Controllable thermal state design method of flexible shapes of piston cooling galleries

Abstract

The uneven temperature field is a great contributor to abnormal deformations and local damages of the piston. The piston gallery cooling is one of the effective ways to achieve an acceptable temperature field of the piston head. However, the traditional design of piston cooling gallery cannot accurately control the local thermal state of the piston, such as the temperature in the piston ring zone. In this paper, a flexible design model is adopted to control local thermal state of piston cooling gallery, and 10 parameters are used to completely determine the cross-section shape and the position of the cooling gallery. The maximum temperature and the maximum temperature gradient of the piston are employed as optimization targets to describe the thermal state of the whole piston. Most importantly, the temperature of the first ring zone is adopted as controllable target to monitor the local thermal state of the piston ring zone. The optimal structure variables and position variables were acquired by a Multi-Objective Genetic Algorithm technique. The results show that support vector machine for regression (SVR) model has a very excellent generalization ability with the determination coefficient greater than 0.90 and the relative deviation less than 7%. The SVR model can quickly obtain the cooling performance of each sample point to save calculation cost. The results illustrate that the controllable thermal state design method is effective, resulting in decreasing thermal load of the piston and adding reliability of the engine. For special focusing on the maximum temperature of the first ring zone, three distinctive optimal solutions (denoted A, B, C) are much lower than original model and other optimization models. Solution A has the lowest the temperature of the first ring zone in three optimal solutions to meet stricter cooling requirements of ring zone. Meanwhile, the temperature of the piston is also affected by the controllable thermal state design method, which have the large reduction with 5.40%. The temperature and temperature gradient are contradictory relationship. Thus, solution A possesses highest temperature gradient, which is 11841.3 °C/m. On the contrary, solution C pay attention to optimize the thermal strength of the piston, its temperature gradient is 11445.7 °C/m. The maximum temperature of the piston and the maximum temperature of the first ring zone are 356.5 °C and 298.3 °C, respectively. Solution B is a compromise between the solution A and the solution C, which is a design solution at which three optimization objectives are sufficiently satisfied. The controllable thermal state design method is adopted to control the design goal of the one or more feature regions of the piston.