An engine is a machine that does work. Automobiles and airplanes have engines that make them move. Other kinds of engines drive machines or lift heavy loads. All engines use energy. Two important sources of energy are gasoline and steam. Image to right: A gasoline engine uses the energy of burning gasoline to push pistons. Credit: World Book illustration

In gasoline engines, the energy of burning gasoline pushes parts of the engine, such as pistons, and causes them to move. This motion then turns the wheels of a car or does some other kind of work. Some steam engines also use pistons. First, coal or another fuel is burned. The energy of the heat boils water, which turns into steam. The energy of the steam pushes on the pistons. How to cite this article: To cite this article, World Book recommends the following format: "Engine." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.