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The undesired Drag Torque (DT) which is developed due to the shearing of fluid film in between the disk and separator plate reduces the efficiency of a transmission and increases the fuel consumption of a car. In order to minimize the transmission loss, the physics of the fluid flow mechanism inside the clutch should be understood well and the factors influencing the DT should be identified. In this paper, a model is proposed to predict the drag torque of a disengaged wet clutch at different rotation speeds, clearances, disk sizes and oil temperatures. The model explains well how the DT changes for the no groove disk, grooved disk and different ATF properties. The proposed model is validated by several experimental results conducted by a visualization tester and images of the fluid film taken during the test. Results show that there is a good degree of agreement between the DT trends derived from the proposed model and the test results for the same condition.

Reduction of drag torque is a crucial demand for improvement of transmission efficiency and fuel economy. In the low speed range, the drag torque at first increases with speed until it reaches a peak point, and then it starts decreasing sharply and finally stays at a minimum level until certain speed limit. Several analytical and simulation models have been presented by the researchers describing the drag torque characteristics at lower clutch speed. However, under certain conditions, the drag torque again starts to rise sharply in the high speed range (6000~10000+ rpm) and even exceeds the peak torque magnitude of low speed. The alarming jump of the drag torque at high rotational speed remains indeterminate to date. In this paper, we presented a simulation model that can predict the high speed torque jump up at different conditions. Simulation result shows that the static pressure decreases very sharply in the oil outlet region as the speed rises beyond 5000 rpm.