Fuel polishing with bleed valve

[Ed Shankle]

Reading the latest post where leaving the bleed valve open a bit for polishing was mentioned, once again had me thinking "really?" Narrow fuel lines, 20 gallons of fuel, how much motoring around would I have to do to actually substantially polish the fuel? That said, I've dutifully followed the recommendation, but in the back of my mind wonder how effective it really is. Does anyone know the flow rate from the return hose to the tank? I don't recall that being defined previously.
Figured I'd start a new post on this as I'm sure there will be discussion😁
Regards,
Ed

[Stu Jackson]

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Does anyone know the flow rate from the return hose to the tank? I don't recall that being defined previously.
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It would be the difference between the fuel pump rate and the fuel use of the engine.  The fuel pump rate is constant.  The engine use varies. 

Fuel pump flow rates are discussed here:  http://c34.org/bbs/index.php/topic,2515.15.html

Engine fuel "use", i.e., consumption, has been measured over the years at 0.5 gph for M25 series engines to around 1.0 gph for the M35 series.

[KWKloeber]

Stu,

Not exactly, because that formula assumes no losses/restrictions (thru the 5/16" fuel line, secondary filter, the injector pump ports, the bleed valve, and (more significantly) the tiny (~~1/8"?) injector banjos and hoses (~~3/16"?) in between the 3 injectors, and the 1/4" return line -- all of which restrict the bypass flow.)

The Facet pump is rated (open air, no elevation or friction losses) at ~32 gal/hr, ~7 psi -- and so in comparison the engine fuel use is insignificant (why the pump basically stops ticking once the lines are pressurized.)  But subtracting even 1.0 gph for the injectors and injector return flow, opening the bypass valve does not pump 31 gal/hr to the tank (e.g., turn the tank over in less than an hour.)  Even halving the pump rate to 16 gph due to losses AHEAD OF the pump, the tank doesn't turn over that quickly.  The conclusion is that the way the bleed is plumbed, the flow is SEVERELY restricted ahead of the Facet pump.


Ed,

This is TMI (or as Stu says, "Explaining how a watch works"), but if it helps a couple owners who have never looked at their fuel system, get a feeling about what goes on under the hood -- it's a good thing.

Depending on RPM the engine uses 1/2 or more gal/hr, and there's already some (minimal) return flow to the tank (what the injector pump supplies above what the injectors need.  It's probably so little that it can be ignored, but let's assume the engine use and fuel return equals 1 gph.

So, that leaves (theoretically) about 31 gal/hour "excess" capacity that the pump could have available to return fuel to the tank.  That's highly theoretical of course, ignoring the restrictions I describe above.  The most critical restriction on the XP would be that, with the bleed valve open, the bypass goes to the #1 injector banjo, travels through the #2 and #3 banjos, and the two tiny (3/16"?) lines between them.  That has to be the most significant of all the restrictions.  Then the fuel goes from the #3 injector banjo to the 1/4" return line to the tank.

On the M-25, the bleed is a screw valve that's open to air (below)





M-25'ers who have switched to the XP bleed knob, have tee'd the valve directly into the 1/4" return line, which bypasses all the restrictions of the banjos and tiny injector lines.  Instead, I left my bleed valve open to air:





As I have said before, I catch the bleed in a cup so that (w/a false sense of security) I can eyeball my fuel quality:





32 gal/hr is (4) 16-oz plastic cups/minute.  I haven't timed it, but guessing from memory that with my valve probably 1/2 - 3/4 open I can fill a 16-oz Solo cup in about 1/2 minute. (Or 32 oz/min, or 15 gal/hour.)

1/2 minute per fill is a guess -- maybe more-maybe less, but certainly within the range of the pump considering restrictions and losses ahead of the bleed valve.   

SO, THEORETICALLY, if I tee'd my valve directly into the return line, I might be able to turn over the tank pretty quickly.  But NOT the way that the M-25XP is plumbed. 

HOWEVER, BIG HOWEVER - I do not know if leaving my bleed full open (without the banjo etc restrictions that are on the XP) would starve the injector pump. 


kk

[britinusa]

Wow, I understood most of that Ken 

On my boat, the bleed valve is literally just cracked open, I doubt it's 1/8th of a turn but I'll check it tomorrow when we load up for an 8 day trip down to Biscayne Bay.

Paul.

[Ed Shankle]

Not TMI Ken. Much of what you described for the 25xp is exactly what I've been ruminating on, questioning significant polishing if the engines aren't run a lot of hours. If they are run a lot then additional fuel (presumably clean) would be added anyway!
Sounds like the simplest way to determine the flow is detaching the return hose from the banjo, replace it with another length of fuel line into a cup/bucket/whatever and run a test. Maybe 2200 rpm's and time how long it takes to get a pint.
Not a burning issue, just looking to determine if it is real or wishful thinking.
regards,
Ed

[KWKloeber]

Ed

Yep that would be an easy way to measure it, both w/ and w/o the bleed open. 
If you get right into it, you could also check whether bypassing w/o the restriction starves the injector pump/affects the engine -- by directing the bleed knob bypass to a bucket.  You'd have to block off the jumper hose to the #1 banjo.

ken


Paul:

Darn, I was trying to make that as confusing as I could.