An engine is a vehicle's main source of power. An engine produces mechanical power out of fuel by burning it. The created pressure is then used to drive a mechanical device. An engine is usually used in cars, planes, boats and more.
Also, and engine depends on what kind of fuel we use. Currently, the most popular is gasoline but diesel is used. What's the difference? Well, diesel contains more heat energy than gasoline, making it more economical but it is thicker, heavier, and does not vaporize as easily as gasoline.