Fuel injection control apparatus and method of direct fuel injection-type spark ignition engine

Abstract

Fuel vapor is introduced into an intake passage of a direct fuel injection type engine by a fuel vapor purge apparatus. The intake passage is provided with an intake oxygen concentration sensor for detecting the amount of fuel vapor in intake air. An ECU corrects the amount of fuel injection from each direct fuel injection valve in accordance with the amount of fuel vapor detected, and changes the fuel injection starting timing and the fuel injection ending timing in accordance with the amount of fuel vapor.

Description

INCORPORATION BY REFERENCE[0001] The disclosure of Japanese Patent Application No. 2000-278681 filed on Sep. 8, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.[0002] 1. Field of the Invention[0003] The invention relates to an apparatus and a method for controlling fuel injection in an internal combustion engine and, more particularly, to a fuel injection control apparatus and a fuel injection control method for an engine having a direct fuel injection valve for injecting fuel directly into a cylinder.[0004] 2. Description of the Related Art[0005] A widely known evaporated fuel purge apparatus prevents release of evaporated fuel (fuel vapor) from a fuel tank into the atmosphere by temporarily adsorbing fuel vapor from the tank to a canister containing activated carbon or the like and supplying (purging) fuel vapor adsorbed to the activate carbon into an engine intake passage during operation of the engine so that the fuel v...

AI Technical Summary

Benefits of technology

[0017] According to the first form of the invention, the fuel vapor detecting means for detecting the amount of fuel vapor in engine intake air, so that the amount of fuel vapor in intake air can be accurately detected. The injection timing correcting means corrects the fuel injection timing so that the state of combustion in each cylinder becomes optimal, in accordance with the amount of fuel vapor detected. For example, if the amount of fuel injection (duration of fuel injection) is reduced for correction in accordance with the amount of fuel vapor, the conventional art adjusts the fuel injection duration by adjusting one of the fuel injection starting timing and the fuel injection ending timing while fixing the other timing. In this invention, however, both the fuel injection starting timing and the fuel injection ending timing are changed so that fuel injection is performed at a timing optimal with respect to the stroke (the intake stroke or the compression stroke) during which fuel injection is performed, the position of the piston, etc. More specifically, if the amount of fuel injection is to be reduced in accordance with the amount of fuel vapor, the fuel injection duration is shortened in the following manner. That is, neither the fuel injection starting timing nor the fuel injection ending timing is fixed, but both the timings are changed; for example, the fuel injection starting timing is retarded and, at the same time, the fuel injection ending timing is advanced. Therefore, the state of formation of mixture in each cylinder at the time of ignition can be optimized, and therefore the combustion in each cylinder can be optimized. Furthermore, the conventional art reduces the amount of fuel injection by the amount of fuel vapor so as to maintain a combustion air-fuel ratio regardless of the presence / absence of fuel vapor, so that the state of combustion in each cylinder becomes close to an optimal state. According to the invention, however, a more appropriate state of combustion can be achieved since the fuel injection starting timing and the fuel injection ending timing are corrected in accordance with the amount of fuel vapor. Therefore, the reduction of the amount of fuel by the amount of fuel vapor is no longer essential, and the degree of freedom in the fuel injection control increases.

[0032] Therefore, the compression stroke fuel injection injects fuel into the relatively low air-fuel ratio mixture formed by the intake stroke fuel injection, so that a dense mixture layer is formed. As a result, the relatively low air-fuel ratio (intermediate air-fuel ratio) mixture formed by the intake stroke fuel injection exists between the lean homogeneous mixture formed by fuel vapor and the fuel-rich mixture layer formed by the compression stroke fuel injection. Hence, the air-fuel ratio of mixture smoothly changes from the dense mixture layer to the homogeneous mixture, so that flames smoothly propagate from the dense mixture layer to the homogeneous mixture. If in this case, the injection of a small amount of fuel during the intake stroke is performed during a latest-possible period of the intake stroke, the fuel injected by the intake stroke fuel injection does not diffuse into the homogeneous mixture, so that a mass of mixture of an intermediate air-fuel ratio can be formed in the homogeneous mixture and therefore the propagation of flames becomes smoother.

Problems solved by technology

Therefore, if the amount of fuel injected into the engine when the purging is not performed is maintained when the purging is performed, the engine air-fuel ratio changes (decreases), so that the state of combustion in the engine may deteriorate in some cases.

According to the conventional art, the engine capable of performing the stratified charge combustion has a problem that the aforementioned purge cannot be performed during the stratified charge combustion mode.

Thus, the air-fuel ratio of the combustible mixture gas layer would excessively shift to the fuel-rich side, resulting in degraded combustion.

However, despite the design for the localization of fuel vapor in each cylinder and the correction of reducing the amount of fuel injection in accordance with the amount of fuel vapor, the engine of the Japanese Patent Application Laid-Open No. 2000-27716 has a problem of being incapable of completely preventing disturbed stratified charge combustion.

Thus, there is a problem of being incapable of performing fuel injection that is optimal in view of the amount of fuel injection, the engine operation state, etc.

Although the above-described problem is related to the stratified charge combustion, similar problems also occur in conjunction with an engine operation in which fuel is injected into each cylinder during the intake stroke to form a homogeneous mixture gas in the cylinder (homogeneous mixture combustion), and an engine operation in which fuel injection is performed in a divided manner during the intake stroke and during the compression stroke, and in which fuel injected during the compression stroke is stratified in a homogeneous lean mixture formed by the fuel injected during the intake stroke so that the fuel injected during the compression stroke forms a combustible mixture layer around a spark plug in each cylinder (weak stratified charge combustion).

That is, similar problems occur if only one of the fuel injection starting timing and the fuel injection ending timing is changed in accordance with a change in the amount of fuel injection.

More specifically, in direct fuel injection type spark injection engines, the problem of failing to accomplish optimal combustion occurs during not only the stratified charge combustion operation but also the homogeneous mixture combustion operation and the weak stratified charge combustion operation if only the amount of fuel injection is corrected at the time of execution of a purge.

However, since the formation of a mixture of fuel injected during the intake stroke and the formation of a mixture of fuel injected during the compression stroke are completely different, optimal mixtures cannot be formed in each cylinder if the two amounts of fuel injection are merely reduced at equal rates at the time of execution of a purge.

In some cases, therefore, combustion may deteriorate.

Therefore, if an amount of fuel vapor drawn into each cylinder in the form of a homogeneous mixture is subtracted from the amount of the compression stroke fuel injection for forming a dense mixture layer as well, the formation of a dense mixture layer may be impeded, and combustion may deteriorate in some cases.

Hence, the air-fuel ratio of a mixture layer formed by the compression stroke fuel injection shifts to a leaner air-fuel ratio, thus leading to the problem of disturbed stratified charge combustion that results in degraded combustion.

Therefore, if there is a great difference between the air-fuel ratio of the dense mixture layer and the air-fuel ratio of the homogeneous mixture, flames do not smoothly propagate from the dense mixture layer to the lean homogeneous mixture in some cases.

However, if the piston is an upper position at the time of execution of fuel injection, injected fuel may deposit on the piston, thereby impeding formation of a homogeneous mixture.

In reality, however, if the amount of fuel injection is relatively great, the aforementioned conditions cannot be fully met because of the lengthened fuel injection duration.

Furthermore, during high-speed engine operation, the duration of the intake stroke becomes relatively short in comparison with the fuel injection duration, so that a similar problem arises even if the amount of fuel injection is small.

Still further, in reality, the piston speed also greatly affects the formation of a homogeneous mixture.

Therefore, if the fuel injection timing is delayed, the stratified mixture becomes excessively dense so that ignition and combustion may fail.

Thus, with regard to both the intake stroke fuel injection and the compression stroke fuel injection, optimal fuel injection timing is affected by many factors, so that it is difficult to obtain an optimal injection timing in a real engine.

Therefore, in a case where the amount of fuel injection is corrected taking into account the amount of fuel supplied to each cylinder by purging fuel vapor, mere correction of the amount of fuel injection with the fuel injection starting or ending timing being fixed will degrade the mixture formation state set by the aforementioned conforming operation, and will result in a failure in accomplishing good combustion in some cases.

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