Effect of nitric oxide on ignition of multiple fuels in a spark ignition and compression ignition engine

Nitric oxide (NO) is a significant component of exhaust gas recirculation (EGR) that, by itself, can change the ignition tendency of fuels. This can have major implications in spark ignition (SI) engines in which knock limits efficiency, and in homogeneous charge compression ignition (HCCI) in which combustion is driven by the chemical kinetics of the fuel. Therefore, the presence of NO in EGR can impact knock limits and combustion phasing in both engine modes. However, whether NO acts as an ignition enhancer or suppressor strongly depends upon the fuel’s chemistry. To isolate the influence of NO from the thermal and dilution effects associated with the other chemical species in EGR, controlled concentrations of NO and N 2 were doped to intake air prior to entering the engine. First, NO was doped in an SI engine fueled with ethanol, gasoline with 10 % ethanol (E10), and iso -octane to study how knock tendency changed. With ethanol and E10, a combustion phasing retard was required at all doped NO concentrations due to increased knock. With iso -octane, there was no change to knock-limited combustion phasing until ∼ 200 ppm, after which knock was suppressed, which enabled a phasing advance. NO was then mixed with intake air in an HCCI engine fueled with ethanol, iso -octane, and n-heptane. At a constant combustion phasing, 1120 ppm of NO enabled a 43 K decrease in intake temperature in HCCI fueled with ethanol. With iso -octane, NO lowered the intake temperature by ∼ 35 K, but there was a saturation effect after ∼ 20 ppm of NO was seeded. With n-heptane, a small concentration of NO initially advanced the onset of low temperature heat release (LTHR), but high concentrations of NO caused LTHR to retard. Overall, fuels that displayed two-stage ignition behavior such as iso -octane and n-heptane saw a decrease in reactivity with increasing concentrations of NO, whereas single-stage ignition fuels experienced a uniform reactivity increase. Chemical kinetic modelling further highlighted that NO can be consumed with a hydroperoxyl radical to promote ignition or react with an alkylperoxyl radical to suppress ignition.