A required driving force for propelling a vehicle is varied during running. Basically, an energy efficiency of an engine of the vehicle would be degraded if operated at an operating point deviated away from an optimum operating point. In the conventional vehicles, therefore, a transmission is disposed on an output side of the engine to improve the energy efficiency by adjusting the driving force in accordance with a speed ratio of the transmission. To this end, for example, a geared transmission adapted to change a gear stage stepwise, and a continuously variable transmission adapted to vary a speed ratio continuously have been used in the conventional vehicles. In the vehicle having the transmission, a torque converter is disposed between the engine and the transmission to allow the engine to rotate even when the vehicle stops.
As widely known in the art, the torque converter is a fluid coupling adapted to transmit torque by rotating a turbine runner by a spiral flow created by a pump impeller. Given that a speed difference between the pump impeller and the turbine runner is large, that is, a speed ratio is small, oil flow returning from the turbine runner to the pump impeller is altered by a stator interposed therebetween. Consequently, torque transmitted through the torque converter is multiplied to establish a creep torque when the speed ratio is small. That is, a torque delivered to the transmission disposed on the output side of the torque converter is altered in accordance with the speed ratio of the torque converter. Therefore, hydraulic pressures governing torque transmitting capacities of clutches and brakes of the transmission are controlled in accordance with an operating state of the torque converter.
For example, Japanese Patent Laid-Open No. 11-325232 describes a hydraulic control device and method of a transmission having the torque converter and clutch devices. According to the teachings of Japanese Patent Laid-Open No. 11-325232, an engagement state of at least one of the clutches of the automatic transmission is hydraulically controlled to carry out a speed change operation. To this end, specifically, an output torque of the engine is calculated based on a parameter representing an engine load and an engine speed, and a required transmission torque of the clutch is calculated based on the calculated engine torque and a parameter representing the engine speed. Then, a command value of the hydraulic pressure applied to the clutch is determined based on the calculated transmission torque.
The automatic transmission taught by Japanese Patent Laid-Open No. 11-325232 is provided with the torque converter connected to an output shaft of the engine. According to the teachings of Japanese Patent Laid-Open No. 11-325232, a torque ratio of the torque converter is calculated based on the engine speed and an output shaft speed of the torque converter (i.e., a turbine speed), and an output shaft torque of the torque converter (i.e., a turbine torque) is calculated based on the calculated torque ratio and an output torque of the engine. The required transmission torque of the clutch may also be calculated based on the calculated turbine torque and the turbine speed.
In recent years, in order to improve fuel efficiency and to reduce emission, a stop and start control (abbreviated as the “S & S” control hereinafter) and a coasting control have been developed. For example, under the S & S control, the engine is automatically stopped when vehicle speed is zero or when an accelerator pedal is released, and the engine is automatically restarted when the accelerator pedal is depressed or a brake pedal is released.
However, during execution of the S & S control, an output torque of the engine would be fluctuated when stopping or starting the engine automatically. In order to prevent propagation of such fluctuation in the engine torque to a powertrain of the vehicle, a clutch device disposed between the engine and the powertrain is brought into disengagement. That is, an actuation control of the clutch disposed between the engine and the powertrain is carried out in conjunction with the S & S control.
One example of such clutch control to be carried out in conjunction with the S & S control is described in Japanese Patent Laid-Open No. 2010-151226. According to the teachings of Japanese Patent Laid-Open No. 2010-151226, fluid is delivered promptly to the clutch when restarting the engine by an idle stop function so that the clutch is allowed to be brought into engagement promptly. To this end, specifically, the vehicle drive unit taught by Japanese Patent Laid-Open No. 2010-151226 is comprised of an oil pump driven by a prime mover to generate hydraulic pressure, a clutch that is hydraulically controlled to transmit torque from prime mover to an output shaft, a hydraulic pressure controller that controls the hydraulic pressure generated by the oil pump at a predetermined pressure to engage the clutch, an accumulator that accumulates the hydraulic pressure generated by the oil pump, and an open/close valve that switches between a blocked state and a communicated state of an oil passage connecting the accumulator and the clutch. The hydraulic pressure accumulated in the accumulator is supplied to the clutch when or before the oil pump is activated.
The vehicle drive unit taught by Japanese Patent Laid-Open No. 2010-151226 is further provided with a clutch pressure control valve that controls hydraulic pressure for operating the clutch, a clutch control valve that is provided in a first oil passage between the clutch pressure control valve and the clutch, and that controls hydraulic pressure in an engagement state between full engagement and disengagement of the clutch, a second oil passage that bypasses the clutch control valve and that is arranged parallel to the first oil passage, and a shift valve that connects, to an oil passage connected to the clutch, the first oil passage when controlling the engagement state between full engagement and disengagement of the clutch, and the second oil passage when not controlling the engagement state between full engagement and disengagement of the clutch. The accumulator is connected to an oil passage connecting the shift valve and the clutch through the open/close valve, and the second oil passage is provided with a one-way valve that allows oil to flow only in the direction from the oil pump to the shift valve.
According to the teachings of Japanese Patent Laid-Open No. 2010-151226, therefore, the hydraulic pressure stored in the accumulator is delivered to the clutch when activating the oil pump or before activating the oil pump. In this situation, since the engagement state between full engagement and disengagement of the clutch is not controlled, the oil passage connected to the clutch is communicated with the second oil passage by the shift valve, and the oil is prone to flow from the accumulator to the oil pump side through the second oil passage. However, since the second oil passage is provided with the one-way valve, the oil does not flow from the accumulator to the oil pump side through the second oil passage. For this reason, the hydraulic pressure from the accumulator is supplied only to the clutch, whereby the hydraulic pressure can be efficiently supplied from the accumulator to the clutch in a short period of time.
However, the clutch control may not be carried out properly in the vehicle using the automatic transmission having the torque converter as taught by Japanese Patent Laid-Open No. 11-325232 during execution of the above-mentioned S & S control or the coasting control for automatically stopping and starting the engine. Specifically, if the engine is stopped or restarted while keeping the clutch in engagement during execution of the S & S control, an abrupt change in an input torque to the powertrain from the transmission to drive wheels (i.e., a turbine torque) may result in vibrations and shocks. Especially, the input torque to the powertrain would be changed significantly when starting the engine thereby amplifying vibrations and shocks. In order to suppress such change in the input torque to the powertrain, according to the conventional art, the clutch is brought into engagement while slipping when restarting the engine.
For this purpose, according to the conventional art as taught by Japanese Patent Laid-Open No. 11-325232, a turbine speed that is easy to be detected is employed as a target control amount of the clutch during the slip engagement of the clutch. Specifically, while the clutch is causing a slip, a speed of an engagement element of the torque converter side is different from a speed of an engagement element of the transmission side. That is, the turbine speed may be different from the input speed of the transmission. Specifically, when the engine is started and an output torque thereof is wasted by the torque converter, such speed difference can be expressed as the following inequality: “engine speed>turbine speed>input speed”.
If the vehicle is stopping or running at an extremely low speed and hence the input speed is low during causing a slip of the clutch based on a target control amount of the turbine speed, a difference between an actual turbine speed and a target turbine speed would not be significantly wide. However, if the S & S control or the coasting control is executed during running, the input speed would exceed the target turbine speed as expressed by the following inequality: “target turbine speed<input speed”. In this case, the clutch would be brought into engagement before the turbine speed is increased sufficiently. If the clutch is brought into engagement before the turbine speed is increased sufficiently to be approximated to the input speed, the input torque would be changed abruptly to cause shocks to provide a driver with an uncomfortable feeling.