Dynamic behaviour of pneumatic suspension systems with auxilliory volumes: Modelling, vehicle simulations and potential

For the purpose of optimising both the handling behaviour and the ride comfort, especially of upper class vehicles, it is common practise to apply pneumatic suspension systems. In comparison with classical coil spring suspensions they have several advantages: a ride-height adjustment can easily be realised by supply or dissipation of air, the vertical eigenfrequency of the body is nearly independent of the load and the stiffness behaviour of the suspension can be influenced by shaping of the piston of the pneumatic spring. The most interesting property is that the dynamic stiffness and damping behaviour is frequency-selective and can be modified by an appropriately coupled auxiliary volume. Since such complex systems can only be rated in interaction with a full vehicle, it is necessary to develop mathematical models and to apply modern CAE methods. In the current essay, a thermomechanically based approach for pneumatic springs with auxiliary volumes is developed and presented. To understand the physics in detail, the model is implemented into the mathematical software system MATLAB and into a full vehicle model. The model of the pneumatic spring takes the heat exchange between the gas and the environment into account and considers the dissipative flow losses in the connection channel between the main and the auxiliary volume. As we show, this leads to heating effects. The state variables of the model are the temperatures, the pressures and the numbers of moles of gas in the two volumes. We demonstrate that the model represents the experimentally observed behaviour of the dynamic stiffness. The simulation of a quarter car shows that it is possible to dimension the flow resistance of the connection channel such that the magnitude of the resonance peak at 1 Hz of the body becomes minimal. This result is in accordance with experimental data from recent literature. Full vehicle simulations on uneven stochastic roads confirm this behaviour: the vertical comfort behaviour of the vehicle can be favourably influenced by an appropriate splitting of the total volume into a main and an auxiliary volume and an optimal adjustment of the flow resistance of the channel.