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(Fuel) Pump physics question? #5103422 05/12/19 01:10 PM
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Ed_Flecko Offline OP
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I *think* this is a physics question?

smile

Whether we're talking about a fuel pump, or any other pump that's submersed in a liquid, I've heard that "head pressure" will make the pumping of said liquid easier which in turn means your pump in question will last longer? I've heard the deeper the pump is submerged, the easier it is to pump the liquid.

I never run my gas tank below 1/4 tank, for other reasons, but I've also heard that by doing so your fuel pump will last longer by not overheating (the gasoline lubricates and cools the pump) and by not having to work as hard to pump fuel due to head pressure.

If what I've heard is true, then maybe it's smart to try and keep your tank above 1/2 full to make the work of your fuel pump as effortless as possible?

Thoughts?

Thank you,
Ed


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Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103431 05/12/19 01:26 PM
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RayCJ Offline
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Yes, it's a physics questions. No, submerging a pump deeply does not make it easier to pump the fluid. Actually, quite the opposite. As the fluid delivery lines are routed higher above the pump it must work against more gravity to deliver the fluid.

Ray

Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103435 05/12/19 01:29 PM
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MrMoody Offline
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I don't think a fuel tank is deep enough to make any real difference.

What does make a difference is that in-tank electric pumps are cooled by the liquid around them, so running nearly empty deprives them of some of their cooling. If this is done habitually, it can shorten the pump life. I've seen this firsthand.


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Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103440 05/12/19 01:38 PM
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eljefino Offline
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In physics, I have a dug (shallow) well, and the pump is in my basement. This only really works down to 25 feet, because at 30 feet, you can pull a perfect vacuum and the water will just stay down there.

If I had a deeper well, I could put the pump down there and it could pump upwards a nearly unlimited number of feet, depending on its design.

An in-tank fuel pump can dead-head 150 psi, though only 60 are needed on your typical port injection setup.

A carbureted "lift" pump makes around 5-6 psi, same as an engine-mounted mechanical pump. The lift pump won't vapor-lock, though, and is "better" since it's not drawing a partial vacuum to get fluid (with a propensity for evaporation) where it needs to go.

Lifting gasoline 2 or 3 feet to get up to carburetor/ injector level is no challenge, so an empty vs full tank doesn't really matter in that regard.

Re: (Fuel) Pump physics question? [Re: MrMoody] #5103442 05/12/19 01:46 PM
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MolaKule Offline
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Originally Posted by MrMoody
I don't think a fuel tank is deep enough to make any real difference.

What does make a difference is that in-tank electric pumps are cooled by the liquid around them, so running nearly empty deprives them of some of their cooling. If this is done habitually, it can shorten the pump life. I've seen this firsthand.



I think it is a "washout" as well in terms of a fuel pump

While you may have some fluid pressure sitting on top of a pump, the pump must provide enough pressure for the 'uphill" pumping to the fuel supply system.

Last edited by MolaKule; 05/12/19 01:46 PM.

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Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103471 05/12/19 02:16 PM
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RDY4WAR Offline
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I third the notion that it won't make any distinguishable difference.

As some anecdotal info, I have experienced 3 fuel pump failures in my life. All 3 occurred right after filling up the tank.


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Re: (Fuel) Pump physics question? [Re: MrMoody] #5103488 05/12/19 02:36 PM
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benjamming Offline
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Originally Posted by MrMoody
I don't think a fuel tank is deep enough to make any real difference.

What does make a difference is that in-tank electric pumps are cooled by the liquid around them, so running nearly empty deprives them of some of their cooling. If this is done habitually, it can shorten the pump life. I've seen this firsthand.


I'm pretty sure that they're primarily cooled by the fluid going through them.

Re: (Fuel) Pump physics question? [Re: benjamming] #5103513 05/12/19 03:12 PM
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The_Eric Offline
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Originally Posted by benjamming
Originally Posted by MrMoody
I don't think a fuel tank is deep enough to make any real difference.

What does make a difference is that in-tank electric pumps are cooled by the liquid around them, so running nearly empty deprives them of some of their cooling. If this is done habitually, it can shorten the pump life. I've seen this firsthand.


I'm pretty sure that they're primarily cooled by the fluid going through them.



You could well be right.

I'll also say that basically all pumps after the early 2000s or thereabouts are sitting in their own module which has a check valve to let fuel in but not back out. Therfore the tank can be nearly empty but the pump will still be immersed in fuel. My understanding is that it was done to prevent lean misfires from low fuel pressure when maneuvering with a low fuel level. Even a lean misfire will still release extra hydrocarbons into the cat, thus shortening its life.


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Re: (Fuel) Pump physics question? [Re: RayCJ] #5103576 05/12/19 04:41 PM
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ammolab Offline
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Originally Posted by RayCJ

Yes, it's a physics questions. No, submerging a pump deeply does not make it easier to pump the fluid. Actually, quite the opposite. As the fluid delivery lines are routed higher above the pump it must work against more gravity to deliver the fluid.

Ray



The question doesn’t move the pump position deeper but just raises the level of fluid above it. The pump remains stationary in the same position below its delivery lines, so no more gravity exists as you fill the tank.

But yes, Not enough to matter.


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Re: (Fuel) Pump physics question? [Re: eljefino] #5103606 05/12/19 05:26 PM
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Rand Offline
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Originally Posted by eljefino
In physics, I have a dug (shallow) well, and the pump is in my basement. This only really works down to 25 feet, because at 30 feet, you can pull a perfect vacuum and the water will just stay down there.



I'd agree except your example is flawed shallow and deeper jet pumps... you can go way below 30ft with a jet pump..


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Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103621 05/12/19 05:49 PM
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mk378 Offline
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In a simple well with a pump at the top of the well and a suction pipe to the bottom, atmospheric air pressure on the surface of the water is the only force available to bring water up to the pump. This does indeed stop working around 25 feet.

The "deep well" version of the jet pump cycles some pressurized water down the well to operate an ejector below the surface. This can bring water up from a much greater depth, though still not unlimited. The system requires installing two pipes from the pump to the well, though they are occasionally installed as a small pipe inside a larger pipe.

Last edited by mk378; 05/12/19 05:52 PM.
Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5103783 05/12/19 09:12 PM
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Shannow Offline
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The principal is "Nett Positive Suction Head", maintaining a pressure such that at the inlet of the pump, the column of fluid doesn't "separate", in the worst case (eljefino's example), or cavitate (still pump but have the fluid form bubbles (cavities)) that cause a loss of volume and damage.

It's referenced in terms of absolute vacuum, not gauge, so yes a pump can provide a degree of lift if situated above the surface of the fluid and properly primed.

It has to take into account the strainers and lines leading to the pump suction, the vapor pressure of the fluid, and how that changes with temperature.

there's some really cool examples of NPSH control in power stations (dearator tanks located on a floor 100 feet up, condensate pumps having a 5 metre extension piece in the middle, and the impeller located 20 feet below ground level so that there's enough "weight" of water to provide NPSH in a vacuum)


If it's the truth....it can handle the pressure !!!
Re: (Fuel) Pump physics question? [Re: Ed_Flecko] #5104462 05/13/19 05:01 PM
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red7404 Offline
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let me see. gasoline with electric wires in there? what can go wrong?

Re: (Fuel) Pump physics question? [Re: red7404] #5104475 05/13/19 05:29 PM
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The_Eric Offline
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Originally Posted by red7404
let me see. gasoline with electric wires in there? what can go wrong?

Obviously not an issue since there's millions of them out there. The mere presence of electricity in a gas tank isn't an issue. Even if they were arcing it probably wouldn't do anything since theres likely not enough oxygen to support combustion.


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Re: (Fuel) Pump physics question? [Re: MrMoody] #5105081 05/14/19 11:48 AM
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supton Offline
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Originally Posted by MrMoody
I don't think a fuel tank is deep enough to make any real difference.

What does make a difference is that in-tank electric pumps are cooled by the liquid around them, so running nearly empty deprives them of some of their cooling. If this is done habitually, it can shorten the pump life. I've seen this firsthand.

I've always heard that, just haven't seen it--but the only high mileage car I had did puke its fuel pump around 250k, and I routinely go down to 2 gallons or so in the tank. But that car also did have a fuel cooler so it truly did have some high fuel temps I suspect.


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