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In View of the performance of high output torque moment, good response and continuously variable transmission of hydrostatic transmission, a new concept of Hydraulic Energy Storage Transmission System (HESTS) of city bus was put forward, although the hydraulic-mechanical split power hybrid city bus had run effectively in some cities, such as Copenhagen, Stockholm and Berlin, etc.[1][2]. HESTS was comprised of a variable displacement pump, a variable displacement pump/motor unit, two accumulators and an Electrical Control Unit (ECU). The high pressure accumulator could be controlled to absorb braking energy in hydraulic energy and reuse the stored energy to restart the prime mover or accelerate the vehicle. HESTS could operate in seven modes while the ECU outputs different combined control signals to pump, pump/motor and directional valves. The mathematical model of a HESTS for a city bus was established.

A Hydrostatic Energy Storage Transmission System(HESTS) designed for vehicle is presented in this paper*. A bench-scale HESTS was built in the Fluid Power and Control Laboratory of Harbin Institute of Technology. The laboratory testing HESTS was powered by a DC. motor which was controlled by a Silicon-Controlled Rectifier(SCR). Loads adding to the HESTS were regulated by a proportional valve control system. The HESTS is more complex because there are not only continuous variables-the shaft speed of the hydraulic axial piston variable displacement pump/motor unit and the position of the displacement-varying cylinder, but also discrete variables-the on/off states of clutch, 2/2 solenoid valve and 2/4 solenoid valve in the HESTS to be controlled. So a fuzzy logic controller with a mode selector is designed to perform the role of control to the HESTS. A personal computer stored control rules executes the tasks of data acquisition, fuzzy logic inference and control.

A low cost, anti-pollution pump/motor displacement-varying mechanism was studied in this paper. In the hydrostatic energy storage transmission system(HESTS), a variable pump (motor) unit was used to transfer the energy. A new pump/motor displacement-varying mechanism was obtained based on low price manual manipulating variable pump with high-speed on-off solenoid valves supporting pressure oil. Simulation results showed that the position precision and the ability of following sinusoidal wave signal of the new mechanism met the requirement of the pump/motor unit. Experiment results proved that the new pump/motor displacement-varying mechanism operated precisely and reliably.

A novel hydraulic transmission system-Hydraulic Energystorage Transmission System(HESTS) which contains a Hydraulic Continuously Variable Transmission(HCVT) and a flow controllable accumulator is proposed. The power flows of the HESTS has been analyzed. Computer simulations show the potentialities of improving fuel economy of a vehicle with a HESTS. A fuzzy logic controller with two regulating factors is developed to improve the performance of the HESTS with variable loads.

Recent advances in hydrostatic transmission efficiency and accumulator technology make the hydropneumatic energy storage automobile appear quite attractive as a means of improving fuel economy. The system examined in this paper utilizes a conventional internal combustion engine, and two hydrostatic pump/motor units with an accumulator between them. The accumulator allows regenerative braking and permits the engine operation to be uncoupled from the road load. Detailed, second-by-second driving cycle simulations have been used to study the fuel economy possible with various combinations of component parameters. The design can provide excellent fuel economy with a moderate size accumulator.