Ignition system with overrun prevention

Abstract

An ignition system of the capacitor discharge type and capable of preventing overrunning of an internal combustion engine includes a control circuit to render a switching transistor connected to a gate-cathode circuit of a main thyristor conductive for a predetermined period of time to interrupt a backward or reverse phase voltage generated by a magneto and serving as an ignition signal for the main thyristor from being applied thereto. The control circuit includes an integrator circuit connected in parallel with a constant voltage source to develop a voltage increasing with time and includes a voltage divider of the constant voltage source to provide a reference voltage. A transistor circuit of the control circuit compares the output voltage of the integrator circuit and the output voltage of the voltage divider and the transistor circuit maintains a control signal to be applied to the transistor circuit until the output of the integrator circuit reaches the reference voltage of the divider. The period of time during which the control signal is applied to the switching circuit to maintain the switching circuit being conductive is precisely constant irrespective of a variation in rotational speed of the engine. When the rotational speed of the engine increases and exceeds a normal rotational speed region, the occurrence of the ignition signal or the backward voltage of the magneto becomes early to cause the leading portion of the ignition signal to overlap with the end portion of the predetermined period of time resulting in retardation of the ignition timing and preventing overrunning of the engine.