Direct injection gasoline engines

Details Direct injection gasoline engines Direct injection gasoline engines

1999-09-23

2001-04-17

Yuen, Henry C. (Department: 3747)

Internal-combustion engines

Combustion chamber means having fuel injection only

Injected fuel spraying into whirling fluid

C123S276000, C123S279000

Reexamination Certificate

active

06216662

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to direct injection (DI) gasoline engines, that is to say spark ignited gasoline engines in which the fuel is injected directly into the cylinder, and is concerned with that type of engine which includes at least one cylinder, a piston reciprocably mounted within the cylinder, a cylinder head closing the cylinder, at least one inlet valve adapted to cause swirl of the inlet air in the cylinder substantially about the axis of the cylinder, the piston crown being provided with a recess which constitutes at least part of the combustion chamber, the recess having a floor and a side wall, a spark plug extending close to or into the recess, at the top dead centre position of the piston, at a position adjacent to the side wall of the recess and a fuel injector which is arranged to inject fuel into the recess and is situated adjacent the side wall of the recess, that portion of the side wall of the recess which is between the fuel injector and the spark plug being arcuate, when viewed in the axial direction of the cylinder.
DESCRIPTION OF THE PRIOR ART
DI engines are becoming increasingly popular for a number of reasons, one of which is that they permit satisfactory operation under low load with stratified charging and thus achieve increased fuel economy at low load. In a conventionally carbureted engine running at low load the amount of fuel supplied is reduced to a low level and the amount of air admitted into the cylinder is also reduced to a correspondingly low level by virtue of the throttle valve being virtually closed. However, this results in substantial pumping losses and thus in a reduction in efficiency. It would of course not be possible to reduce the amount of fuel whilst leaving the throttle valve totally or largely open because this would result in the air/fuel ratio (AFR) within the cylinder rising above the maximum value of about 20:1 at which the air/fuel charge in the cylinder can be satisfactorily ignited by the spark plug. However, if stratified charging is used at low loads the air/fuel mixture within the cylinder is arranged to be non-homogeneous and thus it is possible for the overall AFR within the cylinder to be very much greater than 20:1 but for the AFR in the immediate vicinity of the spark plug at the instant that sparking occurs to be less than 20:1 whereby satisfactory ignition and combustion of the air/fuel mixture is possible. This can be done without substantially throttling the inlet duct and thus without incurring the efficiency penalty associated with pumping losses.
A DI engine of the type referred to above is disclosed in EP-A-0694682. The piston crown in this engine is provided with a recess which constitutes the combustion chamber. A spark plug extends a certain distance into the recess, at the top dead centre (TDC) position of the piston, adjacent one side wall of the recess and a fuel injector is provided adjacent the opposite side wall of the recess and arranged to spray fuel downwardly towards the floor of the recess and laterally towards the wall of the recess in the direction in which the air is caused to swirl in the cylinder by the swirl inducing port. The air and fuel is thus caused to swirl around within the recess and the fuel is forced predominantly, at least initially, into close proximity with the wall of the recess by virtue of centrifugal force and its greater density. Under high load conditions, a substantial amount of fuel is injected into the combustion chamber. This is mixed with the swirling air to form a substantially homogeneous mixture which is subsequently ignited by the spark plug. Under low load conditions, air and exhaust gas mixture is admitted into the cylinder with a significantly reduced amount of fuel. This is achieved by not significantly throttling the inlet duct and commencing injection of the fuel substantially later in the compression stroke. Due to the much shorter time between the commencement of fuel injection and ignition there is insufficient time for homogeneous mixing of the fuel and air to occur. Instead, a small relatively rich “pocket” of fuel and air swirls around within the combustion chamber in close proximity to the chamber wall. After this mixture has swirled through about 180° it will be in the vicinity of the spark plug and the spark timing is such that sparking then occurs. This ignites the fuel/air mixture, whose AFR is locally 20:1 or lower and which thus burns satisfactorily, despite the fact that outside the rich “pocket” of air/fuel mixture the gaseous charge is very much leaner than 20:1.
However, it is found that the engine disclosed in EP-A-0694682 does not work particularly satisfactorily in that there is only a relatively small range of engine load and speed within which stratified charge operation is satisfactory and that if the load and/or speed moves outside this range unsatisfactory combustion or misfiring occurs, thereby necessitating a return to conventional homogeneous charge operation. The engine runs substantially unthrottled during stratified charge operation, as discussed above, but when the engine reverts to homogeneous charge operation the engine inlet throttle again becomes operative and the engine efficiency drops due to the throttling losses referred to above. Thus the prior engine is obliged to run throttled and thus at lower efficiency over a relatively broad range of speeds and loads at which stratified charge operation would otherwise be appropriate, whereby the prior engine does not achieve the full benefit of stratified charge operation which is theoretically available.
It is therefore the object of the present invention to provide a DI engine of the type referred to above in which stratified charge operation is possible over a wider range of speeds and loads than the engine disclosed in EP-A-0694682 and thus achieves overall a higher efficiency and thus a lower overall specific fuel consumption.
SUMMARY OF THE INVENTION
According to the present invention, a DI engine of the type referred to above is characterised in that the floor of the recess adjacent the said portion of the side wall rises progressively towards the cylinder head over at least a part of its length in the direction towards the spark plug.
The invention is based on the recognition that the problem suffered by the engine disclosed in EP-A-0694682 is caused by the fact that it is only at low speeds and loads that the AFR in the vicinity of the spark plug is below the crucial level of about 20:1 and that when the speed rises above a low level the fuel is not satisfactorily ignited. This is thought to be due to the fact that the relatively rich “pocket” of air/fuel mixture which travels around adjacent the side wall of the recess stays very close to or “hugs” the floor of the recess whilst the spark gap of the spark plug is not situated particularly close to the floor of the recess. It is therefore only at particularly low speeds that the spark gap of the spark plug is within an area of the air/fuel mixture which is sufficiently rich to be ignited satisfactorily. However, if the speed of the engine increases somewhat it will be appreciated that the spark is produced after a slightly shorter period of time after the termination of the injection of fuel than at lower speeds. This means that the rich region of the air/fuel mixture has not quite reached the spark gap and thus that unsatisfactory ignition occurs. The rich region of air/fuel mixture does of course not have a homogeneous AFR but tends to be richest in the centre and to become progressively weaker with increasing distance from the centre. However, the fact that the centre of the “pocket” of fuel is spaced in the axial direction of the cylinder from the spark gap means that the tolerance to circumferential spacing is relatively limited. It is not possible for the spark plug to extend a very substantial distance down into the recess in the piston crown and because the rich air/fuel mixture tends to hug the floor of the recess there is only a very limited range of speeds at which the

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