved by charge stratification may need to be sacrificed in the interest of maximising output power, substantially the same regions of the combustion chamber are occupied by the core and dilution streams and the charge density in the two regions is increased as engine load is increased. When the engine is running at its normal warmed up temperature, the fuel should be stoichiometrically related to the first regulated air mass flowing through both manifolds and dilution is provided by the second regulated mass. The second regulated mass of gases may contain ambient air, EGR gases or crankcase gases. Under these conditions, only the stoichiometric fraction of the air in the total charge, corresponding only to the air in the first regulated air mass, reacts with the fuel and this sets the desired load of the engine. Under cold operating conditions, a greater quantity of fuel may be metered so that the mixture strength for the entire combustible charge including the second regulated mass of air is stoichiometric. In this case, the mixture within the first regulated air mass will be over-rich and will not burn completely. This will release into the exhaust gases combustible components and unused air that can be reacted completely in the exhaust system so as to heat the catalytic converter. By selecting the correct ratios of the masses of the gases in the core stream and the dilution stream, and correspondingly the correct ratios of their respective entry velocities, it is possible to maintain the appropriate relative velocity conditions within the combustion chamber at the boundary between the streams to minimise mixing so as to conserve the stratification of the streams under all conditions within the regulated load range. In this proposed method of load control, the overall charge dilution and the degree of charge stratification are intentionally maintained constant over a range of engine operation conditions, and load is controlled by progressively throttling all the flow streams in proportion with one another so that the volumes of all the streams, including the rich core, remain substantially unchanged, while the density of the streams are varied to achieve different loads. This distinguishes the invention from the conventional methods of load control for a stratified charge engine where the size and the concentration of the fuel-rich region are varied with the aim of varying the overall charge dilution and the degree of charge stratification in proportion with load, and keeping the throttling to a minimum. In the invention, the volume and the mixture strength of the core, together with the effective flow cross-section of the core channel through the intake valve, are design parameters for determining the mean velocity of the core stream as it enters the combustion chamber. Smaller core volumes, richer core mixtures and larger core flow cross-sections all yield a lower mean velocity for the core stream, and vice-versa. The invention makes use of these parameters to preset the core stream velocity in relation to the dilution stream velocity with the aim of maintaining minimum mixing between the streams within the combustion chamber for a range of engine loads. The invention is based on the realisation that while the degree of stratification at ignition is dependent on the initial relative masses of the core stream and the dilution stream, it is equally dependent on the relative velocities of these streams while entering the combustion chamber. If these relative velocities are incorrectly selected, the resulting mixing within the combustion chamber during the intake and compression periods will significantly reduce or destroy the initial stratification. In order to maximise the degree of charge stratification at ignition, the mixing should therefore be kept to a minimum in order to conserve the stratification. This can be achieved by controlling the relative angular velocity between the core and the dilution streams to be substantially zero at their boundary as they move together within the combustion chamber. Since the charge motion within the engine cylinder typically follows a single vortex velocity field, a solid body rotation about a horizontal (tumble) or vertical (swirl) axis will have the lowest mixing across the field. This can be achieved by arranging the flow stream entering the cylinder further away from the axis of rotation to have a higher mean velocity than the flow stream entering nearer the axis, the mean velocities at the time of entry into the combustion chamber being in substantially the same ratio as the respective mean radii of rotation within the vortex velocity field. Thus for a given intake port entrance geometry into the combustion chamber and for a given level of dilution of the intake charge, the optimum mean velocity ratio between the streams to produce minimum mixing is a derived design parameter and is substantially constant. A direct consequence of this is that the optimum relative volumes of the flow streams and the way they are distributed within the combustion chamber are also derived design parameters and these should be kept constant even when the engine load is varied. To find the best charge stratification which enables the engine to operate at the highest level of gas dilution while maintaining good combustion stability, an iterative process may be used to establish, for a series of different levels of gas dilution, the optimum velocity ratios between the core and the dilution streams for minimum mixing and then to choose from the series the best set of parameters for pre¬ setting the engine load control system so as to maintain the correct flow ratios under all conditions within the regulated load range. According to a second aspect of the present invention, there is provided a multi-cylinder spark ignition stratified charge internal combustion engine in which the intake charge is drawn into each combustion chamber through at least one intake valve supplied in parallel by at least two flow streams, one being a core stream and the other at least one dilution stream, each stream passing through a respective flow channel in the intake port that is connected to a separate intake manifold regulated by a respective throttle valve, charge stratification being achieved by adding fuel to the core stream, and controlling the relative volumes and flow directions of the core and dilution streams as they enter the engine combustion chamber such that the fuel contained in the core stream is concentrated near the spark plug, characterised by • means for operating the throttle valves to supply to the combustion chamber a combined first regulated mass of air that passes through both intake manifolds, the proportions of the regulated air mass passing along the two manifolds being in a fixed ratio to one another, • means for supplying fuel in an amount which, when the engine is warm, is stoichiometrically related to the first regulated air mass flowing through both manifolds, and • a third throttle valve for supplying a second regulated mass of gases into the dilution stream, the second regulated mass being in a fixed ratio to the first regulated mass, the ratios being such as to minimise mixing within the combustion chamber between the core stream and the dilution stream during the intake and compression periods of the engine. The third throttle valve may control the supply of additional air, EGR gases or crankcase gases to the dilution stream. The three throttle valves can be controlled electronically to maintain the desired velocity ratios entering the combustion chamber so as to conserve the stratification conditions. Moreover, because the entire manifold system is under the same manifold pressure, the same can be achieved by mechanically linking or ganging together throttle valves of similar geometry, whereupon the ratios of the maximum cross sections of the throttle valves will set the mass flow ratios in the different streams. Ideally the gases in the combustion chamber should be arranged to rotate at all times as a solid body to avoid shear at the boundary between the core stream and the dilution stream. In this way, the stratification achieved during the intake stroke is substantially retained during the compression stroke until the instant of ignition. It is possible to fit a two-position valve in series with the second throttle valve that controls the. proportion of the first regulated air stream supplied into the dilution stream, to inhibit selectively the flow of the first air stream into the dilution stream and force all the first regulated air to enter the combustion chamber through the core stream. This two-position valve is advantageously closed under moderately high load conditions and its effect will be to reduce the charge stratification within the combustion chamber. Instead, the core stream will enter the combustion chamber with a higher velocity thereby promoting mixing and homogeneity within the combustion chamber. If desired, a further two-position valve may be fitted in series with the third throttle valve that controls the supply of the second regulated gas stream, for selectively inhibiting the flow of dilution gases and preventing the addition of dilution gases into the dilution stream. In this way, it is possible to enable the combustion chamber to receive a full stoichiometric charge under high load conditions . Such disablement will also interfere with the charge stratification and increase the mixing of the total charge, but under high load conditions the resulting more homogeneous mixture is to be preferred. Preferably, a fourth throttle valve may be connected to the first intake manifold, in parallel with the first throttle valve, for supplying more ambient air to the first intake manifold under high engine load conditions. The fourth throttle valve may be linked indirectly with the other three throttle valves such that it is closed under low and part load conditions and begins to open after the other throttle valves have reached a predetermined opening position. Preferably, the two two-position valves may be operated to disable the flows through the second and the third throttle valves at the same time as the fourth throttle valve begins to open. In the stratified charge engine of the type described, because the separate intake manifolds are interconnected at the intake valves, the manifold pressure is common to all the throttle valves and consequently the ratios of the flows through the throttle valves would remain in the ratios of their respective flow cross-sectional areas. By designing the throttle valves to be of similar geometry and by ganging them to operate in unison, one control step will suffice to vary all the flows in fixed ratios to one another. The preferred engine load control system of the present invention takes advantage of this to achieve simplicity, low cost and high reliability. It eliminates the need of using a multiplicity of electronically controlled throttles which must be operated continuously to maintain the required ratios between the respective flow streams, as would be the case had the individual throttles not been sized and linked together. 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 an intake system for an internal combustion engine, Figure 2 is a diagram showing the possible range of values of the volumes and mixture strengths in the separate streams entering the combustion chamber for a given level of charge dilution, the line S indicating where stratification is best conserved, and Figure 3 is a diagrammatic representation of one cylinder of Figure 1 and contains the key to the cross-hatching employed in Figure 2. Detailed description of the preferred embodiment In Figure 1 there is shown a cylinder head 12 of an engine having four cylinders each having two intake valves 14, a spark plug 16 and two exhaust valves 18. All the exhaust valves are connected to an exhaust pipe 80 through a common exhaust manifold. On the intake side, the two intake valves 14 have separate intake ports one of which has a butterfly valve 20 for port deactivation which remains closed for all but high load operation. This is conventional and intended to increase the speed and swirl of the intake charge during idle and part load operations. The intake valve associated with the other intake port is connected to receive the intake charge through separate intake manifolds 24 and 34 having individual branches 22 and 32 respectively. Each branch 22 contains a non-return valve 23 that only allows flow towards the intake port. Near the intake port a fuel injector 21 is provided for introducing fuel into the air drawn in through the manifold 24. The plenum of the intake manifold 24 is connected to a mass air flow meter 54 through a main throttle valve 50. Similarly the plenum of the intake manifold 34 is connected to the same mass air flow meter 54 through a second throttle valve 60 that is ganged with the main throttle valve 50. The dimensions of the throttle valves 50 and 60 are such that the ratio of the air mass flowing through the two intake manifolds is fixed. A further throttle valve 70 ganged with the main throttle valve 50 allows dilution gases to be introduced into the manifold 34. The dimensions of the throttle valve 70 is such that the gas mass flow through it is in a fixed ratio with the air mass flow through the main throttle valve 50. These dilution gases, depending on the position of a changeover valve 76 may either be ambient air drawn in through a pipe 74 or exhaust gases drawn in through an EGR pipe 78 that leads to the exhaust pipe 80. The throttle valves 60 and 70 can be rendered inoperative by the two-position valves 62 and 72 respectively connected in series with them. The drawing also shows a supplementary throttle valve 40 that is connected in parallel with the main throttle valve 50. To describe the operation of the engine of Figure 1, it will first be assumed that all the various valves are in the positions illustrated. This corresponds to part load operation wherein the intake charge is drawn in only through one intake port. Air is drawn past the three throttle valves 50, 60 and 70 in fixed ratios to one another and, because the valves are ganged together to operate in unison, these ratios remain constant over the load range regulated by the main throttle valve 50. At the intake port, air and fuel are drawn in through the manifold branches 22 and air only is drawn in through the manifold branches 32. Because of the geometry of the intake port, which is designed to promote swirl, the air from the branch 32 is directed towards the periphery of the combustion chamber while the fuel and air mixture from the branch 22 is directed towards the centre of the combustion chamber. The intention of this configuration is to create a radially stratified charge that retains a concentration gradient within the combustion chamber throughout the compression stroke up to the time of ignition. However, though one can create the stratification initially with a wide range of stream velocities, densities and mixture strengths, the stability of the charge stratification will depend strongly on the entry velocities of the two streams. If the two velocities are correctly preset relative to one another by appropriate dimensioning of the intake throttle valves then the intake charge will rotate as a solid body within the combustion chamber and retain the initial radial stratification. On the other hand, if either stream is flowing with a greater angular velocity than the other at the common boundary between the streams, then eddies are created that increase the mixing between the streams and the initial stratification is dissipated. This point can be better understood by reference to Figures 2 and 3. In Figure 2, the core mixture at the centre of the combustion chamber is shaded with diagonal lines . Moving horizontally across the diagram in vertical columns, the same amount of fuel can be introduced by reducing the volume of the core mixture at the same time as increasing the core mixture strength represented by a decreasing core relative air/fuel ratio λ. The corresponding volume of the dilution gases will be increasing as the volume of the core is decreasing, these dilution gases being formed of air drawn in through the throttle valve 60 (shaded light grey) and air or EGR gases drawn in through the throttle valve 70 (shaded dark grey) . The small triangle at the left of the drawing represents internally recirculated air and fuel that is carried across from one cylinder to another. At a given engine load, although the same amount of fuel is injected, it is possible to operate along any vertical line in Figure 2, only at the example line S will the angular speeds of the two streams be correctly matched to achieve solid body rotation. If a larger proportion of air is introduced into the core stream to mix with the injected fuel resulting in a weaker core mixture strength, then the core stream will move faster than the dilution stream and will spread outwards in the combustion chamber. Conversely, if a smaller proportion of air is introduced into the core stream to mix with the injected fuel resulting in a richer core mixture strength, then the core stream will move slower than the dilution stream which will then spread inwards and displace the core mixture. The present invention therefore proposes to take steps to preset the relative mass flows in the different manifolds according to an optimum vertical line S in Figure 2, so as to conserve the charge stratification within the combustion chamber up to the instant of combustion, over the designed part-load control range. The load control range over which stratification is conserved does not extend up to full load because the valve 20 are opened at full load and the concern is then to maximise charge homogeneity rather than charge stratification. As described so far, the dilution gases comprise air drawn in through the pipe 74 and this is the setting to maximise lean burn. However, if there are N0 X emission problems associated with the after-treatment of the lean exhaust gases, then the changeover valve 76 can be used to recirculate exhaust gases instead of drawing ambient air into the dilution stream. This allows the overall mixture strength to be kept at stoichiometry with EGR stratification and a three-way catalytic converter may be used for after- treatment of the exhaust gases. The valve 72 is closed at full load operation. The effect of closing this valve is to stop the dilution supply and to eliminate the fraction represented in dark grey in Figure 2 giving rise to a column H as shown in the left hand side of Figure 2 with an overall mixture calibrated at stoichiometry. The valve 62 can be switched to its closed position at the same time as the valve 72 in order to destroy any remaining stratification, as is desirable under full load operation. The valve 40 is intended to compensate for the small size of the main throttle valve 50 when the engine is operating at full load with the valves 62 and 72 closed. This valve will be initially closed when the valves 62 and 72 are closed and will thereafter progressively open as the demand pedal is depressed. The invention relies on selecting the sizes of the throttle valves 50, 60 and 70 and the design of the intake ports to achieve a stable stratification in the combustion charge over a wide range of engine loads. The ratios of the throttles could be maintained by electronic control but because one needs only to maintain the ratios fixed, the expensive electronic control can be avoided by resorting to the simple alternative of rigidly connecting the throttle valves to one another. Stratification can provide advantages not only in terms of lean burn and reduced pumping losses, but also in assisting rapid light-off of the exhaust catalytic converter when the engine is started from cold. In this case the charge volume ratios employed correspond to the same preset vertical line S at which the stratification is optimised, but greater concentration of fuel is introduced into the core stream sufficient to make the mixture strength of the whole charge stoichiometric including the dilution gases. This will result in incomplete combustion within the combustion chamber because of the well conserved stratification, but the exhaust gases will contain combustible components and unused air that can react completely in the exhaust system to heat the catalytic converter. This reaction can either be by catalytic conversion or by ignition. The embodiment of the invention described so far refers to an engine in which the intake port is designed to produce swirl and the entry velocities of the streams are arranged in solid body rotation about an axis parallel with the axis of the engine cylinder to produce a radially stratified charge across the cylinder bore. However, the invention is equally applicable to an engine in which the intake valves are designed to produce tumble and the entry velocities of the streams are arranged in solid body rotation about an axis perpendicular to the cylinder axis to produce another stratified charge across the height and diameter of the engine cylinder with the fuel-rich region centred in the middle of the cylinder.

Claims: CLAIMS 1. A method for varying the engine load while maintaining optimum charge stratification in a stratified charge engine of the type herein described, the method comprising the steps: • supplying to the combustion chamber a combined first regulated mass of air that passes through both intake manifolds, the proportions of the regulated air mass passing along the two manifolds being in a fixed ratio to one another, • supplying fuel in an amount which, when the engine is warm, is stoichiometrically related to the first regulated air mass flowing through both manifolds, and • supplying a second regulated mass of gases into the dilution stream, the second regulated mass being in a fixed ratio to the first regulated mass, the ratios being such as to minimise mixing within the combustion chamber between the core stream and the dilution stream during the intake and compression periods of the engine. 2. A method as claimed in claim 1, wherein the second regulated mass of gases contains ambient air, EGR gases or crankcase gases. 3. A method as claimed in claim 1, wherein under cold operating conditions the quantity of fuel metered is stoichiometrically related to the entire combustible charge supplied to the engine consisting of both the first and the second regulated air masses. 4. A multi-cylinder spark ignition stratified charge internal combustion engine in which the intake charge is drawn into each combustion chamber through at least one intake valve (14) supplied in parallel by at least two flow streams, one being a core stream and the other at least one dilution stream, each stream passing through a respective flow channel (22,32) in the intake port that is connected to a separate intake manifold (24,34) regulated by a respective throttle valve (50,60), charge stratification being achieved by adding fuel to the core stream, and controlling the relative volumes and flow directions of the core and dilution streams as they enter the engine combustion chamber such that the fuel contained in the core stream is concentrated near the spark plug (16) , characterised by • means for operating the two throttle valves (50,60) to supply to the combustion chamber a combined first regulated mass of air that passes through both intake manifolds, the proportions of the regulated air mass passing along the two manifolds (24,34) being in a fixed ratio to one another, • means (21) for supplying fuel in an amount which, when the engine is warm, is stoichiometrically related to the first regulated air mass flowing through both manifolds (24,34), and • a third throttle valve (70) for supplying a second regulated mass of gases into the dilution stream, the second regulated mass being in a fixed ratio to the first regulated mass, the ratios being such as to minimise mixing within the combustion chamber between the core stream and the dilution stream during the intake and compression periods of the engine. 5. An engine as claimed in claim 4, wherein the charge motion within the engine cylinder follows a single vortex velocity field, and wherein the flow stream entering the cylinder further away from the axis of rotation is arranged to have a higher mean velocity than the flow stream entering nearer the axis, the mean velocities at the time of entry into the combustion chamber being in substantially the same ratio as the respective mean radii of rotation within the vortex velocity field. 6. An engine as claimed in claim 4 or 5, wherein the three throttle valves (50,60,70) are controlled electronically to maintain the desired velocity ratios entering the combustion chamber so as to conserve the stratification conditions. 7. An engine as claimed in claim 4 or 5, wherein the three throttle valves (50,60,70) are geometrically similar and rigidly connected to operate in unison to maintain the desired velocity ratios entering the combustion chamber so as to conserve the stratification conditions. 8. An engine as claimed in any of claims 4 to 7, wherein a two-position valve (62) provided in series with the second throttle valve (60) that controls the proportion of the first regulated air stream supplied into the dilution stream, to inhibit selectively the flow of the first air stream into the dilution stream and force all the first regulated air to enter the combustion chamber through the core stream. 9. An engine as claimed in any one of claims 4 to 8, wherein a two-position valve (72) is provided in series with the third throttle valve (70) that controls the supply of the second regulated gas stream, for selectively inhibiting the flow of dilution gases and preventing the addition of dilution gases into the dilution stream. 10. An engine as claimed in any one of claims 4 to 9, wherein a fourth throttle valve (40) is connected to the first intake manifold (24) , in parallel with the first throttle valve (50) , for supplying more ambient air to the first intake manifold under high engine load conditions. 11. An engine as claimed in claim 10, wherein the fourth throttle valve (40) is linked indirectly with the other three throttle valves (50,60,70) such that it is closed under low and part load conditions and begins to open after the other throttle valves (50,60,70) have reached a predetermined opening position. 12. An engine as claimed in claims 7, 8 and 10, wherein i means are provided to operate the two two-position valves (62,72) to disable the flows through the second and third throttle valves (60,70) at the same time as the fourth throttle valve (40) begins to open.

Inventors: MA THOMAS TSOI-HEI, MA, THOMAS, TSOI-HEI
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, MA THOMAS TSOI HEI, FORD FRANCE S.A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, MA, THOMAS, TSOI-HEI
Application year: 1996
Main classification: F02B17
IPC codes: F02B17, F02B27, F02B29, F02B31, F02B75, F02M25, F02M35