The effect of air-fuel ratio control strategies on nitrogen compound formation in three-way catalysts

The ability of three-way catalysts (TWCs) to effectively remove CO and NO x from the exhaust is directly controlled by the air-fuel ratio at which the accompanying engine is operated. In a stoichiometric engine, small variations in the air-fuel ratio have large effects on the catalyst performance. These effects include wide variations in removal efficiencies and catalytic production of ammonia. The effect of the air-fuel ratio on catalysts has been well studied on automotive engines; these studies show the importance of maintaining an air-fuel ratio close to stoichiometric conditions. In automotive systems a ‘dithering’ technique is used in which the air-fuel ratio is modulated to widen the window of control. The effect of dithering on industrial engines has not been studied. A research programme was conducted to evaluate the effects of the air-fuel ratio on the performance of three-way catalysts operating on natural gas-fuelled industrial engines, the test programme aims at optimizing the engine based on the performance of the catalyst. This project has shown that dithering is an effective technique for enhancing the performance of TWCs on industrial engines. These results show that the allowable air-fuel ratio deviations are much larger with dithering and that the production of ammonia is significantly reduced.