Biphasic sensitization effect of NO2 on n-C4H10 auto-ignition

Abstract

NOx in the exhaust gas can influence the auto-ignition properties, thus are important for the design and operation of HCCI engines and predicting engine knock in spark ignition engines. More work is clearly required to improve the understanding on fuel/NOx chemistry for both reaction mechanism construction and engine application. In this study, n-C4H10, as a representative hydrocarbon fuel with the negative temperature coefficient (NTC) behavior, was selected for test. Ignition delay times of n-C4H10/air mixtures with varying NO2 addition (0, 500 ppm, 1%, and 5%) have been measured behind reflected shock waves. The sensitization effect of NO2 on n-C4H10 auto-ignition was investigated at engine relevant conditions covering temperatures from 700 to 1200 K, pressure of 20 atm, and equivalence ratios from 1.0 to 2.0. Besides, the mixtures with excessive amounts of NO2 (5% and 10%) were also studied at relatively high temperatures and 10 atm, aiming at highlighting the interactions between NO2 and n-C4H10. Results indicated the effect of NO2 on n-C4H10 oxidation exhibits prominent temperature and NO2 concentration dependences. Trace NO2 addition (500 ppm) promotes low-temperature reactivity of n-C4H10 and reduces the ignition delay times. While higher levels of NO2 no longer promote ignition, instead, it turns gradually to be an inhibiting effect. At high temperatures, NO2 dramatically accelerates the n-C4H10 oxidation, and the promoting effect peaks at around 5% NO2 then saturates with further NO2 addition. In the negative temperature coefficient region, the NTC behavior of n-C4H10 is weakened and even disappears by the introduction of NO2. A preliminary detailed kinetic model was proposed to describe the n-C4H10/NO2 chemistry, and in general, is capable of capturing the NO2 sensitization behaviors under different conditions. Kinetic analyses were then performed to clarify the sensitization mechanisms of NO2 and the chemical interactions between NO2 and n-C4H10.