1. Field of the Invention
The present invention relates generally to control systems for internal combustion engines, and more particularly, to a system for controlling idling speed of an internal combustion engine and an air-fuel ratio of a fuel mixture in the internal combustion engine, by which an actual idling speed of the internal combustion engine is controlled to be kept at a predetermined target idling speed and the air-fuel ratio of the fuel mixture is controlled to be of a value in a relatively narrow range including a predetermined value.
2. Description of the Prior Art
There has been proposed an internal combustion engine used for an automobile equipped with an idling speed control system by which a feedback control for intake air mass flow is carried out for controlling the opening degree of an air flow adjusting valve which is disposed in a bypass provided to an inlet passage of the engine for detouring a throttle valve provided therein or for controlling the opening degree of the throttle valve which is opened by an actuator to vary the intake air mass flow supplied to the engine in such a manner that idling speed of the engine is kept at a predetermined target idling speed. It has been also proposed to provide the feedback control for intake air mass flow with a learning function in determination of a feedback control value, as disclosed in, for example, the Japanese patent application published before examination under publication No. 56-44431.
In the idling speed control system by which the feedback control for intake air mass flow is carried out and a learning operation is performed for determining a feedback control value under that feedback control, a feedback correction value is produced in accordance with a difference between an actual idling speed of an engine and the target idling speed and then a learning control correction value is calculated based on the feedback correction value in addition to a fundamental control value used for controlling the intake air mass flow. Each learning control correction value is stored in a memory and renewed in the memory whenever a new one is obtained. The feedback control for intake air mass flow is performed with the fundamental control value, the feedback correction value and the learning control correction value, so that the actual idling speed of the engine is kept at a predetermined target idling speed.
In an internal combustion engine in which the feedback control for intake air mass flow is performed with the fundamental control value, the feedback correction value and the learning control correction value as mentioned above, an actual idling speed of the engine is caused to coincide expeditiously with a target idling speed at the beginning of the control without being easily influenced by secular change or variations in the characteristic of the engine, compared with another internal combustion engine in which the feedback control for intake air mass flow is performed with only the fundamental control value and the feedback correction value.
Then, it has been further proposed to make the internal combustion engine in which the idling speed control is performed as described above be equipped with an air-fuel ratio control system by which an air-fuel ratio of a fuel mixture provided for combustion in the engine is subjected to a feedback control so as to be of a value in a relatively narrow range including a predetermined value. The feedback control for the air-fuel ratio of the fuel mixture is performed to vary quantity of fuel actually supplied to a combustion chamber of the engine in accordance with a fundamental control value and a feedback correction value obtained based on the output of an air-fuel ratio sensor, such as an oxygen sensor disposed in an exhaust passage of the engine.
In the internal combustion engine wherein the feedback control for the air-fuel ratio of the fuel mixture is carried out in addition to the feedback control for intake air mass flow which is performed with the fundamental control value, the feedback correction value and the learning control correction value as described above, when the air-fuel ratio sensor comes to be in malfunction due to breaking of wire, short-circuit or other cause so that the output of the air-fuel sensor does not represent exactly the air-fuel ratio of the fuel mixture provided actually for combustion, the feedback correction value used in the feedback control for air-fuel ratio is not obtained properly and therefore it is feared that the fuel actually supplied to the combustion chamber is increased or decreased excessively. In the case where the fuel actually supplied to the combustion chamber is increased or decreased excessively in the idle operation of the engine, the actual idling speed of the engine becomes higher or lower than the target idling speed to vary the feedback correction value used in the feedback control for intake air mass flow by a relatively large margin and the feedback control for intake air mass flow is conducted to cause the actual idling speed to decrease or increase to the target idling speed with the feedback correction value which is apart from a normal range thereof.
When the feedback correction value used in the feedback control for intake air mass flow is varied in the above mentioned manner owing to the malfunction of the air-fuel ratio sensor, the learning control correction value which is set based on the feedback value is apart from an appropriate range thereof. Therefore, after the air-fuel ratio sensor recovers to operate properly, the feedback control for intake air mass flow is carried out with the learning control correction value stored in the memory under a condition wherein the air-fuel sensor is in malfunction and consequently a relatively long time is necessitated to cause the actual idling speed to coincide with the target idling speed.
The above described troubles or disadvantages arise similarly in an internal combustion engine in which the speed thereof is varied through a feedback control for ignition timing or the air-fuel ratio of a fuel mixture provided for combustion therein is controlled through a feedback control for intake air mass flow.