Dual fuel PFI rails....

[Shannen Durphey]

Barry Tisdale wrote:
> 
> I understand the single rail, 2 fuels deal.  BUT, like you said, changeover will take a bit of time (WAY too long), plus dilute the two fuels, bit by bit, by the volume contained in the rails.  Common volume is maybe 120ml; would take several seconds to completely clear.
> 
> Ok, with 2 rails feeding each injector, there's maybe 2cc or so "common" volume between any rail and the injector.  The fuel pump prevents backflow; otherwise, it wouldn't hold pressure w/ the engine off - no check valve needed there.  

No, this is not true.  The pumps can do anything from prevent backflow
for a fair amount of time to not at all.  Don't let the fact that
there's a check valve in the pump convince you there's no need to
isolate the supply sides of the fuel system.  The check valves are
usually only a neoprene or rubber disc held against a plastic seat by
a small spring.

Each rail is fed by its own pump, & fuel exits via its own pressure
regulator.  Also, each rail 'out' line has a solenoid valve in
parallel with the *same* fuel pump; normally closed valves.  SO - when
FP#1 is ON, retrun line #1 is also open = normal operation.  Ditto for
setup #2 - separate FP, reg, solenoid valve, takk, the works.
> 
> Sure, you will get *some* mixing of fuel, but quite minimal, maybe 12cc or so per switchover.  Pump gas is returned to its tank, race gas to its.  Hardly any mixing.  As soon as the electrical switchover takes place, the rail NOT being used is OFF, pressure-tight.  Remember liquids are incompressable; the fuel in the unused rail has *nowhere* to go; will get *slight* mixing @ the interface between the 2 fuels due to turbulence, but again, minimal.

What about the temperature of the engine heating the fuel in the
"closed" rail?   With double rails you could experience problems with
vapor buildup in the unused rail, leading to the need to purge the Hi
Octy rails before the boost comes up.  There's also the possibility
that this system will force vapor or low octane fuel into the
injectors from the sealed and heated rail during boost.  I'm not going
to say that a small amount of cheap stuff forced through the injector
will cause a problem, but vapors will.  So, if you're cautious and
design the purge cycle into the changeover, you're back to the ability
to use one rail, and energize the high octane pump before it's
needed.  Really, at 1 psi boost do you need to have a full
concentration of race fuel?  And, like Jason said, delaying the
switching of the return side solenoids will purge the low octane fuel
from the rail.  Dumping small amounts of high-go-gas into the low
octane side will only improve the contents of that tank.

Shannen
> 

> Barry - Syclone #26