Suppression of the heat rejection to the coolant was achieved by the use of an air-gap-insulated piston, an antechamber that was partially insulated by an air-gap, and high-temperature coolant (ethylene glycol at 120°C). In comparison to the standard configuration (STD) of the engine, the low-heat-rejection configuration (LHR) resulted in a small increase in brake thermal efficiency for light-load conditions, in a reduction in volumetric efficiency, in an increase in the exhaust energy, and in an increase in the heat rejection to the lubricating oil. Heat-release analysis performed on the two engines showed higher overall fuel burning rates, and consequently shorter combustion durations, in the LHR engine than in the STD engine. This is believed to cause the observed higher nitric oxide emissions. Also, the LHR engine was found to have higher hydrocarbon emissions but slightly lower particulate emissions. The increase in hydrocarbon emissions was attributed to the increase in the contribution of lubricating oil which resulted because of higher cylinder-liner temperatures in the LHR engine.