Minor species production from lean premixed combustion and their impact on autoignition of diesel surrogates

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Formation of minor species, including NO, NO2, and OH, during the premixed burn and cool-down in a constant-volume combustion vessel (CV) was modeled to investigate the effect of these species on the chemical kinetics portion of the ignition delay of n-heptane used as a diesel surrogate. Control parameters included ambient temperature, pressure, and diluent level (EGR) matched to typical diesel engine conditions. For the preburn model, the GRI 3.0 mechanism was used with experimentally determined heat loss from the CV. Subsequently, the cool-down premixed burn products served as reactant inputs and were mixed stoichiometrically with n-heptane, modeled using a reduced reaction mechanism modified to include NO and NO2. Results computed with premixed burn constituents were compared to those using dry air and air plus ideal combustion residuals with the impact of dilution on ignition delay examined. A sensitivity analysis was performed to characterize the influence of OH and NOx levels on ignition delay.The preburn kinetics simulation showed OH concentrations above equilibrium; however, OH was below 100 ppb during the cool-down when fuel spray and ignition would occur. In contrast, the slow chemistry due to the low temperature (1750 K) prevents NO formation from reaching equilibrium levels; rather, levels are frozen in the 10-30 ppm range as the cool-down proceeded. This NO level is of the same order for cylinder charge concentrations in modern diesels when using 20-50% EGR rates producing 100-200 ppm in the exhaust. The ignition delay predictions showed that minor species of NO, NO2, and OH shorten the ignition delay by 3% relative to dry air, while being 6% longer when compared with the simulated dilution of 7.6% residuals (19% O2), typical of internal residuals in an engine. These kinetics effects are small in comparison to changes in oxygen concentration (from 21 to 15%) associated with EGR, which show a 170% increase in ignition delay. © 2011 American Chemical Society.

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Energy and Fuels