Positive displacement pumps explained (hopefully).

Originally Posted By: OVERKILL
The pressure relief valve is a spring-loaded bypass valve in the oil pump, engine block, or oil filter housing. The valve consists of a small piston, spring, and cylinder. Under normal pressure conditions, the spring holds the relief valve closed. All the oil from the oil pump flows into the oil galleries and to the bearings.

However, under abnormally high oil pressure conditions (cold, thick oil, for example), the pressure relief valve


Seems to contradict jrustles theory, no? [/quote]

Yes, it does. Later on in your source's own article, he contradicts himself.

Quote:

pressure setting can be altered.


How can that be? How can you raise normal temperature operating pressure by changing the spring/shim if ONLY under abnormally high oil pressure conditions (cold, thick oil, for example)?

More pressure on a seated valve won't do that, would it
wink.gif
 
Originally Posted By: jrustles

How can that be? How can you raise normal temperature operating pressure by changing the spring/shim if


If the relief is fully closed with hot oil there is no pressure to be gained with a stiffer spring. People shim the stock spring all the time to get more pressure through most of the rpm range.

Great argument JR
 
Originally Posted By: Shannow
Makes CATERHAM's test results impossible if it is a regulator,and always operating


It's not always off it's seat, that's for sure.

But I still insist that one cannot tell by a pressure ramp observed on a pressure gauge exactly when it lifts, and even less so by what degree, that's true. One needs to see a flow chart for that. Melling's got plenty, and those plots confirmed that on one of their high pressure, high flow SBC units, the valve lifted off about 1800ish engine RPM. The chart has been posted somewhere on here.

Originally Posted By: KrisZ

And? Are you suggesting that the positive displacement pumps and their relief valves found in cars behave differently? If so let's hear it, and a verifiable source would be nice as well.


Steam is gas phase and compressible, hydraulics are not and hydraulic pressure is unforgiving. The bypass valve is the only thing that moves in the system, the only thing that can take shocks and make (intentionally designed-in) excess volume 'disappear'.
In fact, the supply side of the oil system will see hydraulic pulses caused by the pump gear lobes themselves, until the bypass opens, then it becomes the 'shock absorber' for that little hydraulic frequency. If there was no bypass valve, an oil pump could hydraulically split itself, if the oil filter didn't rupture first.

The backpressure is as mentioned before, an artifact. One could remove the engine from the pump and get 0psi backpressure, and theoretically 100% of the pump's displacement output. To observe a pressure gauge to determine flow, and where the flow was being directed would be futile in such a case.


The observed pressure gauge behavior seen by some (with stock pump configs) of a lowering of backpressure across the board and at redline, from say running 0w20 on an unchanged pump and engine, is tantamount to opening up engine clearances and lowering bypass spring pressure with a 5w40.

This is due to not only the engine, but bypass also being able to "accept" more oil volume at a lower pressure. In other words, the same excess displacement of the pump is being handled by all leakage points at a lower net backpressure. The backpressure equilibrium as a whole is lowered, and the pressure gauge behavior could be misconstrued as complete inoperation of bypass in the upper RPM range! And this is all before considering internal pump leakage, which is naturally increased. I'm not persoanlly worried, however considering pump leakage from below-spec vis is really only an issue at hot idle, and 8-10psi in my case@600rpm is fine!

Originally Posted By: Shannow
ASME's take on the issue...

https://www.asme.org/engineering-topics/...better-oil-pump

Quote:
As the market and government regulations push automakers to improve emissions and fuel consumption, they are evaluating all opportunities in the engine system to reduce losses. The oil pump is one important component that consumes engine power as it protects engine components from frictional wear and overheating by delivering oil at the correct pressures.

Fixed-displacement oil pumps currently circulate oil in most automobiles. Designers typically oversize the pumps to handle the harshest engine operating conditions. Most of the time, they consume more power and deliver significantly higher oil pressure than needed. They contain pressure-relief valves as a crude, cost-effective, and reliable way to avoid excessively high oil pressures. But these designs are inefficient, losing significant amounts of energy at high oil flows typical in internal-combustion engines.



Nice
thumbsup2.gif
If I'm reading that correctly, they confirm that auto oil pumps are intentionally designed to displace well more than required for the 'harshest engine operating conditions' which I take to mean hot idling at 600rpm with the hottest, most sheared, viscosity broken oil in the crankcase
wink.gif


Originally Posted By: turtlevette
People shim the stock spring all the time to get more pressure through most of the rpm range.



Indeed!

Originally Posted By: OVERKILL
As the engine wears, yes, that # will go down. That doesn't mean that the engine is receiving inadequate lubrication though.


Sure, even 7psig is fine for basic lubrication! With relation to backpressure, increasing clearances is tantamount to reducing pump displacement. High flow pumps are great for engines with high oil demanding features like piston oil cooling jets. Some manufacturers (BMW) have other funky oil supply pipes for the valve train in the rocker cover etc. as well as turbo bearings on any given turbocharged vehicle, which is very common today.

But so too, oil pumps are changing. This discussion will soon be made obsolete. And who can imagine how lengthy, ambiguous, theoretical and heated discussions about variable pressure, variable displacement oil pumps will get here on BITOG!
 
Last edited:
Originally Posted By: turtlevette
And it's more probable that a well designed oiling system on a new engine will be into relief early.


I wouldn't call eternally wasting shaft power on every revolution of the engine for 200,000 miles necessarily good design.

If the relief is open all the time, then there's 100W or so of additional load wasting your fuel, sapping your performance, and heatign your oil...hardly "good"

The old SBC "10psi per 1,000RPM" is apparently sufficient to protect that particular engine family by providing sufficient volume to protect it under nearly any operating condition.

Clearly the relief valve ISN'T regulating the entire range, or there's be no oil pressure "curve", a regulated system would provide a constant pressure, or nearly that.

Originally Posted By: turtlevette
Why do you think they went to all the trouble to develop high volume oil pumps?


Because when they went hi-po, opened up the bearing clearances, and sheared the heck out of the oil during a race they needed more volume ?

It wasn't to satisfy the needs of the relief valve.

You state that it's not a relief, as it serves no "safety" function...well it does, it relieves excess pressure from being applied to the weakest point...the oil filter.
 
Originally Posted By: jrustles
But so too, oil pumps are changing. This discussion will soon be made obsolete. And who can imagine how lengthy, ambiguous, theoretical and heated discussions about variable pressure, variable displacement oil pumps will get here on BITOG!


Dunno if you recall one of my discussions with CATERHAM re oiling systems, but increasingly, stuff like squirters and oilers are being used, which is more than just feed to bearings etc. and DO rely on pressure for effective operation, plus move/lose considerable volume...as per the ASME document, there's enough oil flow that alterations in squirters can affect NOx production.

Part of (probably most of) the reason for going variable displacement.

http://www.kspg.com/fileadmin/media/Bros...rbrennung_e.pdf

Fig 4 shows how the power requirements for modern engine would equate to significantly more auxiliary energy wasted. were a high volume traditional pump installed to meet the new needs.

Note also, that I don't take the 800rpm point as the point at which the relief is operating...the interesting information contained is worth more to the discussion than that point.
 
Originally Posted By: turtlevette

It's misused a bit in the auto industry. In power plants and such a relief normally means that it provides a safety function. Say a pipe has a burst pressure of 5000 psi. Its decided to run the system at a 2/1 safety factor at 2500psi. The relief is there to prevent system overpressure to failure.

The engine block has drilled passages in solid iron or aluminum and are very robust. The failure pressure is probably on the order of thousands of psi.

So in theory, I could shim the heck out of my pump and let it run at 500 psi. Its not going to break anything, just waste a lot of power.




Like I said...relief...

Youll spaghettify your driveshaft and distributor drive if you try 500psi.

Yet another safety function brought to you by the system relief.
 
Originally Posted By: jrustles
Originally Posted By: jrustles
Originally Posted By: jrustles
Originally Posted By: OVERKILL
The pressure relief valve is a spring-loaded bypass valve in the oil pump, engine block, or oil filter housing. The valve consists of a small piston, spring, and cylinder. Under normal pressure conditions, the spring holds the relief valve closed. All the oil from the oil pump flows into the oil galleries and to the bearings.

However, under abnormally high oil pressure conditions (cold, thick oil, for example), the pressure relief valve


Seems to contradict jrustles theory, no?


Yes, it does. Later on in your source's own article, he contradicts himself.

Quote:

pressure setting can be altered.


How can that be? How can you raise normal temperature operating pressure by changing the spring/shim if ONLY under abnormally high oil pressure conditions (cold, thick oil, for example)?

More pressure on a seated valve won't do that, would it
wink.gif



Because pressure climbs with RPM just like volume? At some point the relief opens. With a heavier spring/adjustable spring, you raise that point. So instead of hitting the relief at 5,000RPM and capping pressure at 65psi, you raise the pressure to 80psi and the pump would go into relief at like 8,000RPM.

Your "point" seems to forget that oil pressure and volume increases with RPM
21.gif
 
Originally Posted By: Shannow
Throw some dimensional analysis into the equations that you were using, and you'll find with your method of equating torque (fxl) with "f", you are out by an "l" component...therefore the formula as you applied it is incorrect...formula is, but can't use any number or colour that you like, or it becomes nonsense.

Agreed. What a mess. The torque vector is the cross product of the displacement vector and the force vector. Hence, it is measured in N-m, and not newtons.

This is the kind of thing that drives mathematicians and physicists crazy. Physics without math is a bunch of hand waving, and that's what we've got here if we equate torque and force, since they're clearly not the same. And as you point out, dimensional analysis would reveal that error immediately.
 
what is the point of this thread? was there an initial question trying to be answered?
what was the initial point of talking about centrifugal pumps?

a positive displacement pump (gerotor like on an LS1 or the original gear style in a sbc) output a fixed volume per revolution. you can calculate that. as engine rpm increases your main leakage point is through the lifters which meter oil to the valve train (only talking about traditional sbc/sbf). as rpm increases, more oil goes to the valve train. you size the gears on the oil pump to account for that, if the oil pump is sized right then that's why oil pressure can stay at a given pressure as rpm increases- the increase in volume output of the oil pump nearly matches the loss up to the valve train. as rpm increases, the amount of leakage would not increase through the main, rod, and cam bearing clearances if oil pressure was somehow held constant. the reason you do get more leakage is because of an oil pump outputting more volume as rpm increases than metered by lifters thus increasing pressure and that pressure increase results in more leakage from wherever there's clearance. and the amount of oil lifters throw up the push rod will vary in the real world because of dirt, wear, and lifter bore wear.
 
Originally Posted By: Garak
This is the kind of thing that drives mathematicians and physicists crazy. Physics without math is a bunch of hand waving, and that's what we've got here if we equate torque and force, since they're clearly not the same.


My physics teacher in the late 60's was fond of saying this! So very true and funny, too...
 
Originally Posted By: OVERKILL
I have heard (though do not have first hand experience with) of an engine's bearings at higher RPM consuming an increasingly greater volume of oil resulting in an observable loss of oil pressure. Example, say you have an engine with a 7,000RPM redline and a pump with a 65lb relief spring. Oil pressure at 4,500RPM is 50psi. Relief pressure is reached at 5,500. But when closing in on 7,000RPM, observed pressure on the gauge reduces from 65psi to 55psi, which is counter-intuitive given that volume moved by the pump increases with engine RPM in a linear fashion. The explanation being that the bearings self-pumping nature, which also increases with RPM, effectively eclipsed the pump/RPM relationship. Any thoughts on this?


OVERKILL, that statement stuck with me a bit while looking at other things....

http://www.mellingengine.com/Portals/5/pdf/pdf_catalog/pressure-vs-flow.pdf

Second last paragraph reckons that the centrifugal effect of a spinning crank will drag oil out of the system if the rod bearings are worn excessively versus the cranks.

Needs more pondering, but I did the calcs (and proved myself wrong) when discussing cross drilling with molakule a while back...3" throw at a lot of revs could shift a LOT of oil.
 
Originally Posted By: SteveSRT8
So very true and funny, too...

Yep, that's basically it. Torque and force "seem" much the same, as do mass and weight, until you actually get right down to things.

By the way, all the talk here using non-SI units is giving me a headache.
wink.gif
 
Originally Posted By: 1 FMF
what is the point of this thread? was there an initial question trying to be answered?
what was the initial point of talking about centrifugal pumps?


The point of this thread was to clarify how positive and centrifugal pumps work because their effect on flow and pressure is being confused in the oil section, specifically in this thread:
http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3369155/3

I posted this thread in the oil section, but it was moved to maintenance.
 
Originally Posted by Boxnuts
Originally Posted by gregk24
Originally Posted by ZeeOSix
Originally Posted by gregk24
So if they are truly positive displacement would a 15w40 be fine in a car requiring 0w20? It should not make a difference right?


A higher viscosity oil will make the oil pressure higher at any given RPM before the pump goes into pressure relief.

But ... it will make the oil pump go into pressure relief easier and with less volumetric output from the pump. And when the pump does go into pressure relief there is less fixed pump volumetric output from that point with increased engine RPM.

So what that means is that with a higher viscosity oil, you could be reducing the oil volume delivered to the engine, especially at higher RPM when it counts to have as much oil volume as possible.


But as long as the pump is NOT in pressure relief, the 15w40 would still be pumped around the same as a lighter weight..right?

No -- oil volume will not reduce.

With increasing RPM the pressure rises until the pressure relief valve operates, from that point on the pressure remains constant. The flow into the engine is then constant only dependent on resistance to flow. As I understand it from what has been said in other posts the resistance to flow reduces as bearing speed increases, and this will lead to an increase in oil flow. This increased flow will tend to reduce the pressure at the pump but the relief valve will close to compensate and maintain the pressure at the valve setting.
Viscosity also affects resistance to flow so that if the oil temperature rises the viscosity drops and flow increases.


And how does oil pressure influence the oil flow through the unloaded side of the bearing, given the same viscosity? In other words, todays engines can come with variable flow oil pumps, is there a difference with 15 psi vs 75 psi of oil pressure, in the flow the pump must provide?
 
Originally Posted by Jetronic


And how does oil pressure influence the oil flow through the unloaded side of the bearing, given the same viscosity? In other words, todays engines can come with variable flow oil pumps, is there a difference with 15 psi vs 75 psi of oil pressure, in the flow the pump must provide?



This is one of those threads I vaguely remember and almost joined way back to comment on but had other priorities. The bulk of what's here is incorrect, incorrectly framed or misunderstood/misrepresented leading to a lot of confusion.

I'll answer your inquiry from a pump design engineer perspective but at a very basic level (which will leave out a lot but make for an easier read)

Baselines:

No pump made pumps "pressure"- "pressure" (resistance to flow) comes from the system and is all the "things' the flow must overcome (SH, TDH, friction, length of run, fittings and so forth) A properly designed pump will deliver design flow to the point of use in all conditions relative to the system curve which defines all system losses. The pumps ‘head" is the max pressure it can pump before forces equalize.

Centrifugal pumps move media by adding energy (velocity) by volute structure relative to the cutwater, vane design and so forth. They can vary flow via internal recirculation and "generally' are higher volume and lower pressure (compared to PD) compared to the same class. The relationship of RPM to output CAN be decoupled due to recirculation

Positive displacement pumps ‘displace" a fixed volume of media per revolution. These pumps are generally lower volume but higher pressure than a CP of the same class. The RPM to output relationship in this pump CANNOT be decoupled and this type of pump can pump till it pops. (or worse)

Flow is a bit trickier because it has more than one usage. Without going deep into CFD- GPM flow is a measure of volume and FPS flow is a measure of fluid velocity.

Relief- there are many ways relief devices are engineered for a given system. Some are used to make sure a maximum is never reached (a safety). Others are used as a "flow control" to make a baseline pressure available at all times when the pump is always run in a steady state (basically a fixed RPM) whereas other systems vary output by pump RPM to a point.

So to get a usable answer- your question would have to be reframed in terms of volume flow or velocity flow.

If all things are equal, the difference between the 15 psi and the 75 psi is in the system resistance- not a design point of the pump (notice the part about all other things being equal- that's what makes that statement correct in its context). Assuming a steady state, the PD flow (volume) would not change relative to system pressure but a CP "could" depending on its internal characteristics.

That information is correct at the very basic level on pumps in general but there's a lot more to it.
 
I know all that but modern pumps have variable flow to control the oil pressure and keep it lower in certain conditions for fuel economy. So, does a bearing have significantly less side leakage when the pump is regulated to say 15 psi vs typical 75 psi bypass valve setting? or another way, can you push more oil through it with higher pressure.

Theres a few different designs for variable output pumps and the simplest just have an extra electronically actuated bypass. But none are centrifugal pumps.

I guess its actually a bearing question, not a pump question but they were talking about bearing speed and leakage.
 
Last edited:
Originally Posted by Jetronic
I know all that but modern pumps have variable flow to control the oil pressure and keep it lower in certain conditions for fuel economy. So, does a bearing have significantly less side leakage when the pump is regulated to say 15 psi vs typical 75 psi bypass valve setting? or another way, can you push more oil through it with higher pressure.

Theres a few different designs for variable output pumps and the simplest just have an extra electronically actuated bypass. But none are centrifugal pumps.

I guess its actually a bearing question, not a pump question but they were talking about bearing speed and leakage.


I can do the bearings too.

Let's talk a basic system: Pick up-Pump-service line-bearing- sump (repeat cycle)

They are "reading" the pressure (system pressure) to control the flow (vary) based on whatever design points are in place.

The rest is somewhat backwards ( read up on Couette flow) The bearing ( the combination of flow path, total volume and exit path) "is" the restriction which "defines" pressure in terms of flow: restriction (backpressure is simply looking at system pressure from the other end back to the pump discharge)

That's a fixed parameter design point range based on clearance, thermal expansion etc.

So, to create more pressure in that basic system for fluid film lubrication, volumetric flow is increased and pressure will rise on the supply side as the fluid increases in density and velocity in the bearing then exits (Bernoulli and Poisson)

On the exit side, it will be at whatever pressure the sump is. That's two different pressures for two different conditions.

This is why to measure pump efficiency we have a gauge on both sides and examine the delta.

So, yes you can push more fluid through the bearing (provided the pump, system and bearing will handle it) and yes the supply side pressure will increase accordingly in relation to viscosity, volume etc.

I'm not quite sure how you are defining "side leakage" but my term "exit path" would be all avenues that the worked fluid could escape from so I think I'm including what you are referencing)- so there velocity will increase but "pressure' will normalize to whatever prevailing conditions are in the sump.

In this case, "bypassing" is basically being used as a flow regulator running off of pressure ( a very common thing)

That help any?
 
That helps a lot. Any clues or ballpark figures how much more flow would be needed to rise the pressure from 15 psi to 75 psi? My car has a pump which regulates oil pressure by those amounts.

I am asking because when I drive on track the car can pull quite a bit over 1G in long turns. No problem when turning left as the oil pickup is on the right side and all flow will be pushed to that side, but for other turns the oil supply is limited to 1 quart angled up 45 degrees or more and only half of the oil coming out of the bearings will make it back to oil pick up until lateral G drops. So oil flow can make a big difference to getting damage or not. Dry sump isn't an option but I could possibly put some extra baffles and traps in the sump to buy some extra reserve oil. Got a feeling the lower pressure and higher than specced oil viscosity is what has averted disaster until now. Turbo wont like running dry either.

Luckily on the street I take roundabouts in the correct way (big lefthand turns essentially and at just about 1G).

By side leakage I meant the oil that gets out of the bearing without actually getting to the part thats lubricated, on the side with the most play
 
Originally Posted by Jetronic
That helps a lot. Any clues or ballpark figures how much more flow would be needed to rise the pressure from 15 psi to 75 psi? My car has a pump which regulates oil pressure by those amounts.


In truth that cannot be calculated with any degree of accuracy without knowing the pump flow range, suction side volume (Coriolis Effect and possible vortexing going into the pump causing aeration or cavitation), regulator range and set point and the total volume of lubricated surface to overall exit path

In basic terms a pump with bigger gears (volume) and possibly a redux in the supply line (velocity) would do better provided the regulator could be adjusted for the higher pressure.

Originally Posted by Jetronic
No problem when turning left as the oil pickup is on the right side and all flow will be pushed to that side, but for other turns the oil supply is limited to 1 quart angled up 45 degrees or more and only half of the oil coming out of the bearings will make it back to oil pick up until lateral G drops. So oil flow can make a big difference to getting damage or not. Dry sump isn't an option but I could possibly put some extra baffles and traps in the sump to buy some extra reserve oil.


On the pick-up, I deal with centrifuging on a few occasions. There's no hard and fast answer to that. Baffles work to a point but so do angle weirs that force a "pool" close to the suction. (Basically an anti vortexing plate like in a mixer) A soft tube with a machined pick up on a rail for sliding also works too.

May take some experimenting but there are ways around centrifuging.
 
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