PISTON HEAT TRANSFER IN AN AIR-COOLED ENGINE

Abstract

Thermal energy flow within the piston of an air cooled engine was investigated by measuring the heat flux in the engine cylinder liner in the region of travel of the piston rings. Engine sump oil temperature and oil viscosity were varied to change the piston ring heat flow by altering the oil film between the cylinder wall and piston ring. Engine load was varied to change heat flow into the piston. Also, the crankcase oil splash cooling of the piston was varied to change heat flow leaving the piston. When sump oil temperature was reduced, more heat flowed to the cooler sump. Changes in oil viscosity had little effect on the cylinder wall heat flow but increased cylinder wall temperature due to an increase in frictional heating with lower oil viscosity. It was thus determined that this change in heat flow was due to the change in sump oil temperature and was not due to a change in oil film characteristics. A decrease in engine load decreased the average temperature and heat flux in the cylinder wall. The largest decrease in heat flux was on the exhaust side of the engine where the largest fraction of cylinder heat is convected to the external airflow. It was found that the change in heat flux caused by the decrease in engine load was likely due to a change in the piston ring pack heat flow. For changes in the crankcase oil splash cooling of the piston, there were only small changes in average temperature and no changes in the heat flux. Although the piston was still believed to lose heat to the crankcase oil splash, its percentage of piston heat loss was too small to affect piston ring pack heat flow. Superposition was evaluated for sump oil temperature and engine load as well as sump oil temperature and oil viscosity. In both cases, significant error was found with the linear superposition assumption. Linear superposition was assumed not to hold for these complex heat flows.