Glow plug control circuit

Apparatus controlled by an ignition switch for energizing a normally-open power switch to enable application of power from an associated power source to a heater filament of a glow plug in a diesel engine, the filament being one which is heated to a predetermined design temperature when a specified voltage is applied thereto, the power source having a supply voltage greater than the specified voltage, the ignition switch being operable in a first position to prevent the application of the supply voltage to the apparatus, operable in a second position to apply the supply voltage at a first terminal of the apparatus, and operable in a third position to apply the supply voltage at the first terminal and at a second terminal of the apparatus and to the starter for the engine, the ignition switch further being operable, when turned to the third position and released, to return to the second position, said apparatus comprising:

• means operable, after the ignition switch has been turned from the first to the second position, to energize the power switch to cause preheating of the filament for a time period which varies as an inverse function of available supply voltage and equals the time required to raise the temperature of the filament from ambient temperature to the design temperature,
• means operable, after the preheat time period, to maintain the filament at the design temperature for a prestart time period by alternately energizing and then de-energizing the power switch in repetitive fashion, and
• means operable, after the ignition switch has been turned from the second to the third position, to continue alternately energizing and then de-energizing the power switch for an afterglow time period predetermined by the amount of time required for smooth engine idling and to minimize engine noise and white smoke emission.