Study On The Piston Thermal Load Of An OPOC Engine

As the internal combustion engine trends toward the higher power density, higher reliability, lower fuel combustion and lower emission, the combustion temperature has been increased significantly. However, as a key component in an engine, working in poor conditions where the piston should bear high-temperature and high-pressure and high-temperature gas which changes periodically, the durability and reliability of the piston is critical for the performance of the engine. To make sure that the piston can work safety and reliably, it is crucial to find an effective way to cool it. However, the most popular method widely used is forced cooling by injecting oil. The studying of the flow and heat-transfer characteristics of the oil in the inner cooling channel of the piston can not only provide a basis of the rational design of the inner cooling channel, but also offer a relatively accurate heat transfer boundary condition for the piston’s thermal load calculation.This thesis took an OPOC engine with inner cooling gallery structure as the research object, conducting a study on the flow and heat-transfer characteristics of the oil in the inner cooling channel of the piston by applying the method of CFD simulation, also through combining computer simulation and experiment, carrying on the research of the thermal load of the piston at different engine speeds. The concrete contents include:(1)The characteristics of cooling oil flow and heat transfer in internal combustion engine are studied with the selection of VOF two-phase flow model and dynamic grid technique by CFD(Computational Fluid Dynamics) software. Transient oil distribution, transient heat transfer coefficient distribution, the heat transfer coefficient in different location (the upper surface, the lateral aspect of the middle side, the middle and the lower surface) of oil gallery is obtained by simulation under one particular condition. Then the tendency of the oil filling rate in oil chamber along with the change of the crankshaft angle and the relationship between averaged wall heat transfer coefficient and distance in circumferential direction of the oil cavity is raised. Finally the relationship between the oil gallery average oil filling ratio and engine speed, the injection pressure is analyzed under different working conditions.(2)The storage type piston temperature measuring system was adopted in the whole machine of the piston and the temperature of the five key location in the top of piston was measured, engine dynamic response curves was got by recording the temperature change of each measurement point with different working condition, so this paper could provide the basis for studying the thermal load of piston and fixing temperature field calculation model accurately.(3)The thermodynamic cycle process of the internal combustion engine was simulated using the GT-POWER software, and the unsteady heat transfer coefficient and unsteady temperature at the top of piston were obtained. After achieving the cycle average heat transfer coefficient and average temperature, the steady-state heat transfer boundary conditions of the top of the piston was determined combining with the experience formula. At the same time, the other heat transfer boundary conditions of the piston were determined according to the result of the cooling oil CFD simulation in the inner cooling gallery and experience formula. At last, the piston temperature field of various operating conditions were calculated using the finite element software, and the results were compared between the calculated and the testing at each measured point and exists the maximum error of 8.0%,it shows that the calculation results were more reasonable.(4)In view of the problem that the temperature of the piston top was more higher in the high speed running than other conditions, the paper studied the cooling oil’s flow and heat transfer characteristics under different engine speed and injection pressure and the influence on the piston temperature field when the injection pressure of cooling oil changed, and found that increasing the injection pressure could obviously reduce the temperature of the piston top and ring groove, it could be used as an improvement plan to enhance the piston cooling of the high power density engine.