Modeling and Analysis of the Formation of Oxides of Nitrogen and Formaldehyde in Large-Bore, Lean-Burn, Natural Gas Engines
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Emissions from large-bore, spark ignition engines running on natural gas pose a serious problem, especially in non-attainment areas. These engines emit substantial amounts of oxides of nitrogen, unburned hydrocarbons and carbon monoxide. The pathways of formation of oxides of nitrogen (NOx) and formaldehyde (HCHO) have been explored in this study. This is done by using UWSI, a computer model of gas phase combustion in spark-ignition engines previously developed at the University of Washington, and through chemical reactor modeling using CHEMKIN. The UWSI model is set up and calibrated using data from a single test case. Further matching of NOx emissions for varying fuel-air equivalence ratios is also performed and the model is then used to predict NOx emission for three leanest cases beyond the range of test data. The HCHO emission from the model for each test case is examined. Several scenarios which may lead to HCHO formation in the engine are also modeled. NOx formation pathways at lean conditions in these engines are studied through chemical reactor modeling. A simplified NOx model based on the results of the chemical reactor modeling is developed and validated by comparing its results against those obtained from the UWSI code. Results obtained indicate that incomplete propagation of the flame across the cylinder is the most likely pathway to HCHO formation in these engines. Engines experiencing borderline auto-ignition may also contribute to HCHO emission, while unburned charge trapped in cracks and crevices and released late in the cycle is not indicated to have a significant effect. The Zeldovich and nitrous oxide pathways are shown to be the predominant contributors to NOx formation at lean conditions in these engines. A simplified NOx predictor based on these pathways is developed and validated against data obtained from the UWSI model.
- Mechanical engineering