Development of a Thermal Model for a Twin-Tube Shock Absorber: Experimental and Modeling Approach

Abstract

The automotive suspensions play a crucial role in vehicle safety and driving comfort. Their function is to damp the vibrations produced in the spring due to external excitation. As the shock absorber is compressed and extended, hydraulic fluid starts to flow between the upper chamber and lower chamber through the piston orifice. Piston orifice restricts the flow of hydraulic fluid through it, thereby providing the required damping action. Therefore, the geometrical configuration of piston orifice decides the amount of damping. Further there is a rise in the temperature of hydraulic fluid. This heat needs to be dissipated to the surrounding; otherwise the performance of shock absorber gets reduced. Keeping in view the role of heat generation and dissipation, the aim of this dissertation is to develop a thermal model for the twin-tube hydraulic shock absorber using bond graph method. In this thesis, experiment is conducted on a rear shock absorber of Toyota Innova. Temperature at the upper and lower part of the shock absorber is measured with the k-type thermocouples. Bond graph model for the twin-tube hydraulic shock absorber is developed. Simulation results are compared with the experimental results to validate the model. Both the experimental and simulation results show that the volume flow rate of hydraulic fluid is more in the upper chamber as compared to that in the lower chamber. Temperature of hydraulic fluid in the upper and lower chamber is calculated by taking enthalpy as flow variable in the bond graph. Temperature rises linearly with respect to time. There is a saturation of temperature after some time. At this point, the surface temperature and hydraulic fluid temperature become equal and shock acts as a single entity transferring heat to the surroundings. This point marks the point of maximum heat transfer to the surroundings. For any shock absorber, it is always desirable to achieve the saturation point as early as possible. The bond graph model developed in this thesis work may be used by the manufacturers to know the hydraulic and thermal performance of their product. Further, it will help the manufacturers in deciding the size and number of piston orifices which is deciding factor for the comfort level of the vehicle.