A diesel engine is an INTERNAL COMBUSTION ENGINE  which operates using the DIESEL CYCLE.

Diesel engines use Compression IGNITION , a process by which FUEL is injected after the air is compressed in the combustion chamber causing the fuel to SELF IGNITE . By contrast, a   GASOLINE ENGINS utilizes the OTTO CYCLE , in which fuel and air are mixed before ignition is initiated by a SPARK PLUG. .

Diesel engines are more efficient than gasoline (petrol) engines of the same power, resulting in lower fuel consumption. A common margin is 40% more MILES PER GALLON  for an efficient TURBODIESEL.

The worldwide usage of the diesel engine is highly dependent on local conditions and the specific application. Applications which require the diesel's reliability and high torque output (such as SMALL GENERATORS, PUMPSETS FOR FARMS ,TRACTORS .TRUCKS, HEAVY EQUIPMENTS , most BUSES  etc.) are found practically world-wide