An air-cooled piston engine has greater combustion efficiency than a diesel engine.

• A. True
• B. Not sure
• C. False
• D. Both are equally efficient

Answer: (C)

Combustion efficiency is about how completely the fuel is burned inside the cylinder, which depends on how the air-fuel mixture is prepared, the ignition method, and the combustion chamber design—not on whether the engine is air- or liquid-cooled. Diesel engines use compression ignition with much higher compression ratios and direct fuel injection, which promotes more complete combustion and the ability to run lean. An air-cooled piston engine is typically a spark-ignition design, where the mixture has to be ignited by a spark and the burn can be less complete under certain operating conditions, such as at part-throttle or with throttling losses. Because of that, the claim that an air-cooled piston engine has greater combustion efficiency than a diesel engine isn’t correct.