Effect of CNG on High-Cycle Fatigue Life of the Piston in a Bi-Fuel Engine Based on Stress Gradient Analysis

The piston is a vital component in the internal combustion engine, subjected to complicated loads. This complex component transfers the pressure resultants from engine combustion into the connecting rod and crankshaft, and due to its exposure to intensive gas temperature and pressure alterations, it is considered a critical component. One purpose of piston design is to ensure durability with a long fatigue life. Thus, simulation and analysis of fatigue cracks are essential. Regarding various bi-fuel vehicles, the effect of compressed natural gas (CNG) on the distribution of piston stress and fatigue should be studied, as well. Therefore, this study evaluates the CNG impact on the high-cycle fatigue (HCF) life of the piston in a bi-fuel engine, with a focus on the stress gradient. For this purpose, the temperature and stress were predicted by finite element analysis (FEA) and ANSYS software. Then, nCode Design Life software and Goodman's criterion were applied to obtain an estimation of HCF life. The piston's mechanical properties were determined using tensile tests at various temperatures. As the FEA showed, the upper areas of the piston pin and ring grooves are critical. According to thermo-mechanical analysis results, the piston in the CNG condition shows a higher tolerance of 30 °C temperature and 7.3 MPa stress than the gasoline. Fatigue analysis results showed that substituting CNG for gasoline reduces piston fatigue life by approximately 1.557×109 cycles or 41%. Investigating the HCF safety factor showed no critical area in the piston. No rupture in different parts of the piston was observed through the 800-hour durability test. According to the 800-hour durability test, the piston will not be ruptured in any sections.