1. Field of the Invention
The invention relates to a line pressure control apparatus, and a line pressure control method for an automatic transmission. More particularly, the invention relates to an apparatus and a method that controls a line pressure when a slip control for a lock-up clutch is being executed.
2. Description of the Related Art
In general, an automatic transmission includes a torque converter and a gear transmission mechanism. By selectively operating a plurality of frictional engagement elements such as clutches and brakes, a power transmission passage is switched in the gear transmission mechanism so that a predetermined gear ratio is automatically selected. In such an automatic transmission, a hydraulic circuit is provided to control the supply/discharge of the hydraulic pressure to/from actuators for the frictional engagement elements. More specifically, in the hydraulic circuit, a regulator valve is provided, and various other valves, such as a manual valve and a shift valve, are provided downstream of the regulator valve. The regulator valve regulates the hydraulic pressure discharged from an oil pump driven by an engine, to a predetermined line pressure.
In such an automatic transmission, excess hydraulic oil discharged from the regulator valve is supplied to the sliding portion of the gear transmission mechanism as lubricating oil. Japanese Patent Application Publication No. JP-A-5-118417 describes a control unit that avoids the lack of lubricating oil. The control unit receives a signal from a throttle-valve opening amount sensor that detects the opening amount of a throttle valve, a signal from an idle switch that determines whether the throttle valve is completely closed, and a signal from an oil-temperature sensor that detects the temperature of hydraulic oil. Thus, the control unit controls the line pressure using a duty solenoid valve provided in a hydraulic circuit. If a predetermine condition is satisfied when the temperature of hydraulic oil is high, the control unit decreases the line pressure.
FIG. 6 in Japanese Patent Application Publication No. JP-A-5-118417 shows that the line pressure for the automatic transmission is set to different values when the shift is being performed, and when the shift is not being performed. When the downshift is performed while the accelerator pedal is released, the rotational speed of the input shaft of the gear transmission mechanism needs to be increased by the hydraulic pressure for the frictional engagement elements. Therefore, the line pressure needs to be set to a high pressure to perform the shift. If the line pressure is set to a high pressure, fuel efficiency deteriorates. Accordingly, it is determined whether the shift is being performed. When the shift is not being performed, the line pressure is set to a low pressure, as compared to when the shift is being performed. This may improve fuel efficiency during idling.
In a known technology, when controlling the lock-up clutch that directly connects the input side to the output side of the torque converter, a feedback control (slip control) is executed to adjust the engagement force for the lock-up clutch to a predetermined value according to the difference between the rotational speed of a pump on an input-side (that corresponds to an engine speed) and the rotational speed of a turbine (that corresponds to the rotational speed of the input shaft of the automatic transmission). Thus, the slip state of the lock-up clutch of the torque converter is appropriately controlled to prevent vibrations and noise, and to improve fuel efficiency. That is, the slip control for the lock-up clutch is executed when fuel supply is cutoff to improve fuel efficiency during deceleration of a vehicle. When the shift is performed while the deceleration slip control is being executed, the line pressure is increased for the reason described above. The hydraulic circuit is configured to execute the slip control for the lock-up clutch using the line pressure as an original pressure. Therefore, when the line pressure is suddenly increased while the slip control is being executed, the lock-up clutch is engaged, and a shift shock occurs.
However, Japanese Patent Application Publication No. JP-A-5-118417 does not describe the line pressure used for executing the slip control for the lock-up clutch.