Study of influence of boundary conditions on deformation and stresses in a cooled piston of a diesel engine - Part A - models

Abstract

The paper presents influence of boundary conditions on mechanical and thermal stresses of an oil cooled piston in diesel engine Scania DC09 074A and its deformation due to high combustion pressure and variable temperature. Due to different fixing of the pin in the piston hub the deformation of the piston shape and von Misses stresses differ at the same boundary conditions. This part of paper describes mathematical model of the piston load especially of heat transfer to wall taking into account three elements: convection, radiation and conductivity of the piston material. Calculations of the piston loads were carried out by using 0D mathematical model at assumption of the Vibe combustion model and the Woschni heat transfer model for homogenous mixture. For precise determination of the gas temperature in the combustion chamber the CFD technique recently is used and for that case the simulation of compression, combustion and expansion processes was carried out by using the program Fluent. The numerical calculations were done for two kinds of fuel: diesel oil and CNG due to possibility of changing the oil fuelling system into gaseous fuelling system. In order to proper work of CNG engine the compression ratio should be decreased. This was done by reducing the volume of the combustion chamber. Most calculations have done by researchers at assumption that the piston hubs are fixed, but it is not right assumption. In summary the paper gives indications how to set boundary conditions for "floating" piston pin. The paper presents shortly mathematical model of heat balance in the piston and FEM solution of stresses and deformation which are based on strains. This study is an introduction to calculations showed in the part B.