The Redstone Engine is the Tier 1 engine from the BuildCraft mod which allows you to power tools and other mod tools. It is primarily used to pump from pipes, as using it is very slow, about 1.5 seconds between each pump. Because it's so slow, it shouldn't be used to power large things like fillers or Quarries, as it would go at one block per 1.5 seconds. Additionally, Redstone Engines cannot be connected via Wooden Conductive Pipes to machines; they must be connected directly. The Redstone Engine requires no fuel, however, only a Redstone signal like all other Engines.
The color of the device shows its power output. There are four stages:
Blue, meaning either that it's off or producing low power .
Green, meaning that it's at medium efficiency.
Yellow, which is the peak power production of 1 MJ per second.
Yellow - Red, meaning that it will make more energy at every second pumping. Even at this stage it will not explode.
A redstone engine reaches only the flashing red with yellow color if it is connected to a device or pipe. If it is not connected to a device or pipe, then the redstone engine can turn red. It will stay red for a little while if you connect it to a device, but if you do it will eventually cool down to the previous color stage.
Red, the redstone engine makes the most energy.
If you connect a redstone engine to another one to supply more power the original redstone engine will turn red, but it will explode if you leave the second one on for too long. Fortunately, it only blows up itself, nothing more.