What fuel do diesel engines primarily use?

Diesel engines primarily use diesel fuel. This fuel is formulated for compression-ignition engines, which ignite fuel by injecting it into air heated by high compression rather than using a spark plug.

Most conventional diesel fuel is a petroleum distillate made during crude-oil refining. It is often called petrodiesel when it needs to be distinguished from alternatives such as biodiesel, biomass-to-liquid diesel, or gas-to-liquid diesel. Modern road diesel is commonly treated to reduce sulfur and meet national fuel standards.

Diesel fuel is not the same as gasoline, even though both are petroleum products. Gasoline is designed for spark-ignition engines and is judged largely by its octane rating; diesel fuel needs strong compression-ignition performance, commonly expressed through its cetane number. Some diesel engines can burn other fuels, including heavy fuel oils or biodiesel, but ordinary high-speed engines in cars, trucks, and equipment primarily use diesel fuel.