Long-term efficiency of catalytic converters operating in Mexico City

A 1999 ordinance by the Government of Mexico City bans 1993 model-year vehi cles from on-road operation if their catalytic converters are not replaced with new ones. To validate the benefits of this action, we examined three i ssues related to exhaust emissions of vehicles equipped with catalytic conv erters. After selecting representative fleets of in-use vehicles, a compari son between emissions and catalyst efficiency in cars with two categories o f exhaust emission limits was carried out. For that purpose, two fleets wer e selected, each made up of 10 vehicles run under similar conditions. A thi rd, larger fleet with emissions control systems was used to evaluate and si mulate real-world conditions of vehicles in a controlled laboratory. Finall y, the aging effect on the catalytic converter was studied on vehicles run for 100,000 km, replacing their old emission control devices for new ones. The 1991-1992 model-year vehicles showed a high percentage of compliance wi th the corresponding emissions standard (90%) in comparison with 1993 model -year and later vehicles (Tier 0). However, NOx emissions were higher for t he newer vehicles. Fifty percent of the 1991-1992 model-year vehicles evalu ated under the official inspection/maintenance (I/M) procedure did not meet the regulated emissions standard when the results were compared with those of the U.S. federal Test Procedure FTP-75. Our results suggest that the re placement of old catalytic converters with new ones will have little effect on decreasing polluting emissions because these vehicles were in very bad mechanical condition. Results of catalytic activity as a function of mileag e indicated inefficient catalyst performance for the fleets tested. All pol lutant conversions were below 90% efficiency, and they deteriorated by an a verage of 30% after the vehicles were run for 100,000 km.