A study has been made of potential methods for controlling SO4= emissions from oxidation catalyst-equipped vehicles. The methods considered included operating condition and catalyst changes, as well as the use of a vehicle trap for SO4=. Emissions of SO4= from non-catalyst cars were also measured.
The only engine operating variable we found to significantly lower SO4= emission was exhaust gas O2 level. Limiting air pump use reduced SO4= emissions by factors of 5 to 7 over the FTP, and by factors of 2 to more than 10 at 96 km/h. Some increase in CO and HC emissions was observed when the greatest SO4= reductions were achieved, but it appears that properly modulated carburetion could overcome this problem. Limited excess air shows great promise as a means of minimizing SO4= emissions. Use of a three-way or oxidation catalyst system with closed-loop fuel metering control achieved very low levels of SO4=, while simultaneously controlling CO and HC, at the stoichiometric A/F ratio and slightly leaner than stoichiometric.
Pelleted catalysts emitted lower amounts of SO4= during low speed operation than did monoliths, because of their greater storage capacity for sulfur oxides. However, during subsequent high speed operation, the sulfur oxides were driven off, resulting in higher SO4= emissions from pellets than from monoliths. There appears to be no clear cut overall choice between the two catalyst configurations as far as SO4= emissions are concerned.
Catalyst age was found to be an important factor, Catalysts with 40,000 km of service gave about 65% less SO4= emissions over the FTP, and 25-40% less at 96 km/h, compared to relatively fresh catalysts. These results show that emission inventories which are predicted from tests with relatively fresh catalysts will tend to overestimate the buildup of atmospheric sulfate from catalyst-equipped vehicles, since these cars would have an average age greater than the test vehicles.
Pt catalysts gave somewhat higher SO4= emissions than Pt-Pd catalysts, while concentration of Pt-Pd on the catalyst had no effect in the range studied. Gasoline powered vehicles without catalysts produced only negligible quantities of SO4=. A Diesel powered vehicle gave higher than expected SO4= emissions, but high carbonaceous particulate emissions may have caused inaccurate measurements.
Vehicle SO4= traps containing CaO-based pellets achieved almost complete SO4= removal from the exhaust for 40,000 km, but pressure drop through the trap became excessive as the sorbent swelled with age. Efforts to lower the pressure drop resulted in poorer SO4= removal and capacity. Additional work is necessary to achieve satisfactory properties for all three parameters. Suggestions for further investigation are discussed.