Quasi-dynamic and thermal analysis of a diesel engine piston under variable compression

Abstract

Due to ever increasing demand for high speeds of automobiles and high gas loads, the piston of an IC engine is subjected to large thermal and structural stresses. The authors have therefore embarked upon evaluating the stresses induced in the piston and also study the effect of compression ratio on it. An experimental setup consisting of a computerized variable compression ratio diesel engine test rig with necessary instrumentation was established. A comprehensive load history in terms of pressure vs. crank angle(p-Θ) diagram and (pressure vs. gas volume), speed, fuel and air consumption rates, turning moment etc., were obtained at compression ratios of 16.5,17.5 and 18.5 from the experimentation. Part one is heat transfer analysis for evaluating the thermal boundary conditions like heat transfer coefficients and heat flux. Further the mean temperature of the working fluid is also obtained from thermodynamic analysis. Part two is dynamic analysis by using the equations of equilibrium for slider-crank mechanism. Part three is stress analysis using finite element method coupled field at critical angles of crank rotation where either inertia forces or wrist pin forces are maximum. The results in the form of von-mises stresses and deformation are presented. Further, factor of safety is evaluated. It was observed that increasing compression ratio has an appreciable effect on the stresses. It was interestingly observed that the mean temperature of gases is higher at compression ratio of 17.5. Stresses developed due to differential thermal expansion in the piston form a significant part of the total stresses.