Modeling the Detailed Chemical Kinetics of NOx Sensitization for the Oxidation of a Model fuel for Gasoline

At temperatures below 1100 K, the oxidation of nitric oxide (NO) impacts the oxidation of hydrocarbons, causing a sensitization effect in fuel combustion. This effect can be important in engine operations, especially those involving high levels of exhaust-gas recirculation (EGR). Many researchers have observed this NO sensitization for the oxidation of hydrocarbons in HCCI engines as well as stirred reactors. They used several model-fuel components relevant to gasoline, such as n-heptane, iso-octane, and toluene. As found in stirred reactor experiments, NO tends to increase the extent of oxidation for high-octane fuel components, such as isooctane and toluene. However, for the low-octane component n-heptane, NO has an inhibiting effect on hydrocarbon oxidation, particularly at low temperatures corresponding to the negative temperature coefficient (NTC) region. In this study, a detailed reaction mechanism for the combustion of complex gasoline surrogates has been extended to incorporate the sensitization effect of NO_x on the oxidation of hydrocarbons. The NO_x sub-mechanism incorporates recent updates in the kinetics literature for the hydrogen cyanide and related chemistry, as well as various production pathways that lead to NO_x emissions from fuel combustion. The gasoline-NO_x mechanism contains 1833 species and 8764 elementary reaction steps, including formation of several polycyclic aromatic hydrocarbons (PAH) species.

The extended self-consistent surrogate mechanism has been validated against available stirred-reactor measurements that cover a range of pressures, temperatures, and equivalence ratios for various small and large hydrocarbon components included in the mechanism. It successfully captures NO's inhibiting effect for n-heptane at temperatures below 650 K as well as its promoting effects at higher temperatures. Though validation data are not available for all the components of a complex gasoline surrogate, self-consistency of the mechanism that is built on rate-rules should guarantee the predictive capability for other components as well as their blends. In addition to the validation using the limited fundamental experimental data available, modeling using the detailed reaction mechanism has been performed for a typical gasoline HCCI engine using an eight-component gasoline surrogate. Higher levels of NO are predicted to significantly advance the combustion phasing due to the sensitization effect. The expected effect of exhaust gas recirculation (EGR) on combustion phasing and emissions has also been discussed.