A fuel-based approach to estimating motor vehicle cold-start emissions

The temporary ineffectiveness of motor vehicle emission controls at startup causes emission rates to be much higher for a short period after starting than during fully warmed, or stabilized, vehicle operation. Official motor vehicle emission inventories estimate that excess emissions during cold-sta rt operation contribute a significant fraction of all hydrocarbon, carbon m onoxide (CO), and nitrogen oxide (NOx) emissions from California vehicles. In an effort to verify these estimates under real-world conditions, vehicle emissions were measured in an underground parking garage in Oakland, CA, d uring March 1997. Hot stabilized emissions were measured as vehicles arrive d at the garage in the morning, and cold-start emissions were measured as v ehicles exited in the afternoon; the incremental, or excess, emissions asso ciated with vehicle starting were calculated by difference. Composite emiss ions from similar to 135 vehicles were sampled during each of six morning a nd six afternoon periods. Measured stabilized exhaust emissions were 19 +/- 2 g nonmethane hydrocarbons (NMHC), 223 +/- 17 g CO, and 8.6 +/- 1.3 g NOx per gal of gasoline consumed. Cold-start emissions of 69 +/- 2 g NMHC/gal, 660 +/- 15 g CO/gal, and 27.8 +/- 1.2 g NOx/gal were measured for vehicles spending an average of similar to 60 sec in the garage after starting in t he afternoon. Using second-by-second emissions data from California's light -duty vehicle surveillance program, average fuel use during cold start was estimated to be similar to 0.07 gal, and the cold-start period was estimate d to last for similar to 200 sec. When cold-start emission factors measured in the garage were scaled to represent the full 200-sec cold-start period, incremental start emission factors of 2.1 g NMHC, 16 g CO, and 2.1 g NOx p er vehicle start were calculated. These emission factors are lower than tho se used by California's motor vehicle emission inventory model (MVEI 7G) by 45% for NMHC, 65% for CO, and 12% for NOx. This suggests that the importan ce of cold-start emissions may be overstated in current emission inventorie s. Overall, the composition of volatile organic compound (VOC) emissions me asured during cold start was similar to that of hot stabilized VOC emission s. However, the weight fractions of unburned fuel and acetylene were higher during cold start than during hot stabilized driving.