A spark-ignition engine requires that air and fuel be premixed and vaporized before combustion. Poor vaporization leads to long cranking times when the engine is cold, rough idle and stall, poor driveability during warm-up, and increased exhaust emissions resulting from misfire or incomplete combustion. Finally, evaporation of raw gasoline from the tank and fuel system contribute to air pollution. Reducing gasoline volatility helps control these emissions, and it also reduces fuel vapor losses when the tank is being filled. Because of these conflicting requirements, and because of the variety of climatic conditions, gasoline volatility must be carefully controlled in the manufacturing process.
Volatility is adjusted seasonally and geographically to account for weather extremes.
There are several ways to characterize volatility. One is distillation, which determines at what temperature specific fractions of liquid gasoline turn to vapor. Distillation properties can predict both hot and cold temperature performance. Another way is vapor pressure, which is a measure of the force on a closed container at a specific temperature. Vapor pressure relates to hot-temperature performance and evaporative losses.