Diesel Engines (producing power)
Diesel Engines burn diesel to produce power. They are the cheapest way of producing power.
Electric Motors (producing power)
Electrical motors differ from other engines in that they have an adaptive power output, meaning that it can provide high Torque or high RPS depending on what is needed without requiring a gearbox. They can also supply mechanical power from 0 RPS and so do not require a clutch like the diesel engines to avoid stalling.
Electric Motors consume large amounts of electrical energy and can not (except in very unique cases) be used without another power generating component connected to a generator. At the current state of the game smaller motors seem to be more efficient but even several of them will not provide sufficient output for larger vessels.
Jet Engines (producing power)
More information here
Each gearbox has 2 ratios. When the gearbox is given an on signal, the second ratio is engaged. When the gearbox arrows travel with the flow of power it will act as a step down gearbox. By rotating the gearbox through 180 degrees, it can be used as a step up gearbox. A ratio of 1:-1 is also available to allow for reversing the direction of rotation.
A clutch is used to control the transmission of force transmitted through piping. It has a range from 0 to 1. At value 1 the clutch is fully engaged and will transmit the maximum amount of power from its input to its output. At 0 the clutch is completely disengaged and will transmit no power. A clutch allows you to separate your engine from the drive train to accommodate multiple gears or to let the engine remain running while the load is idle. There is a small power loss associated with using a clutch.
Air (consuming power)
Used to control yaw of helicopters.
Water (consuming power)
Solid Rocket Fuel blocks provide the fuel, Solid Rocket Boosters consume that fuel, and act as an engine.
You can combine multiple fuel blocks with a single engine, just place all of them on top of a booster (top=opposite site of the exhaust).
You can only start the boosters with a Boolean tick. The booster can not be throttled at will run at full thrust until the fuel source is used up.
Solid Rocket Booster Huge
Solid Rocket Booster Large
Solid Rocket Booster Medium
Solid Rocket Booster Small
Train (consuming power)
Wheels (consuming power)
See control surfaces.