Fuel efficiency improvement measures are focusing on both cold and hot conditions to help reduce CO2 emissions. Recent technological trends for improving fuel economy such as hybrid vehicles (HVs), engine start and stop systems, and variable valve systems feature expanded use of low-temperature engine operation regions. Under cold conditions (oil temperature: approximately 30°C), fuel consumption is roughly 20% greater than under hot conditions (80°C). The main cause of the increased friction under cold conditions is increased oil viscosity.
This research used the motoring slipping method to measure the effect of an improved crankshaft bearing, which accounts for a high proportion of friction under cold conditions. First, the effect of clearance was investigated. Although increasing the clearance helped to decrease friction due to the oil wedge effect, greater oil leakage reduced the oil film temperature increase generated by the friction. Consequently, the friction reduction effect was less than that predicted by the lubrication calculation.
Next, the rate of friction used as heat energy was analyzed based on the oil film temperature and the leakage amount. In the test engine, approximately 20% of the total friction was used to increase the journal oil film temperature. Since the analysis found that raising the oil film temperature by reducing leakage may help to decrease friction under cold conditions, bearings with a sealing function were evaluated for verification. This sealing method can potentially reduce overall friction by 5%.
