Hot-Start Driveability of Low T ₅₀ Fuels

A two-phase test program designed to investigate the effects of Reid vapor pressure, T₅₀, and oxygenates on hot-start and driveability performance of vehicles operated at high and low altitude in high and intermediate ambient temperature ranges was conducted during July and August of 1992. Twenty 1983 - 1992 model-year vehicles were tested on a set of eighteen fuels which included six hydrocarbon-only fuels, six gasoline-ethanol blends, and six gasoline-MTBE blends. Fuel-injected vehicles showed very few demerits and were insensitive, in most cases, to the fuel variables studied. By comparison, carbureted vehicle's demerit levels were three times the level associated with fuel injected vehicles. Statistically significant degradation of driveability was demonstrated in these vehicles when tested on fuels with low T₅₀ and low RVP. Driveability problems related to low T₅₀ fuels were frequently symptomatic of vapor lock. In carbureted vehicles using high-RVP fuel at high altitudes, gasoline-oxygenate blends showed significantly improved driveability relative to pure hydrocarbon fuels. The effect of ambient temperature was greater than that of any other variable.