return line to gas tank

[Greg Hermann]

>Vapor pressure of various components of a liquid are dependent on
>temperature only.  When the vapor pressure is raised (by heating) to the
>pressure of the local atmosphere, it will boil.  Because some constituents
>of gasoline have a very low vapor pressure in order to aid in cold
>starting, they will indeed boil at pressures commonly seen in a FI system
> -- IF the rail temperature gets high enough, such as when pulling a  10
>mile 7% grade at 10,000 ft (remember, most of these systems are referenced
>to atmosphere, so at 10,000 ft you've lost about 3 psi of atmospheric
>pressure, which compounds the problem).  Many areas of the country utilize
>two or more blends of fuel, a summer blend when the extra volatility would
>only increase emissions and cases of vapor lock, and a winter blend when
>the volatility is necessary for starting.  At 40 below, even gasoline needs
>all the help it can get.

Very well stated, thanks!
>
>Aviation fuel is missing most of those 'lighter' constituents in order to
>preclude  obviously undesirable vapor lock.  Thus, carb'd piston aircraft
>engines are notoriously hard to start when stone-cold.

And, because a lot of the lighter components of gasoline are of higher
octane rating, their absence in av-gas helps to explain why high octane
av-gas is so bloody expensive--heavier components which also have high
octane are what are costly.

Regards, Greg