PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 1
Volatility in Gasoline
Volatility, as applied to gasoline, is its tendency to change from liquid to
vapor at any given temperature. The volatility of gasoline affects ease of
starting, length of warmup period, and engine performance during normal
operation. The rate of vaporization increases as the temperature increases
and as pressure decreases. The volatility of gasoline must be regulated
carefully so that it is volatile enough to provide acceptable cold weather
starting, yet not be so volatile that it is subject to vapor lock during
normal operation. Refiners introduce additives to gasoline to control
volatility according to regional climates and seasons.
a. Starting Ability. To provide satisfactory cold weather performance
and starting, the choke system causes a very rich mixture to be delivered to
the engine. Gasoline that is not volatile enough will cause excessive
Because unvaporized fuel does not burn, it is wasted. This reduces fuel
economy and causes a condition known as crankcase dilution.
b. Crankcase Dilution. Crankcase dilution occurs when the fuel that is
not vaporized leaks past the piston rings and seeps into the crankcase. The
unvaporized fuel then dilutes the engine oil, reducing its lubricating
A certain amount of crankcase dilution occurs in all engines during warmup.
It is not considered harmful in normal quantities because it vaporizes out
of the oil as the engine warmsup. The vapors are then purged by the
crankcase ventilation system.
c. Vapor Lock. Vapor lock is one of the difficulties experienced in
hot weather when using highly volatile fuels. When fuel has a tendency to
vaporize at normal atmospheric temperature, it may under higher temperature
form so much vapor in the fuel line that the action of the fuel pump will
cause a pulsation of the fuel vapor rather than normal fuel flow. Heat
insulating materials or baffles are often placed between the exhaust pipe
and fuel line to help avoid vapor lock. Hot weather grades of gasoline are
blended from lower volatility fuels to lessen the tendency toward vapor