Gasoline Distillation Effect on Vehicle Cold Start Driveability

Cold start vehicle driveability performance depends on many parameters, one of which is the distillation character of the fuel. In the late 90's, a gasoline driveability index (DI) was developed for spark ignited combustion vehicles by a consortium of automotive and petroleum industry scientists based on correlation studies between controlled fuel quality matrices and vehicle performance under specific ambient conditions. The DI equation uses a weighted sum of gasoline distillation temperatures at the 10, 50 and 90 percent evaporation volumes, commonly called T₁₀, T₅₀ and T₉₀. These three distillation volatility points are specified by the ASTM International D 4814 fuel specification and are seasonally adjusted. This paper studies the cold start driveability performance of Federal EPA Bin 5 and Bin 8 vehicles with respect to fuel distillation characteristics at temperatures other than T₁₀, T₅₀ and T₉₀. Fuels studied include regular Summer-grade unleaded gasoline, high-DI (1255°F), high-DI with high-T₇₀, and high-DI with both high T₃₀ and T₇₀. The results demonstrate that, although the latest vehicles can tolerate high-DI fuel in cold start driveability, the performance is deteriorated in some vehicles with the high-T₇₀ and the combined high T₃₀ and T₇₀ test fuels. The degree of poor cold start performance also depends on the ambient conditions. Summer weather with high-dew point temperatures experienced more cold start driveability performance demerits with the two test fuels as compared to normal Summer quality gasoline. Gasoline survey data shows that high T₇₀ fuels are available in the market place, but appear to be regionally isolated. When these high T₇₀ or high in both T₇₀ and T₃₀ gasoline blends show up in the field it has resulted in severe vehicle owner dissatisfaction. The mechanism of the poor cold start performance with such fuels is discussed.