The Effects of NOx Addition on the Auto Ignition Behavior of Natural Gas under HCCI Conditions

Controlling start of ignition in Homogenous Charge Compression Ignition (HCCI) engines remains a major challenge. Here we have investigated changes in intake charge composition and its effects on ignition delay for natural gas based HCCI engine operation. In particular, we have investigated the effects of small amounts of nitrogen dioxide (NO₂) on operating characteristics. Previous research had shown that NO_x presence might attenuate natural gas ignition. The hypothesized catalytic effect of NO_x on methane ignition at HCCI conditions was experimentally confirmed in a custom built engine. The problem was further studied in both zero and multidimensional numerical engine simulations with detailed chemistry. The simulations were used to complete a reaction rate sensitivity analysis to elucidate the controlling chemistry, and further confirm that a significant shift in ignition phasing is produced with the addition of just several ppm by volume of NO₂ or NO_x (NO + NO₂). The results suggest that for natural gas even the small amounts of NO_x present in trapped and the recirculated charge fractions may be important in HCCI operation. The use of varying NO₂ addition by injecting a chemical fuel additive may even offer a control strategy, although the impact of the additional nitrogen species on engine-out emission quality deterioration remains to be evaluated. However, the more general principle of varying the concentration of ignition inhibitors or accelerators typical in controlling octane or cetane character of fuels clearly deserves additional exploration as a control strategy.