1. Field of the Invention
The present invention relates to an electronically controlled fuel injection system for an internal combustion engine.
2. Description of the Prior Art
In an electronically controlled fuel injection system for an internal combustion engine, the amount of intake air is generally detected at a predetermined timing for each predetermined cylinder or for each predetermined cylinder group so that the amount of fuel to be injected may be calculated on the basis of the detected value.
To supply fuel by an amount corresponding to the amount of air actually led into a combustion chamber of each cylinder, it is necessary to detect the amount of intake air at a timing close to the timing at which fuel is actually injected. If the timing for detecting the amount of intake air deviates too much from that of the actual fuel injection, the change of the amount of intake air between these timings is not properly reflected on the amount of fuel to be injected. Accordingly, it becomes impossible to properly control the air-fuel ratio in a transition stage. Because of this, it is considered that the fuel injection had better be performed during the intake stroke of each cylinder and it is preferable to perform, immediately before the fuel injection, the detection of the amount of intake air and the calculation of the amount of fuel to be injected on the basis of such detection.
The fuel injection during the intake stroke hardly causes any problem when the engine load is low. However, when the engine load becomes high and requires an increased amount of fuel and the period required for the fuel injection is prolonged, there arises a phenomenon that the injected fuel can not completely enter the combustion chamber during the intake stroke. Accordingly, the detection of the amount of intake air and the calculation of the amount of fuel to be injected are usually performed prior to the intake stroke, thus making it possible to set the fuel injection timing in compliance with the period of time required for the fuel injection at a high load. In this case, the injection timing is occasionally rendered to be variable according to the engine load. However, uniformly advancing the timing for detecting the amount of intake air in such a manner cannot achieve fuel injection corresponding to the actual amount of intake air at the time of transition as described above and is followed by a problem in that the air-fuel ratio momentarily becomes lean, resulting in lowered drivability.
The Japanese Patent Laid-open Application No. 60-116840 discloses a fuel injection system in which the amount of air to be actually led into the combustion chamber of each cylinder is accurately detected. In this system, the fuel injection is performed in compliance with the amount of intake air which changes with time, even in the intake stroke, particularly at the time of transition. To this end, the calculation of the amount of fuel to be injected is performed a plurality of times in the intake stroke of each cylinder so that fuel may be injected on the basis of each calculation. However, this system also involves a problem in that the injected fuel cannot completely enter the combustion chamber during the intake stroke when the engine load is high and the injection pulse width becomes long.