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. However, it will not usually explode.