checking fuel pressure - mechanical pump - a question

deQuincey

deQuincey

Quousque tandem...?
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any hydraulic technician on board ?

i would like to check the pressure that the fuel pump is able to give, there is this or other similar kits that contains the elements to be able to do so



the usual setup for a CS is this one




obviously there is not too much room to put the pressure gauge at the output of the pump, moreover, once the air filter cover is in place you can see nothing

but the hose that goes to the front carb is free and in a good position to hold the gauge



i learnt, from the specs book that mechanical pump gives 0,25 bar at 4000 rpms

so my questions are:

1- is there a chance of meassuring fuel pump pressure output ? you must consider that when the carbs are refuelling their reservoirs, and thus their floater valves opened, Pressure = 0

2- provided that both carb floaters are closed, do you think that the three pressures are equal ? i mean: ( P = Pr = Pf ), i think so

3- wich are the odds of those two floaters to be closed simultaneously ? very rare / almost all the time / once every x engine cycles ?

thanks for your help
 
deQuincey said:
wich are the odds of those two floaters to be closed simultaneously ? very rare / almost all the time / once every x engine cycles ?
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I don't think it works that way; the float valves in the carburetors aren't digital, they're analog. By that I mean that they aren't either "open" or "closed". Instead, the floats hold them at an intermediate position that creates some back pressure while allowing some flow through. Think of the carburetor float valves working like the regulators on an oxygen-acetylene welding rig - not like a toilet.

The hoses between the fuel pump outlet and the two carburetors are short, gently curved and fairly large in diameter, so the pressure should be consistent throughout. In other words, what you call "P", "P f" and "P r" should be the same (at least to an accuracy that's >> the resolution of your gauge). So putting the gauge near the front carburetor should work OK.
 
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DeQ: As you state, the pump pressure will vary with RPM and at 4000, the floats will be open to some extent all the time. T-connector for in-line pressure with compensating for the pressure drop across the connecting tube which could be long enough to clear and see the gauge?............

I believe these pumps have a check valve in which case there would be a baseline static pressure that could be determined.
 
bavbob said:
...the pump pressure will vary with RPM ....
Click to expand...

A mechanical fuel pump puts out a constant pressure equal to the force of the spring behind the diaphragm X the effective area of the diaphragm. That simple system has nothing to do with RPM. At higher engine speeds the diaphragm gets pushed back against the spring more frequently, allowing the pump to deliver a greater flow at higher RPM - but not a greater pressure.

A centrifugal pump (like a water pump) will put out higher pressure at higher speeds.
 
As JMackro stated, the output pressure in the three lines is the same, so you can install your gage in any of them, choose one for convenience.

If you want to know the full static output pressure: the float bowls will hold enough fuel to run the engine for more than long enough to check the pump pressure. You can verify this by disconnecting the fuel hoses from the carbs (make sure to route them to a can that will safely and without splashing catch the gas output from the pump first) then run the engine until it dies.

Once you are satisfied there will be enough time, reconnect the hoses, run the engine for a minute to re-fill the carbs, then again disconnect and install the gage. Run the engine at whatever speed you want, there should be more than enough time to check the gage. That will show the static output of the pump.

I think the pressure on a running engine would be more meaningful and I would install the gage in one of the lines and run the engine. If there is any variation in output pressure it should be due to the float needle valves opening slightly, but I don't think the trickle of gas going into the carbs will show up on the gage except possibly at high rpm.

More simply, the pump output should be much more than enough to fuel the engine through all ranges of operation.

Nice clear pictures, clean engine!
 
deQuincey said:
any hydraulic technician on board ?

i learnt, from the specs book that mechanical pump gives 0,25 bar at 4000 rpms
Click to expand...

I assume it is 0,25 bar above atmospheric pressure, not absolute pressure.

It seems like a system worth servicing as diaphragm leaks and float valve leaks could ruin the day. Is there any inspection maintenance recommended? Do pump check valves also fail?
 
I once had a problem with the fuel pump on one of my cars - may have been the '58 VW. A tiny bit of grit had lodged in the reed style check valve, so the pump wasn't putting out. I didn't know anything about filters then, just pinged that little grit into the weeds where it lies today, reassembled the pump and went on my way. To be fair, the check valve itself didn't "fail" but it didn't work either with that grit in there.

CS fuel pumps I've seen have crimped covers and can't be opened, too bad.
 
Let us know what you find. Since Volumetric flow has to increase with RPM and flow is x-sectional tube area X velocity, by default the pressure in the line has to increase as shown by the Hagen–Poiseuille equation since the line area is constant.
 
bavbob said:
Since Volumetric flow has to increase with RPM and flow is x-sectional tube area X velocity, by default the pressure in the line has to increase
Click to expand...

If the first clause in that sentence were true, then I'd agree with the second. That is, if a mechanical fuel pump always delivered greater flow with greater RPM then sure, the pressure would go up accordingly.

But it doesn't work that way. The motion from the eccentric on the camshaft serves to push the diaphragm back to draw in more fuel from the tank - it does not push the diaphragm out to expel fuel to the carburetors. The spring is what expels the fuel and it does that only at the flow rate the carbs demand. And that fuel is always at a constant pressure = diaphragm area X spring force. (OK, the spring force will vary slightly as the diaphragm moves, but that effect is minor.)

If you still don't believe me, do this thought exercise: When you are going up a steep hill in 3rd gear at 4,000 rpm with your foot on the gas, the engine needs a relatively large amount of fuel. But when going down that hill and using the engine for braking, you might also be in 3rd at 4,000 rpm, but your foot is off the gas and the engine is using very little fuel. A fuel pump whose rate of delivery was proportional to rpm couldn't handle these two situations.
 
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For clarification, is the point of this thread to determine if the fuel pressure is constant at a certain PSI level or is there some other intellectual exercise being pursued?
In my experience, fuel pressure in a carbureted engine is a balance between not enough and too much. For example, on my son's American V8 hot rod, the aftermarket Holley electric fuel pump is externally regulated to 5.5 psi so that the force of the fuel knocking at the float bowl door does not overpower the float and "sink" it thereby allowing too much fuel into the float bowl when it is not needed. Likewise, when the fuel needs of the engine are high (such as a 1/4 mile drag race) the fuel demands of the engine increase and a reliably regulated fuel flow helps to minimize the risk of running lean during a high demand duty cycle.

Disclaimer, I am not a hydraulic technician or anything remotely close. My knowledge of laminar flow is limited to the speed of beer coming out of a beer tap faucet !!! :wink:
 
jmackro said:
If the first clause in that sentence were true, then I'd agree with the second. That is, if a mechanical fuel pump always delivered greater flow with greater RPM then sure, the pressure would go up accordingly.

But it doesn't work that way. The motion from the eccentric on the camshaft serves to push the diaphragm back to draw in more fuel from the tank - it does not push the diaphragm out to expel fuel to the carburetors. The spring is what expels the fuel and it does that only at the flow rate the carbs demand. And that fuel is always at a constant pressure = diaphragm area X spring force. (OK, the spring force will vary slightly as the diaphragm moves, but that effect is minor.)

If you still don't believe me, do this thought exercise: When you are going up a steep hill in 3rd gear at 4,000 rpm with your foot on the gas, the engine needs a relatively large amount of fuel. But when going down that hill and using the engine for braking, you might also be in 3rd at 4,000 rpm, but your foot is off the gas and the engine is using very little fuel. A fuel pump whose rate of delivery was proportional to rpm couldn't handle these two situations.
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Fascinating, I have been doing thought experiments since yesterday. Another thought is that the flow rate is about the same at the time one upshifts even though the revs drop down. The float valve is the spoiler in my thoughts, when it is closed the fuel stays in the pump and the diaphragm is pushed down to BDC but the spring never pushes it up again until the valve opens. One can have many revs of the pushrod without the diaphragm moving any fuel then.

As a worrier type of person I would prefer a pump that is on the low pressure than high pressure side as the symptoms would be more benign.

The last thought experiment is to imagine a thick roof used to store fuel instead of the tank in the trunk. Gravity is the ultimate fuel pump, and it has never failed me. Who is in for a group buy?
 
C'mon this is a great exercise to get the cobwebs out. I went the next step and starting thinking about the function of the fuel pressure regulator in those of us with motronic systems. We all know that as usual, DeQ will be the definitive word.
 
late reply

I am a professional fluid dynamicist and this is a great thread.

I think the pressure gauge is a fine idea, but I am a data junkie. For those concerned about where to put it, that is where you can use the Poiseuille equation to convince yourself it doesn't matter. Anywhere between the pump outlet and carbs that is convenient. At 10-gallons/hour, the pressure drop through 8mm tubing is about 0.006 millibars/cm, so go nuts.

DP/L =q*128*mu/pi/D^4

q=10.5 cc/s
mu=6e-4 Pa*s
D=0.8 cm
(1e5 Pa/bar)
 
vraned said:
I am a professional fluid dynamicist and this is a great thread.
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I did not know! Now I have a sounding board for all my whacky initiatives:

In addition to putting the gas tank in the E9 roof to use gravity as a fuel pump I would like to put an alternator in series with my backyard sprinklers so I can generate electricity when I am watering the lawn. What kind of pressure differential do I need to run the alternator? Imagine that, a house fully powered by the water company. Better than solar.
 
That will cost you about $6.50/kWh. Of course, that is assuming you have an ideal alternator.
 
nice theoretical approach, here comes the praxis, after some empirical moments...



0,2 bar at iddle



and 0,2bar at any rpm,....no variation

and obviously, when engine is stopped pressure remains,



the setup adds a bit of control illusion to the engine bay ;-)

 
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Well, I will admit when I am wrong...........but what about when your engine actually has some dirt on it?
 
Nice! Another fine addition to an impressively composed engine bay.
 
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