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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.

A significant reduction of C02 emission can be achieved by improving the efficiency of transmission of cars. A reduction of drag torque or spin loss of disengaged clutches can improve the efficiency of transmissions. Generally, the drag torque is measured by conducting drag test which needs making samples, manpower, power and wastage of raw materials. In this paper, an analytical model is proposed to predict the drag torque of a disengaged wet clutch at different rotation speeds, clearances, disk sizes and oil temperatures without making any samples and conducting any drag tests. Various assumptions are made from the results of visualization investigations. Visualization results show that the volume of oil existed in between the clutch disk and clutch plate decreases with increasing speed due to centrifugal force. It is also noticed that several visible air bubbles are formed in the oil film at lower speeds and the size of the bubbles increase with increasing speeds.