Engine/Transmission (1998.5 - 2002) why is it better to push fuel rather than pull?

[THenningsen]

I've heard everyone say that the lift pump functions best when it is pushing fuel rather than pulling it. Why is that so? Either way the fuel has to travel just as far. Can someone explain to me why this is?

 

[Mhannink]

its easier to push the fluid and pressurerise it then it is to pull it up hill and then push it higher yet

 

[TAbbott]

All fuel has a tiny amount of air entrained within it. Some more than others. I've seen claims by manufacturers of fuel/air separators that as much as 2% to 3% of diesel fuel by volume is actually entrained or dissolved gases. Anyway, when you put a vacuum on it, some of that air comes out of solution, whereupon it exists in the form of minute bubbles in the fuel rather than being dissolved within it. When those bubbles are subjected to the pressures inside a high pressure pump and injector they compress and minutely change the timing and the liquid volume of the injection pulse --- liquids don't compress, while gases do -- and they also over time will help erode away your nozzles when they burst at the instant the pressure on them is released. What's different is that when you push the fuel instead of pull it, you leave whatever amount of the air that exists in the fuel in solution because it's never subjected to a vacuum. That's the way it was explained to me at any rate.

 

[THenningsen]

okay... . that helps. It seemed to me that the pump was pushing fuel the same distance and so it wouldn't matter.

 

[DZink24]

All fuel has a tiny amount of air entrained within it. Some more than others. I've seen claims by manufacturers of fuel/air separators that as much as 2% to 3% of diesel fuel by volume is actually entrained or dissolved gases. Anyway, when you put a vacuum on it, some of that air comes out of solution, whereupon it exists in the form of minute bubbles in the fuel rather than being dissolved within it. When those bubbles are subjected to the pressures inside a high pressure pump and injector they compress and minutely change the timing and the liquid volume of the injection pulse --- liquids don't compress, while gases do -- and they also over time will help erode away your nozzles when they burst at the instant the pressure on them is released. What's different is that when you push the fuel instead of pull it, you leave whatever amount of the air that exists in the fuel in solution because it's never subjected to a vacuum. That's the way it was explained to me at any rate.

That being said, does my 04 push or pull?

 

[Rebel Ram]

Along with what TABBOT wrote, In any hydraulic system positive pressure is the most efficient manner in which to work. To try to pull a liquid all you are doing is creating a low pressure area and then relying on atmospheric pressure to push your fuel. The most efficient system would only be capable of pulling down to zero PSI. atmospheric pressure is about 14 PSI at sea level.

So that in a nut shell is the theory. Remember this though. Gasses and liquids are NEVER sucked. They are ALWAYS pushed. If you can modify your thinking in this way trouble shooting flow problems of all sorts are much easier to solve.

 

[NoSeeUm]

Good responses. But I would like to add a bit more to them.



Your questions about pushing or pulling are valid. I think I know where you are coming from, because the motive force to do either is almost exactly the same. IE: It takes so much energy for a XX flowrate, wether you are pushing or pulling. And in the OEM case the vane type (positive displacement) Carter pump should pump a set volume every revolution reqardless if it were pulling or pushing.



It just never quite works out quite that good. What changes in pulling or pushing is the pump used to move the liquid, the amount of restriction in the suction / discharge line and the head pressure of the liquid at the suction point.



But.....



If you are pulling with a pump you have to remember that the most suction (vacuum) you will ever be able pull any where at any time at any place with any pump is equal to atmospheric pressure.



Consider that atmospheric pressure is 14. 7 psia. At those conditions the best vacuum you will ever pull with any pump is 14. 7 psi of vacuum. So you could use a 1 Hp pump or a 1,000,000 Hp pump and the most suction either could ever pull is 14. 7 psi.



So consider the first case of a pump pulling. That is the equivelant to like pressurizing the fuel tank with air to 14. 7 psi and seeing how much fuel the fuel lines could flow.



Now consider a second case of a pump pushing. Say the same pump we used in pulling case is capable of producing 40 psi if it is used to push. This is the equivelant of pressurizing the tank to 40 psi, and then seeing how much fuel will flow through the same fuel lines. You can see that there will be likely more fuel flowing with 40 psi vice 14. 7 psi.



So in both cases, what is really the limiting factor?



It is the fuel line restriction. In the pulling case you get a maximum of 14. 7 psi to work with. In the pushing case you get the pressure capability of the pump. And so you start out from the get go giving the fuel system an advantage if the pump can be used to push vice pull.



Jim

 

[ScottLewis]

Pretty good answer Jim.



Consider that on well systems, virtually all deep wells use a submersible pump rather than one on the surface. Pumps can develop much more pressure than vacuum.



Dan

 

[obert]

Rebel and NoSeeUm really nailed it. In physics, nothing pulls, there is always a pushing force. When any pump turns, in this case a vane pump, the inlet port expands, creating a "lower pressure area", and atmosphereic is used to fill that lower pressure area. Moving the pump closer to the tank reduces the "head" that the air has to push to the pump, making it a more eficcient (reliable?) pump.

On the push side, most fluid pumps are psitive displacement, meaning it closes the area around the outlet port, "pushing" the fluid out because for practical purpouses the fluid will not compress and has nowhere else to go but out to the system.

 

[THenningsen]

Thanks guys, that was the kind of answer I was looking for. So the obvious question is... . in their infinite wisdom, why would cummins put the pump where they did?

 

[NVR FNSH]

So why didn't the 12V engines with mechanical pumps mounted on the engine experience similar failures to the engine mounted electrical pumps on 24V's?



Brian

 

[NoSeeUm]

So why didn't the 12V engines with mechanical pumps mounted on the engine experience similar failures to the engine mounted electrical pumps on 24V's?



Brian

The mechanical 12V pump is a piston style, and the electric 24V pump is a vane style. The mechanical pump is more robust. FWIW there is really nothing wrong with pulling using a positive displacement pump. But the pump would typically be more heavy duty and designed more towards pulling in mind. The suction side piping would need to be sized and configured accordingly so that it would create only a minimal pressure drop (headloss) at rated flow.



Another thing that is important to know is that every pump pulls regardless of the what kind it is. There are no exceptions from pumps 1 Hp to 1,000,001 Hp. Please keep in mind that my discussion above was really only concerned about pumps pulling from a very shallow tank, like a diesel truck fuel tank.



Further explanation, going back to DanVilla's comment about deep water well pumps. The suction pressure (suction head) on that pump would be equal to atmospheric pressure + the weight of the water column. If the pump were turned off and lets say the well was about 100 feet deep. Then the pump suction pressure would be 14. 7 psi (atmosphere) plus 50 psi (water column) for a total of about 65 psi. So if the pump is running at full flow and it could pull lets say 25 psi of suction pressure, then the net positive suction pressure (head) on the pump would be 65 psi minus 25 psi or 40 psi.



If you subtracted the 50 psi of water column, by hooking the pump up to a shallow tank, even though the pump could pull 25 psi suction down deep, it could only pull 14. 7 psi under those conditions. That pump would likely really be hating life.



The critical part about maintaining a net positive suction pressure, as was said earlier above, is that it prevents cavitation at the pump suction port or inside the pump itself. Go below a certain value of positive suction pressure and the pump will cavitate. Plain and simple, the value of the net positive suction pressure varies from pump to pump and from design to design.



Jim

 

[Rebel Ram]

Thanks guys, that was the kind of answer I was looking for. So the obvious question is... . in their infinite wisdom, why would cummins put the pump where they did?

That was a Dodge thing. And they (the Dodge engineers) Suck! :-laf

Had to do it.

 

[obert]

So far the 24v pumps I've had fail were due to the motor failing, or the drive between the motor and pump breaking. The one I just took off was getting "weak", maybey that one really is pump failuer.

... . Another thing that is important to know is that every pump pulls ... .





... . The critical part about maintaining a net positive suction pressure, as was said earlier above, is that it prevents cavitation at the pump suction port or inside the pump itself... .

A question, jim, just for my curiosity to see another point of view. Look like we use different jargon (perhaps due to geography or industry), my head pressure, it seems, is your suction pressure. We split where you say the pump still pulls. Not arguing, just tryin' to clear the water on how that compares to what I might already know, only in other words maybe?

Does that make any sense?

 

[GCULP]

even though the question has already been answered, think of it this way, is it easier to suck a milk shake through a straw, or blow it out all over the dash? :-laf :-laf :-laf

 

[cojhl2]

Another thing that is important to know is that every pump pulls regardless of the what kind it is. There are no exceptions from pumps 1 Hp to 1,000,001 Hp. Please keep in mind that my discussion above was really only concerned about pumps pulling from a very shallow tank, like a diesel truck fuel tank.



Further explanation, going back to DanVilla's comment about deep water well pumps. The suction pressure (suction head) on that pump would be equal to atmospheric pressure + the weight of the water column. If the pump were turned off and lets say the well was about 100 feet deep. Then the pump suction pressure would be 14. 7 psi (atmosphere) plus 50 psi (water column) for a total of about 65 psi. So if the pump is running at full flow and it could pull lets say 25 psi of suction pressure, then the net positive suction pressure (head) on the pump would be 65 psi minus 25 psi or 40 psi.



If you subtracted the 50 psi of water column, by hooking the pump up to a shallow tank, even though the pump could pull 25 psi suction down deep, it could only pull 14. 7 psi under those conditions. That pump would likely really be hating life.



The critical part about maintaining a net positive suction pressure, as was said earlier above, is that it prevents cavitation at the pump suction port or inside the pump itself. Go below a certain value of positive suction pressure and the pump will cavitate. Plain and simple, the value of the net positive suction pressure varies from pump to pump and from design to design.



Jim

I take exception to nothing "sucks". My bank balance sucks.



Little confusion re the 100ft well. Water can only be drawn up about 33 ft. . 0. 433 ibs/sqin holds 1 ft of water. so with only 14. 7 lbs to work with the total column can be only that, and then at that there would be 0 flow.



Is this still consistant with what you are saying noseeum, or does my thinking suck?

 

[NoSeeUm]

I take exception to nothing "sucks". My bank balance sucks.



Little confusion re the 100ft well. Water can only be drawn up about 33 ft. . 0. 433 ibs/sqin holds 1 ft of water. so with only 14. 7 lbs to work with the total column can be only that, and then at that there would be 0 flow.



Is this still consistant with what you are saying noseeum, or does my thinking suck?

It sounds about right to me. Shallow water wells use an inductor style pump. Which means that the pump is mounted at the surface and has to pull the water up. It also means that 33 feet of water is about 14. 7 psi. Amazing huh? It all comes back around doesn't it. In all cases you can't lift water more than about 30 feet. Diesel fuel is a bit lighter so FWIW you could lift it a bit farther.



The case of the deep water well is different. The pump is located near the bottom of the well and pushes vice pulling it up. Used this way the pump can push the water up to any height that essentially the pump discharge pressure can rise too. So if the pump rating is at 100 gpm @ 200 psia, then it could pump about 100 gpm pushing the water up about 370 feet. In other words 370 feet of water column (185 psi) plus 14. 7 psi (atmosphere) or about 200 psi.



Sorry about your bank account... . :{



Jim

 

[NVR FNSH]

The mechanical 12V pump is a piston style, and the electric 24V pump is a vane style. The mechanical pump is more robust. FWIW there is really nothing wrong with pulling using a positive displacement pump. But the pump would typically be more heavy duty and designed more towards pulling in mind. The suction side piping would need to be sized and configured accordingly so that it would create only a minimal pressure drop (headloss) at rated flow.

I realized the error of my ways this morning on my way into work... ... ... Completely forgot that they were different style pumps.



Brian

 

[NoSeeUm]

So far the 24v pumps I've had fail were due to the motor failing, or the drive between the motor and pump breaking. The one I just took off was getting "weak", maybey that one really is pump failuer.









A question, jim, just for my curiosity to see another point of view. Look like we use different jargon (perhaps due to geography or industry), my head pressure, it seems, is your suction pressure. We split where you say the pump still pulls. Not arguing, just tryin' to clear the water on how that compares to what I might already know, only in other words maybe?

Does that make any sense?

Yeah, I would normally use "suction head pressure or suction head" but for the crowd here it did not seem to fit. Re-reading my posts it found it a tad confusing as well, but hopefully I got the point across.



Jim

 

[NoSeeUm]

I realized the error of my ways this morning on my way into work... ... ... Completely forgot that they were different style pumps.



Brian

Yeah... .



I have never had a 12V pump apart, but likely it is not a piston pump, close enough though.



I am guessing it is a diaphram type?



BTW the RASP is another example of a pump that seems to hold up pulling fuel.



Jim