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. This "configuration" can also be adjusted by using varying plug designs for a more exact thermal control. Of all internal combustion engine types, the glow plug engine most resembles the hot bulb engine, since on both types the ignition occurs due to a "hot spot" within the engine combustion chamber.