Mechanisms of enhanced heat transfer of piston oscillating cooling in a heavy-duty diesel engine

The thermal load and reliability of pistons become major concerns with the increase of power density of internal combustion engines. The oscillating cooling technology is often adopted to enhance the heat transfer of pistons in heavy-duty diesel engines. To understand the heat transfer mechanisms, this study developed an optical oscillating cooling piston based on a real heavy-duty engine. The characteristics of two-phase oscillating flows were optically observed under different conditions. As engine speed increased from 100 to 700 r/min or oil supply pressure decreased from 0.2 to 0.8 bar, the oscillation intensity of the two-phase flow gradually increased and the oscillating flow pattern developed from slug to intermittent mist flow. Furthermore, computational fluid dynamics simulations were performed to analyze the heat transfer mechanisms of oscillating cooling and optimize the structure of the piston. The results showed that the heat transfer coefficient of oscillating cooling can be decomposed into three components, namely conventional flow, oscillation enhanced and jet flushing parts. The conventional flow part accounts for the largest proportion of the total heat transfer coefficient, whose value is positively correlated with oil flow rate. The oscillation enhanced part is optimal when the oil filling rate is between 40 % and 60 %. The jet flushing part can lead to a higher temperature gradient in the outer cooling gallery and thus increased thermal stress. The cooling capacities of inner and outer galleries are mainly influenced by the diameters of the oil outlet and distributor pipes, respectively. The jet flushing part can be controlled by changing the inclination angle of the distributor pipe. Finally, optimization achieved an improvement of 8.3 % in cooling capacity of inner cooling gallery and a reduction of 27.5 % of heat dissipation inhomogeneity in the piston.