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A liquid motor fuel, heavier and less volatile than gasoline.

Diesel fuel is classified as a combustible liquid, and not as a flammable liquid. It is widely used as a fuel in locomotives, large trucks, emergency generators, and other industrial applications.

In a gasoline engine, fuel entering the carburetor is ignited by a spark from the engine’s spark plugs. A diesel engine operates on a different principle: a spray of fuel is introduced into a chamber in which air has been compressed, raising the air to a temperature of 1,000ºF. This heat ignites the fuel.

The most common is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted. To distinguish these types, petroleum-derived diesel is increasingly called petrodiesel. Ultra-low sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents.