A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber during the final stage of compression. This is in contrast to a petrol engine (known as a gasoline engine in North America) or gas engine, which uses the Otto cycle, in which a fuel/air mixture is ignited by a spark plug.
It operates using the diesel cycle (named after Rudolf Diesel). Diesel engines have the highest thermal efficiency of any internal or external combustion engine, because of their compression ratio. Low-speed diesel engines' thermal efficiency exceeds 50%.
Diesel engines are manufactured in two stroke and four stroke versions. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships. Use in locomotives, large trucks and electric generating plants followed later. In the 1930s, they slowly began to be used in a few automobiles. Since the 1970s, the use of diesel engines in larger on-road and off-road vehicles in the USA increased. As of 2007[update], about 50 percent of all new car sales in Europe are diesel.