No subject

the pump is OK for the standard capacity fuel pump but possibly not for the 
flow from a larger capacity pump. The pressure drop between the fuel tank 
and pump increases with the square of the flow - doubling the flow causes 
four times the pressure drop. As the fuel tank is at atmospheric pressure 
(and if we ignore the fuel level in the tank), the fuel in the line to the 
pump will be under vacuum and the more volatile components will start to 
vaporize, causing bubbles.

The ideal pump capacity is the smallest pump required to provide the flow of 
fuel for full load operation, plus a minor excess amount.  This results in 
the minimum flow from the fuel tank to the pump.  Any pump with larger 
capacity provides no benefit but results in higher flow from the fuel tank 
to the pump compounding the problems you are having.


If the existing pump is to be retained, the following suggestions made by 
others should work :

a) install a transfer tank between the fuel tank and pump - the top of the 
transfer tank should be connected to the fuel tank vent and, most 
importantly, the return flow from the engine must be rerouted from the fuel 
tank to the transfer tank. The flow from the fuel tank to the transfer tank 
is then equal to the fuel flow to the engine;

b) connect the return flow from the engine directly to the inlet of the pump 
(in principle, this is the same as the transfer tank option but not as 
robust a design - apart from the temperature build up issue, any air 
displaced from the return line has no easy vent path and may become trapped 
in the pump inlet tubing);

c) add a second outlet to the fuel tank and tie this into the existing 
tubing close to the pump inlet.

The piping and tubing from fuel tank to pump, in all cases should slope 
downwards from tank to pump to avoid pockets where air or vapor can collect. 
To minimize pressure drop the internal diameter should be at least equal to 
the pump inlet connection.

Do you know the fuel flow required by the engine?  If so, and it can be done 
safely, why not determine the flow from the tank by temporarily 
disconnecting the tubing at the pump and measuring the volume of fuel 
drained over a known time interval.  Ideally the tank level would be low. 
If the measured flow is less than the flow required, repeat the measurement 
but disconnecting the tubing as close to the tank as possible (to eliminate 
any restriction between tank and pump).  If still too low, the tank will 
have to be modified, I suspect, as you have eliminated tank internal issues 
already.

Alan

85 urq








----- Original Message ----- 
From: "Ben Swann" <benswann at verizon.net>
To: "'Claus Vegener'" <vegener at post7.tele.dk>; <a.sigal at bluewin.ch>
Cc: <urq at audifans.com>
Sent: Thursday, April 17, 2008 1:36 PM
Subject: Re: [urq] Fuel Pump surge - still having problems


> Claus,
>
> I sure hate to have to get rid of this tank.  I don't think I'll be able 
> to approach 20
> gal capacity and fit into this space like this tank does.
>
> I have to admit the feed is horrible.  I took the car around this 
> afternoon and when I
> got home the problem was so bad the car was starving for fuel at even very 
> low power.
> Seems once a little air/vapor gets in the feed line it gets worse and 
> worse.
>
> It does seem weird there are bubbles forming when the tank is even over 
> half full.  I am
> pretty sure the problem begins to occur even if not driving, so I don't 
> think it is due
> to slouching around.  The pickup on this tank is down low in a separate 
> well, so always
> the intake is covered with fuel, unless empty past the lowest guage 
> reading.
>
> I think this problem is caused by the restrictive feed line - the pump 
> "suction" is
> causing the fuel to vaporize (cavitate) it is not necessarily air, but 
> fuel that has
> vaporized.  Your description below is somewhat descriptive.  The slope of 
> the line makes
> a difference.  If I lift the part of the line going into the pump, it will 
> even cause
> the car to stall - pump just sucks vapor and not fuel.
>
> I looked at my lines again - I may have a few too many fittings and some 
> line size
> changes - these don't help.  I can rework, but still would be good to 
> divert the fuel -
> maybe can be cooled somewhat by an in-line accumulator - similar to a 
> small surge tank.
>
> So the only way you could get rid of the bubbles is using a fuel cell?
>
> Ben
>
> -----Original Message-----
> From: Claus Vegener [mailto:vegener at post7.tele.dk]
> Sent: Thursday, April 17, 2008 4:09 PM
> To: Ben Swann; a.sigal at bluewin.ch
> Cc: urq at audifans.com
> Subject: Re: [urq] Fuel Pump surge - still having problems
>
> Recirculation the fuel in this way will heat up the fuel.
>
> That's why it is supposed to go to the tank, or an fuel cooler had to be 
> at
> the return line.
> I had also problems with air bubbles in the supply line to the pump; but 
> the
> issue is a bit different.
> But I agree, the bubbles is from fuel slouching in corners in the original
> Urq tank.
> I installed a fuel cell instead of the tank (in fact 2 small ones to have
> differnt fuel)
> just flat in the trunk and have still less gravity to help
> as the fuel pump is inside the trunk at the right side.
> 1) return line must go to bottom of tank to prevent air bubbles
> 2) I have first one 200 l/h Bosch pump, but that is not enough for 500 Hp
> even it should be.
>    So 2 in parallel - the second started from a boost/rpm matrix in the
> 034EFI
> 3) 2 inline fuel filters with glass for visibility - and I noticed, that 
> if
> they catch bubbles,
> they could stand just before the pump intake or the top of the filters
> (sloping downwards to the pumps a bit) for 30 km making troubles.
> Systems from a house radiator central system for freeing air don't work ;
> but raising the pumps a bit, so bubbles could run upwards as air want, 
> make
> the pumps to "eat" the bubbles at once.
> And solve the problem. So not so much concern about gravity. The pumps
> should be able to suck a little - but not with air.
> Claus
> ----- Original Message ----- 
> From: "Ben Swann" <benswann at verizon.net>
> To: <a.sigal at bluewin.ch>
> Cc: <urq at audifans.com>
> Sent: Thursday, April 17, 2008 8:27 PM
> Subject: Re: [urq] Fuel Pump surge - still having problems
>
>
>> I'm beginning to like this idea as a solution - it would also tend to 
>> keep
>> the filter
>> sock blown free of any restriction and apply additional positive pressure
>> to the intake.
>>
>> I'm not quite pictureing exactly what you are describing, but kind of 
>> like
>> the idea of a
>> hard 1/2" Cu line going directly into the tank - coupled by a small
>> Fernco(rubber pipe
>> joiner).  The return line could feed into the tee.  Another tee into the
>> hard line could
>> use the return line to tank fitting to flow in the opposite direction -
>> providing
>> additional fuel feed instead of acting as a return.
>>
>> Doing this would be a lot better than performing any surgery on the
>> otherwise perfectly
>> good tank.  Maybe need to feed a tiny surge tank to prevent any voids - 
>> is
>> that what you
>> are suggesting.
>>
>> I think there is a solution in the works - some Cu pipe, fittings, tees,
>> Mapp torch - it
>> has to at least result in improved flow into the pump.
>>
>> Now if I can just drive the car enough to clear out remaining 14 gal of
>> fuel -
>> problematic with sloppy shifter that has also been driving me nuts - at
>> least I found
>> the problem was a loose ball on the shift link.
>>
>> Appreciate any input into this scheme.
>>
>> Ben
>>
>>
>> -----Original Message-----
>> From: Ado Sigal [mailto:a.sigal at bluewin.ch]
>> Sent: Thursday, April 17, 2008 1:50 PM
>> To: Ben Swann
>> Cc: urq at audifans.com
>> Subject: Re: [urq] Fuel Pump surge - still having problems
>>
>> I believe that your problem is that under acceleration, the pump manages
>> to draw the air from the fuel surface. Below 1/2 tank suggests this to
>> be the case. A proper surge baffle of decent capacity, which Ur and most
>> tanks with outside pumps are missing, would be a solution. (reason why
>> in tank pumps don't have this problem, is because there is always decent
>> surge baffle pool, and large pump intake close to the bottom). If you
>> don't want to open the tank, other solution would be to return the fuel
>> directly in front of the pump intake, instead of elsewhere in the tank.
>> Simple longitudinal small capacity self bleeding cylinder (top line from
>> the tank, bottom to the pump, and fuel return on the side) directly
>> before the pump, should solve the cavitation and fuel starvation
>> problem. In this scenario, the amount of fuel drawn from the tank would
>> be only the amount of fuel used by the engine.
>>
>> Regards,
>>
>> Ado
>>
>>
>>
>> Ben Swann wrote:
>>
>>>Essentially what you say that I'm "flowing way too much fuel to return it
>>>to the pump
>>>unused" is how it is done usually.  If not I'd like to know.
>>>
>>>Still - I'd like to find out how to keep the cavitation from 
>>>appening.  -
>>>some point during running, the fuel starts to bubble and cavitation 
>>>during
>>>running.
>>>
>>
>> I do notice it seems to happen more as the tank goes to below 1/2 -
>> still a lot of fuel in the tank tho.
>>
>>>Ben
>>>
>>>
>>>
>>
>>
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>
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