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No question, Charlie - the more of your fuel
line kept under EFI pump level pressures , the less chance for vapor lock.
In tank pumps certainly do that - but, as you point out there are other
considerations. Wing root sounds like a pretty good compromise in
luie of in-tank pumps. I considered that but in the end decided
against it as it would have high pressure fuel lines inside my cockpit - which I
personally do not favor {:>)
Don't know for certain (and may never) the cause of the
latest sputtering engine event - but, in absence of in-tank pumps and/or return
to tank - I still believe that pressure applied by a boost pump can prevent
vapor lock (based on my own personal experience with my unique
installation)
Ed
Sent: Saturday, August 13, 2011 9:06 AM
Subject: [FlyRotary] Re: vapor lock
The really frustrating thing about all this is that every
installation is different (not to mention that it's only speculation as to what
caused this particular power loss).
Sixties-era cars (carb, engine
mounted fuel pump) had vapor lock problems all the time in hot weather. Modern
cars, almost never (in-tank high pressure pump). Keeping the pumps as low &
as close to the tank(s) as practical would seem to be the best path. Van
recommends mounting pumps on the floor in the cabin. That means that the max
lift would be maybe 3-4 inches, through a -6 line, and nowhere near the high
under-cowl temps. There's a guy flying an injected Lyc on ethanol-laced mogas
who never has a problem with vapor lock. He removed the mech pump and uses
wingroot mounted electric pumps.
If it weren't for the maintenance
related inconveniences, I'd seriously consider in-tank pumps, as others have
done.
But we still don't know whether this is what caused the recent
power loss....
Charlie
On 08/13/2011 06:20 AM, Ed Anderson
wrote:
I should have added - the best overall approach -
returning hot fuel to the heat-sink tanks and drawing new cooler fuel into the
lines.
Ed
Sent: Saturday, August 13, 2011 6:47 AM
Subject: [FlyRotary] Re: vapor lock
Ok, Finn, that was my guess as
well.
That then brings up a question - my understanding
of "vapor lock" is that it is caused by a low pressure area/combined with hot
fuel on the EFI pump intake which cause the gasoline to flash to vapor -
naturally the EFI pumps can not pump vapor - therefore as fuel is injected
from the high pressure side of the pump (reducing pressure on that side),
vapor can form there as well. In any case, insufficient fuel is injected
into the engine.
Since the injectors are still clicking open, it would
seem that any vapor on that side of the pump already has a chance to vacate
the line (through the injector) - so my assessment is that it is not the
relief of vapor/gas from the high pressure side that remedies the problem,
it's removing the gas from the low pressure side (pump inlet) and thereby
permitting liquid fuel to be pumped that "cures" a vapor lock situation.
So I am puzzled why a gas vent on the high pressure side
would have much (if any) effect on vapor lock. IF there is pressure on
the injector side - I question whether it would be as high as pump pressure -
and even if it were, the injector opening would provide a path for it to be
release - not to mention the pressure regulator. So as I said -I'm a bit
puzzled as to the mechanism that a vent in the high pressure side prevents
vapor lock.
In my opinion, there are two ways to reduce/eliminate
the vapor in the low pressure side - either cool the fuel sufficiently
(somewhat difficult to do) or to increase the pressure in the low pressure
line forcing the vapor back into the liquid - ergo - use a boost
pump.
FWIW
Ed
Sent: Saturday, August 13, 2011 2:53 AM
Subject: [FlyRotary] Re: vapor lock
Hi Ed,
I believe it's simply a return to the tank from
the high-pressure side via a very small orifice. How small I do not
know.
Finn
On 8/10/2011 9:28 AM, Ed Anderson wrote:
The one I potential preventive measure/fix I have not
looked into is the vapor by-pass/dump that I know a few folks are
using. I search the archive but could not find a description of this
method - anyone care to provide
one?
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