Abstract
An automatic power transmission has some pressure control valves for wet-clutch engaging or band-brake tightening control. Recent transmissions are being changed from hydromechanical control systems to electrohydraulic systems for improvement of shifting quality and control flexibility. A conventional hydromechanical system has a large accumulator for the smooth pressure control.; however, the size is sometimes a problem during installation on vehicles.
In this paper, a two-stage pressure control valve is applied to the control system, consisting of a PWM control switching valve and a secondary pressure control valve. The secondary valve is provided with a hydraulic position limiting mechanism for reducing the pressure surge. The electrohydraulic pressure control system is applied to advanced automatic transmissions. The system reduced the torque change at gear-stage shifting by half.
