To pre-heat the cylinder head and engine block before cold starts, the diesel uses glow plugs. They look like short, stubby spark plugs and are connected to the car's electrical system. Elements inside heat up very quickly once power is applied.
Glow plugs are activated either by an auxiliary position on the steering column switch, or by a separate switch. On the latest designs they automatically switch themselves off once the engine has fired and is accelerated above idling speed.