A dual fuel engine concept with lean premixed methane–air charge ignited by a diesel pilot flame is highly promising for reducing NOx and soot emissions. One drawback of this combustion method, however, is the high nitric dioxide (NO2) emissions observed at certain operating points. The conditions leading to increased NO2 formation have been investigated using a batch reactor model in cantera. It has been found that the high emission levels of NO2 can be traced back to the mixing of small amounts of quenched CH4 with NO from the hot combustion zones followed by postoxidation in the presence of O2, requiring that the temperatures are within a certain range. NO2 formation in the exhaust duct of a test engine has been modeled and compared to the experimental results. The well-stirred reactor model has been used that calculates the steady-state of a uniform composition for a certain residence time. An appropriate reaction mechanism that considers the effect of NO/NO2 on methane oxidation at low temperature levels has been used. The numerical results of NO–NO2 conversion in the duct at low temperature levels show good agreement with the experimental results. The partial oxidation of CH4 can be predicted well. It can be shown that methane oxidation in the presence of NO/NO2 at low temperature levels is enhanced via the reaction steps CH3+NO2CH3O+NO and CH3O2+NOCH3O+NO2. In addition, the elementary reaction HO2+NONO2+OH is the important NO oxidizing step.

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