A Redstone Engine is the first tier of engine in Buildcraft. They produce low amounts of energy for no cost. Apply a redstone signal to turn it on. This engine will almost never explode due to heat. If eight or more redstone engines are lined up in a row and turned on without something to send power to (even an inactive object will work to prevent explosions, such as a wooden pipe that is attached to an empty chest) and one of them reaches maximum heat (engine turns red) it can explode. However, with proper management, explosions are highly unlikely. As with all engines, it generates the most power at the red stage.
The engine will run slowly at the blue stage, and as the heat builds up it will speed up. A way in which you can speed up the process of heating is to place a steam engine next to the redstone engine and allow it to "help". This will get the engine to the red/orange stage quite quickly, but make sure to turn off the steam engine when it hits that stage or else it will explode.
You can build up to eight redstone engines behind each other in a line to increase the energy/productivity output.
To craft the engine you will need 3 wooden planks, 2 wooden gears, a piston, and glass. This engine is very cheap in terms of resources and is usually used to extract items from chests (using a wooden pipe ) or 4 of them are used to run a pump. (they have to all be at the red stage for the pump to run effectively, so give it time to heat up).
Power Output (From Buildcraft wiki)Edit
The Redstone Engine has 4 different power outputs, depending on how hot it is, it will eventually heat up and reach the last (best) output.
Blue: Makes 1 Energy Pulse every 5 seconds (0.01MJ/t).
Green: Makes 1 Energy Pulse every 2.5 seconds (0.02MJ/t).
Orange: Makes 1 Energy Pulse every 1.25 seconds (0.04MJ/t).
Orange-blinking-Red: Makes 1 Energy Pulse every second (0.05MJ/t). This is the max safe power output.