1993-03-01

Investigation of the Time- and Temperature Dependency of Hydro-Pneumatic Suspension Systems 930265

Hydro-pneumatic suspension systems have been used increasingly in recent times because of the well known advantages they offer. It is equally well known that the spring force is time- and temperature dependent, resulting in varying ride height and ride comfort of vehicles equipped with hydro-pneumatic suspension systems.In the present study, the temperature- and time dependent characteristics of hydro-pneumatic springs are modelled mathematically. This is done in order to investigate the effect of heat transfer on the spring characteristic so that it can be taken into account during the design of such systems.The mathematical analysis is based on the solving of the energy equation of a gas in a closed container by using the Benedict-Webb-Rubin equations for real gas behaviour. This differs from the traditional method which makes use of the ideal gas approach and polytropic processes to determine the spring characteristic.The mathematical model is verified against experimental data and good correlation is achieved between the predicted and measured spring characteristics. It is shown that although an isothermal process predicts the static spring force accurately, the same is not true for the dynamic spring force which can deviate considerably. The dynamic spring force can be described more accurately with the proposed analysis of using the real gas approach and solving the energy equation.The predicted and measured force - displacement diagrams clearly show a hysteresis loop which is dependant on the excitation frequency and amplitude. This indicates that energy is dissipated while the spring is compressed and expanded because of the irreversible heat transfer between the gas and its surroundings. A hydro-pneumatic spring thus has an amount of inherent damping which can dissipate up to 10% of the energy needed to compress the spring.This analysis can now be used to determine and compare the performance of different hydro-pneumatic suspension units at the design stage to find an optimum design, therefore avoiding costly experimental work.

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