Evaluation of NO_x Production Rate in Diesel Combustion Based on Measurement of Time Histories of NO_x Concentrations and Flame Temperature

摘要

In this study, we evaluated NO_x production rates of diesel combustions occurred in a constant volume chamber of a rapid compression machine in order to investigate relationship between flame behaviors and NO_x emissions. A total gas sampling device was used to measure the NO_x concentration in total gases existing in the chamber at a designated time. An EINO_x (Emission Index of NO_x) production rate was evaluated on the time history of NO_x concentration. Temporal temperature distributions in the chamber were measured with a high speed 2-color thermometry. Gas oil (JIS #2) was used as the fuel. The EINO_x production rate increases with increasing injection pressure through temperature rises in flames due to enhanced mixing of fuel vapor with ambient air. An increase in the ambient pressure causes overlaps between flames formed around the nozzle, which reduces the flame temperature. Nevertheless, high reactant concentrations resulting from the elevated ambient pressure slightly increase the net EINO_x production rate. A reduction of number of injection holes decreases the total surface area of flames formed in the chamber, and then decreasing the EINO_x production rate. In this study, the EINO_x production rate was modeled in terms of the ambient air density, flame volume and reaction rate coefficient of an elementary reaction in the Zel'dovich mechanism. There is a correlation between the results obtained from the prediction model and the EINO_x production rate even though a large derivation remains in the result obtained under the high ambient pressure condition.