A model is presented which makes it possible to predict power losses in a 6-speed manual gearbox. The following sources of dissipation, i.e., power inputs in the model, are considered: i) tooth friction, ii) rolling element bearings, iii) oil shearing in the synchronizers and at the shaft-free pinion interfaces and iv) oil churning. Based upon the first principle of Thermodynamics for transient conditions, the entire gearbox is divided into lumped elements with a uniform temperature connected by thermal resistances which account for conduction, convection and radiation. The numerical predictions compare favorably with the efficiency measurements from the actual gearbox at different speeds and torques. The results also reveal that, at lower temperatures (about 40°C), power loss estimations cannot be disassociated from the accurate prediction of temperature distributions.

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