The possibility whether brake thermal efficiency could be over 50% in reciprocated engines for passenger cars is examined. Maximum possible indicated thermal efficiency was estimated from fuel-air cycle calculation in which cooling loss and timeloss were considered. It is concluded that over 50% brake thermal efficiency is attainable in both gasoline and diesel engines with compression ratio of over 16 and air excess ratio of over 1.5. under condition that mechanical efficiency is improved andas much as possible and exhaust energy recovery system such as turbo-compound system is applied.
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