Experimental and kinetic study of NO/NO2 chemical effects on n-heptane high temperature auto-ignition

Nitrogen oxides can be introduced into the combustion engine through exhaust gas recirculation (EGR) and therefore change the reaction channel of hydrocarbon fuels. In this paper, we firstly evaluate the conversion of NO to NO2 with the presence of oxygen and later apply the results into the simulation. Shock tube experiments are carried out at equivalence ratio of 0.5, 1 and 2, and total NO/NO2 concentration of 100, 500 and 1000 ppm. Experimental results show that for n-heptane equivalence at 0.5 and 1 and temperature lower than 1250 K, the addition of NO/NO2 promote the ignition and the promoting effects strengthen as the concentration of NO/NO2 increases. At a higher temperature , however, promoting effects of NO/NO2 are negligible. The kinetic analysis suggests that reactions: NO + HO2 ⇔ NO2 + OH and NO2 + H ⇔ NO + OH may work as a “reaction cycle” to effectively produce the OH radicals and promote the ignition.