The torque converter transmits power through a circulating fluid flow, using hydraulic fluid (ATF) as its working medium, which circulates within a closed working chamber. The engine drives the pump impeller, converting mechanical energy into the kinetic energy of the fluid. High-speed fluid impacts the turbine blades, driving the turbine to rotate and transferring energy to the output shaft. At startup, the large speed difference between the pump impeller and turbine causes the stator to change the fluid flow direction, generating a torque-boosting effect (maximum torque increase of 1.8 to 2.5 times). At this time, the stator is locked to the housing via an internal one-way clutch, thus changing the direction of the fluid flowing out of the turbine and generating a reverse force on the turbine, achieving torque amplification.
As the turbine speed increases, when the speed ratio (turbine to pump impeller) reaches approximately 0.8 to 0.85, the torque-boosting effect disappears. If the turbine speed continues to increase, the fluid flow may impact the back of the stator blades, causing the turbine output torque to be less than the pump impeller input torque, i.e., a torque reduction phenomenon occurs. At high speeds, the lock-up clutch engages, switching to mechanical transmission. The purpose of locking is to eliminate slippage and energy loss caused by hydraulic transmission, rigidly connecting the pump impeller and turbine to achieve a 1:1 direct power transmission, thereby improving transmission efficiency. Locking typically occurs when the turbine speed and pump impeller speed are nearly synchronized (e.g., a speed ratio of approximately 0.8 or higher). The electronic control unit regulates the lock-up state based on data such as engine speed and vehicle speed.
