Temperature rise and suspension dynamic characteristics of semi-active hydro-pneumatic suspension based on thermodynamics and heat transfer

The temperature of the oil has a decisive impact on the vehicle's dynamics as it significantly affects the viscosity, influencing the damping force of the hydro-pneumatic suspension (HPS), and subsequently affecting the dynamics of the suspension. Therefore, this paper proposes a more accurate semi-active HPS model to investigate the HPS temperature rise characteristics through thermodynamics and heat transfer. According to the flow equation of small holes, flow models for normally open holes, check valves and solenoid valves were established by combining the viscosity-temperature equation of the oil and the conservation law of mass, HPS thermodynamic model reflecting the viscosity-temperature characteristics was deduced. The real gas equation of state, the Redlich-Kwong (R-K) equation, was used to model the elastic forces of nitrogen. Besides, based on the first law of thermodynamics, combined with the differential equation of internal energy, Newton's cooling equation, and Fourier's law, models of semi-active HPS heat transfer and heat transfer coefficients were constructed, with nitrogen, oil, and cylinder being respectively taken as the objects of study. Combining the suspension thermodynamic model and heat transfer model, MATLAB was utilized to solve the variation of suspension nitrogen and oil temperatures over time under sinusoidal excitation at different frequencies, amplitudes, and currents. The results show that the temperatures of the oil and nitrogen eventually stabilize, reaching thermal equilibrium, with the oil temperature consistently higher than that of the nitrogen. Furthermore, the results also demonstrate that the increased excitation frequency and amplitude, as well as decreased current significantly raise the thermal equilibrium temperatures of oil and gas. The elastic and damping characteristic curves of the suspension at different temperatures demonstrate that as the temperature increases, the damping force provided by the suspension decreases, and elastic forces increase.