Turbo-compound engine
A Turbo-compound engine is a compound engine that uses a blowdown turbine to recover energy from the exhaust gases of a piston engine. The turbine is usually connected to the crankshaft mechanically but also electric and hydraulic systems have been investigated. 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).
When a blowdown turbine is attached to an engine it generally reduces the engine power somewhat due to the restriction of the exhaust flow but the output of turbine is greater than the power lost from the engine thus increasing total output.
Turbocompound was used on on a few airplane engines during and after WW2 such as Napier Nomad and the Wright R-3350. Turbo-compound versions of the Napier Deltic and Allison V-1710, were constructed but neither exited the prototype stage. It was realized that in many cases the power produced by the simple turbine was approaching that of the enormously complex and maintenance intensive piston engine it was attached to. Eventually the piston engine was deleted and the turboprop was created.
Alternatives
Thermo-electric systems are using the exhaust heat to generate electric energy.
Future
Hybrid electric power trains with downsized engines do have higher average engine power in parctice. Thus the turbo compound engine may be more efficiently used in those cases and propably have an introduction with parcel services and urban public transport.