An engine is a part to accelerate a craft. There are three different type of engines available which have different properties.
- → Main article: Reaction engine
A reaction engine (also rocket engine) simply pushes mass (usually called fuel) out of a nozzle. Because of Newton's third law it propels the craft with the same force it pushes the mass out. There are four different type of rocket engines which only differ in the kind of fuel they use:
Those engines work in vacuum and weightlessness. They don't have a specific speed limit except for the speed of light, which isn't implemented in KSP.
- → Main article: Jet engine
A jet engine pushes air from an intake out of the nozzle. They could work in weightlessness but need surrounding air to work which is impossible to recreate in KSP. They are more effective than rocket engines as they mostly gain acceleration by pushing the air through the engine and not only by pushing fuel away from the craft. These engines use only liquid fuel.
A completely different type of engine are rover wheels. They can accelerate a craft in vacuum but they need friction with an surface to move along this surface, so they won't work in weightlessness but with artificial gravitation they can drive on the inner side of a centrifuge in weightlessness. Usually they are very slow compared to the fuel consuming engines but run only on electricity so that they can run forever as long as there is a generator on board.