GB2321669A

GB2321669A - Direct injection spark ignition engine

Info

Publication number: GB2321669A
Application number: GB9704549A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1997-02-01
Filing date: 1997-03-05
Publication date: 1998-08-05
Also published as: GB9704578D0; GB9702109D0; GB9704549D0; GB2321670A

Abstract

The engine has a variable volume combustion chamber defined by a cylinder head, a cylinder bore and a piston reciprocating within the cylinder bore, a spark plug, intake and exhaust ports connected through intake and exhaust valves to the combustion chamber and a fuel injector for injecting fuel directly into the combustion chamber. The injector 50 is positioned in an unswept part of the combustion chamber and is oriented with the axis of its spray pattern pointing directly towards the spark plug 40. An engine management system 60 is provided to operate the injector 50 in a predetermined phase relationship with the spark timing to direct towards the vicinity of the spark plug electrodes at least a proportion of the desired total fuel quantity for the cylinder charge, the proportion being only sufficient to generate at the time of ignition a region of readily ignitable mixture having it centre in the immediate vicinity of the spark plug 40.

Description

DIRECT INJECTION SPARK IGNITION ENGINE The present invention relates to a spark ignition internal combustion engine having direct (in-cylinder) fuel injection.

According to a first aspect of the present invention, there is provided a spark ignition internal combustion engine having a variable volume combustion chamber defined by a cylinder head, a cylinder bore and a piston reciprocating within the cylinder bore, a spark plug, intake and exhaust ports connected through intake and exhaust valves to the combustion chamber and a fuel injector for injecting fuel directly into the combustion chamber, wherein the injector is positioned in an unswept part of the combustion chamber and is oriented with the axis of its spray pattern pointing directly towards the spark plug, and an engine management system is provided to operate the injector in a predetermined phase relationship with the spark timing to direct towards the vicinity of the spark plug electrodes at least a proportion of the desired total fuel quantity for the cylinder charge, the proportion being only sufficient to generate at the time of ignition a region of readily ignitable mixture having its centre in the immediate vicinity of the spark plug.

According to a second aspect of the present invention, there is provided a method of operating a spark ignition internal combustion engine having an injector for injecting fuel directly into the combustion chamber, the injector being located in an unswept part of the combustion chamber and being aimed directly at the spark plug, in which, throughout the idle and part load operating range of the engine, a first proportion of the desired total fuel quantity that is only sufficient to generate at the time of ignition a region of readily ignitable mixture having its centre in the immediate vicinity of the spark plug is injected by means of the said injector towards the end of the compression stroke, and the remaining proportion of the fuel is introduced earlier into the intake charge to mix homogeneously with the intake air.

When only a proportion of the desired total fuel quantity is used to generate an ignitable mixture in the vicinity of the spark plug electrodes at ignition, the remainder may be introduced using another fuel injector. In such a case, the other injector may either be mounted within the cylinder but with its spray aimed away from the spark plug or it may be mounted in the intake port or further upstream in the intake system.

As an alternative, it is possible to use the same injector to introduce the remainder of the desired total fuel quantity into the charge without wetting the spark plug by suitably positioning the injector in the combustion chamber and timing a separate earlier injection to coincide with the opening of the intake valve. In this case, if the fuel spray is directed to intercept the air curtain in the intake port and/or exiting from the mouth of the open intake valve, then it will be carried by the air stream away from the spark plug and avoid flooding the electrodes of the spark plug.

Whereas in prior art direct injection engines, the spark timing was dictated by the fuel mixing process in that the spark had to occur at an instant when the mixture gradient near the edge of the fuel spray in the vicinity of the spark electrodes would be ignitable, the present invention allows this priority to be reversed and enables the spark timing to be set at will. No matter when the engine management system determines the spark should occur, provided that the directly aimed injection of the present invention is used to create an ignitable cloud centred in the vicinity of the spark plug electrodes to coincide with the timing of the spark, then reliable ignition will be achieved. This means always initiating the fuel injection a short time before the desired spark timing, the phase difference allowing for the time it takes for fuel to reach the vicinity of the spark plug following the operation of the injector. The duration of operation of the injector will generally be short as only a brief puff of fuel is required to ensure ignitability and avoid flooding the spark plug electrodes.

In essence the combined spark and fuel timings can be regarded as a timed plasma generator that can be fired at will at any instant in the engine cycle to ignite the remainder of the charge which in itself would not be ignitable by means of a spark alone.

In normal engine operation, the ignition timing will generally be optimised for best thermodynamic efficiency.

This is common practice with homogeneous charge engines but the invention allows the same flexibility in selecting the spark timing even when operating with a stratified charge.

In prior art stratified charge engines, the spark timing is restricted to a narrow window in order to achieve stable ignition taking into account the fuel mixing process. The resulting ignition timing is however not automatically optimum for best thermodynamic efficiency nor lowest exhaust emissions. There is a conflict in that the engine requires one spark timing for stable ignition but different ignition timings for best thermodynamic efficiency or lowest emissions. Because stable ignition must be the first priority, the other requirements would have to be compromised. The present invention reduces the dependence on the fuel mixing process, which is least sensitive in a directly aimed fuel spray, thus significantly widening the spark timing window for stable ignition and allowing the ignition timing to be positioned almost at will for optimisation of different criteria.

Depending on the engine operating conditions, other criteria may be allocated a higher priority than maximising thermodynamic efficiency. For example after cold starts, it is more important to minimise the light-off time of a catalyst in the exhaust system in order to meet legal emissions requirements. In this case, the invention allows the spark timing to be retarded within wide limits, even when operating with an overall lean mixture, to maximise the exhaust gas temperature when idling after a cold start.

In a similar vein, it has been proposed to reduce emissions when operating at light loads with a very lean mixture by inducing auto-ignition in a homogeneous charge.

This was previously achieved by increasing charge temperature and pressure using preheating and/or high compression ratios. The invention can if desired be used to allow the spark timing to be advanced to such an extent as to cause auto-ignition in the remaining lean charge.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a cylinder of an engine of the invention, taken along the segmented section line I-I in Figure 2 during the intake stroke, Figure 2 is a view of the combustion chamber in Figure 1 looking from the piston towards the cylinder head.

Figure 3 is a similar section to that of Figure 1 showing the piston near the top of the compression stroke, and Figure 4 is a view of the combustion chamber in Figure 3 looking from the piston towards the cylinder head.

The engine in the drawings has a variable volume combustion chamber 10 defined between a piston 16 reciprocating in a bore 14 and a cylinder head 12 having intake ports 20 and exhaust ports 30. The intake ports 20 are controlled by two intake poppet valves having stems 24 and heads 26 and the exhaust ports 30 are controlled by two exhaust poppet valves having stems 34 and heads 36. A spark plug 40 is located centrally in the combustion chamber.

The engine is controlled by a management system that comprises a so-called EEC (Electronic Engine Control) unit 60. The EEC unit 60 is connected to receive signals from various sensors that measure the relevant operating parameters such as engine speed, load, temperature and crankshaft position and appropriately controls the ignition timing, the fuel quantity and the fuel injection timing in accordance with stored algorithms and calibration data.

Conventional design would place a high pressure direct fuel injector in the cylinder head pointing downwards away from the spark plug electrodes and towards the piston 16. It is such geometry that is responsible for the wetting of the piston crown that in turn causes soot and hydrocarbon emissions. In the described preferred embodiment of the present invention, a fuel injector 50 controlled by the EEC unit 60 is located at the periphery of the cylinder bore 14.

A small scallop or recess 52 is formed in the cylinder head 12 and/or cylinder block to accommodate the injector 50. In this position the injector 50 lies above the top piston ring in its top-dead-centre position, that is to say above the swept volume of the cylinder and in a permanently exposed position.

The orientation of the injector 50 is such that the axis of the spray pattern of the injector is directed straight towards the spark plug 40. At the same time, the injector is aimed at the open mouths of the intake valves 26 to intercept the air curtain represented by the arrows 28 in Figures 1 and 2 when the intake valves are open. Hence as shown in Figure 1 when fuel is injected during the intake stroke, the spray passes through the mouths of the intake valves 26 back into the very ends of the intake ports 20.

The bulk of the spray does not land on the back of the intake valves 26 nor on the walls 22 of the intake ports 20 because during the intake stroke high velocity air 28 being drawn into the cylinder will carry the fine spray from the injector 50 with it into the combustion chamber 10. Any wetting that does take place at the end of the intake port 20 is minimal because of the small area exposed to the spray and the fine atomisation of the spray by the high pressure injector. The part of the spray that manages to traverse the down draught of the intake air will reach the cylinder head in the vicinity of the spark plug 40 but it will be again minimal and will be evaporated by the motion of the trapped charge during the compression stroke. The fuel injected while the intake valves 26 are open will therefore be substantially homogeneously distributed throughout the intake charge and will cause minimum wetting of the walls of the intake ports 20 and the combustion chamber 10.

As so far described the preferred embodiment of the invention provides an improvement over conventional engines in which the injector is directed towards the piston, as it avoids wetting of the piston which gives rise to soot and high hydrocarbon emissions in the exhaust gases. It does not yet enable the engine to be optimised for ultra lean burn because the charge is homogeneous and may not be ignitable.

To optimise lean burn, one needs to stratify the charge so that an ignitable pocket is present near the spark even though the bulk of the charge does not have sufficient fuel concentration to be ignitable on its own.

In accordance with the invention, such charge stratification is achieved when required in an engine with the illustrated injector geometry by separately introducing a proportion of the desired total fuel quantity into the combustion chamber while the intake valve is closed near the end of the compression stroke, as shown in Figures 3 and 4.

This injection is timed by the EEC unit 60 that also sets the ignition timing in such a manner that an ignitable cloud of fuel and air will be present in the vicinity of the spark plug 40 at the instant that a spark is fired. The fuel injection occurs a brief period before the spark, to allow for the time taken for fuel to reach the vicinity of the spark plug following operation of the injector. This separately injected proportion of the fuel, which need only be a very small amount, will glance over the head of the intake valves 26 and create a rich pocket centred on the spark plug 40. Even if the remainder of the charge is too lean to be ignitable, this pocket will be ignitable and once it has lit its flame will propagate to consume the entire charge.

Under idling condition, this late injection near the end of the compression stroke could supply all the fuel needed. Under part load conditions, a combination of early and late injections can be used to achieve an overall lean mixture with a rich pocket next to the spark plug. Under high load conditions, one can dispense with the late injection and rely on a stoichiometric homogeneous charge achieved only by early injection.

In known stratified charge engines, the mixture at the centre of the rich pocket resulting from the total injected fuel quantity is too rich to be ignitable and the spark must be located in a region near the envelope of the pocket where there is a fuel concentration gradient. Conventionally, the spark is either located at the edge of the fuel spray or the spray is directed onto the piston and then carried round back to the spark plug. In both cases, the geometry of the fuel rich region will vary with operating conditions and it is difficult, especially when operating at the limits of lean burn, to ensure that an ignitable mixture is present in the vicinity of the spark at the instant of ignition. In the present invention, when operating with a stratified charge, only a small amount of fuel is aimed directly at the spark plug 40 and any other fuel required is homogeneously distributed throughout the charge. Consequently, it is possible to direct the axis of the spray straight at the spark plug during the second injection as the centre of the spray will not in this case be too rich to be ignitable.

Furthermore the same amount of fuel can be aimed towards the spark plug under all conditions, to achieve reliable ignition throughout the operating load range.

It is inevitable that some wetting of the cylinder head will take place. When the engine is hot this fuel will be evaporated before the exhaust stroke and will be burnt with the remainder of the charge, but when the engine is cold some fuel may remain on the cylinder head wall. To avoid such fuel flowing into the exhaust port during the exhaust stroke, it is possible (though not shown in the drawings) to form a small protruding lip around the circumference of the exhaust valve seat to prevent such flow from taking place.

The protrusion need not be pronounced as it is only a thin film of fuel that must be prevented from flowing into the exhaust port, and for this reason it will not disturb the gas flow in the combustion chamber nor will it cause any turbulence to detract from the charge stratification.

Though the invention has been described by reference to an engine with two intake and two exhaust valves per cylinder, this is not an essential feature of the invention and it can be applied with other valve configurations.

Furthermore, while it is preferred to use the same injector to introduce the remainder of the desired fuel quantity into the cylinder, it is alternatively possible to use a separate injector for this purpose, whether positioned in the intake system or in the combustion chamber, in which case the illustrated injector need not be positioned near to an intake valve and may instead aim its spray at the spark plug from any point on the circumference of the combustion chamber.

The spark plug 40 may sit within a recess in the cylinder in order to avoid flooding of the electrodes, especially when the engine is cold, but this is not essential to the invention as the amount of fuel sprayed directly at the spark plug is not sufficient to cause such flooding and the remainder of the fuel will be distributed uniformly in the intake air charge and also not likely to wet the electrodes.

Claims

1. A spark ignition internal combustion engine having a variable volume combustion chamber defined by a cylinder head, a cylinder bore and a piston reciprocating within the cylinder bore, a spark plug, intake and exhaust ports connected through intake and exhaust valves to the combustion chamber and a fuel injector for injecting fuel directly into the combustion chamber, wherein the injector is positioned in an unswept part of the combustion chamber and is oriented with the axis of its spray pattern pointing directly towards the spark plug, and an engine management system is provided to operate the injector in a predetermined phase relationship with the spark timing to direct towards the vicinity of the spark plug electrodes at least a proportion of the desired total fuel quantity for the cylinder charge, the proportion being only sufficient to generate at the time of ignition a region of readily ignitable mixture having its centre in the immediate vicinity of the spark plug.

2. A spark ignition internal combustion engine as claimed in claim 1, wherein the fuel injector is located within the combustion chamber at the periphery of the cylinder bore and in such a position that when fuel is sprayed into the combustion chamber while an intake valve is open, a proportion of the injected fuel passes through the mouth of the intake valve towards the back of the intake valve head before being drawn into the combustion chamber along with the intake air charge.

3. An internal combustion engine as claimed in Claim 1 or Claim 2, wherein the fuel injector is mounted in the cylinder block near the top of the cylinder bore and opens into the combustion through a cut-out provided partly or wholly on the wall of the cylinder bore or the wall of the cylinder head.

4. An internal combustion engine as claimed in Claim 2 or Claim 3 when appended to Claim 2, wherein each cylinder has two intake valves and the injector is positioned to direct its spray along the plane of symmetry between the intake valves to introduce fuel into the mouths of both intake valves.

5. A method of operating a spark ignition internal combustion engine having an injector for injecting fuel directly into the combustion chamber, the injector being located in an unswept part of the combustion chamber and being aimed directly at the spark plug, in which, throughout the idle and part load operating range of the engine, a first proportion of the desired total fuel quantity that is only sufficient to generate at the time of ignition a region of readily ignitable mixture having its centre in the immediate vicinity of the spark plug is injected by means of the said injector towards the end of the compression stroke, and the remaining proportion of the fuel is introduced earlier into the intake charge to mix homogeneously with the intake air.

6. A method as claimed in claim 5, wherein the combined timings of the spark and of the injection of the first proportion of the fuel are selected such that the ignition timing is optimised for best engine thermodynamic efficiency.

7. A method as claimed in claim 5, wherein the combined timings of the spark and of the injection of the first proportion of the fuel are selected such that the ignition timing is optimised for maximum exhaust gas temperature.

8. A method as claimed in claim 5, wherein the combined timings of the spark and of the injection of the first proportion of the fuel are selected such that the ignition timing is advanced to such an extent as to cause auto-ignition in the remaining charge.