The present invention relates to apparatuses and methods for controlling engines that inject fuel directly into their cylinders. More particularly, the present invention relates to apparatuses and methods for changing combustion modes in engines in accordance with the load applied to the engine.
In a typical engine, fuel is injected into an intake passage from a fuel injector. The fuel is then homogeneously mixed with air passing through the intake passage and sent into a combustion chamber. In the combustion chamber, the homogeneous air-fuel mixture is ignited by a spark plug and burned to generate engine power.
The burning of air-fuel mixture, which is distributed homogeneously in the combustion chamber, is generally referred to as homogeneous charge combustion. An engine performing homogeneous charge combustion has a throttle valve arranged in its intake manifold. The size of the throttle valve opening is adjusted to vary the amount of air supplied to the combustion chamber and thereby control engine power.
However, homogeneous charge combustion produces strong vacuum pressure in the intake manifold when the throttle valve throttles the intake manifold. The vacuum pressure increases pumping loss, which occurs when the air-fuel mixture is drawn into the combustion chamber from the intake manifold. This decreases the efficiency of the engine.
To solve this problem, an engine capable of performing stratified charge combustion has been proposed. Stratified charge combustion is performed by injecting fuel directly into the combustion chamber such that the fuel is concentrated about the spark plug. Ignition by the spark plug burns the stratum of rich air-fuel mixture surrounding the spark plug. This, in turn, burns the stratum of lean air-fuel mixture surrounding the rich mixture. The power of a stratified charge combustion engine is basically controlled by adjusting the amount of fuel injected toward the vicinity of the spark plug. Accordingly, pumping loss is decreased since the throttle valve need not throttle the intake manifold. This improves the efficiency of the engine. Furthermore, stratified charge combustion improves fuel efficiency since the engine can be driven with an air-fuel mixture having a lean air-fuel ratio.
Japanese Unexamined Patent Publication No. 5-52145 describes an engine that switches combustion modes between stratified charge combustion, homogeneous charge combustion, and semi-stratified charge combustion depending on the load applied to the engine. When the engine load is relatively low, stratified charge combustion is performed by injecting fuel into the combustion chamber during the compression stroke. When the engine load is relatively high, homogeneous charge combustion is performed by injecting fuel into the combustion chamber during the intake stroke. When the engine load shifts from low to high or from high to low, semi-stratified charge combustion is performed before proceeding to homogeneous charge combustion or stratified charge combustion. Fuel is injected twice, once during the intake stroke and once during the compression stroke, to perform semi-stratified charge combustion. Accordingly, semi-stratified charge combustion is an intermediate combustion mode performed between stratified charge and homogeneous charge.
In addition to a throttle valve, which adjusts the amount of intake air, the engine includes a swirl control valve, which adjusts the intensity of the swirling motion of air in the combustion chamber. When the combustion mode is shifted between stratified charge combustion and homogeneous charge combustion in accordance with changes in the engine load, the positions of these valves are varied to adjust the state of the intake air (flow rate and amount) in the combustion chamber. However, when varying the positions of the valves, a certain length of time is required for the valves to reach their target position. In other words, the valves have a response delay. This further delays the adjustment of the intake air conditions (amount and swirl). During such delays, the conditions of the intake air in the combustion chamber remain inappropriate for the combustion mode that is to be performed. This may cause unstable combustion of the air-fuel mixture.
Accordingly, the engine described in Japanese Patent Publication No. 5-52145 performs semi-stratified charge combustion for a certain length of time before switching between stratified charge combustion and homogeneous charge combustion. In other words, semi-stratified charge combustion is performed for a predetermined time period when switching combustion modes between stratified charge combustion and homogeneous charge combustion to achieve an intake air state that is optimal for the combustion mode that is to be performed. Thus, semi-stratified charge combustion stabilizes combustion and allows smooth switching of combustion modes.
However, the time required for semi-stratified charge combustion is not necessarily constant. For example, if the difference between the actual position of the valves and the target position is large or if the state of the intake air is changing dramatically, the duration of semi-stratified charge combustion may be insufficient. On the other hand, if the difference is small, the duration of semi-stratified charge combustion may be too long.
If the semi-stratified charge combustion time period is too short, the combustion mode is switched before the intake air conditions are appropriate for the new combustion mode. This may lead to power fluctuation and misfires. If the semi-stratified charge combustion time is too long, fuel efficiency will deteriorate. Accordingly, if the time for semi-stratified charge combustion is constant, unstable combustion may occur when switching combustion modes between stratified charge and homogeneous charge and there will be a loss in fuel efficiency.