US Patent 7225907 Hydraulic pressure control system

A hydraulic pressure control system for an automatic transmission is comprised of an engagement element engaged when the vehicle runs and a first engagement control actuator controlling the engagement element by supplying the controlled hydraulic pressure to the engagement element when the vehicle starts running. An effective cross sectional area of the engagement element is set at a minimum area which has an engagement capacity capable of transmitting a maximum torque inputted from the engine when the engagement element receives a maximum pressure controlled by the first engagement control actuator.