SAE Technical Paper Series Characterization of Recent-Model High-Emitting Automobiles

In-use vehicles which are high emitters make a large contributionto the emissions inventory. It is not known, however, whether high-emitting vehicles share common emissions characteristics. We study this by first examininglaboratory measurements of second-by-second engine-out and tailpipe emissions from a small number ofMY90-97 high-emitting vehicles. We distinguish highemitter types by the behavior of six ratios in low- andmoderate-power driving: the engine-out emissions indices (engine-out pollutant to fuel-rate ratios) and the catalystpass fractions (tailpipe to engine-out ratios) for CO, HC, and NOx. Four general types of high emitter areobserved: 1) fuel-air ratio excessively lean, 2) fuel-air ratio excessively rich, 3) partial combustion such as misfire,and 4) severe deterioration in catalyst performance in vehicles where malfunctions of Types 1, 2 or 3 are notpredominant. We also find that these behaviors may be chronic, or may only occur transiently. The second stepis to determine the prevalence of the four different types of high emitter in the on-road fleet. For this we analyzeIM240 tailpipe emissions from a large sample of cars measured in the Arizona inspection and maintenanceprogram. We find that all four types of failure are observed with roughly comparable probabilities.