Relative Effects of Catalyst and Fuel Formulation on Gasoline Vehicle Exhaust Emissions

The effects of catalyst and fuel formulation changes were investigated on vehicles meeting European Stage II (94/12/EC) emission limits when tested over the modified European test cycle. The OEM standard Pt/Rh catalyst formulation was compared with advanced Pd/Rh catalysts, at nominally the same PGM cost, and with Pd/Rh catalysts at increased PGM loadings. No other changes were made to the vehicles. The largest relative emissions benefits for the advanced Pd/Rh catalysts at equivalent PGM cost were 28% for THC, 30% for CO and 22% for NOx. Pd/Rh catalysts with higher PGM loadings gave further improvements in emissions, with total reductions of 38% for THC, 40% for CO and 31% for NOx compared to the standard OEM catalyst. In addition, one of the vehicles was tested with a Pt/Pd/Rh catalyst formulation. The performance of this catalyst was comparable with the Pd/Rh catalyst at similar PGM loading.

The effect of six different fuel parameters was studied on regulated and unregulated exhaust emissions with the different catalyst formulations. The first fuel in the series contained high aromatics, olefins and sulphur, no MTBE and had low mid-range and back-end distillation. The sequential changes in individual fuel properties resulted in a significant change in fuel composition and distillation across the test matrix, with the final fuel in the series being a severely reformulated gasoline. The change in emissions for changing fuel formulation was generally independent of catalyst formulation.