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This paper deals with an energy-saving electro-hydraulic power steering (EHPS) system using a center-closed servo valve without an accumulator. During non-steering operation, the pump and motor in this EHPS system operate at an extremely low speed with a low voltage. At neutral position in non-steering condition, the pump delivery flow leaks through the clearances in the hydraulic components, raising the pump pressure because the center-closed servo valve does not pass oil-flow. With a lower delivery flow rate, the pump pressure rises slightly and the motor current is also low, resulting in low electric energy consumption. When the steering wheel is turned, the pump and motor operate at a high speed and supply as much oil as needed to produce sufficient assist force. This EHPS system was tested on a conventional engine-driven vehicle with a front weight of 720kg and its effectiveness was revealed.

This paper deals with an advanced electro-hydraulic power steering (EHPS) system compactly integrating a hydraulic pump with an electric motor, a steering gearbox with a center-closed servo valve and an accumulator. In this EHPS system, the hydraulic pump with an electronically controlled motor works only during steering operation. Stopping the hydraulic pump during non-steering operation is extremely effective for reducing energy consumption in power steering because steering operations are infrequent. The EHPS system described in this paper is suitable for pollution free vehicles such as electric vehicle having no engine-driven accessories and has the advantages of reduced power consumption and ease of installation. This EHPS system may be also used for engine-driven vehicles and has been tested on a conventional gasoline engine vehicle with an 810 kg front weight. Test results demonstrate its effectiveness.

This paper deals with energy-savings in a hydraulic power steering systems by the reduction of the driving torque of a hydraulic pump in non-steering operation. In normal driving, steering operation is infrequent although the pump is continuously driven by the engine. Therefore, for energy-savings in power steering, it is effective to reduce the pump driving torque during non-steering operation. A reduction of the pump driving torque in non-steering operation is possible by decreasing the pump outlet flow rate regulated by a flow control valve. The authors have investigated a prototype pump with an additional device to minimize the regulated flow rate under a non-steering condition and to restore the flow rate during steering operation. The mechanism is a kind of pressure reducing valve that changes the operation of the flow control valve in response to the pump outlet port pressure which is an indication of steering operation.