It consumes lava to produce Minecraft Joules (Buildcraft Energy)
Power Minimum / Maximum: 0.4 MJ/T / 4.0 MJ/T
When the engine is producing 4 MJ/t, one lava cell or bucket will produce 18,000 MJ, a Thermal Generator will make 30,000 EU per bucket.
8 engines are needed to provide the necessary power to run one Magma Crucible at full speed (Fewer will work, but slower).
Instead, you can pump lava directly from the Nether or a lava ocean age for massive amounts of free energy. Simply hookup a buildcraft Pump to an Ender Tank and place a chunk loader (optional) to keep it running. In your main base/ generator room, place another ender tank with the same color configuration and pump lava out from there.