The Effect of Nitric Oxide (NO) on the Reactivity of Ethanol and Gasoline: An Experimental Study

<div class="section abstract"><div class="htmlview paragraph">Achieving stable HCCI combustion requires specific in-cylinder boundary conditions. Trace residual species, such as nitric oxide (NO), can have an impact on the reactivity, and thus the combustion stability, of different fuels in HCCI. This study investigates the effects of nitric oxide (NO) on the reactivity and combustion stability of ethanol and gasoline in a single-cylinder HCCI engine. The promoting and inhibiting impact of NO on iso-octane’s ignition delay time are available in the literature; nevertheless, as a baseline study, these effects on the autoignition of gasoline were documented in this work. For ethanol, the NOx concentration seeded in the intake air varied from 0-1000 ppm while maintaining a constant combustion phasing (CA50 at 7.5 CAD) and a global equivalence ratio of 0.34.</div><div class="htmlview paragraph">Ethanol exhibited a linear reduction in intake temperature, decreasing by 47 K with 927 ppm NO. For gasoline, a 225-ppm increase in NO reduced the intake temperature required for HCCI by 40 K. However, gasoline showed a non-linear response, attributed to different autoignition characteristics of the fuels. Ethanol's reactivity enhancement is linked to NO's role in converting less reactive <span class="formula inline"><math display="inline" id="M1"><mi>H</mi><msub><mover accent="true"><mi>O</mi><mo>̇</mo></mover><mn>2</mn></msub></math></span>​ radicals into more reactive <span class="formula inline"><math display="inline" id="M2"><mi>O</mi><mover accent="true"><mi>H</mi><mo>̇</mo></mover></math></span> radicals in a chain propagation step. Therefore, NO significantly influences combustion stability, introducing a potential “runaway” effect on combustion phasing in advanced combustion modes. In traditional mixing-controlled combustion, NO affects the kinetically controlled pilot heat release, altering the premixed heat release spike. However, the ignition delay correlation for ethanol widely used in the literature failed to capture these reactivity enhancement effects, as the Livengood Wu integral predicted the same autoignition threshold for ethanol with varying NO concentration.</div></div>