Turbo-compound engine

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).

When a blowdown turbine is attached to an engine it will reduce power due to exhaust gas flow restriction. However, the energy recovered will exceed the energy lost and total output will increase.

Turbo-compounding was used on on a few airplane engines during and after [World War II] , the Napier Nomad and the Wright R-3350 being examples of the application. Turbo-compound versions of the Napier Deltic and Allison V-1710 were constructed but neither was developed beyond 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 to which it was attached. As a result, turbo-compound aero engines were soon supplanted by turboprop and turbojet engines.

Alternatives

Thermo-electric systems (Thermogenerators) are using the exhaust heat to generate electric energy [Grandeur, Eder, Vehicle Fuel Economy Improvement through Thermoelectric Waste Heat Recovery DEER Conference, Chicago 2005 [http://www.eere.energy.gov/vehiclesandfuels/pdfs/deer_2005/session6/2005_deer_crane.pdf] ] .

Future

Hybrid electric power trains with downsized engines do have higher average engine power in practice. Thus the turbo compound engine may be more efficiently used in those cases and probably have an introduction with parcel services and urban public transport.

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