Hydraulic control system for an automatic transmission

A hydraulic control method comprising:

• determining whether or not driving conditions detected in a non-variable line pressure control mode satisfy variable line pressure control entrance conditions;
• determining whether or not gears are in an in-gear state by calculating a gear ratio using an engine rpm and turbine rpm when the driving conditions satisfy the variable line pressure control entrance conditions;
• entering a variable line pressure control mode in an in-gear state, and performing the variable line pressure control by calculating a minimum line pressure and variable line pressure gradient wherein the variable line pressure control is calculated based at least in part on an engine brake torque and a torque converter torque ratio, wherein the engine brake torque is determined on a basis of engine torque loss, and wherein a minimum line pressure duty is determined based on the calculated required line pressure and a preset minimum line pressure;
• determining whether or not a present line pressure is less than the calculated minimum line pressure;
• performing a normal line pressure control when the present line pressure is less than the minimum line pressure and then determining whether a gear shift starts or a damper clutch is in a direct coupled state;
• performing a gear shift to a target gear ratio after a predetermined time from a point when a line pressure control duty reaches 100% if it is determined that the gear shift starts or the damper clutch is in the direct coupled state in normal line pressure control;
• and
• performing variable line pressure control according to the driving conditions by entering the variable line pressure control mode for determining another shift begin point after delaying for a predetermined time if the gear shift to the target gear ratio is completed.