Impact of piston secondary motion on oscillating flow and heat transfer of oil inside piston cooling gallery of diesel engine

Abstract: Because of the stringent emission and fuel economy standards, automotive engineers are forced to develop engines with much higher power densities. Pressure and temperature levels within a modern internal combustion engine cylinder have been pushing to the limits of traditional materials and design. Piston cooling is a critical measure for achieving designed engine performance especially for heavy-duty internal combustion engines. The various piston cooling gallery structures have been widely applied in piston design to provide high cooling efficiency. In previous research of achieving high cooling efficiency of the piston gallery, only the reciprocating motion of piston has been considered and investigated fully. However, the secondary motion is another important quantity due to the inevitable gap between piston and cylinder liner. For its tiny displacement, the impact of piston secondary motion on oscillating flow and heat transfer of cooling oil inside the piston gallery has not been investigated or recognized. In order to obtain the secondary motion, a piston dynamics model was established in this study. And then, a simulation model named model-B was established with the computational fluid dynamics simulation method and a relative displacement method with a consideration of the reciprocating motion as well as the secondary motion. The piston secondary motion was directly applying on the boundary of the piston gallery. The relative displacement method allows the cooling gallery to be treated as a rigid body, and the original constant boundary conditions could be translated into varying conditions that change as a function of engine crank angle. As a contrasting model, another model named model-A was established without the secondary motion in order to find out the degree of impact. In order to validate the accuracy of the computational fluid dynamics simulation model-A, a recognized test of a cube cavity was borrowed to contrast with the simulation results. The simulation results were difference with the experiment values by ±15 %. In other words, the computational fluid dynamics simulation model has certain ability for predicting the rules of the oscillating flow and the heat transfer processes. The result of this study showed that the secondary motion could be regard as an impact load for the gallery with a radial acceleration of 2,450 m/s2. The interface between the oil phase and the air phase was more unstable due to radial displacement. The oil flow inside the fluid mixing zone was more disorder. The results showed that the radial displacement of the secondary motion had the main influence on the oscillating flow of the cooling oil inside the gallery. A dimensionless number called Reynolds number is used to characterize the oscillating flow of the cooling oil inside the piston gallery. The tilting angle of the secondary motion had the main influence on the instantaneous convention heat transfer performance of the piston gallery. The instantaneous oil charge rate of the gallery was reduced by the secondary motion, and the cycled averaged oil charge rate was reduced by 4.6%. The instantaneous convention heat transfer performance was affected by the secondary motion, and the biggest change ratio was 24.9%, which appeared in exterior region. For the whole heat transfer process, although the oil charge rate was reduced by the effect of the secondary motion, the cycle averaged heat transfer coefficient changed a little. The heat transfer efficiency of the cooling oil inside the gallery was improved, but the impact of the secondary motion on the comprehensive heat transfer performance of the gallery can be neglected.