Validation of a combined CFD/FEM methodology for the evaluation of thermal load acting on aluminum alloy pistons through hardness measurements in internal combustion engines

This work presents the results of a multidisciplinary research project, carried out in close collaboration with Ducati Motor Holding S.p.A., for the development of an integrated methodology to design engine components in aluminum alloy under high thermal loads. The results refer to the study of an AA2618 (Al-Cu-Mg) alloy piston for high performance motorcycle engines. The piston has been selected as the pilot component for the development and validation of an advanced CFD-FEM simulation methodology for the prediction of the inner thermal diffusion. The subsequent validation has been achieved through both the mechanical and microstructural characterization of the component. The methodology here presented consists of close interaction between fluid-dynamics (CFD) simulations of the combustion process and Finite Element (FEM) simulations of the thermal diffusion inside the components. Combustion is the main engine heat source and is simulated by means of a three-dimensional CFD code for reactive flows (FIRE v2008-AVL), with the use of advanced combustion (ECFM) and wall interaction models. The temperature map on the surfaces is based on the results of the iteration with FEM simulation of thermal diffusion. The FEM model used for the diffusion analysis receives the results of combustion analysis as input. Two different methods have been tested for the transfer of the CFD thermal load to the FEM models: a) imposition on the piston crown of a spatial distribution of heat flux averaged over the mean engine cycle; b) imposition on the piston crown of both heat flux coefficients and temperatures. The latter option allows the reduction of the number of iterations for the convergence of the thermal map inside the piston. The dissipation of the thermal load is accomplished by applying heat coefficients and temperatures, on the remaining parts of the piston surface. The validation of the CFD/FEM methodology is carried out through hardness measurements in different piston locations after bench tests. The identification of the hardness curves, as a function of temperature and time, for the T6 heat-treated AA2618 allowed the assessment of the local temperature reached by the component from the knowledge of the operating time of the engine and local hardness.