Thin vs Thick Discussion Chapter 1

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482
Location
Cheshire, England
"assuming the relief is closed" is being used as a qualifier to the statement that a PD pump will give the same flow regardless of viscosity oil but in the real world the relief will not be closed. We have a chart showing the relief opening at 2000 rpm and I'm assuming that is with the oil up to temperature. I have an engine where I know the relief is designed to open at or before.2000 rpm even with hot oil. When the oil is cold I believe the relief will be open from idle onwards. So particularly for cold oil it seem clear to me that while a PD pump can deliver the same flow rates at it's output irrespective of viscosity, some of that flow is being lost to the relief valve such that the flow rate to the engine galleries is not the the same with practical consequences for example time to deliver oil to the top end at start up.
 

OVERKILL

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46,520
Location
Ontario, Canada
"assuming the relief is closed" is being used as a qualifier to the statement that a PD pump will give the same flow regardless of viscosity oil but in the real world the relief will not be closed. We have a chart showing the relief opening at 2000 rpm and I'm assuming that is with the oil up to temperature. I have an engine where I know the relief is designed to open at or before.2000 rpm even with hot oil. When the oil is cold I believe the relief will be open from idle onwards. So particularly for cold oil it seem clear to me that while a PD pump can deliver the same flow rates at it's output irrespective of viscosity, some of that flow is being lost to the relief valve such that the flow rate to the engine galleries is not the the same with practical consequences for example time to deliver oil to the top end at start up.

The relief is in fact closed most of the time, even on cold start north of 1,000RPM. Driving around, even with a cold engine on a day like today, my relief stays closed. I have to push it to north of 2,000RPM with a cold engine, to get on the relief.

As the oil warms, it takes a significantly higher RPM to get to the relief pressure, and this has been consistent with every engine I've ever owned that I've tracked this on.

This video is an 8C cold start, oil is 6C, oil is 0w-40:

After I took the cold start video, I left the page open and drove normally for a couple of blocks, initial oil pressure stayed around 60-62psi, below the relief pressure of 65. This dropped the further I went and by the time i got home I was at 39psi with the oil not up to temp.
UNADJUSTEDNONRAW_thumb_11df.jpg
 
Messages
28,358
Location
PNW
"assuming the relief is closed" is being used as a qualifier to the statement that a PD pump will give the same flow regardless of viscosity oil but in the real world the relief will not be closed. We have a chart showing the relief opening at 2000 rpm and I'm assuming that is with the oil up to temperature. I have an engine where I know the relief is designed to open at or before.2000 rpm even with hot oil. When the oil is cold I believe the relief will be open from idle onwards. So particularly for cold oil it seem clear to me that while a PD pump can deliver the same flow rates at it's output irrespective of viscosity, some of that flow is being lost to the relief valve such that the flow rate to the engine galleries is not the the same with practical consequences for example time to deliver oil to the top end at start up.
When the pump starts going into pressure relief depends on a lot of things ... one of them being the design of the oiling system and pump itself including the pressure relief valve, and of course the oil viscosity at any given moment. Most normal vehicles on the road even at high RPM can't achieve max oil pressure with hot oil to trigger pump pressure relief - maybe if the oil filter is clogged is one case where pump pressure relief could happen with hot oil.

And as mentioned, even though a PD is in pressure relief, there is still going to be increased oil flow as the RPM increases like the chart shows. No mechanical spring loaded relief valve is going to regulate a perfect rock steady maximum oil pressure after it starts opening as RPM keeps increasing, and how well it regulates the max pump output pressure and flow also depends on the flow performance/size of the relieve valve.

WRT to your last sentence, it all depends on the temperature and viscosity of the oil. If it's super cold and thick, and barely pumpable then the pump might have a hard time getting oil to its inlet and pumpability will suffer. If the oil is pumpable, then the relief valve could/might open momentarily depending on many factors, but most engines don't rev much above 1500 RPM on cold starts, so keeping the RPM down until the oil warms up some helps mitigate pump relief. Bottom line is PD pumps are used for a reason, and that is to ensure adequate oil lubrication to the engine over a very broad range of use conditions.
 
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2,524
Location
South Carolina
When the pump starts going into pressure relief depends on a lot of things ... one of them being the design of the oiling system and pump itself including the pressure relief valve, and of course the oil viscosity at any given moment. Most normal vehicles on the road even at high RPM can't achieve max oil pressure with hot oil to trigger pump pressure relief - maybe if the oil filter is clogged is one case where pump pressure relief could happen with hot oil.

And as mentioned, even though a PD is in pressure relief, there is still going to be increased oil flow as the RPM increases like the chart shows. No mechanical spring loaded relief valve is going to regulate a perfect rock steady maximum oil pressure after it starts opening as RPM keeps increasing, and how well it regulates the max pump output pressure and flow also depends on the flow performance/size of the relieve valve.

WRT to your last sentence, it all depends on the temperature and viscosity of the oil. If it's super cold and thick, and barely pumpable then the pump might have a hard time getting oil to its inlet and pumpability will suffer. If the oil is pumpable, then the relief valve could/might open momentarily depending on many factors, but most engines don't rev much above 1500 RPM on cold starts, so keeping the RPM down until the oil warms up some helps mitigate pump relief. Bottom line is PD pumps are used for a reason, and that is to ensure adequate oil lubrication to the engine over a very broad range of use conditions.
Sounds like a little back peddling...
 
Messages
9,619
Location
Virginia
‘A useless point to debate in reality.... but....
Interesting that the 167 page governing document:
Engine Oil Licensing and Certification System
API 1509
EIGHTEENTH EDITION, JUNE 2019
never once mentions the word “Winter.”
I spoke to an SAE engineer some years ago (when I was a member) and asked what the W stood for. He said no word in particular, that it could have been any other letter.


Useless to debate because it is exceptionally obvious to any casual observers that you are not correct in your ideas of what "w" means.
 
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14,373
Location
Kendall, FL
Same PDP, same RPM...

Thicker, higher viscosity oil (more resistant to flow) may flow the same volume at the same RPM but it will make the pump work harder to do so (hence resistance). The phrase "more resistant to flow" is in relation to an ICE and the level of resistance each viscosity demonstrates while in use.
 

OVERKILL

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46,520
Location
Ontario, Canada
Same PDP, same RPM...

Thicker, higher viscosity oil (more resistant to flow) may flow the same volume at the same RPM but it will make the pump work harder to do so (hence resistance). The phrase "more resistant to flow" is in relation to an ICE and the level of resistance each viscosity demonstrates while in use.

Correct. And the artifact of this is higher pressure. It will also of course require more power to pump.
 
Messages
28,358
Location
PNW
Same PDP, same RPM...

Thicker, higher viscosity oil (more resistant to flow) may flow the same volume at the same RPM but it will make the pump work harder to do so (hence resistance).
Yes, and also why oil pressure goes down at the same RPM with the same oil as the oil warms up and becomes less viscous. When the pump not in relief of course.
 
Messages
342
Location
USA
I wonder what Molakule's opinion is of the "W"? I was taught it was for winter, but it's not my field of expertise. So, I only know what I read. Please understand I'm not advocating for turmoil or arguing among members. Only clarification...Ty
 

AEHaas

Thread starter
Messages
1,444
Location
Sarasota, Florida
“With a PD pump, all of the oil leaving the pump will go through the engine, and only If the PD starts going into pressure relief, then that is when not all the oil leaving the pump will go through the engine. With increased RPM there is an increase in flow output to the engine oiling system, even if the pump is in pressure relief - see graph below. Exactly how much pump "slip" the pump has, the viscosity of oil being pumped (thicker oils pump better due to less slip) and the overall pumping efficiency of the pump will determine just how much flow roll-off there is before the pump hits pressure relief, and also while in pressure relief dependent on the relief valve design.”



I agree with the need for more flow with increasing RPM as depicted by your graph. It is a very good point. In later chapters I will show how thicker and or cooler oils can impede the necessary flow as shown in your graph.

I disagree that after hitting the pressure relief point the engine oil flow can still increase with increasing RPM. A fluid can only move if there is a difference in pressure. If the system is limited to say 80 PSI then no amount of pump output will increase the flow unless the pressure is allowed to also increase. If there is no increase in the pressure then no increase in flow is possible. In all my vehicles the pressure hits the relief point and increases no further, regardless of the RPM. Therefore flow cannot increase further. Oil flow is limited to that amount when the maximum pressure is encountered, sometimes at just a few thousand RPM. Again, later chapters will make this clear.
 

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OVERKILL

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46,520
Location
Ontario, Canada
I disagree that after hitting the pressure relief point the engine oil flow can still increase with increasing RPM. A fluid can only move if there is a difference in pressure. If the system is limited to say 80 PSI then no amount of pump output will increase the flow unless the pressure is allowed to also increase. If there is no increase in the pressure then no increase in flow is possible. In all my vehicles the pressure hits the relief point and increases no further, regardless of the RPM. Therefore flow cannot increase further. Oil flow is limited to that amount when the maximum pressure is encountered, sometimes at just a few thousand RPM. Again, later chapters will make this clear.

The relief in the pump is a fixed size orifice and it is quite possible to overwhelm it, which will drive up pressure beyond what is required to open it. I believe I remarked on this in response to a similar statement made in another thread and even provided an example. A very common example of this is an HVSP pump in a standard clearance SBC or SBF where you can drive pressure well beyond the relief because the relief orifice cannot bypass enough oil back to the feed side of the pump.
 
Messages
28,358
Location
PNW
I disagree that after hitting the pressure relief point the engine oil flow can still increase with increasing RPM. A fluid can only move if there is a difference in pressure. If the system is limited to say 80 PSI then no amount of pump output will increase the flow unless the pressure is allowed to also increase. If there is no increase in the pressure then no increase in flow is possible. In all my vehicles the pressure hits the relief point and increases no further, regardless of the RPM. Therefore flow cannot increase further.
As I mentioned, the roll-off of flow throughout the RPM range is dependent on many things: 1) Pump slip/efficiency, 2) oil viscosity, 3) Design and size of the pressure relief valve. Look at the graph, and you can see even as more and more flow is being shunted and not flowing into the engine, there is still increasing flow going through the engine. It's not as much increase as if the pump wasn't in relief, but flow is still increasing with RPM. As far as your Ferrari, who know for sure. Are you revving the engine to near redline with relatively cold oil when you're looking at how the pressure increases with RPM? If not, then it's an incomplete test. And what oil viscosity are you using while doing that experiment?

I imagine if someone designed a computer controlled pressure relief valve based on a sensitive pressure gauge, then the pressure could be controlled dead nuts. Oil pump pressure relief valves are just a spring loaded device, with a linear force spring.

Look at the video OVERKILL posted in post #23. He says the pressure relief valve is set to 65 PSI, yet as he keeps revving the engine when the oil pressure hits 65 PSI, the oil pressure keeps increasing some because the pressure relief is unable to max the pressure at 65 PSI. This is a typical control characteristic of a spring loaded relief valve that isn't super sized to bypass the shunted oil around the pump without some flow loss.

Oil flow is limited to that amount when the maximum pressure is encountered, sometimes at just a few thousand RPM. Again, later chapters will make this clear.
You might want to do some more studies on PD pumps used in an ICE, and how the pressure relief valve come into play and operates.
 
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Messages
2,235
Location
Muncie, Indiana
I imagine if someone designed a computer controlled pressure relief valve based on a sensitive pressure gauge, then the pressure could be controlled dead nuts. Oil pump pressure relief valves are just a spring loaded device, with a linear force spring.

Look at the video OVERKILL posted in post #23. He says the pressure relief valve is set to 65 PSI, yet as he keeps revving the engine when the oil pressure hits 65 PSI, the oil pressure keeps increasing some because the pressure relief is unable to max the pressure at 65 PSI. This is a typical control characteristic of a spring loaded relief valve that isn't super sized to bypass the shunted oil around the pump without some flow loss.
So part of what you're saying is people don't get that spring loaded relief valves don't just necessarily open wide up and stay open, they constantly modulate open and shut as the valve will begining springing open when the inlet and outlet pressures are at a certain delta, but by opening the relief valve that delta decreases and the valve starts shutting but as the restriction grows stronger it starts opening back up more and more or less the valve constantly modulates while in relief, which on a similar note is why in Fram marketing materials they make such a big deal out of their oil filter bypass valves surviving millions of cycles.
 

4WD

Messages
17,479
Location
Texas
Well, one problem with this debate is it keeps coming back to pump output rather than oil intake of the engine … and yes that component can max out as more oil is recirculating via the PRV … despite pump output increases with RPM
So part of what you're saying is people don't get that spring loaded relief valves don't just necessarily open wide up and stay open, they constantly modulate open and shut as the valve will begining springing open when the inlet and outlet pressures are at a certain delta, but by opening the relief valve that delta decreases and the valve starts shutting but as the restriction grows stronger it starts opening back up more and more or less the valve constantly modulates while in relief, which on a similar note is why in Fram marketing materials they make such a big deal out of their oil filter bypass valves surviving millions of cycles.
Sure … for example cars today will have 8 speed to 10 speed transmissions - so the oil pump and PRV must repeat this pattern several times during spirited acceleration …
 
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3,199
Location
Western S.C.
... Look at the graph, and you can see even as more and more flow is being shunted and not flowing into the engine, there is still increasing flow going through the engine. It's not as much increase as if the pump wasn't in relief, but flow is still increasing with RPM. ...
I imagine if someone designed a computer controlled pressure relief valve based on a sensitive pressure gauge, then the pressure could be controlled dead nuts. Oil pump pressure relief valves are just a spring loaded device, with a linear force spring.
That graph and its title are not clear whether it's about actual flow to each component, or optimum flow.

Inability of a typical relief valve to maintain its exact nominal setting isn't the only reason flow rate varies with speed.
 

4WD

Messages
17,479
Location
Texas
That graph and its title are not clear whether it's about actual flow to each component, or optimum flow.

Inability of a typical relief valve to maintain its exact nominal setting isn't the only reason flow rate varies with speed.
Indeed … it’s a hydraulic system these days … VVT … AFM … piston jets … variable output oil pumps … ECM controls/sensors …
Things have been simplified in this “discussion“ in attempts to reach common ground.
 
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28,358
Location
PNW
So part of what you're saying is people don't get that spring loaded relief valves don't just necessarily open wide up and stay open, they constantly modulate open and shut as the valve will begining springing open when the inlet and outlet pressures are at a certain delta, but by opening the relief valve that delta decreases and the valve starts shutting but as the restriction grows stronger it starts opening back up more and more or less the valve constantly modulates while in relief, which on a similar note is why in Fram marketing materials they make such a big deal out of their oil filter bypass valves surviving millions of cycles.
Correct, a spring loaded valve (like a PD pump pressure relief valve, or a filter bypass valve) just doesn't open all the way when they start to crack open. They gradually open, and require more and more force (delta PSI across the spring loaded valve) to open farther in order to flow more volume. It's not a "step function", and therefore you will see something similar to the graph in post #16. In order to have a step function pressure relief valve, it would have to actuated (computer controlled) to open the correct amount based on the delta PSI across it to keep the pump outlet pressure dead nuts constant.
 
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