Nitric oxide interactions with hydrocarbon oxidation in a jet-stirred reactor at 10 atm

The purpose of the present work is to better define the influence of trace amounts of NO on the oxidation of model fuels such as n-heptane, iso-octane, toluene, and methanol. This information is of interest for understanding and modeling autoignition whether for engine knock or for engines operating under compression ignition modes such as HCCI (homogeneous charge compression ignition) or CAITM (controlled autoignition). The experiments were performed in a jet-stirred reactor at 10 atm over a temperature range of 550 to 1180 K with a residence time of 1 s for stoichiometric mixtures highly diluted in nitrogen. The carbon content was about 1 molar percent and the added NO ranged from 25 to 500 ppmv. The effects of NO vary with the temperature regime. At the lowest temperatures NO inhibits the reaction. As temperature rises beyond 675 K, NO can considerably accelerate the reactivity of all fuels to an extent that can supercede the NTC behavior in the case of n-heptane. Modeling work indicates that in this temperature region at 10 atm the promoting effect of NO is largely due to the catalyzed production of OH, involving the dissociation of HONO, with the latter formed from reactions between NO2 and HO2, CH3O, or CH2O. In the intermediate temperature regime the intensity of the accelerating effects is observed to rise with the octane number of the fuel, with the exception of methanol. For toluene, the onset of oxidation drops down from 900 to 800 K with as little as 50 ppmv NO.