Control system for a direct injection-spark ignition engine

Details Control system for a direct injection-spark ignition engine Control system for a direct injection-spark ignition engine

1998-11-25

2003-02-04

Dolinar, Andrew M. (Department: 3747)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C123S299000

Reexamination Certificate

active

06513320

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an engine control system for a direct injection-spark ignition type of engine equipped with a lean NOx conversion catalyst in the exhaust line, and, in particular, to a direct injection-spark ignition engine control system which operates the engine with a fuel charge changed between lean and rich.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line a NOx adsorbing type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application WO93/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased, This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
A lean NOx conversion catalyst disposed in an exhaust line as described in the International Patent Application WO93/07363 declines its NOx conversion performance as the temperature of exhaust gas rises. In particular, a NOx adsorption type of lean NOx conversion catalyst shows significant aggravation of NOx conversion performance. In order to keep or enhance the NOx conversion performance, it is desirable to increase combustion velocity of fuel by improving the homogeneity of fuel distribution in the combustion chamber and drop the temperature of exhaust gas as low as possible by rising the combustion efficiency of fuel mixture. However, these demands are not satisfied in the present circumstances.
SUMMARY OF THE INVENTION
It is an objective of the invention to provide an engine control system for a direct injection-spark ignition type of engine which rises a combustion velocity of fuel by providing an improved homogeneous distribution of the fuel with an effect of rising combustion efficiency so as to drop the temperature of exhaust gas as low as possibly.
The foregoing object of the present invention are achieved by providing an engine control system operative to control a direct injection-spark ignition type of engine which is equipped with a fuel injector for delivering fuel directly into a combustion chamber and an exhaust system having a lean NOx conversion catalyst for lowering an emission level of nitrogen oxides (NOx) in exhaust gas at an air-fuel ratio of &lgr;>1 such that the engine makes stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a zone other than the lower engine loading zone. While the engine is monitored to be in a homogeneous charge combustion zone, the engine control system divides a given amount of fuel into two parts, desirably two substantially equal parts, and energizes the fuel injector to intermittently deliver through early and late split injection respectively in a intake stroke and controls the fuel injector to such that a midpoint between points at which said early and late split injection are timed respectively to start is before a midpoint of a intake stroke.
According to the engine control system, while a part of fuel delivered through the early split injection is sufficiently diffused sufficiently homogeneously in the combustion chamber whose volume increases as the piston moves down before the late split injection is started, another part of fuel subsequently delivered through the late split injection is diffused and made homogeneous in the combustion chamber whose volume has sufficiently increased. In consequence, a homogeneous fuel mixture is generated in the combustion chamber with an effect of increasing combustion velocity and combustion efficiency in a combustion stroke, providing a drop in exhaust gas temperature. The control in which fuel injection is made such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke provides various prominent effects described below.
On condition that a given amount of fuel has to be injected within an entire intake stroke, the early split injection can be timed to be caused at a point at which the piston moves down at a relatively high speed, which is accompanied by generation of a strong intake air stream, accelerating accomplishment of a homogeneous distribution of fuel mixture and evaporation of fuel. The early and late split injection is off as one whole to the early side of a intake stroke and, in consequence, fuel delivered through the late split injection sticks to a cylinder wall near when the piston reaches its bottom-dead-center (at the end of a intake stroke), so as to evade tardy accomplishment of a homogeneous distribution of fuel mixture. In addition, there is certainly provided a long period of time allowed for a fuel mixture to evaporate before spark ignition of the fuel mixture and a rise in intake air temperature by recirculated exhaust gas, which are always desirable for acceleration of fuel evaporation. These effects mutually affect one another to produce a greatly improved homogeneous distribution of fuel and significantly improving combustion efficiency of fuel mixture due to a shortened combustion time, lowering the temperature of exhaust gas.
In the zone other than the lower engine loading zone, the engine control system may control the engine to operates with a fuel mixture enriched to be &lgr;<1 while the engine is in a cold condition as well as in a warm condition. Even while the engine is in a cold condition where fuel is slow of evaporation and combustion, the split injection in a intake stroke makes fuel acceleratingly evaporate with an effect of improving combustibility of an enriched fuel mixture. Further, in the zone other than the lower engine loading zone, the engine control system may control the engine to operates with a fuel mixture of &lgr;>1 while the engine is in a warm condition. This provides an improved homogeneous distribution of fuel sufficient to expand a limit of air-fuel ratio for a lean fuel mixture, lowering specific fuel consumption and the amount of NOx formation. The split injection may be made during an engine start, which is desirable to accelerate evaporation of fuel and prevents th

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