Technically a glow plug engine is fairly similar to a diesel engine and hot bulb engine in that it uses internal heat to ignite the fuel, but since the ignition timing is not controlled by fuel injection ( as in an ordinary diesel engine ), or electrically ( as in a spark ignition engine ), it must be adjusted by changing fuel / air mixture and plug / coil design ( usually through adjusting various inlets and controls on the engine itself . ) A richer mixture will tend to cool the filament and so retard ignition, slowing the engine.
