Diesel fuel sprays were visualized in the main chamber of a single-cylinder, air-cooled prechamber diesel engine. The experiments were conducted while the piston was positioned at various locations with respect to the ceiling of the main chamber. A ceramic disk with an embedded nickel-chrome heating coil was bolted on the top of the piston. The temperature of the ceramic surface was varied in the 300-500°C range. It was observed that the impinging fuel spray was associated with the formation of a boundary zone (B.Z.) between the hot surface of the piston and the turbulent flow in the main chamber. The spray appeared to have been deflected away from the piston by this boundary zone. It was found that the thickness of this zone increases with increasing both the piston temperature and the chamber height, and decreases with increasing the quantity of fuel per injection. Performance tests were conducted on an operating single-cylinder, air-cooled prechamber diesel engine. The results show that a ceramic-topped piston improved the brake thermal efficiency by 11% over that of the same engine when fitted with an ordinary aluminum-alloy piston. The carbon monoxide emissions were reduced by 15% while nitrogen oxides increased up to 100%.