Volatility

Volatility is a fuel's ability to change from a liquid state to a vaporized state. Liquid gasoline must vaporize before entering the combustion chamber, since liquid gasoline will not burn. The rate at which the gasoline vaporizes determines the amount of evaporative emissions that are released from the fuel system.

Volatility can be determined through three different tests:
^ The Vapor-Liquid Ratio
^ The Distillation Curve
^ The Reid Vapor Pressure Test (RVP)

The Vapor-Liquid Ratio test determines what temperatures must exist in order to create a vapor-liquid ratio of 20.

The distillation curve is a graph showing the relationship between the temperature and the percentage of evaporated fuel. The fuel components that boil at low temperatures (below about 90°F) are known as the light ends and the fuel components that boil at about 300°F are known as the 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 provide a 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 (kPa/psi) that the vaporized fuel exerts within a sealed container when the fuel is heated to 100°F. The volatility increases proportional to the RVP. While the RVP can easily be measured in the field, this may be misleading because it is possible for two fuels with the same RVP to have different distillation curves, and therefore, different driveability characteristics.

As stated, an 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, the combustion chambers and the spark plugs. A volatility that is too high could cause high evaporative emissions and purge canister overload, vapor lock, and hot weather driveability conditions. Since the volatility is dependent on temperature, different fuels are used during certain seasons of the year, thus creating problems during sudden temperature changes.