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Volatility
Gasoline is more volatile than diesel oil, Jet-A or kerosene, not only because
of the base constituents, but because of the additives that are put into it. The
final control of volatility is often achieved by blending with butane. The Reid
Vapor Pressure test is used to measure the volatility of gasoline. The desired
volatility depends on the ambient temperature: in hotter climates, gasoline
components of higher molecular weight and thus lower volatility are used. In
cold climates, too little volatility results in cars failing to start. In hot
climates, excessive volatility results in what is known as "vapour lock" where
combustion fails to occur, because the liquid fuel has changed to a gaseous fuel
in the fuel lines.
In the United States, volatility is regulated in large urban centers to reduce
the emission of unburned hydrocarbons. In large cities, so-called reformulated
gasoline that is less prone to evaporation, among other properties, is required.
In Australia summer petrol volatility limits are set by State Governments and
vary between capital cities. Most countries simply have a summer, winter and
perhaps intermediate limit.
Volatility standards may be relaxed (allowing more gasoline components into the
atmosphere) during emergency anticipated gasoline shortages. For example, on 31
August 2005 in response to Hurricane Katrina, the United States permitted the
sale of non-reformulated gasoline in some urban areas, which effectively
permitted an early switch from summer to winter-grade gasoline. As mandated by
EPA administrator Stephen L. Johnson, this "fuel waiver" was made effective
through 15 September 2005 . Though relaxed volatility standards may increase the
atmospheric concentration of volatile organic compounds in warm weather, higher
volatility gasoline effectively increases a nation's gasoline supply because the
amount of butane in the gasoline pool is allowed to increase.
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