Herein disclosed is a hydrostatic power transmission system of the split power path type having a constant fluid displacement unit as a pump unit driven by a driving member such as a transmission input shaft of a motor vehicle and a variable fluid displacement unit as a hydraulic motor unit driven by the pump unit by means of fluid power and connected to a driven member such as a transmission output shaft of the motor vehicle. The two fluid displacement units are interconnected by a fluid circuit, which in the prior art transmission systems is opened to drain off the working fluid when the motor vehicle stops and closed to restore the fluid pressure in the fluid circuit when the motor vehicle is to start. A fluid pressure control mechanism is thus connected to the fluid displacement units so that the fluid pressure in the fluid circuit is automatically reduced to zero level when the vehicle is brought to a full stop and gradually increased from the zero level as the motor vehicle is started and driven at a low speed whereby the shocks otherwise resulting from the abrupt variation in the driving torque transmitted through the transmission system can be avoided. The transmission system is also provided with an arrangement adapted to interrupt the fluid path leading from the constant to the variable fluid displacement units during direct-drive condition in which fluid pulsation otherwise takes place in the latter unit to produce noises.
A hydraulic continuously variable speed transmission comprises a hydraulic closed circuit for hydraulically connecting the hydraulic pump and the hydraulic motor, a direct clutch valve placed in the closed circuit to block it, a speed reduction ratio control actuator to control the speed reduction ratio, a direct clutch actuator for moving said direct clutch valve from a full-open position to a full-closed position, ratio detecting means which detects the speed reduction ratio and valve position detecting means which detects position of the direct clutch valve to find if the direct clutch valve is moved to a slight-open position. The direct clutch valve is moved quickly from the full-open position to the slight-open position when it is found by the ratio detecting means that the speed reduction ratio has become "1", and then the direct clutch valve is moved slowly from the slight-open position to the full-closed position when it is found that the direct clutch valve has been moved to the slight-open position.
A method of controlling clutch operation, for a vehicle having a transmission and clutch means comprises setting a reference capacity of said clutch means corresponding to a parameter representing vehicle speed and an accelerator opening representing the driver's intention of acceleration or deceleration: and controlling said degree of the engine power transmitted to the wheels by adjusting the capacity of said clutch means so as to coincide with said reference capacity. An embodiment of the method includes a method of determining clutch engagement completion in continuously variable speed transmission comprises setting a lower limit of an expected clutch engagement completion vehicle speed corresponding to an accelerator opening representing operator's intention of acceleration and deceleration, determining, in vehicle start, that said clutch valve is closed to engage the clutch completely when it is detected that vehicle speed has been below the lower limit and the opening of said clutch valve has been below a given opening, and permitting successive speed ratio control.
A continuously variable speed transmission in which a bypass line is formed between two hydraulic lines disposed between a hydraulic pump and a hydraulic motor to constitute a hydraulic closed circuit, and a opening control valve is disposed to open or close the bypass line. The opening control valve is opened when the reduction rate of the engine speed or the vehicle speed exceeds a given in order to short-cut the two lines and create a neutral condition.
