Fuel consumption potential of gasoline engines in an electrified powertrain: homogeneous and heterogeneous lean combustion in comparison

This paper evaluates the fuel consumption of homogeneous and heterogeneous lean combustion in the WLTC test cycle. A lean combustion engine is combined with an electrified powertrain and the combustion processes are compared with each other. There is also a distinction with regard to the degree of electrification. First, investigations are carried out with an “engine in the loop” test bench. It turns out that, as expected, the best fuel consumption results can be achieved with heterogeneous lean combustion in combination with homogeneous lean combustion. In addition, it is shown that, in combination with P1 hybridization, low-load heterogeneous lean combustion becomes less important, but continues to contribute to an improvement in fuel consumption. Additionally, P1 hybridization increases the percentage of homogeneous lean combustion by 13%. Thus, the cycle fuel consumption is improved through electrification disproportionately for homogeneous lean combustion by 7.5%, for stoichiometric combustion by 6%. Furthermore, electrification contributes to reducing nitrogen oxide emissions by about 50% in the test cycle to 9 mg/km. The reduction can be achieved by shifting the load points from high loads with higher NOx raw emissions to lower loads with lower NOx raw emissions and by omitting heterogeneous lean combustion. In the second step, the combustion processes for two different engine displacements are compared in calculations. This allows further investigations. It turns out that, with increasing degree of electrification and decreasing engine displacement, heterogeneous lean combustion can no longer contribute to an improvement in fuel consumption and rather an expansion of homogeneous lean combustion at high loads becomes necessary. In general, thanks to the electrification of the powertrain in combination with lean combustion, the cycle fuel consumption can be greatly reduced by up to 33% to 3.76 l/100 km. Electrification does not compete with the advantages of lean combustion, but complements them. The presented results show the potential for improvement in fuel consumption for future developments in gasoline engines in hybridized powertrains.