Emission reduction using three-way catalyst on methane-fueled engine

Emissions of NOx (nitrogen oxides), CO (carbon monoxide), and HC (hydrocarbons) are measured on a CFR (Co-operative Fuel Research) engine equipped with a three-way catalyst. The catalyst is a general-purpose three-way catalyst with a ratio of Pd:Rh (Palladium: Rhodium) of 9:1. Emissions are measured before and after the catalyst using a Horiba gas analyzer. Attention is given to the hydrocarbon emissions, which are mostly CH4, and therefore difficult to be reduced in the catalyst. Different equivalence ratios, spark timings, catalyst temperatures, and compression ratios are tested. It is found that higher compression ratios lead to higher engine-out emissions of HC, which not only leads to higher post-catalyst emissions, but also makes reduction of CO and NOx more difficult. Tests are conducted with steady engine operation as well as with fuel dithering (modulation of the equivalence ratio with using a square wave). The fuel dithering did not improve the emission levels significantly, but provided some qualitative advantages. For example, a post-catalyst narrow-band lambda sensor could be better monitored which would otherwise be pegged to its limit. A control strategy is investigated by using such a lambda sensor between two catalysts to prevent CO or NOx excursions when the overall operation drifted toward rich or lean. The strategy is found to work better for preventing NOx excursions than for CO excursions, and the lowest emissions were found during the phase when NOx declined from a peak.