Advances In Heavy Vehicle Dynamics with Focus on Engine Mounts and Individual Front Suspension

Doctoral thesis, 2012

The main objective of the research presented in this thesis is to enhance driver comfort and handling of a heavy truck that in turn leads to better vehicle safety and stability. The focus has been put on studying two suspension systems of the truck, engine and front axle suspensions, due to their significant impact on the dynamic performance of the vehicle. At first, a computational model that evaluates the nonlinear behavior of dynamic stiffness and damping of the elastomeric engine mounts is developed and successfully verified and validated against measurement data. This model is then utilized to examine adaptronic mounting systems in which actuators, sensors and controller are incorporated in the passive engine suspension. Widely used conventional engine mounts in commercial vehicles cannot fulfill the conflicting requirements for the best isolation concerning both road and engine induced excitations. Hence, to improve the noise and vibration suppression it is necessary to go beyond passive isolators and use semi-active/active suspension. The results of simulations of engine vibration dynamics and transmitted forces to the vehicle structure have shown good potential for adaptronic engine mount setup. Subsequently, the effects of front axle suspension design, i.e. Individual Front Suspension (IFS) as well as semi-active damper, on the comfort and handling characteristics of the vehicle are discussed. Employing the model of the tractor semitrailer combination, the study presents the results of comparison between the trucks with IFS and rigid front axle by assessing the responses of the vehicles to various road excitations and steering input. The obtained results show enhanced comfort and steering feeling for the truck with IFS. Moreover, the capabilities of a semi-active front axle suspension are investigated through control strategies such as skyhook, Linear Quadratic (LQ) and Model Predictive Control (MPC). The developed controllers are verified by simulation with respect to the identified road inputs and maneuvers. The outcome of the semi-active damper compared to that of the passive suspension that are presented quantitatively and qualitatively clearly show the great influence of the semi-active dampers on the vehicle dynamic performance. With continuous semi-active dampers accelerations in the cab are decreased particularly up to wheel hop frequency. Also, tire forces are either unchanged or improved. Hence, unlike passive suspension that is a compromise between ride comfort and handling, semi-active suspension facilitates enhancing both target criteria.