Diesel engine
Most other engines need a system called an ignition system – one that uses an electric spark – in order to burn the mix of fuel and air and make power. Other types of ignition systems use compressed air from an outside source, such as an air compressor.  A diesel does not.  It burns a diesel fuel (similar to heating oil) through very high compression or pressing together of the mix. A tiny bit of fuel is injected, or forced, into the engine's cylinders at just the right moment. Since gases heat when they are pressed together, the compression of the air and fuel mix cause the mix to explode within the cylinder.