In response to a predetermined engine crankshaft position a reference pulse is generated having a leading edge advanced with respect to that predetermined position as a function of the rotational speed of the crankshaft. At low engine speeds the reference pulse is directly used as the ignition current. A start-of-dwell-time data word is derived in response to the trailing edge of the reference pulse and an end-of-dwell-time data word is derived in response to the leading edge of a subsequent reference pulse. These data words are used for engine operations at constant high speeds to advance the beginning and end of the dwell time of ignition current by an amount commensurate with an increase in engine speed. When the engine is rapidly accelerated the dwell time is started in response to the leading edge of the reference pulse and terminated when the end-of-dwell-time data word is decremented to zero.