Numerical investigations of piston cooling using oil jet in heavy duty diesel engines

Abstract

Thermal loading of diesel engine pistons has increased dramatically in recent years owing to applications of various technologies to meet low emission and high power density requirements. Control of piston temperatures by cooling of these pistons has become one of the determining factors in a successful engine design. The pistons are cooled by oil jets fired at the underside from the crankcase. Any excessive piston temperature rise may lead to engine seizure because of piston warping. However, if the temperature at the underside of the piston, where the oil jet strikes the piston, is above the boiling range of the oil being used, it may contribute to the generation of mist. This mist significantly contributes to the non-tailpipe emissions in the form of unburnt hydrocarbons (UBHCs). The problem of non-tailpipe emissions has unfortunately not been looked into so seriously, as the current stress of all automobile manufacturers is to meet the tailpipe emission legislative limits. A numerical model has been developed using finite element methods for studying the oil jet cooling of pistons and has been validated experimentally on a flat plate cooled by oil jet. Using numerical modelling, the heat transfer coefficient (h) at the underside of the piston is predicted. This predicted value of heat transfer coefficient significantly helps in selecting the correct oil type, oil jet velocity, oil jet diameter, and distance of oil nozzle from the underside of the piston. It also helps to predict whether the selected grade of oil will contribute to mist generation. Isotherms of the predicted temperature profiles in a production grade piston have been plotted.