Evaluation of advanced air-fuel ratio control strategies and their effects on three-way catalysts in a stoichiometric, spark ignited, natural gas engine

Jones, Andrew Lawrence, author
Olsen, Daniel B., advisor
Marchese, Anthony, committee member
Johnson, Jerry, committee member
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Colorado State University. Libraries
Engine emissions are a growing concern in the 21st century. As the world works to combat rising pollution levels, engine emissions are under scrutiny. Natural gas engines are increasing in popularity over diesel engines, due to the high availability of fuel and fewer pollutant emissions than comparable diesel engines. Pollutants such as NOx, CO, and THCs (total hydrocarbons) are harmful to the environment and are currently regulated, and limits for these pollutants are expected to decrease further in the future. A three-way catalyst (TWC) is a cost-effective exhaust after treatment system can be used to reduce pollutant emissions through a series of reactions that are catalyzed by special conditions within the catalyst. Using TWCs, emissions can be drastically reduced using simple chemical reactions, without affecting engine performance. Air-fuel ratio dithering is a strategy that can be used to increase catalyst reduction efficiency by utilizing the oxygen storing properties of ceria, a material in the catalyst washcoat. Dithering is a method of periodically varying the air-fuel ratio of the engine around an optimum point. The focus of this work is understanding how dithering affects oxygen storage in a catalyst, as well as how dithering amplitude and frequency can be tuned to maximize catalyst efficiency. Experiments were performed on a CAT CG137-8, a stationary natural gas engine used for gas compression. Three different catalysts were tested, including the standard catalyst for the test engine, a custom catalyst with one half of the oxygen storage capability of the standard catalyst, and the standard catalyst artificially aged to 16,000 hours. Emissions data were collected across a dithering parameter sweep where a large number of amplitude and frequency combinations were tested. Additionally, steady state and dithering air-fuel ratio sweeps were performed to investigate the emissions window of compliance across a wide range of air-fuel ratios. It was found that dithering with optimized amplitude and frequency can significantly reduce pollutant emissions with a fresh catalyst. However, dithering does not have a large effect on aged catalysts. Additionally, dithering was shown to improve the window of emissions compliance on a standard catalyst by 100% but showed a smaller improvement on a catalyst with ½ oxygen storage capability. The window of compliance with an aged catalyst was unimproved by dithering. Optimized dithering has the potential to significantly reduce engine emissions, allowing for compliance with more stringent emissions requirements or for less expensive catalysts to be used.
2021 Spring.
Includes bibliographical references.