Positive displacement pumps explained (hopefully).

[turtlevette]

Originally Posted By: jrustles
, which is certainly almost always open above idle, even on a hot engine. Your observed slower/lower pressure build to plateau is simply due to the increased volume of leakage, particularly through the bypass.


Did you write that? Or is it from a source? This is what I've been saying for months and everyone here thinks I'm a moron.

To not have bypass operation all the time is to not have any reserve capacity to compensate for engine wear.

Thank You!

 

[KrisZ]

Originally Posted By: turtlevette
Originally Posted By: jrustles
, which is certainly almost always open above idle, even on a hot engine. Your observed slower/lower pressure build to plateau is simply due to the increased volume of leakage, particularly through the bypass.


Did you write that? Or is it from a source? This is what I've been saying for months and everyone here thinks I'm a moron.

To not have bypass operation all the time is to not have any reserve capacity to compensate for engine wear.

Thank You!


Well, that's not what this paper is saying, but keep in mind it's not automotive specific.
http://hydraulicspneumatics.com/other-technologies/chapter-9-relief-and-unloading-pressure-controls

Quote:
The main advantage of direct-acting relief valves over pilot operated relief valves is that they respond very rapidly to pressure buildup. Any relief valve does not know there is a problem until pressure is very near or at its setting


But it also says

Quote:
The main disadvantage of direct-acting relief valves is that they open partially at about 150 psi below set pressure. (In a 1500 psi setting)


So you guys are partially right. A spring operated direct relief pressure valve will start to, what the article calls, crack below the relief setting, but it will not happen the moment the system sees pressure, but close to the maximum setting.

 

[turtlevette]

Originally Posted By: KrisZ
Well, that's not what this paper is saying, but keep in mind it's not automotive specific.



NO [censored] Sherlock. That like steam blow off valves on a power plant. Ever hear one of those go on full load reject?

 

[OVERKILL]

http://www.waybuilder.net/sweethaven/Mec...=3&modNum=6

Quote:
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy


Quote:
As a safety factor to assure sufficient oil delivery under extreme operating conditions, the oil pump (gear or rotary) is designed to supply a greater amount of oil than is normally required for adequate lubrication. This requires that an oil pressure relief valve be incorporated in the pump to limit maximum oil pressure.

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 opens. Oil pressure pushes the small piston back in its cylinder by overcoming spring tension. This allows some oil to bypass the main oil galleries and pour back into the oil pan. Most of the oil still flows to the bearings and a preset pressure is maintained. Some pressure relief valves are adjustable. By turning a bolt or screw or by changing spring shim thickness, the Oil on the inlet side of the pump is caught in the gear teeth and carried around the outer wall inside the pump housing. When oil reaches the outlet side of the pump, the gear teeth mesh and seal. Oil caught in each gear tooth is forced into the pocket at the pump outlet and pressure is formed. Oil squirts out of the pump and to the engine bearings. pressure setting can be altered.


Seems to contradict jrustles theory, no?

 

[OVERKILL]

Doesn't really help with the discussion but a neat read from Melling:

http://www.mellingengine.com/Portals/5/pdf/pdf_catalog/high-volume-pumps.pdf

 

[Shannow]

Makes CATERHAM's test results impossible if it is a regulator,and always operating

 

[KrisZ]

Originally Posted By: turtlevette
Originally Posted By: KrisZ
Well, that's not what this paper is saying, but keep in mind it's not automotive specific.



NO [censored] Sherlock. That like steam blow off valves on a power plant. Ever hear one of those go on full load reject?



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.

 

[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.

 

[ac_tc]

What you have there is a P-controller, but we call it a pressure
relief valve. A p- controller cannot meet a setpoint, it will always deviate from the set point. Low amplifing " Low P" will
give it steady running and no oscilation but a bigger deviation compared to a hig P setting.
-Were are the settings??
Valve seat/ type- the rate of the valve opening vs travel.
Spring rate- the installed spring pressure will set opening point
and the rate will set opening rate.
What you will se is a oilpressure that rises to opening point quickly and then slowly raises with engine speed until you reached.
full open valve.

 

[Oil Changer]

So who's winning?

 

[SteveSRT8]

Originally Posted By: KrisZ
Originally Posted By: turtlevette
Originally Posted By: KrisZ
Well, that's not what this paper is saying, but keep in mind it's not automotive specific.


NO [censored] Sherlock. That like steam blow off valves on a power plant. Ever hear one of those go on full load reject?


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.


No generic description is going to account for the variables in design and implementation.

Engine design and its resulting effects are impossible to ignore. Too much variation.

I can change bearings and change my oil pressure without changing anything else.

And it is indeed a relief valve as it relieves excess pressure...

 

[Shannow]

Originally Posted By: SteveSRT8
I can change bearings and change my oil pressure without changing anything else.


Gets back to what Kingsbury say on my earlier PDF...need more flow, you have to change the bearing design (more radial clearance)...oil pressure reflects that.

 

[KrisZ]

Originally Posted By: SteveSRT8


No generic description is going to account for the variables in design and implementation.

Engine design and its resulting effects are impossible to ignore. Too much variation.

I can change bearings and change my oil pressure without changing anything else.

And it is indeed a relief valve as it relieves excess pressure...


True, but we are talking about the oil pump pressure relief valve and not the whole system, and even then, any hydraulic system, no matter the application, will have to obey certain generic principals. And it is the generic traits of a positive displacement pump and the type of relief valve implemented, that makes them suited for automotive applications. There are other type of relief valves, if you guys bothered to read more on the link I provided, but obviously the direct acting, spring loaded relief valve design was chosen as best.
A hydraulic system is a hydraulic system and certain design criteria, like fast reaction to pressure spikes, will implement similar measures no matter the application.

Dismissing that link and info in it simply on the basis that it does not discuss automotive application is a rather emotion driven move than a logical one.

 

[SteveSRT8]

Originally Posted By: KrisZ
Originally Posted By: SteveSRT8


No generic description is going to account for the variables in design and implementation.

Engine design and its resulting effects are impossible to ignore. Too much variation.

I can change bearings and change my oil pressure without changing anything else.

And it is indeed a relief valve as it relieves excess pressure...


True, but we are talking about the oil pump pressure relief valve and not the whole system, and even then, any hydraulic system, no matter the application, will have to obey certain generic principals. And it is the generic traits of a positive displacement pump and the type of relief valve implemented, that makes them suited for automotive applications. There are other type of relief valves, if you guys bothered to read more on the link I provided, but obviously the direct acting, spring loaded relief valve design was chosen as best.
A hydraulic system is a hydraulic system and certain design criteria, like fast reaction to pressure spikes, will implement similar measures no matter the application.

Dismissing that link and info in it simply on the basis that it does not discuss automotive application is a rather emotion driven move than a logical one.


There was no intent to disparage your contribution. I found it quite illuminating.

Be careful, lest you become as the Turtle with a derogatory reference in every post! I read quite well, thank you.

But many others are posting based on particular experiences with a particular engine or family of engines. It seemed both pertinent and relevant to me as the variables are many and we are primarily automotive around here...

 

[OVERKILL]

Originally Posted By: jrustles
There is no fixed numerical bypass backpressure point. It WILL change if you change your viscosity. Seeing a lower backpressure on low vis oil does not necessarily mean greater flow through the engine- yes, there may be- but greater flow through all other, immediate, closer outlets including housing/gear clearances and a given backpressure-determined position of the bypass valve, which is certainly almost always open above idle, even on a hot engine. Your observed slower/lower pressure build to plateau is simply due to the increased volume of leakage, particularly through the bypass.


That hasn't been my experience with SBF's and SBC's, which, with hot oil of basically any viscosity, will hit the relief point on the pump and stay there. That pressure is always the same on the oil pressure gauge regardless of whether the oil is 0w-20 or 5w-50. Now, it GETS to that point faster (at a lower RPM) with the heavier oil than the thinner oil, but the end result with my SBF was always 65psi. This behaviour would graph the same as the LSx shown earlier in the thread.

That does however bring up a good point about some particular engines, one of which was a Dodge engine mentioned in another thread that was in fact apparently on the relief at much above idle. It had very high idle oil pressure, which climbed fast, and ultimately meant that the relief was bypassing oil internally much of the time.

I think it is foolhardy for us to think that these systems operate the same in all engines as SteveSRT8 pointed out. The information I posted from Melling in the other thread regarding the spring pressures available for SBC and BBC engines mirrors my experience with them and the SBF. You fit a given spring and that is the pressure at which the pump starts bypassing. And with an HV pump on a relatively tight SBC, you can over-run the relief spring and see pressure well above what you've fitted the pump for.

I am sure it is quite possible for some Japanese engines to have light springs that do in fact result in the oil pump bypassing some of the oil much of the time, operating in the manner in which you've depicted. That isn't the case with any of the American engines I have experience with though, which are easy enough to find the bypass spring pressure settings for. Some members of the Modular family have a 70psi spring for example. IIRC, one of them has one around 90psi. As noted in the Melling PDF I posted recently, one particular Ford engine family has a 125psi relief spring factory.

I believe I posted this in another thread, but perhaps the answer to the specific question as to how often the pump is on the relief is "it depends"

 

[turtlevette]

Originally Posted By: OVERKILL
but perhaps the answer to the specific question as to how often the pump is on the relief is "it depends"

And it's more probable that a well designed oiling system on a new engine will be into relief early.

A poorly designed oiling system on a new engine will be into relief late.

A well designed oiling system on an old worn out engine will be into relief late.

A poorly designed oiling system on an oil worn out engine may never hit relief.

Its desirable to have more oil flow than necessary, a trait of a good design.

Why do you think they went to all the trouble to develop high volume oil pumps?

 

[OVERKILL]

Well, "back in the day", those pumps were developed (and used exclusively in) high performance builds with looser clearances. They weren't used factory except in engines like the BOSS 302, BOSS 429...etc.

The big change was when pumps went from cam-driven (SBC, SBF, BBF, BBC...etc), which of course operate at half-speed, to full-speed crank-driven types (Modular, the Japanese one that was posted on the previous page...etc). That said, the LSx posted earlier, and what I've seen from the Modular engines, show that they still operate with a high pressure relief that isn't open most of the time. So unless we are calling the LSx and the Modular poorly designed, I think your assertion is a bit too general

As the old saying goes "there is more than one way to skin a cat". Perhaps what we are seeing are two different ways of doing the same thing. A higher volume pump with a loose spring bypassing a lot of the time vs a "regular" volume pump with a heavy spring that doesn't bypass unless we are at high RPM or experiencing very heavy oil on a cold start with elevated RPM.

 

[turtlevette]

Originally Posted By: SteveSRT8

And it is indeed a relief valve as it relieves excess pressure...


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.

So yes, its a relief being applied as a pressure regulator. Whether it operates at 1x or 2x the desired point is of no consequence as the setpoint is no where close to the design failure point.

 

[turtlevette]

Originally Posted By: OVERKILL
Perhaps what we are seeing are two different ways of doing the same thing. A higher volume pump with a loose spring bypassing a lot of the time vs a "regular" volume pump with a heavy spring that doesn't bypass unless we are at high RPM or experiencing very heavy oil on a cold start with elevated RPM.


No to do the same thing, both pumps would need the same spring and deliver the same oil pressure to the engine. The difference is the high volume pump has reserve capacity that is being dumped back into the pan. As the engine wears the hv pump is able to maintain pressure as the oil that was being bypassed before is now flowing thru the engine to maintain pressure.

 

[OVERKILL]

Originally Posted By: turtlevette
Originally Posted By: OVERKILL
Perhaps what we are seeing are two different ways of doing the same thing. A higher volume pump with a loose spring bypassing a lot of the time vs a "regular" volume pump with a heavy spring that doesn't bypass unless we are at high RPM or experiencing very heavy oil on a cold start with elevated RPM.


No to do the same thing, both pumps would need the same spring and deliver the same oil pressure to the engine. The difference is the high volume pump has reserve capacity that is being dumped back into the pan. As the engine wears the hv pump is able to maintain pressure as the oil that was being bypassed before is now flowing thru the engine to maintain pressure.



Well we know pressure is the resistance to flow. So if our "standard volume" pump is causing us to observe 36psi at idle with a 65psi relief, we know there is resistance to flow through the engine, ergo, the engine is resisting the volume of oil it is seeing. As the engine wears, yes, that # will go down. That doesn't mean that the engine is receiving inadequate lubrication though. Power, on a healthy engine, is being wasted creating that pressure head because pumps are oversized to account for wear. So when you have a tired SBC that shows 15psi on the gauge at idle, we know that the engine has "opened up" due to wear. We also know it isn't receiving inadequate lubrication. This is why manufacturers give us a minimum safe oil pressure standard.

With the high volume pump with a lower relief spring pressure, say it starts to crack at 30psi for example, you would get less pressure drop over time as the engine wears, IE, if your idle pressure was 35psi new and was bypassing a fair bit, it would just bypass less and retain the 35psi by virtue of less oil being bypassed. but this doesn't mean this engine is getting "better lubrication" than the SBC.

Both engines are designed with wear in mind and their lubrication systems are part of that design process. This is why I say both designs accomplish the same thing. Both oil pumps are able to provide more oil than the system needs, even as the engine wears. The difference is that in the one system you will observe a loss of backpressure via oil pressure and on the other you may not.

We didn't see Windsors and SBC's dropping dead as they aged, so we know the "old way" works. And the same goes for the Modular, arguably the longest lasting gasoline engine ever made. These engines are obviously receiving adequate lubrication.

Different doesn't inherently mean "better".