Hydrocarbon impact on NO to NO 2 conversion in a compression ignition engine under low-temperature combustion

Abstract

Compression ignition engines can employ high rates of exhaust gas recirculation to realize low-temperature combustion in order to reduce the NO_x emissions. However, a substantial increase in NO₂ contribution to the NO_x emissions is also observed. The relationship between this NO to NO₂ conversion is also affected by the hydrocarbons originating mainly from the fuel. This can have important consequences for the design of the exhaust after-treatment system. Therefore, this article presents an empirical investigation of the impact of hydrocarbon emissions on the in-cylinder NO–NO₂ conversion process. First, engine motoring tests are performed with propane and NO gases dosed into the engine intake manifold. Engines with different compression ratios are employed to study the effect of in-cylinder temperature and intake HC–NO ratio on the NO–NO₂ conversion process. Next, the hydrocarbon impact on the NO_x survivability at different engine combustion modes is investigated using a common-rail diesel engine test platform with independent control of exhaust gas recirculation, intake boost, and exhaust back pressure. Results show that the existence of hydrocarbon has a strong promotion effect of converting NO to NO₂. During compression test, NO–NO₂ conversion rate can reach 95% under certain intake HC–NO concentration ratio, and the minimum HC–NO concentration ratio to sustain a high NO–NO₂ conversion rate is sensitive to peak in-cylinder temperature; engine combustion results also show that hydrocarbon not only can promote the in-cylinder NO–NO₂ conversion process, but also has the potential of decreasing the total NO_x emissions under low-temperature combustion mode.