THERMOMECHANICAL BEHAVIOUR OF A SOLID SKIRT PISTON.

A three-dimensional finite element full analysis of thermomechanical behaviour of a solid skirt piston is presented in this article. The thermomechanical model describes the combined action of the thermal combustion loads, the mechanical load due to combustion pressure and the inertial load due to the important change of direction of the piston in the cylinder bore. The aim is to give a good tool for piston design and allows the piston to run under thermal and mechanical stress limit. The study is applied on a Diesel typical solid skirt piston of the direct injection V-8 air cooled engine. A 3-D finite element method allows to compute the operating temperature distribution in steady-state heat under durability test running engine. The calculated temperatures are introduced as nodal values in the 3-D thermomechanical finite element method. The model was used to calculate thermal expansion, thermomechanical deformation and operating stresses due to the simultaneous action of thermal and mechanical load. The results indicate that the combined effect of the pressure and inertial loads, are very weak in front of the effect of the thermal loads. The highest temperature for solid skirt pistons are localized in the piston head and the correspondent thermal stresses are under the acceptable levels. In the skirt, the obtained temperatures are under the limited value which causes the deterioration of the lubricating oil film. These results give the principals keys in the development of the piston design (clearance, fatigue and piston material). NOMENCLATURE E Young's modulus (N/m 2 ) Fi e Inertia load onto node i for the element e (N) h Thermal convection coefficient (W/m 2 .K) k Thermal conductivity coefficient (W/m.K) L Connecting rod length (m) mi e Lumped mass at node i for the element e (kg) n Normal unit vector Ni Nodal interpolation function PG Maximum pressure of combustion gases (Pa) r Crankcase radius (m) T Temperature (K) TG Average gases temperature (K) Tr Temperature of cooling (K) Th Oil temperature (K) Tp Temperature of inner surface (K) Ts Temperature of surroundings (K) u Displacement vector with components u, v, w (m) V e Volume of the finite element (m 3 ) x, y, z Rectangular frame of reference i j x Co-ordinates of the nodal point i j of the hexahedral element (m)  Thermal expansion coefficient (m/m.K)