1. Field of the Invention
The present invention relates generally to ignition timing control methods and systems for an internal combustion engine, and more specifically to such a method or system which is arranged to provide improved driveability and stable operation especially when the engine operating conditions are rapidly changing, and to alleviate uncomfortable shock inherent in acceleration.
2. Description of the Prior Art
Among recent advances in automotive electronics is an electronic ignition timing control system in which ignition timing (spark advance or retard) is determined, using a data processing device such as a microcomputer, depending upon engine operating variables such as engine speed and engine load. Such an electronic ignition timing control system provides a wide flexibility of control and affords an optimal ignition timing adapted to the engine operating conditions, compared with a conventional mechanical ignition control device such as a centrifugal advance mechanism and a vacuum advance mechanism.
However, such an electronic ignition timing control system determines the ignition timing depending upon the engine operating conditions by performing a calculation and, therefore, the resulting ignition timing changes abruptly and discontinuously when the engine operating conditions are changing from one state to another. This is true especially in an electronic ignition timing control system using several ignition timing tables each prepared for one of several divided categories of the engine operating conditions. For example, (1) during a cranking period, ignition timing is determined by selecting a proper value from a table containing a collection of ignition timing values as a function of engine coolant temperature, (2) when a throttle switch for detecting a throttle position is in its on position (i.e. the throttle valve is fully closed), ignition timing is determined depending upon engine speed from another table, and (3) during normal operations, the ignition timing is selected from still another table containing ignition timing values as a function of engine speed and engine load which is detected from intake vacuum, incoming air quantity or width of fuel injector pulses. In such a system, the specific ignition timing table to be applied is replaced by a different table when the engine operating conditions change from one state to another among the states (1) to (3), which leads to a discontinuous change of ignition timing. A sudden, abrupt change of ignition timing causes an abrupt or violent change of the engine output torque, thus deteriorating the driveability and also causing unstable variation of dwell angle, making the length of time current can flow through the ignition coil too long or too short. If the current flows for too long of time, a power transistor is often damaged, and, if the current flows for an insufficient time, insufficient ignition energy supply induces misfire and backfire. Furthermore, just after starting of acceleration when the throttle valve begins to open from its fully closed position, ignition timing changes greatly and rapidly in a spark advance direction to follow the change of the engine operating conditions. Consequently, the engine output torque suddenly increases, presenting uncomfortable shocks and vibrations.