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Modelling of the Hydro-pneumatic Suspension System of a Rally Truck

The aim of this project is to develop a simulation model containing the hydro-pneumatic suspension  system of the competition class GINAF Dakar Rally Truck. The model is validated by measurement data on both component level as well as full vehicle level. The model can be used for further research with respect to active damping control or implementing the system in other groups of vehicles like commercial delivery vans.

The hydro-pneumatic system uses an accumulator to generate spring force (similar to the HydrActive  suspension system of Citroën) and a remote valve block to generate damping force. A hydraulic cylinder replaces the damper strut and springs of the vehicle. The cylinder generates oil volume displacement towards the accumulator.

The oil is assumed to be incompressible and the volume of the air chamber inside the accumulator is diminished which creates a pressure increase by means of the “ideal gas law”. Higher pressure results in a higher reaction force and so a spring is established. The flows from the piston chambers and rod chambers of the cylinders are led through  the tubing system and a remote damper manifold. In the damper manifold the oil flows create a pressure loss over the solenoid valves and other appendages in the tubing system. The valves are placed parallel for each flow and therefore the total flow-through area changes when one of the valves opens or shuts.

Due to the pressure losses energy is dissipated and damping is generated. Using the displacements of the four cylinders all system pressures can be derived with respect to spring and damping behavior. All appendage models are combined in the remote valve block of this suspension system model. The model of the hydro-pneumatic system is validated and appears to be a very good approximation of the system in real life. Some special concerns are built in such as oil fluid division over the valves, overshoot behavior of the pressure relieve valves and the interaction of  the valve models.

Finally the full vehicle model is introduced and validated. The full vehicle model uses a multi-body model to relate the pressures within the suspension system with the load forces on the cylinders created by the vehicle mass and road profile. The multi-body model consists of four wheels, two independent suspended axles and a chassis with fixed loading space and cargo compartment.

The suspension in this multi-body model consists of a lower triangle, an upper triangle, a cylinder and a tierod for each wheel separately. As a steering mechanism a steering box at the front axle is modeled. The suspension arms in the model create the appropriate installation ratio between wheel displacement and cylinder displacement for any given condition caused by road conditions and weight transfer. The cylinders are connected to the chassis at the top end and to the lower triangle at the bottom end, generating cylinder displacement and velocity.

In the full vehicle model some additional mechanical forces are incorporated as well, such as friction force and weight transfer. Position dependent damping reduces the spiky spring behavior created by the bump stops when the full cylinder stroke is reached. Friction created over the cylinder and the rotation points of the suspension reduces the number of spring movements needed before the vehicle returns to steady state driving.

The inertia of the vehicle has a huge influence at the rear axle behavior but implies further tuning of the friction parameters as well to be sure the number of spring movements are kept equal. The height of the centre of gravity of the vehicle and a horizontally translational mass to represent the moving fuel inside the tank do not contribute to the total vertical vehicle driving dynamics much.

At the end of this project a full simulation model is available, validated and suitable for upcoming projects of V.S.E. After validation it appears that the original idea for splitting the damping behavior and spring behavior of the system is allowed to be modeled separately and be added to each other afterwards.
Source: TU/e
Author: J.A. Razenberg

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