1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, and more particularly to a misfire-detecting system of this kind, which is adapted to detect a misfire attributable to the fuel supply system.
2. Prior Art
In an internal combustion engine having spark plugs, a misfire can occur, in which normal ignition does not take place at one or more of the spark plugs. Misfires are largely classified into ones attributable to the fuel supply system and ones attributable to the ignition system. Misfires attributable to the fuel supply system are caused by the supply of a lean mixture or a rich mixture to the engine, while misfires attributable to the ignition system are caused by failure to spark (so-called mis-sparking), i.e. normal spark discharge does not take place at the spark plug, for example, due to smoking or wetting of the spark plug with fuel, particularly adhesion of carbon in the fuel or unburnt fuel to the spark plug, or abnormality in the sparking voltage supply system.
The present assignee has already proposed a misfire-detecting system for detecting misfires attributable to the fuel supply system, which comprises sparking voltage detecting means which detects sparking voltage, i.e. voltage across electrodes of the spark plug, and misfire-determining means which determines that a misfire has occurred when a time period over which the detected value of the sparking voltage exceeds a predetermined reference value (Japanese Patent Application No. 3(1991)-326507 and corresponding U.S. Ser. No. 07/846,238 filed Mar. 5, 1992).
In the above proposed system, the time period over which the detected value of sparking voltage exceeds the predetermined reference value corresponds to a time period over which a predetermined amount of electric charge or more is stored in floating capacitance in the vicinity of the spark plug. Depending upon the behavior of discharge caused by sparking of the spark plug, the charge can be discharged within a short time period even if a misfire has occurred. This phenomenon can take place when the sparking voltage assumes a considerably high voltage value at the end of an inductive discharge caused by sparking, due to occurrence of a misfire. In such an event, discharge again takes place between the electrodes of the spark plug and terminates within a short time period so that the misfire is not detected to have occurred.
Further, even in the case where the spark plug has just started smoking due to adhesion of carbon, etc. to the electrodes of the spark plug and hence has decreased insulation resistance between the electrodes, or in the case where the spark plug has just started recovering from its smoking state due to its own purifying action, dielectric breakdown is likely to occur so that there is no significant difference in the time period over which the sparking voltage exceeds the predetermined reference value between when normal firing has occurred and when a misfire has occurred.