What type of fuel does a diesel engine primarily use?

A diesel engine primarily uses diesel fuel.

Unlike a typical gasoline engine, a diesel engine ignites its fuel through compression rather than a spark plug. The engine compresses air until it becomes extremely hot, then injects finely atomized diesel fuel into that hot air. The fuel ignites without the conventional spark-ignition system used by gasoline engines.

Diesel fuel is a distillate fuel oil commonly refined from crude petroleum. Its relatively high energy density and the efficiency of compression ignition make it useful for heavy-duty applications such as trucks, buses, trains, ships, farm equipment, construction machinery, and generators.

The answer does not mean every engine called “diesel” must burn ordinary petroleum diesel. Many diesel engines can use biodiesel blends, and some specialized designs have operated on fuels such as vegetable oil, kerosene, or heavy fuel oil. Nevertheless, diesel fuel is the standard and defining fuel associated with modern diesel engines. The engine is named after German inventor Rudolf Diesel, who patented his design in 1892.