a time period during which current is conducted through a primary winding of an ignition coil of an internal combustion engine is calculated in response to signals corresponding to an actual rotational speed of the engine and the magnitude of a power supply voltage which is applied to the primary winding , respectively . a time for initiating the primary winding current is controlled in accordance with the calculated time period . such current is conducted for a maximum period of time , which is determined in response to the magnitude of the power supply voltage , so as to avoid excessive heat build - up in the ignition coil . the rotational speed of the engine is determined by counting clock signals during time intervals which correspond to predetermined angular segments of rotation .