The feasibility of frame vibration control in automotive vehicles using active hydraulic engine mounts is investigated. The focus of the study is on engine idle conditions. A 38th dynamic model of a simplified vehicle system is used. The simplified system consists of a powertrain and a bare chassis supported by air pillow mounts at four corners. A modified LQG (linear-quadratic-Gaussian) control design method was used to synthesize a reduced order controller with improved robustness to harmonic disturbances. This modified LQG controller design is compared to a standard LQG controller.First, the conceptual implementation of active hydraulic mounts is proposed. The proposed mounts use a close-coupled servo-valve for high-speed fluid injection. The active mounts are capable of producing either attractive or repulsive forces between the powertrain and chassis.Predictions are made for levels of frame vibration reduction and actuator power consumption from simulation of closed-loop system responses. These results provide an estimate of hardware requirements for the implementation of an active mount system.Various issues in control design for flexible structures are addressed. Chief among these are the issues of model order reduction and controller order reduction. Other issues addressed are disturbance robustness and parameter robustness.