Comparison of analytical and multibody dynamic approaches in the study of a V6 engine piston

摘要

Two methods for the modelling and analysis of the piston/cylinder interaction are compared in this paper with reference to a V6 turbocharged gasoline engine piston. The first method is a traditional analytical method based on the equation of motion derived for the axial movement of the piston, written according to the kinematics of the crank mechanism. The second method is based on a multibody analysis, performed with the software AVL Excite Power Unit after a modal reduction of the system. The lateral force between the piston and the cylinder calculated using the analytical method is compared to the one calculated with a simplified multibody model. The results are well correlated, but in order to identify the piston secondary motion, an advanced multibody model is necessary since the consideration of the hydrodynamic effects, the flexibility of the bodies and the real profiles of the contacting surfaces is crucial. With regard to the latter aspect, a finite element model of the system under thermal boundary conditions has been implemented to evaluate the radial deformation and proper constraints have been considered to take into account the engine block stiffness. The obtained results are important because they have an impact on the radial clearance between the piston and the liner. When the multibody model takes into account these advanced effects, the piston slap events are clearly identified after the top dead centre and corresponding secondary peaks in the lateral force acting on the thrust side are calculated. At the end, a sensitivity analysis to the change of the piston-pin offset is also performed. Apart from the expected result of a lower lateral force peak when the offset is taken towards the anti-thrust side, some interesting differences are detected about the slapping behaviour. These events are clearly recognisable only if the offset is taken towards the thrust side because of the unfavourable piston rotation just prior to their occurrences. The pronounced anti-clockwise rotation of the piston prior to the lateral force peak occurrence in case of anti-thrust side offset, on the other hand, does not allow for a proper piston slap, even if the lateral acceleration is higher due to the centre of mass position with respect to the piston pin axis.