Diesel fuel is a type of liquid fuel burned in diesel engines, which power many trucks, buses, trains, ships, and heavy construction equipment. Unlike gasoline engines that use spark plugs to ignite the fuel, diesel engines compress air so tightly that it becomes extremely hot, and when diesel fuel sprays into that super-heated air, it ignites on its own.

Rudolf Diesel, a German engineer, invented the diesel engine in the 1890s, and the fuel was named after him. Diesel fuel is thicker and oilier than gasoline, almost like thin cooking oil, and it contains more energy per gallon. This makes diesel engines more efficient for moving heavy loads over long distances, which is why semi-trucks hauling goods across the country almost always run on diesel.

You can often recognize diesel fuel at gas stations by its green pump handles (in the United States) and its different nozzle size. Diesel engines sound different too: they make a characteristic deep, rumbling noise that's louder and more clattery than gasoline engines. That distinctive sound comes from the way the fuel ignites inside the engine.

While diesel engines are powerful and efficient, diesel exhaust produces more soot and pollution than gasoline, which is why engineers continue working on cleaner diesel technology and alternative fuels.