BuildCraft's three engines, (from left to right) the Redstone Engine, Stirling Engine, and Combustion Engine.
As of Buildcraft 2.0.1 there is the option to use pneumatic engines to power the various machines in BuildCraft. Engines may be picked up by breaking them with a stone pick or better. The energy output from an engine only occurs on the top side. Engines can be rotated if they are not facing the right way using a wrench. When right clicked with a wrench, an engine will automatically face towards something it can power. If there is nothing to power, it will not change. Engines must be switched on by a lever or redstone current to function.
Each has various strengths and requirements to function.
MJ (Minecraft Joules)
Engines output power in terms of MJ/t (Minecraft Joules per tick). One second is equivalent to 20 ticks (assuming no lag). Knowing how much MJ/t an engine generates or a machine uses is key in creating the best system.
Temperature
Steam engines and Combustion engine connected to pipes
You can monitor the temperature of the engine by the color of its core. Blue represents a cold engine, green represents an engine that is warming up, yellow represents an engine that is running at the optimum efficiency , red represents an engine that is in danger of overheating and an engine that is bright red has overheated and will explode if not turned off or cooled down very quickly. Combustion engines only go to yellow mode if they are starting to overheat, as they have run out of an internal supply of water or they are heating up faster than water is being supplied, so keeping them in the green stage is recommended.
In SMP for beta 2.2.1 of buildcraft, the energy folder should be installed last or the engines won't work properly.
Redstone Engine
A Redstone Engine.
Redstone engines are the most basic engine. When applied with redstone current, it will be powered forever until you turn it off. As of version 3.2 they can no longer be used to allow transfer of power through wooden conductive pipes.
Steam/Stirling Engine
Two Steam Engines.
This mid-range engine is significantly more powerful than the Redstone Engine but uses burnable fuel, such as wood or coal.
Combustion Engine
The highest tier of engine. You can attatch a conductive pipe or kinesis pipe (same thing, different versions) to transport the energy elsewhere. Combustion engines are expensive, and can overheat and explode unless you pump water into it to cool it off. A Combustion Engine can be fueled by pumping oil or fuel into it
For a more specific view on different engines and their energy consumption, see engines - in depth.
Creative Engine
Creative engines are only spawnable in creative.
The power of these can be selected with the wrench.