Positive displacement pumps explained (hopefully).

[edhackett]

Originally Posted By: Shannow
Originally Posted By: turtlevette
Shannow stated that torque is not a force. I disagree with that. Torque is a force acting on a lever. Get out the statics books.


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.


Imagine a bare crank resting in its bearings. Attach a lever to the end of the crank. Push down on the lever. The crank will rotate. The force it took to rotate the crank is torque. Load as used for the flow calculations is the weight the crank places against the bearings. Torque applied to the crank in low friction bearings has virtually no effect on load.

Turtlevette, does that help you see the difference?

Ed

 

[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?

 

[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.

 

[edhackett]

Originally Posted By: KrisZ

A positive displacement pump is used when a certain amount of fluid has to be delivered, or constant amount of flow maintained. In these pumps the flow is constant and the pressure varies. That is why these pumps have pressure relief valves, to protect the system and the pump from over pressurization. Because these pumps will keep on moving the fluid until something gives way.


Right. Let's simplify by ignoring the efficiency and say the oil pump delivers the exact same volume over the range of viscosities used in an engine.

Higher viscosity fluids have a greater resistance to flow, so greater pressure is developed as the volume is moved against the resistance of the lubrication system.

Lower viscosity fluids have less resistance to flow so lower pressure is developed as the volume is moved against the resistance of the lubrication system.

When the pressure generated by the flow exceeds the relief pressure setting, flow is diverted to prevent pressure from rising above the desired level.

Ed

 

[turtlevette]

Originally Posted By: edhackett
Originally Posted By: Shannow
Originally Posted By: turtlevette
Shannow stated that torque is not a force. I disagree with that. Torque is a force acting on a lever. Get out the statics books.


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.




Imagine a bare crank resting in its bearings. Attach a lever to the end of the crank. Push down on the lever. The crank will rotate. The force it took to rotate the crank is torque. Load as used for the flow calculations is the weight the crank places against the bearings. Torque applied to the crank in low friction bearings has virtually no effect on load.

Turtlevette, does that help you see the difference?

Ed


Your example Ed is how I was viewing it. Doing a simple statics analysis. You push down with 400 pounds in steady state the bearings have to push up with 400 pounds. The mistake I made was using 1 ft for the lever. An SBC has a 3.48" throw. Or 3.48/12 =.29 foot throw. 400/.29=1379 pounds force divided by 5 bearings = 276 pounds force.

Does that make sense? I think you have to average the force. Using the impulse force of 8000 pounds throws things off.

 

[turtlevette]

Originally Posted By: JAG
Here you go, turtlevette...see page 8: http://turbolab.tamu.edu/proc/turboproc/T34/t34-16.pdf



I'm looking through it. The example they use is a fairly large bearing of 3" supporting an 1100 pound load. It needs a makeup of about 1 gal/min at 5000rpm. I'm reading more...

 

[OVERKILL]

Originally Posted By: turtlevette
Originally Posted By: JAG
Here you go, turtlevette...see page 8: http://turbolab.tamu.edu/proc/turboproc/T34/t34-16.pdf



I'm looking through it. The example they use is a fairly large bearing of 3" supporting an 1100 pound load. It needs a makeup of about 1 gal/min at 5000rpm. I'm reading more...



Just an FYI, but the 351W has 3" mains

 

[turtlevette]

Originally Posted By: ZeeOSix
Here is real data from the on-board pressure and oil temperature sensors on my Z06. The oil temperature was a constant 200 deg F, and this is how the oil pressure changed with engine RPM.


That must be well worn. My POS 5.3 Suburban 110k miles with half and half 0w-20,0W-30 AFE has around 42 psi at hot idle. The bypass comes in much sooner too. I'll have to play with it the next time I'm out and take note where the pressure levels off.

 

[turtlevette]

Originally Posted By: OVERKILL
Originally Posted By: turtlevette
Originally Posted By: JAG
Here you go, turtlevette...see page 8: http://turbolab.tamu.edu/proc/turboproc/T34/t34-16.pdf



I'm looking through it. The example they use is a fairly large bearing of 3" supporting an 1100 pound load. It needs a makeup of about 1 gal/min at 5000rpm. I'm reading more...



Just an FYI, but the 351W has 3" mains

I doubt they're several inches wide. I was looking for the length but couldn't find it. The 351 don't support 1100 pounds each either. It's still a bit apples and oranges.

 

[OVERKILL]

Originally Posted By: turtlevette
Originally Posted By: OVERKILL
Originally Posted By: turtlevette
Originally Posted By: JAG
Here you go, turtlevette...see page 8: http://turbolab.tamu.edu/proc/turboproc/T34/t34-16.pdf



I'm looking through it. The example they use is a fairly large bearing of 3" supporting an 1100 pound load. It needs a makeup of about 1 gal/min at 5000rpm. I'm reading more...



Just an FYI, but the 351W has 3" mains

I doubt they're several inches wide. I was looking for the length but couldn't find it. They don't support 1100 pounds each either.



Correct, I doubt they'd have 1,100lbs on them. IIRC, they are about 1" wide.

 

[edhackett]

Originally Posted By: turtlevette


Your example Ed is how I was viewing it. Doing a simple statics analysis. You push down with 400 pounds in steady state the bearings have to push up with 400 pounds. The mistake I made was using 1 ft for the lever. An SBC has a 3.48" throw. Or 3.48/12 =.29 foot throw. 400/.29=1379 pounds force divided by 5 bearings = 276 pounds force.

Does that make sense? I think you have to average the force. Using the impulse force of 8000 pounds throws things off.


This site helps explain. There is no torque at TDC. The force is straight down on the bearing. In this position there is no lever to generate torque. The full force of combustion is load on the bearing.

I'm looking for the site were I found the 6200 value. I believe peak bearing load is a few degrees after TDC. It was late and I didn't get it bookmarked.
Torque Generation

6200 psi source:
http://mb-soft.com/public2/engine.html

Ed

 

[turtlevette]

I believe there is still some confusion here. I have not found anything that says a flooded bearing has more flow restriction than a starved. Ie there is nothing that comes out and says the supply restriction is dynamic and tied to rpm.

Yes the side leakage needs to be greater to keep the bearing cool but not necessarily mean the bearing is "consuming" oil.

I'll ponder on it tomorrow while working on the Trans Am.

 

[Shannow]

Originally Posted By: turtlevette
Doing a simple statics analysis. You push down with 400 pounds in steady state the bearings have to push up with 400 pounds. The mistake I made was using 1 ft for the lever. An SBC has a 3.48" throw. Or 3.48/12 =.29 foot throw. 400/.29=1379 pounds force divided by 5 bearings = 276 pounds force.

Does that make sense? I think you have to average the force.


First iteration of the bearing design, that's a great first approximation.

Need more detail for the next round, but that gets the show moving.

 

[Shannow]

Originally Posted By: turtlevette
I believe there is still some confusion here. I have not found anything that says a flooded bearing has more flow restriction than a starved. Ie there is nothing that comes out and says the supply restriction is dynamic and tied to rpm.

Yes the side leakage needs to be greater to keep the bearing cool but not necessarily mean the bearing is "consuming" oil.

I'll ponder on it tomorrow while working on the Trans Am.


If you look at the pressure profile around a journal, there is an area of negative pressure away from the loaded portion. That part can actually cavitate, and if, for example there was a drum of oil connected to that point, it would suck as much oil as it needed from there directly without a pump.

The nukes that you worked on would have had flow control orifices in the generator bearings to control the working temperature through the recirculating and mixed flow in the bearings.

As per the Kingsbury pdf that I posted ealier, they state that supply pressure will not materially alter the flow of oil into a bearing...take that statement revised, and a bearing will not swallow any more oil than it needs for a given operational position...the excess of which is the artifact of pressure that we see on the guage.

That's what makes CATERHAM's assertion that his engine is a viscometer, and measured using oil pressure, at constant load/speed/temperature correct (and it measures HTHS retty well by the look of it)...it doesn't mean that his oil pressure is a guarantee of lubrication, but that's another party.

So if a bearing swallows twice as much oil at twice the speed, and the pump delivers twice the volume at twice the speed, then they are a good match.

 

[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?

 

[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.

 

[jrustles]

I'm going to take some pictures of an actual oil pump relief valve a bit later on today. I had no idea such doubts still existed about the way they operate, considering we've gone full nerd pedantic about this issue before

 

[JAG]

Originally Posted By: jrustles
I'm going to take some pictures of an actual oil pump relief valve a bit later on today. I had no idea such doubts still existed about the way they operate, considering we've gone full nerd pedantic about this issue before

That was exactly my thought last night when reading on here. Good lordie, no wonder people don't agree about some lubrication topics when there are some people who don't understand how things work. I think good progress is being made, though.

 

[Ken2]

Quote:
The Positive Displacement Pump has more or less a constant flow regardless of the system pressure or head.
Well, yeah, that's what positive displacement means. It positively displaces the same volume each rotation or each stroke as long as it's moving, less slippage past the necessary tolerances.
Quote:
Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.
It all depends on the design. I've run centrifugal pumps that put out 1500 psi. A positive displacement pump can be designed for higher pressures--hydraulic pumps, for example.
Quote:
In the Positive Displacement Pump the flow is increased when viscosity is increased
In a small way. Going from one hot motor oil viscosity grade to a hot heavier grade is minor, and will make little difference in a pump in good condition. If you're pumping gasoline today and gear oil tomorrow, and the pump isn't like new, yes there might be some output difference.

For clarity, I'd call the relief valve at the engine's oil pump a pressure control valve. I'd call the relief valve at the oil filter a differential pressure bypass valve. And, the oil pump's pressure control valve is a simple device with a wide proportional band. The pressure it begins to open will be a fair bit less than the pressure it takes to push it wide open.

There are few parts of an engine that actually need a certain oil pressure. As long as sufficient oil is delivered to keep each bearing flooded with oil, it is good. A different story might be a drilled conn rod feeding oil to an oscillating wrist pin where the oil pressure is needed to actually lift the pin from the bearing--it can't develop the hydrodynamic oil wedge due to its oscillating motion.

The picture of the Gerotor pump is a difficult example of a positive displacement pump to gain understanding. Here's how it works (and note there is an inlet port (red loop) and outlet port (blue loop) hidden behind the picture):

Look how the gear comes away from the corresponding gear opening a space that pulls oil in from the supply line. It circles, then the gear moves into that space and pushes oil out through the discharge port.

A centrifugal pump is completely different. The rotor spins and throws the liquid into a space. The space is a volute (expanding volume as you move around it) and it converts the high velocity of the thrown out liquid into pressure. The actual pressure potentially developed depends on the speed of the rotor and the shape of the volute. A centrifugal fan, such as a turbocharger, works the same way. The only place I've seen centrifugal oil pumps are the cargo oil pumps on oil tankers. All the fuel oil and lube oil pumps of even the biggest engines I've seen are positive displacement of many different configurations--gear pumps, screw pumps, etc.

 

[jrustles]

Quick review: Oil pumps theoretically move exactly their displacement's worth of volume per revolution. That pumped volume then is simply multiplied by RPM. It is theoretically no different than a refrigerant compressor or an air compressor, regardless of pump-type (piston, rotor etc) except one difference- the oil pump is hydraulic, and oil is virtually uncompressible.

The variables: because the entire operation of the oil pump is variable

Actually, it's simpler to begin with the aspects of the pump that are NOT variable.
-the pump's displacement per revolution
-the bypass/regulator (whatever you want to call it) spring pressure and orifice size and thus varying bypass orifice size to seen backpressure ratio
-the 'orifice-equivalent' of the clearances of the pump gears and housing itself

Everything else varies, including this illustrious "bypass point".

I should note that out of all the parameters of measuring the output of an oil pump and the quality of system lubrication, backpressure is about the most worthless one of all. You cannot tell when the "bypass point" is, or when it begins to open by reading backpressure gauge behavior, you can only tell when the bypass/regulator has reached equilibrium given ALL OF THE VARIABLES, like immediate viscosity (accounting for grade and temp) and engine RPM.

If that doesn't make sense, then the following should help illustrate


Here is a standard 80s Japanese crank-mount, trochoid/gerotor type pump with the back plate removed. The parts of this particular pump are labeled

Here is the bypass circuit close up. The hole is the 'variable orifice" if you will. It is fully closed and spring loaded in operation.

Here, it's cracked open.

opened some more

and pegged, a position i doubt could ever be reached in practice, except maybe a redline winter start

So, questions to consider:
Which one of the three circled bypass positions is the "bypass point"?


Let's use the orange circled bypass position (mid point) for an analogy:
We know that at that position, the spring pressure is constant. But at that position dictated by the spring pressure, a higher volume of low vis oil will 'bypass' than a higher vis oil- if the same backpressure is seen.

So it boils down to this: your bypass spring and relief orifice size determine the regulated oil pressure, and that pressure changes with viscosity.

The bypass value supplied by manufacturers is the designed equilibrium pressure achieved by using the recommended grade at normal operating temperature. If you change the viscosity, you change the 'bypass point' and it ends up being "whatever" the other variables dictate. Because you see 80psi during cold start on TGMO, and see it level out at 65psi at redline at normal operating temp, DOES NOT MEAN THE BYPASS IS CLOSED!!!! To achieve a higher running "bypass point" backpressure reading, you need to change the bypass spring.

It's foolhardy at best to try optimizing anything, let alone viscosity choices,
based on backpressure gauges and logical assumptions thereof. 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.