Influence of the pressure gradient during combustion on the fatigue behaviour of an internal combustion engine piston

In recent years, increasing demands for higher power density, lower emission and lower fuel consumption drive the research on internal combustion engines towards the adoption of innovative types of combustion like Homogeneous Charge Compression Ignition, Spark Controlled Compression Ignition and Gasoline Direct Injection Compression Ignition. All these systems exhibit aggressive combustion strategies and, as a consequence, a high-pressure gradient can be encountered in the combustion chamber at the early beginning of the combustion process. On one hand, the specific value of the pressure gradient marginally affects the kinematic analysis of the engine piston and its primary axial translational motion. On the other hand, it can possibly affect its secondary motions, namely piston transversal motion and piston tilting. Therefore, to investigate the influence of the pressure gradient, a numerical approach capable to capture the whole piston dynamic behaviour is mandatory. In this contribution, different pressure profiles are taken into account and then employed in different Multibody analyses to understand the influence of the pressure gradient on the piston secondary motions. In particular, three different pressure profiles exhibiting an increasing gradient, but with the same total heat released, are considered. In addition, three different radial clearances between the piston and the cylinder liner are simulated for each pressure profile. So that, a total of nine Multibody analyses are performed. The results are then post-processed and the Dang Van high cycle fatigue safety factor contour plot on the piston is obtained for each configuration. The results show a dependency of the fatigue life of the piston on the pressure gradient, albeit no detrimental structural issues are detected for the different configurations analysed. The presented methodology represents a useful tool for the structural assessment of pistons when a first original safe combustion strategy is modified towards more efficient ones characterized by pressure profiles presenting a higher gradient.