Effects of Gasoline Composition and Operating Parameters on the Response of Knock Limit to Nitric Oxide in a Lean-Burn DI-SI Engine

Lean operation of spark-ignition engines can lead to efficiency gains and lower NOx emissions due to reduced combustion temperatures. However, lean operation may still encounter difficulties with end-gas autoignition and knock, which worsen by the presence of trapped NO in the cylinder. This study assesses the impact of intake-seeded NO on end-gas autoignition and knock for two gasolines with similar octane rating but different composition: high cycloalkane fuel (HCA) and high olefin fuel (HO). Experiments were performed at stoichiometric and lean (lambda = 2) conditions and at two engine speeds of 1400 rpm and 2000 rpm to investigate the influence of equivalence ratio and residence time on end-gas autoignition trends. Chemical kinetics simulations revealed that the mechanisms that control the effect of NO on knock are similar at lambda = 2 and lambda = 1, with NO + HO2 = NO2 + OH being the main pathway for enhancing reactivity by promoting low-temperature heat release (LTHR). Compositionally different fuels reacted differently to NO seeding and engine speed. HCA was less knock-limited than HO for low NO concentrations and low speeds but became more knock-limited for high NO concentrations and high speeds. This is because OH produced from NO helped to overcome the OH quenching of cyclopentane, a main species in HCA. This effect was not strongly affected by residence time, with HCA’s LTHR (promoted by NO) mildly changing as speed increased. In contrast, the inherent low temperature chemistry of HO reduced the impact of NO on reactivity, with HO’s LTHR (less reliant on NO) suppressed as residence time decreased and no noticeable LTHR detected at 2000 rpm and low NO concentrations. For the range of NO concentrations tested here, NO increased the knock propensity of the fuel at both stoichiometric and lean conditions, especially for fuels with mild low-temperature chemistry.