When Fuels, Engines and Emissions Research Center researchers at Oak Ridge National Laboratory achieved a 45 percent brake thermal efficiency in a multi-cylinder engine, they demonstrated a new potential for passenger-size diesel engines. They also reached a Department of Energy joule milestone and verified the efficiency potential of recovering thermal exhaust energy that is normally discarded to the environment. The reported efficiency is the fraction of fuel heating value converted to useful work as measured at a shaft. At one time this was done literally with a mechanical brake applied to the engine output shaft, hence the term brake efficiency. Robert Wagner of the Energy and Transportation Science Division noted that there are other ways of measuring efficiency that exclude many of the real-world losses and therefore give considerably higher but less real-world information.
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