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.
Using predictive models of some of these properties, Shell seasonally blends gasolines to assure good driveability performance under all conditions. Sulfur and Corrosion Sulfur compounds occur naturally in crude oils. Since only a portion of these compounds are removed in the refining process, gasolines usually contain trace amounts of sulfur. At excessive concentrations, sulfur compounds may corrode the fuel system and the engine, and degrade the crankcase oil. Excessive sulfur compounds in the exhaust may corrode the exhaust system and reduce the efficiency of the catalytic converter.