Friction Reduction by Optimization of Local Oil Temperatures

The reduction of engine-out emissions and increase of the total efficiency is a fundamental approach to reduce the fuel consumption and thus emissions of vehicles driven by combustion engines. Conventional passenger cars are operated mainly in lower part loads for most of their lifetime. Under these conditions, oil temperatures are far below the maximum temperature allowed and dominate inside the journal bearings. Therefore, the objective of this research was to investigate possible potentials of friction reduction by optimizing the combustion engine’s thermal management of the oil circuit.

Within the engine investigations, it was shown that especially the friction of the main and connecting rod bearings could be reduced with an increase of the oil supply temperature. Furthermore, on a journal bearing test rig, it was shown that no excessive wear of the bearings is to be expected in case of load increase and simultaneous supply of cooler oil. In addition to the test investigations, MBS-(T)EHL simulation models were built up to investigate the behavior of the crankshaft bearings. Different driving profiles were simulated using GT-Suite to determine the achievable reduction of fuel consumption as well as the resulting reduction of CO₂ emissions. It could be shown that a local increase of the oil temperature in the main and connecting rod bearings lead to significant friction advantages in the range of part load. These simulations show a reduction of CO₂ emissions of at least 1.5 g/km by increasing the oil supply temperature from 85 to 140 °C.