Generalised Boundary Conditions for Designing Diesel Pistons

Diesel pistons are so designed that the heat dissipation is adequate to limit the temperatures and to avoid undesirable distortion hence scuffing with the cylinder. The temperature distribution in a piston decides its outer profile at room temperature considering the hot running clearances. The Finite Element Method enables calculation of piston expansion and hence the form to be machined at room conditions. However, the results are only as accurate as the boundary conditions imposed on the piston model, irrespective of mesh size and shape, and the method of solution.

In this work, the heat transfer coefficients at the boundary surfaces were judiciously chosen from the literature and practice and were generalised to apply to pistons widely varying in size, rated bmep and speed. From the simulation of the thermodynamic cycle, the average gas temperature and the peak cylinder pressure acting on the piston were obtained. Finite Element models in two or three dimensions are made for four different pistons to validate the applicability of the boundary conditions. The model for thermal expansion simplifies to a steady state problem because of large thermal inertia at the frequency of firing. The transient deformation is mainly due to the cylinder pressure at its maximum. The predicted temperature distribution in the metal piston is satisfactorily compared with the experimental values obtained using relaxation in the hardness of the hypoeutectic Aluminium. Piston ovality, waviness of the groove and profile of the piston were obtained using the calculated deformation. Favourable comparison of the estimated piston shape with the proven profiles indicates wide applicability of the generalised boundary conditions.