Simple combustion engine

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An engine, or motor, is a machine used to change energy into movement that can be used. The energy can be in any form. Common forms of energy used in engines are electricity, chemical (such as petrol or diesel) or heat. When a chemical is used to produce energy it is known as .

The difference of engine and motor is that an engine creates mechanical energy from heat, while motor creates mechanical energy from other kinds of energy, like electricity. Typical engines are steam engine and internal combustion engine, while typical motors are electric motor and hydraulic motor.

"Engine" was originally a term for any mechanical device that converts force into motion. Hence, pre-industrial weapons such as catapults, trebuchets and battering rams were called "siege engines". The word "gin, " as in "cotton gin", is short for "engine." The word derives from Old French engin, from the Latin ingenium, which is also the root of the word ingenious. Most mechanical devices invented during the industrial revolution were described as engines—the steam engine being a notable example.

In modern usage, the term engine is typically restricted to heat engines, like steam engines and internal combustion engines, that burn or otherwise consume fuel to perform mechanical work by exerting a torque or linear force (usually in the form of thrust). Examples of engines which exert a torque include the familiar automobile gasoline and diesel engines, as well as turboshafts. Examples of engines which produce thrust include turbofans and rockets.

A toy steam engine. Fuel is burn in the tray at the bottom, steam is made in boiler which works the piston (the blue part), which turns the wheel

Early kinds of engine used heat that was outside of the engine itself to heat up a gas to a high pressure. This was usually steam and the engines are called steam engines. The steam was piped to the engine where it pushed on pistons to bring about motion. These engines were commonly used in old factories, boats and trains.

Common fuel types for internal combustion engines are petrol, diesel, autogas and alcohol. There are many other types of fuels.

There are lots of different configurations of piston engines. Their cylinders have pistons in them and a crankshaft. Any number of cylinders can be used but 1, 2, 3, 4, 6, 8, 10 and 12 are common. The cylinders can be arranged in many ways, in a straight line, at an angle to each other or in a circle.

A 'Rotary' engine has no cylinders and uses a triangle shaped rotor spinning in an oval housing which mimics the movement of a piston.

The inside of a turbine showing the fins which are pushed by jets of steam

Hot gas can also be made to push a turbine around rather like the way the wind turns a windmill. Most electric power stations use big steam turbines. Others use water or wind turbines. Smaller turbines use internal combustion. Jet engines used in aircraft are a kind of turbine engine.

Jets of hot gas push a rocket

A rocket causes movement by shooting jets of gas very fast out of a nozzle. The gas may have been stored under pressure or be a chemical fuel that burns to make a very hot gas. Although they are very simple, rockets are the most powerful engines we know how to make. They will work in space where there is nothing to push against.