Modeling of NO and CO Raw Emissions Based on Mixture Inhomogeneities in SI Engines

Stricter pollutant emission regulations and CO₂ reduction requirements for internal combustion engines make further development of gasoline engine combustion processes essential. Homogeneous lean-burn processes, also in combination with high powertrain hybridization, offer great potential for increasing efficiency. However, nitric oxide (NO) emissions increase due to excess air and the use of a conventional three-way catalytic converter is not effective because the excess air prevents the complete reduction of the produced NO. In stoichiometric operation, nitric oxide and carbon monoxide (CO) emissions are also expected to increase in importance with higher degrees of hybridization since engine starts with potentially cooled down exhaust gas after-treatment systems are expected to occur more frequently. Therefore, reliable prediction of NO and CO raw emissions is of crucial importance in engine design and calibration. The formation of NO and CO depends significantly on the air-fuel ratio and the temperatures in the burnt mixture. Accordingly, existing mixture inhomogeneities have a major influence on NO and CO formation, with research showing different behavior in stoichiometric and homogeneous-lean operation. Currently used 0D/1D emission models for NO and CO show large deviations from the measurement, especially in lean operation. Within an FVV project, a quasi-dimensional model based on a distribution function is developed to reproduce the influence of mixture inhomogeneities on emission formation.