Volatility

Volatility is a fuels ability to change from a liquid state to a vapor state. Since liquid gasoline will not burn, gasoline must vaporize before entering the combustion chamber. The rate at which gasoline vaporizes determines the amount of evaporative emissions released from the fuel system, and therefore has made volatility an environmental concern. The federal government has lowered the maximum allowable volatility, but certain driveability conditions have resulted.

A fuels volatility can be determined through 3 different tests: the Vapor-Liquid Ratio, the Distillation Curve, and the Reid Vapor Pressure test (RVP). The Vapor-Liquid Ratio test determines what temperatures must exist to create a vapor-liquid ratio of 20. The distillation curve is a graph showing the relationship between the temperature and the percentage of fuel evaporated. The fuel components that boil at relatively low temperatures of less than about 90°F are known as light ends and those that boil at about 300°F are known as heavy ends. The light ends are important for cold starting and cold weather driveability. The heavy ends provide engine power and are important for hot weather driveability. The proper mixture of these components provides proper operation across a wide range of temperatures. However, the distillation curve of a gasoline usually requires laboratory testing. The Reid Vapor Pressure (RVP) test measures the pressure (psi) a vaporized fuel exerts within a sealed container when heated to 100°F. The volatility increases proportional to the RVP. While the RVP can easily be measured in the field, the RVP may be misleading because it is possible for 2 fuels with the same RVP to have different distillation curves, and therefore, different driveability characteristics.

As stated, improper volatility can create several driveability problems. A low volatility can cause poor cold starts, slow warm ups, and poor overall cold weather performance. A low volatility may also cause deposits in the crankcase, in the combustion chambers, and on the spark plugs. A volatility that is too high could cause high evaporative emissions and a purge canister overload, vapor lock, and hot weather driveability conditions. Since volatility is dependent on the temperature, different fuels are used during certain seasons of the year, thus creating problems during sudden temperature changes.