Study of passive and adaptive hydraulic engine mounts

The primary objective of this study is to comprehensively understand the nonlinear dynamic characteristics of the passive hydraulic engine mount by using analytical and experimental methods, and to develop a new broadband adaptive hydraulic mount system. A low-frequency nonlinear lumped-parameter mathematical model of the hydraulic mount is developed for the first time by measuring nonlinear system parameters such as chamber compliances and the steady-state inertia track fluid resistances in addition to the effective viscosity of glycol fluid, and by formulating the switching mechanism of the decoupler dynamics. When compared with the measured responses in both the time and the frequency domains, this new mathematical model predicts the deflection amplitude and excitation frequency dependent dynamic properties of the hydraulic mount reasonably well up to 20 Hz. However, beyond 20 Hz, some discrepancies are observed due to the unmodeled dynamic characteristics such as the upper chamber fluid inertia and the compliance effect of gas-liquid phase transformation. By analyzing a simple vehicle model which incorporates a nonlinear passive mount and carrying out laboratory mounting system with both harmonic (3-20 Hz) and impulsive excitations, the resonance control, vibration isolation and shock absorption properties of various mounts have been explained clearly. The low-frequency performance limitations of the regular passive mount have also been identified. The high-frequency dynamic characteristics of the passive mount related to the upper chamber fluid mass resonances are studied by measuring the dynamic stiffness spectra of simple-orifice mounts up to 250 Hz. The high-frequency performance problems of the passive mount associated with noise, vibration and harshness criteria are identified by introducing a quasi-linear analysis method. A new adaptive hydraulic mount system with broad bandwidth performance features is developed by implementing the on-off damping control with the engine intake-manifold vacuum system and a microprocessor based solenoid valve controller. The proposed adaptive mount functions as an inertia track mount for the purpose of resonance control and shock absorption, and as a low-damping rubber mount for vibration and acoustic isolation. Although technical prospects of this adaptive system appear promising, its real performance should be evaluated through actual vehicle testing.