The present invention relates generally to a combustion timing detecting system for internal combustion engines and particularly to a system which detects the crank angle at which the maximum pressure occurs within a cylinder or the occurrence of miss fire.
Usually, the maximum torque output of an internal combustion engine can be obtained if the crank angle .theta..sub.pmax at which the maximum cylinder pressure occurs is 15 degrees after top dead center (15 degrees ATDC).
For the purpose of generating maximum torque, the ignition timing may be feedback controlled on the basis of the crank angle .theta..sub.pmax at which the maximum pressure occurs within the cylinder. In this technique, if .theta..sub.pmax is greater than, for example, 15 degrees ATDC, the ignition timing is advanced, while if .theta..sub.pmax is less than 15 degrees ATDC, the ignition timing is retarded.
Highly accurate detection of the crank angle .theta..sub.pmax requires precise monitoring of the pressure within the cylinder. The arithmetic processing of the detected value tends to impose a burden on a Central Processing Unit (CPU). For example, Japanese First Publication (tokkaisho) No. 61-68533 discloses a detection method for detecting .theta..sub.pmax accurately whereby cylinder pressure is detected at a few predetermined crank angle positions and the pressure curve is deduced mathematically.
For example, the detection of .theta..sub.pmax according to the above process is carried out as follows: Cylinder pressure is detected at three detecting angles .theta..sub.1 to .theta..sub.3 in the vicinity of the desired crank angle at which the pressure within the cylinder reaches a maximum pressure (for example, .theta..sub.1 =7 degrees ATDC, .theta..sub.2 =15 =degrees ATDC, .theta..sub.3 =23 degrees ATDC). Cylinder pressure values P.sub.1 to P.sub.3 which are representative of the pressure within the cylinder detected at crank angles .theta..sub.1 to .theta..sub.3 respectively are mathematically processed to obtain an estimated .theta..sub.pmax.
It will be appreciated that when .theta..sub.pmax is between the initial detecting angle .theta..sub.1 and the last detecting angle .theta..sub.3, it may be estimated accurately. On the other hand, when .theta..sub.pmax is outside the range of .theta..sub.1 to .theta..sub.3, it cannot. However, by examining which is greater P.sub.1 or P.sub.3, .theta..sub.pmax may be presumed that when P.sub.1 is greater than P.sub.3, .theta..sub.pmax is advanced from the target value, and when the P.sub.1 is smaller than P.sub.3, .theta..sub.pmax is retarded from the target value. Thus, it may be determined whether the ignition timing is advanced or retarded.
However, when the ignition timing is greatly retarded, the cylinder pressure curve has two peaks. One is defined by compression at the top dead center. Other is defined by pressure due to combustion. In this case, the peak due to combustion occurs after crank angle .theta..sub.3 with the result that the cylinder pressure value P.sub.1 is greater than P.sub.3. It therefore becomes impossible to judge whether the ignition timing is advanced or retarded.
Moreover, in the above method for estimating .theta..sub.pmax when .theta..sub.pmax is outside the range defined between .theta..sub.1 and .theta..sub.3 due to the compression peak, when missing occurs P.sub.1 is greater than P.sub.3, and it is therefore impossible to judge whether the engine is missing or the spark is advanced.