Prediction of power losses in an automotive gearbox incorporating a thermally coupled lubrication model

Abstract

The continuous tightening of environmental standards, and in particular the stricter vehicle CO2 emission regulations, have put increased pressure on passenger vehicle manufacturers to develop more efficient components. One way of improving fuel consumption and reducing emissions in a vehicle is by reducing the friction-related power loss in major components such as the engine and the transmission. Between 5% and 6% of the total fuel energy that the car converts is lost through friction in the transmission and ends up as heat being absorbed by the transmission lubricant. The principal aim of this project was developing a method to accurately predict power losses in the gearbox -the main transmission component- while at the same time having the ability to assess the influence of lubricant properties on the gearbox efficiency. For this purpose, a thermally coupled numerical model was developed, incorporating lubricant parameters extracted from rheological tests. The model calculates the friction coefficient in gear elastohydrodynamic (EHL) contacts and then uses it in an iterative scheme to predict in-contact and bulk temperatures as well as power losses. Finally, the model’s predictions for gears are coupled to current experimentally derived models for bearing and churning losses. The method has been applied to a single speed gearbox and a manual six speed automotive gearbox. Two fully formulated lubricants of nominally same specification were tested and comparisons were made for different input conditions and gear geometry changes. In the second case, a light duty truck fitted with the simulated gearbox has been instrumented and tested under different conditions. Temperature predictions from experimental drive cycles have been compared to model predictions and standardised drive cycles have also been simulated. Results show that the overall gear power loss is heavily dependent on the input torque and that the lambda ratio of the contact affects each component differently. In addition, the distribution of losses between gear friction, bearings and gear churning is not constant but depends on the selected components, the gear geometry, the surface roughness and the specific lubricant rheology. Furthermore, significant power loss differences of up to 11.7% and sump temperature differences of up to 3.1 °C respectively have been predicted between the two oils. Simulations of the six speed gearbox have shown that the model is able to predict bulk oil sump temperatures within 5-10% of measured values and can effectively be used to compare and rank different lubricants in terms of overall efficiency for a given gearbox.