Numerical Investigations Of Piston Cooling Using Oil Jet

Thermal loading of diesel engine pistons has increased dramatically in recent years due to applications of various technologies to meet low emission and high power 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 undesirable piston temperature rise may lead to engine seizure due to piston warping. However, if the temperature at the underside of the piston, where the oil jet strikes the piston, is above the boiling point of the oil being used, it may contribute to the mist generation. This mist may significantly contribute to the non-tail pipe emissions in the form of unburnt hydrocarbons (UBHC). The problem of non-tail pipe emissions has unfortunately not been looked into so seriously, as the current stress of all the automobile manufacturers is on meeting the tail -pipe emission legislative limits.

A numerical model has been developed using computational fluid dynamics (CFD) tools, such as finite elements methods for studying the oil jet cooling of pistons. Using the numerical model developed by Stevens and Webb (1991), the heat transfer coefficient (h) required at the underside of the piston is predicted. This predicted value of heat transfer coefficient significantly helps in selecting right oil type, jet velocity, jet diameter and distance of the jet from the underside of the piston. It also helps to predict whether the oil selected will contribute to mist generation or not and if it contributes to mist generation then it helps in selecting the oil which does not contribute to mist generation. Grid generation for a production grade M & M DI 2500 engine piston has been done using GNUPLOT. Isotherms of the predicted temperature profiles in the piston have been plotted using TECPLOT.