1. Field of the Invention
The present invention relates to a hydraulic control system for automatic transmissions, and more particularly, to a hydraulic control system for automatic transmissions which realizes direct shifting by operating each friction element of a 5-speed automatic transmission, in which one-way clutches are applied to each of the friction elements, using hydraulic pressure duty controlled in actuators.
2. Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction members actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated in an oil pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the oil pump to each valve and actuator.
In such a hydraulic control system, at least one of the friction elements is selectively engaged by the operation of the manual valve, actuators and valves to realize a plurality of shift modes. However, when shifting from a drive D range to a neutral N range by the operation of each friction element using independent duty control pressures of the actuators, since the friction elements are independently operated from a disengaged state to an engaged state by hydraulic pressure supplied from line pressure lines, shift shock is generated.