Effect of lubricant temperature and type on spur gear efficiency in racing engine gearbox across full engine load and speed range

Abstract

Energy losses are of great significance to the automotive and motorsports industries. Many of these losses are incurred during power transmission through the gearbox. There has been considerable research in this area; however, generally gear losses are not calculated at part load condition, nor are so-called dry sump systems considered outside of motor racing. The method developed by Anderson & Loewenthal, which considers efficiency over part-load conditions is used here to calculate geartrain losses with varying speed, load and temperature conditions in a spur gear type gearbox for motorsport application. Both oil bath (wet sump) and oil jet (dry sump) systems of lubrications are considered. The Changenet method is used to calculate the churning losses in the typical oil bath lubrication system. Seventeen different mineral and synthetic oils were evaluated. At 75 kW engine output, 1200 W were lost in the dry sump gearbox whereas 1320 W were lost in the wet-sump gearbox – in first gear at 10,500 r/min engine speed. The oils studied showed a spread of total drive efficiency of 97.8% to 99% in the most extreme temperature case. From the observation of how efficiency and film thickness relate to operating temperatures, it is clear that tight temperature control is critical to obtain the potential benefits available from oil optimisation. The dry sump gearbox is predicted to be more efficient only above 5000 r/min engine speed.