Lubrication & efficiency of rear wheel drive axles in road vehicles

Abstract

The automotive rear axle is a part of the final drive for front engine, rear wheel drive, road vehicles. The axle is an important component in vehicle dynamics. The task of the axle is to transfer drive to the road surface as efficiently and with as low a mechanical loss as possible. Usually, the rear axle consists of a hypoid bevel geared transmission and a differential. A thermally coupled mathematical model of a hypoid axle is developed to calculate the total power loss in the rear axle. The drive cycle and gear and bearing details along with operating conditions are used as input data. The model also considers gradient, tyre pressure, aerodynamics and external temperature for a given drive cycle. The heat liberated due to mechanical losses at each time step and removed by convection is found, leading to the temperature of the bulk oil and components. Buckingham’s approximate analysis is used to define contact conditions for the hypoid gear pair. Elastohydrodynamic theory is applied to calculate film thickness and traction at the gear and bearing contacts. A tribometer (MTM) is used to obtain lubricant rheological parameters. Empirical formulae are used to find churning, seal and bearing losses. The efficiency of the axle is derived for different lubricants for the specified drive cycle. The prediction of the axle power loss is validated through comparisons with extensive experiments performed on the Ford F150 2010 model and a separate axle test rig, over a wide range of operating conditions. The comparisons between modelling results and test measurements demonstrate that the thermally coupled model is indeed capable of predicting the axle efficiency or temperatures reasonably well. The findings showed that the intended use of the vehicle greatly affected the temperature in the axle and hence determines the ranking order of lubricants. Lubricant rheology strongly influenced the overall efficiency of the axle. A lubricant boundary friction additive was only effective for the most severe drive cycle giving a significant reduction in the axle temperature. A simple test rig was built up to study churning losses of a partly immersed spur gear. This is similar to dip lubrication used in the axle. The influence of variation in air pressure within a cylindrical enclosure was investigated. This test was used to investigate the effect of the gear speed, air density and fluid properties, and used as a lubricant ranking method for churning losses. Lubricant base oils, water and aqueous glycerol solutions were tested using the inertia rundown method. The findings showed that the principal effects are those due to inertia and weight of the oil on the churning power loss. High viscosity lubricants impede gravitational reflow reducing overall losses. When air pressure varied, vacuum (0 bar) in the enclosure increased the power loss by up to 4.5 times, while Compressed air (2 bar) reduced the power loss by up to 2.2 times, compared to atmospheric pressure, for the more viscous oils. Glycerol aqueous solutions give similar trend curve for the losses comparable to oils but an effect of surface tension is predominant. Adding a surfactant to water led to a reduction in the power loss possibly resulting from the effect of the surface tension of the fluid.