A Turbo-compound engine is a reciprocating engine that employs a blowdown turbine to recover energy from the exhaust gases. The turbine is usually mechanically connected to the crankshaft but electric and hydraulic systems have been investigated as well. The turbine increases the output of the engine without increasing its fuel consumption, thus reducing the specific fuel consumption. The turbine is referred to as a blowdown turbine (or power-recovery turbine), as it recovers the energy developed in the exhaust manifold during blowdown, that is the first period of the exhaust process when the piston still is on its expansion stroke (this is possible since the exhaust valves open before bottom dead center).