An experimental study of Nitric Oxide (NO) as ignition enhancer for ethanol and gasoline HCCI engine

Controlled HCCI combustion strategy for single stage ignition fuels requires curated in-cylinder boundary conditions for low cycle to cycle variability and sustained combustion. In this work the effects of using NO as a seed gas on the autoignition of neat ethanol was studied. Based on the concentration and initial conditions, the promoting and inhibiting impact of NO doping on iso-octane ignition delay time are available in the literature; nevertheless, the change in autoignition characteristics with NO seeding for gasoline with GCI strategy was also documented as a baseline comparison in this work. The NOx concentration doped with the intake air was varied from 0-1000 ppm while maintaining a constant combustion phasing (CA50) of 7.5 CAD and a global equivalence ratio of 0.34, respectively. The operating conditions was supercharged similarly for both neat ethanol and gasoline to purge the in-cylinder residuals after every cycle. The only difference was in the initial intake temperature requirements for ethanol being higher at 400 K due to its higher-octane number and heat of vaporization, and 350 K for gasoline. The intake temperature requirement for igniting gasoline lowered by 40 K and sustained combustion was recorded at the lowest intake temperature of 310 K at 7bar net IMEP. Temperature requirements reduced linearly for ethanol by 47 K as well however, the drop was not proportionate to the concentration doped in the intake air for the baseline case, which aligns with the autoignition characteristics being different for both fuels. These differences are further explained based on the chemical kinetics from the literature as the reaction pathway for ethanol consumption is primarily fueled by H abstraction by the OH radical. Presence of NO enhances reactivity as it oxidizes the relatively unreactive 〖HO〗_2 radical to the OH radical in a chain propagation step. However, excess concentration of NO has shown inhibitory effects on reactivity as it consumes the reactive OH radical in a chain termination reaction, but such high concentrations were not introduced in these experiments therefore the inflexion point in reactivity for either gasoline or ethanol was not observed.