Understanding the nitric oxide (NO) conversion process plays a major role in optimizing the Selective NOX Recirculation (SNR) technique. SNR has been proven in gasoline and diesel engines, with up to 90% NOX conversion rates being achieved. This technique involves adsorbing NOX from an exhaust stream, then selectively desorbing the NOX into a concentrated NOX stream, which is fed back into the engine's intake, thereby converting a percentage of the concentrated NOX stream into harmless gases.
The emphasis of this paper is on the unique chemical kinetic modeling problem that occurs with high concentrations of NOX in the intake air of a spark ignited natural gas engine with SNR. CHEMKIN, a chemical kinetic solver software package, was used to perform the reaction modeling. A closed homogeneous batch reactor model was used to model the fraction of NOX versus time for varying initial conditions and constants. The molar fraction of NOX was monitored on small time scales, on the order of the time for complete combustion, and on large time scales, on the order of minutes.
Predicting the amount of NOX using a closed homogenous batch reactor model on small time scales allowed estimates of the amount of NOX converted locally during the combustion process. These values were then compared with experimental values, which were acquired from a Cummins 10 liter spark ignited natural gas engine. The model predicted conversion rates varying between 35% and 42%, and experiments showed conversion rates between 18% and 23% for a constant intake NO concentration of 25,000 ppm. Predicting NOX levels on large time scales confirmed that the NOX conversion phenomenon, over the in-cylinder time period, is rate limited rather than equilibrium limited, which gives insight into how to maximize these conversion efficiencies for the SNR process.