Temperature and Frequency Dependent Nonlinearities of an Integrated Hydro-Pneumatic System with Mixed Gas-Oil Emulsion Flow

Abstract

Hydro-pneumatic suspension (HPS) systems is increasingly being implemented on commercial vehicles, mainly attributing to the integration of adaptable nonlinear pneumatic-stiffness and hydraulic-damping properties. The integrated HPS design with shared gas-oil chamber, however, leads to gas-oil emulsion flow within the suspension chambers which intricately affects internal and external properties of the HPS, especially under variations in temperature and excitation frequency. This study experimentally and analytically investigated the temperature- and frequency-dependent properties of the hydro-pneumatic suspension with the gas-oil emulsion. The laboratory experiments are performed under three different near-constant temperatures (30, 40 and 50°C) in the 0.5–8 Hz frequency range. An analytical model of the HPS is formulated considering the effects of temperature on internal fluid properties, gas-oil emulsion flows between coupled chambers, dynamic seal friction, and polytropic change in the gas state. The internal parameters, including the gas volume fraction and discharge coefficient of the emulsion and the dynamic friction components, as well as the external stiffness and damping characteristics are identified. The relationships between these properties and the system temperature, velocity and excitation frequency are further investigated. The simulated responses obtained under different excitations showed reasonably good agreements with the experimental results of the HPS. The results suggested that increased temperature yielded greater equivalent stiffness and comparable damping properties of the system. The gas volume fraction, discharge coefficient and magnitude of seal friction generally tended to increase with increasing temperature. Increased excitation frequency leaded to greater hysteresis in hydraulic damping force and seal friction, and reduced seal friction magnitude and Stribeck effect.

Pressure Dependent Nonlinearities of a Compact Hydro-Pneumatic Strut with Integrated Gas-Oil Chamber

“…Moreover, the reported models have generally assumed constant flow discharge coefficients for modeling fluid flows through bleed orifices and damping valves. The laboratory measured force-displacement and force-velocity characteristics of a HPS involving flows of a gas oil emulsion, however, suggest that flow discharge coefficients may vary considerably with gas volume fraction [11,13]. The study also suggested the dependence of discharge coefficients on pressure difference and the excitation frequency.…”

“…The instantaneous volume of gas within the emulsion is obtained from the flow continuity relations for the piston-and rod-side chambers of the strut, given by: 𝐴 𝑐 𝑥 ˙+ 𝑉 ˙𝑔 + 𝑉 ˙𝑒𝑐 = 𝑄 𝑏 + 𝑄 𝑣 + 𝑄 𝑙 (13) (𝐴 𝑐 − 𝐴 𝑟 )𝑥 ˙− 𝑉 ˙𝑒𝑟 = 𝑄 𝑏 + 𝑄 𝑣 + 𝑄 𝑙 (14) where 𝑉 𝑔 is instantaneous volumes of the undissolved gas and 𝑉 𝑒𝑖 (𝑖 = 𝑐, 𝑟) is emulsion volume in the pistonand rod-side chambers. 𝑄 𝑏 and𝑄 𝑣 are the flow rates through the bleed orifices and check valves, respectively, and 𝑄 𝑙 is leakage flow rate.…”

“…Constant values of discharge coefficients are generally considered adequate for struts involving flow of incompressible hydraulic fluids [7,25]. In the case of two-phase emulsion flows, such coefficients have been expressed as functions of excitation frequency, temperature, and peak pressure difference [11,13]. Considering strong correlations of the pressure difference with the fluid temperature and excitation frequency, the discharge coefficients may be expressed as a function of the pressure difference alone.…”

“…Although the proposed design is significantly simple and cost-effective compared to many earlier HPS designs, it poses complex characterization and analysis challenges due to highly nonlinear properties of the gas-oil emulsion. The effective damping and stiffness characteristics of such a strut have shown highly nonlinear dependence on the gas volume fraction of the emulsion, the operating pressure, and temperature [13].…”

“…Apart from the friction, the reported analytical models of HPS have considered polytropic ideal gas process, incompressible hydraulic flows, and turbulent hydraulic flows through valves and damping orifices [11][12][13]27]. The ideal gas model assumes a constant compressibility factor, which has been reported to vary between 0.92 to 1.5 for the nitrogen gas within a HPS [17].…”