Electronic ignition timing advance circuit

Abstract

An electronic ignition circuit is disclosed which produces a spark timing signal for controlling the firing of the cylinders of an internal combustion engine. A sensor produced input signal having a frequency proportional to engine speed is coupled to an ignition timing means which produces a spark timing signal by controlling the charging and discharging of a capacitor. The produced spark timing signal differs in phase from the input signal as a predetermined function of engine speed and the phase difference preferably varies as function of speed according to a plurality of different slope line segments over a range of engine speeds. An advance means also receives the sensor input signal and produces a digital output signal for the timing means comprising a series of pulses synchronized with the input signal and having predetermined pulse widths related to the magnitude of the vacuum pressure produced by the engine. The ignition timing means charges a capacitor at a first rate during a portion of the period of the input signal, maintains the discharge on the capacitor at a constant level during the existence of the advance pulse and discharges the capacitor at a second rate after the occurrence of the advance pulse. This results in adding a predetermined amount of phase, determined by the magnitude of the vacuum pressure produced by the engine, to the speed dependent phase versus engine speed relationship produced by the ignition timing means.