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RE: [FlyRotary] Returnless Fuel System - Fuel Cooler
Hey Todd,
I think you may have missed the point or lost
the plot good buddy. Firstly, where did the 1/8" line come into
it?? Secondly, there 'ain't no "impellors" in an EFI pump.
They use rollers!! TOTALLY different animals to centrifugal
pumps!.
I AM also aware that the flow through any
orifice is controlled by the differnetial pressure. What I still
don't get is the bit about "pressure recovery", as my issue is the
flow in the open flow return line as it exits rthe
regulator !!! The regulator is providing that back
pressure in the fuel rail, and as the "back pressure"
increases, the pressure drop through the pump outlet orifice
decreases. At any rate, there is NO chance of cavitation unless
there is no fuel head on the pump inlet!
Anyway, let me rephrase the questions
again:
SITUATION #1
I have an EFI pump that has a good head of
fuel on it. It is connected to a fuel rail in the normal way, via a
pressure regulator set at 45 PSI. This means that there
is 45 PSI back pressure on the pump. I observe a given fuel
flow out the return line. When the engine is started, the regulator
brings the rail pressure back to about 32-33 PSI. I have about 25" MAP
(Manifold Absolute Pressure) at idle. I notice the fuel flow out the
return line INCREASES as the rail pressure drops when the engine
starts.
The regulator is connected to an open
flow 1/4" ID line which returns to the surge tank in the normal
way, and the surge tank is vented to atmosphere. The pump is
pumping through a restrictor, say 3/16". The question
is:
QUESTION #1
"What limits the flow of the fuel IN THE RETURN
LINE??. The 3/16" restriction, or the 1/4" ID of dump line
itself. "
By observation, I know that as load
(MAP) is increased on the engine and the RPM rises, and as more &
more fuel goes to the injectors, the fuel pressure in the rail rises in
concert with the increased manifold pressure, (therefore even more back
pressure on the pump up to 45 PSI @ WOT), and the amount of bypass fuel
from the regulator diminishes. So the only time I'm liable to have flow
issues in the return line is at idle, (which would be indicated by the
regulator not being able to control the pressure in the rail).
By observation, fitting a high capacity Bosch
Motorsport or SX pump, I actually have this problem, which is
why I use a 5/16" return line in these instances. The problem then goes
away.
Again by observation, at the
limit, IF the EFI pump is incapable of supplying the necessary fuel
to the engine at WOT and high RPM, the fuel flowing from the return line
ceases completely. Soon after this, as the engine RPM continue to
rise, the pressure in the fuel rail begins to drop, as the pump
cannot pump any more fuel, and the engine begins to run out of fuel.
I have watched this on the dyno MNAY times.
SITUATION #2
I have the same EFI pump, this time on the
bench. It has a good head of fuel on it.
QUESTION #2
"For any given restrictor, say our 3/16" one
again, does the rate of flow from the pump into an open container (vented
to armospheric - no other restrictions - open flow)"
a. increase,
b. stay the same, or
c. decrease
if I attach a length of fuel line of 1/14" ID to
the pump outlet.
QUESTION #3
"What happens if I drill the outlet out to
1/4" and do the same as in Question #2.
QUESTION # 4
"What happens if I drill the restriction out to
9/32" "
Cheers buddy,
Leon
(waiting with my little velvet bag to catch the
pearls).
----- Original Message -----
Sent: Tuesday, February 22, 2005 8:34
AM
Subject: [FlyRotary] Re: For Todd "Vena
Contracta" was Re: Returnless Fuel System - Fuel Cooler
Hi Leon;
I'm a little late responding as I spent
the weekend rebuilding my dirt bike with one of them ole fashioned piston
engines what go up & down to make something go round 'n round? What a
goofy idea!
Anyways, back to the subject at hand.
Pressure recovery ?... so what? Well without pressure differential there
is no flow. None. I realize that jets are used to meter flow and that a
restriction is an impediment to flow, however my point was that if the EFI
pump has an outlet of say 1/8" into a 3/8" line (or in Bob's case, his 1/4"
return line) it does not have the equivalent flow of a 1/8" line. While
it will not have full flow of an unrestricted line due to permanent
pressure loss, it will recover much of its pressure resulting in a much
greater flow than would be provided by a 1/8" tube. To calculate this the Beta
ratio of the "orifice" is required, however it really isn't that important.
Besides, I can't find right now the required formula. I should get back to
studying this as I return to school for 4th year in 2 months and I can be
assured that there will be at least one twisted question concerning this on
the incoming exam.
Someone had mentioned drilling out the
pump outlet to reduce this restriction, but that would be a really bad idea.
The pump outlet is usually sized smaller than the pipe that it is sized for in
order to provide adequate back pressure to the pump to prevent cavitation. If
you drill out this outlet, then bubbles can/will form in the impellor and the
resulting cavitation will result in less flow and quite possibly the vapour
lock that we are trying to avoid. This restriction does not diminish the
requirement for a proper sized tube to be plumbed to it in order to achieve
rated flow.
Just my 2 bits worth
Cheers
Todd
So I just can't see what the size of an
attached pipe has to do with the flow rate through a "vena contracta"
("vena constricta"??). You also say "but much of this pressure is
recovered downstream" Hmmmmmm, ... I say "so what"??. We aren't
interested in pressure drops or pressure recovery, ... it's flow rates
that are the SOLE issue, and the capacity of an open
flow 1/4 inch line to get rid of the fuel dumped by the
regulator.
Leon
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