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Investigations into fuel compositional effects on emissions using model and full range fuels suggest aromatic components promote NOx conversion over the catalyst Steady state results derived from a single engine (Ricardo Gasoline Fuels Consortium data) show that at a typical part load condition, the catalyst removes NOx less effectively with lower aromatic fuels. Neither CO nor H2 contribute significantly to catalyst performance. Two vehicles were tested over a European cycle. Toluene formed more combustion chamber NOx, offset by increased catalyst conversion efficiency giving lower tailpipe NOx than isooctane in the vehicle with the better catalyst light-off and AFR control.

Vehicle-based studies have shown that a reduction in the aromatic content of gasoline fuels can result in increased NOx emissions from catalyst-equipped vehicles. A study with simulated exhaust gas has shown that light paraffins, especially methane, are unreactive and cause substantial breakthrough of unreacted NO over the catalyst. However, unsaturated exhaust components including aromatics are effective reactants and play a large part in converting NO over the catalyst. Engine tests have shown that methane is predominantly produced by fuel paraffins and olefins, but hardly at all by aromatics. Thus it appears that methane generated during combustion of low aromatics fuels may be the cause, wholly or in part, of the poor NO conversion efficiency observed when catalyst-equipped cars are operated on such fuels. However, it cannot be excluded that low aromatics fuels are associated with increased air-to-fuel ratio which will also contribute to poor NO performance.