The Effect of Aromatics, MTBE, Olefins and T90 on Mass Exhaust Emissions from Current and Older Vehicles - The Auto/Oil Air Quality Improvement Research Program

Exhaust emissions were measured as a function of gasoline composition in two fleets of vehicles - 20 1989 vehicles and 14 1983-1985 vehicles. Eighteen different gasolines were tested which varied in aromatic, olefin, and MTBE content and in the 90 percent distillation temperature (T₉₀). Subject to the cautions and qualifications described in the body of this paper, mass exhaust emissions in both fleets of vehicles were affected by changes in fuel composition. Responses to changes in MTBE and olefins were similar in both fleets: adding MTBE reduced emissions of HC and CO, and reducing olefins lowered emissions of NO_x while raising emissions of HC. In the current fleet, reducing aromatics lowered HC and CO, while in the older fleet, reducing aromatics raised HC and lowered NO_x. In the current fleet, lowering T₉₀ reduced HC over 20%, while raising NO_x slightly. In the older fleet, lowering T₉₀ reduced HC by only 6%. Regression equations were developed which satisfactorily describe the variations in the data. In addition, other analyses were carried out which help to understand the differences between the fleets and between technology types.