A diesel engine is an internal combustion engine which operates using the Diesel cycle. Invented in 1892 by German engineer Rudolf Diesel, it was based on the hot bulb engine design and patented on February 23, 1893. 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 engine utilizes the Otto cycle, in which fuel and air are mixed before ignition is initiated by a spark plug. For more info see wikipedia article :-http://en.wikipedia.org/wiki/Diesel_engine