return line to gas tank

[MJones]

Somebody wrote:

>>Im sure you think it ws vapor lock, you were seeing,  my only question is 
that
how in hell at ~40 psi DPFI line pressure are you going to boil gasoline 
off
into vapor, and not log since boil off the coolent at ~16 lbs rad cap
pressure? >>

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.

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 FI'd aircraft 
engines are hard to start hot even with boost pumps-- vapor lock is still a 
problem.)

Mike Jones

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