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An air-gap-insulated piston designed for reduced heat loss was evaluated by examining its influence on the coolant heat rejection, engine performance and exhaust emissions of a single-cylinder divided-chamber diesel engine. At 1000 and 1500 r/min engine speed, use of the low-heat-rejection (LHR) piston resulted in a reduction in total coolant heat rejection ranging from 3% at light load to 5-7% at full load, in a general reduction in hydrocarbons, carbon monoxide and smoke emissions, in an increase in oxides of nitrogen, and in a significant improvement in brake specific fuel consumption only at light loads. It was estimated that the LHR piston design reduced the piston-crown surface heat transfer by an amount equivalent to from 3.5% (full load) to 5.5% (light load) of the input fuel energy at 1000 r/min.