Dynamic friction behavior of a gasoline engine in transient operation

To minimize friction in the piston group of an internal combustion engine, systematic experimental testing must be performed on a number of design variants. A vehicle simulation can be used to determine fuel and CO2 savings of each variant in a driving cycle. Despite the transient operating conditions of the driving cycles, the analysis often uses friction maps measured in steady-state operation. This article therefore investigates whether a dynamic friction behavior occurs in transient engine operation. For this purpose, specific friction measurements with defined changes in operating point are taken. It appears that after changes in engine speed or load, both increases and decreases of friction can occur temporarily in comparison with measurements in steady-state operating conditions. The sign and magnitude of these differences depend on the specific change in operating point and on the engine temperature. The observed dynamic friction effects are characterized qualitatively and explanations of the underlying mechanisms are derived. Because the dynamic friction behavior can lead to a momentary difference of 15 % compared to the stationary friction value, friction measurements in transient engine operation appear to be sensible for evaluating savings potential in a driving cycle. It can be experimentally demonstrated, however, that dynamic friction phenomena have only a relatively low influence on cumulative fuel consumption and CO2 emissions.