Modeling Criteria and Air Toxic Pollutants in Light-Duty Biodiesel Exhaust

Biodiesel fuel use is more widespread, but studies of the fuel's effects on air toxic emissions, especially from-light-duty diesel engines, are largely insufficient for understanding the air quality effects of biodiesel use. This study applied steady-state and transient cycle data on air toxics and criteria pollutant exhaust emissions from a light-duty diesel engine running on biodiesel fuel blends to develop models for pollutant emission rates. Using second-by-second Fourier transform infrared spectroscopy data on formaldehyde, carbon monoxide (CO), carbon dioxide (CO2), nitrogen monoxide (NO), and nitrogen dioxide (NO2) emissions from waste vegetable oil and soybean biodiesel blends, statistical analyses examined how engine operating conditions, biodiesel fuel content, and biodiesel feedstock affected steady-state emissions. Each factor had a significant effect on emissions of all five species. Transient cycle emissions data were used to develop multiple linear regression models for each pollutant on the basis of measurable engine parameters and biodiesel content. Log-linear models of CO2 and NO predicted emission rates very well; transient emissions of NO2, formaldehyde, and CO were modeled only moderately well. Modeling high concentrations in the two species formed by incomplete combustion, formaldehyde and CO, proved especially challenging, which suggested the need for additional variables or separate models for different modes of engine operation. Fuel-normalized formaldehyde emission rates up to one order of magnitude higher than previously reported for a heavy-duty engine suggest the need for more light-duty mobile source air toxic emissions studies of biodiesel to identify the feedstock and operating blend that minimizes air toxic emissions from light-duty diesel engines.