The catalytic reduction of emissions from a natural gas fueled spark ignited engine
Activity in converting gasoline engines to operate on natural gas has increased in recent years due to the passage of The Clean Air Act of 1990 and The National Energy Policy in 1992 (EP ACT). Both policies require companies with large vehicle fleets to purchase a number of alternatively fueled vehicles. Although natural gas, the most commonly used alternative fuel, is considered a "clean" fuel, proposed legislation for the future will require additional technology to reduce engine exhaust emissions further. An additional concern for converted vehicles is whether the conventional catalyst on the gasoline engine, which was designed for gasoline fueling, is effective when the engine is being fueled by natural gas. One possible emissions reduction strategy is the catalytic aftertreatment of the engine-out exhaust for stoichiometric operation. The focus of this work was to develop a catalytic converter test facility, to evaluate the operational and emissions characteristics of a converted natural gas engine, and to evaluate two catalytic converters, one designed for gasoline fueling and the other for natural gas fueling. The experiments were conducted in the Internal Combustion Engines Laboratory at The University of Alabama. The engine used in this work was a 2.8 liter, six-cylinder, spark-ignited engine. The experiments consisted of open-loop and closed-loop fueling controlled tests. The results indicated a slight power loss with the gasoline catalyst compared to the natural gas catalyst. The emissions results showed sensitivity to air-fuel mixtures for the closed-loop controlled operating region (equivalence ratio around 0.99) with gasoline and natural gas fueling. Both catalysts had difficulty reducing the hydrocarbon emissions for natural gas fueling and the N Ox reduction was inadequate for both catalysts with natural gas and gasoline. Open-loop results showed marked improvement in catalyst effectiveness for NOx reduction for rich mixtures. For lean operation, the natural gas catalyst showed significantly better results in hydrocarbon oxidation than the gasoline catalyst. For stoichiometric open-loop operation, the catalyst effectiveness for CO (carbon monoxide), THC (total hydrocarbons), and NOx (oxides of nitrogen) was maximum at slightly rich mixtures. Both catalysts were similar in effectiveness for rich operation.