What is EHPS (Electric Hydraulic PowerSteering)?


Generally, a power steering system uses the power of the engine to drive the vane pump that generates hydraulic pressure. EHPS is a power steering system that uses a motor to generate the hydraulic pressure and reduces the power required to operate the steering wheel. Since this system reduces the load on the engine, it improves fuel economy. The motor rotation rate (pump discharge volume) is controlled by the ECU according to such data as the vehicle speed and steering wheel turning angle.


(1) Pump body

The pump is driven by the engine crank pulley and drive belt, and sends fluid, under pressure, to the gear housing. The discharge volume of the pump is in proportion to the engine speed, but the amount of fluid sent to the gear housing is regulated by a flow control valve, with excess fluid being returned to the suction side.

(2) Reservoir tank

The reservoir tank supplies the power steering fluid. It is installed either directly to the pump body or separately. If not installed to the pump body, it is connected to it by two hoses. Normally, the reservoir tank cap has a level gauge for checking the fluid level. If the fluid in the reservoir falls below the standard level, the pump will suck in air, causing faulty operation.

(3) Flow control valve

The flow control valve controls the flow volume of the fluid from the pump to the gear housing, maintaining a constant flow regardless of the pump speed (rpm).

(4) Idle-up device

The pump produces maximum fluid pressure when the steering wheel is turned fully to the right or left. At this time, there is maximum load on the pump, which causes a decrease in engine idle speed. To solve this problem, almost all vehicles are equipped with an idle-up device which acts to raise the engine idle speed whenever there is a heavy load on the pump. The idle-up device functions to raise engine idle speed when pump fluid pressure acts on the air control valve (installed on the pump body) to control the flow of air. On EFl engines, when the piston of the air control valve is pushed by fluid pressure, the air control valve opens and the volume of air bypassing the throttle valve is increased to regulate engine speed.

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