Diesel is a type of fuel used to power engines, named after Rudolf Diesel, the German engineer who invented the diesel engine in the 1890s. Unlike gasoline engines that use spark plugs to ignite the fuel, diesel engines compress air so tightly that it becomes hot enough to ignite the fuel on its own. This makes diesel engines very efficient and powerful.

You'll find diesel engines in large vehicles that need serious power: semi-trucks hauling cargo across the country, buses carrying dozens of passengers, trains, construction equipment, tractors, and many ships. Some cars and pickup trucks also run on diesel, especially in Europe. Diesel fuel contains more energy per gallon than gasoline, which means diesel vehicles can often travel farther on a tank of fuel.

At gas stations, diesel pumps are usually separate from gasoline pumps and often have green handles. It's important never to put diesel in a gasoline car or gasoline in a diesel car, since either mistake can seriously damage the engine.

The word diesel can also describe the engine itself: a truck might have a powerful diesel under the hood. When you hear the distinctive rumbling sound of a semi-truck, that's the deep growl of a diesel engine at work.