Shock tube experimental studies on the ignition delay of n-heptane/ethanol fuel blends with acetaldehyde (CH 3 CHO) and oxynitride (NO/NO 2 ) additives

Abstract

In recent studies around low-temperature combustion (LTC) engines, it has been found that the incomplete oxidation products (IOP) within exhaust gas recirculation (EGR) presents non-negligible chemical effects. However, IOP generally has complex ingredients, and little literature available conducts quantitative and comparative studies on the effects of these ingredients. In this paper, CH₃CHO, which is considerably abundant, and NO, which is most active, are selected as typical ingredients of IOP, and their effects on ignition delay times of n-heptane/ethanol fuel blends are studied experimentally in a shock tube covering the equivalence ratios of 0.5, 1, 1.5, temperature range of 900–1200 K, and pressure of 10 atm. Experimental results show that both CH₃CHO and NO/NO₂ (part of NO is inevitably converted to NO₂) promote ignition, and the promoting effect of NO/NO₂ is stronger than CH₃CHO . It is consistent for both CH₃CHO and NO/NO₂ that the higher the concentration or, the lower the equivalence ratio, the greater the promoting effect. The difference is that the promoting effect of CH₃CHO weakens with temperature increasing, while the promoting effect of NO/NO₂ strengthens. Besides, the promoting effect of NO₂ is weaker than that of NO, and NO₂ even begins to inhibit ignition at temperatures below 960 K. In addition, it follows chemical kinetic analysis that both CH₃CHO and NO/NO₂ accelerate the decomposition of fuel by producing more OH radicals during the ignition stage. Although the additives are in trace amounts, the OH produced by their early involvement in reactions can effectively accelerate the ignition, which indicates the non-negligible chemical effects of IOP in EGR.