New fuel volatility indices

Cold-start driveability continues to be a concern for both the automotive and oil industries.

From 1995 to 1997, the Coordinating Research Council (CRC) conducted a cold-start driveability program to evaluate the behavior of low-volatility fuels at cold, intermediate, and warm temperatures. Earlier programs had shown that driveability is a nonlinear function of the American Society for Testing & Materials' driveability index (ASTM DI), which is a measure of fuel volatility and defined as 1.5T₁₀ + 3T₅₀ + T₉₀. While the ASTM DI is probably the most widely used index of overall driveability, it was developed using non-oxygenated fuels and carbureted cars. Because the DI does not blend linearly-while a percent-evaporated-based driveability index (EDI) does - there is an advantage to developing an EDI. Another advantage is that percent evaporated is a measure recognized worldwide and will be useful with global gasoline specification development. The U.S. Environmental Protection Agency's complex model is also written in terms of percent-evaporated variables.

The program used 135 vehicles to evaluate 87 hydrocarbon, MTBE-blended, and ethanol-blended fuels. The cold-temperature phase of the program was conducted with run temperatures ranging from −21° to −7°C (−5° to +20°F), the intermediate phase from 2° to 10°C (35° to 50°F), and the warm phase from 18° to 27°C (65° to 80°F). Trained raters conducted all the tests in each phase using similar fleets of late-model vehicles. All the vehicles had automatic transmissions, and the fleets consisted of 70% cars and 30% light-duty trucks and vans. There were 115 port-fuel-injected, 11 singular-point-injection, and 9 central-point-injection fuel systems in the test fleet. No carbureted vehicles were tested.