Explain straight weight oil viscosity recommendations to me, please.

[A64Pilot]

You guys are trying still to make this difficult, no one said anything about energy, power sources, vernatherm or anything else. 

If you restrict the outflow of a hydraulic system, the pressure will rise. Cold oil as I said or thick oil is a restriction and pressure will rise.

Restricting flow will cause pressure to rise in the whole system, prior to the restriction. 

Flow and pressure are energy, get hit by a garden hose and then by a fire hose, they are at the same pressure as they come off the same main, but the fire hose will knock you on your arse from the energy of the greater mass of water.

The pressure relief valve in normal cruise will mostly be closed, if you want to see that look at your oil pressure soon after takeoff with cool oil, it will most likely be higher than it is in cruise, mostly because the oil pressure relief valve is likely open with the cool oil or pressure would be too high and that higher pressure you see is most likely the cracking pressure of the relief valve. If in normal cruise your pressure is lower than it was with the cool oil, the regulator is closed, because you lower than the cracking pressure. It’s often called a cold oil relief valve for that reason.

Now if your takeoff oil pressure remains the same in cruise, you may have an engine that the valve stays open, but I’ve never seen that. Mine runs close to redline on initial takeoff, but in a few minutes is in the middle of the green, and I think that’s common.

But from the beginning all I was trying to point out was that oil pressure is VERY important, without enough pressure you will have metal to metal contact on your bearings and that can quickly lead to engine failure.

[Fly Boomer]

5 minutes ago, A64Pilot said:

energy of the greater mass of water.

You are correct.  It's the mass, not the pressure of the water.

[A64Pilot]

4 minutes ago, Fly Boomer said:

You are correct.  It's the mass, not the pressure of the water.

It’s actually both, it’s sort of a mass x velocity = energy thing

Edited June 7, 2022 by A64Pilot

[Fly Boomer]

5 minutes ago, A64Pilot said:

It’s actually both, it’s sort of a mass x velocity = energy thing

If you think fluid dynamics is hard to understand, do some reading on the fundamentals of particle physics and the standard model.  Regardless of how much you read, it just doesn't make sense.  Welcome to the universe.

[Andy95W]

25 minutes ago, Fly Boomer said:

If you think fluid dynamics is hard to understand, do some reading on the fundamentals of particle physics 

PLEASE don’t make this thread even more ridiculous than it’s already become!

Edited June 7, 2022 by Andy95W

[carusoam]

The fun part…

I think Siri dropped a word out of what A64 was writing…

So…

Telling someone to go back and read it….

May point out some fun quirks of modern writing…

-a-

[Pinecone]

Oil pressure in the engine does NOT establish or re-establish the hydrodynamic film. 

Otherwise the cam/lifter would not have any lubricant film.

AH64Pilot is correct that restricting the flow does increase the pressure.  And the an worn engine, with low oil pressure may have issues.

BUT, the issue is, that the pressure drop is because more oil is flowing out at the first change.  Think of a hose with a number of holes along it.  If the pressure is high enough, water squirts out of all the holes.  If we make the holes bigger, more water leaks out in the early holes, the pressure drops and no water comes out of the holes near the end.

[A64Pilot]

20 hours ago, carusoam said:

The fun part…

I think Siri dropped a word out of what A64 was writing…

So…

Telling someone to go back and read it….

May point out some fun quirks of modern writing…

-a-

I left squared out to keep it simple

[A64Pilot]

On 6/7/2022 at 5:47 PM, Fly Boomer said:

If you think fluid dynamics is hard to understand, do some reading on the fundamentals of particle physics and the standard model.  Regardless of how much you read, it just doesn't make sense.  Welcome to the universe.

There is a reason why I’m not a degreed Engineer and your touching on it, my math skills, well they aren’t skills actually.

[A64Pilot]

17 hours ago, Pinecone said:

Oil pressure in the engine does NOT establish or re-establish the hydrodynamic film. 

Otherwise the cam/lifter would not have any lubricant film.

AH64Pilot is correct that restricting the flow does increase the pressure.  And the an worn engine, with low oil pressure may have issues.

BUT, the issue is, that the pressure drop is because more oil is flowing out at the first change.  Think of a hose with a number of holes along it.  If the pressure is high enough, water squirts out of all the holes.  If we make the holes bigger, more water leaks out in the early holes, the pressure drops and no water comes out of the holes near the end.

Please explain then why high performance engines require higher oil pressure than the stock motors?

I understand the cam lifter interface but that’s hardened steel on hardened steel and the design doesn’t require oil flow, a crankshaft does. By the way that’s called splash lubrication

Do some reading, particularly about how flow, restriction and pressure are related to each other, and how as RPM increases oil pressure need does too. If it was simply a film of oil was all that’s required then as RPM and loads increased it would be fine for oil pressure to remain low.

https://www.chevyhardcore.com/tech-stories/engine/oil-pump-selection-is-high-pressure-or-high-volume-better/

Edited June 8, 2022 by A64Pilot

[Pinecone]

Again, oil pressure is an indication of the delivery of the proper flow to all parts of the engine.

Again, even 100 psi of oil pressure does not counteract over 1000 psi con rod to crank load.

[Fly Boomer]

17 hours ago, A64Pilot said:

There is a reason why I’m not a degreed Engineer and your touching on it, my math skills

I can sympathize.  I have a degree in mathematics, and I can barely balance my checkbook.

[BobbyH]

17 hours ago, A64Pilot said:

There is a reason why I’m not a degreed Engineer and your touching on it, my math skills, well they aren’t skills actually.

Well I am a "degreed Engineer" who worked aerospace for 20 years and want to commend @A64Pilot for doing a good job of explaining what is going on.  The fun part is that it is both simple and complicated at the same time.  Although I haven't played this game (fluid dynamics) for over 20 years, the basics are that heat will lower the viscosity (shearing stress) in a fluid.  This will mean that the oil in the engine will have less resistance flowing through the engine as it heats up, thus increasing the flow while decreasing the pressure.  The art of what the engineers are trying to accomplish is getting all the parts lubricated well, under a variety of conditions, but especially during cruise (steady state) conditions.  The multi-viscosity oils are an attempt to improve the cold temperature properties of the oil to try and flatten the viscosity curve of the Ground-Air-Ground cycle while maintaining decent pressure in the system to reach all the parts.  

Bob

[A64Pilot]

9 hours ago, Pinecone said:

Again, oil pressure is an indication of the delivery of the proper flow to all parts of the engine.

Again, even 100 psi of oil pressure does not counteract over 1000 psi con rod to crank load.

I don’t think I ever said it did, but at 2400 RPM each piston goes through a power stroke 20 times per second, the oil cushion has to be refilled 20 times every second, and the higher the load the greater the cushion has to be. It’s like that airbag, the pressure isn’t what keeps you from hitting the dash, the air cushion does because it takes time to push the air out of the bag even with it full of holes it will stop you in a very short time, if it had to stop you again in 1/20 of a second it going to take significant pressure to refill the bag, the faster the bag has to be refilled the high the pressure required.

‘BTW did you read the article? Do you now understand how restriction is what builds pressure?

You keep saying oil pressure has nothing to do wit it, all is needed is a film of oil, but you won’t explain what we have a min oil pressure, so why do you think we have oil pressure minimums if oil pressure has no role in lubrication?

Edited June 9, 2022 by A64Pilot

[Pinecone]

Hmm, but YOU said, the pistons/rings are splash lubricated.  So where does oil pressure affect that?

 

I have said MULTIPLE times that our oil pressure indicates that the oil is getting to all parts of the system.  If the oil pressure drops, it is likely that gaps have opened (or oil thinned out) to the point where the far reaches of the system are not getting enough oil.

As my analogy previously, the oil system is like a pipe or hose with holes along it.  The pressure is measured before the first hole.  If the holes are small enough, the pressure is high and the water squirts out of the last holes.  If the holes get larger, the pressure drops and the water doesn't squirt from the far holes.

And I keep asking, and you have not answered, what does 100 psi of oil pressure mean to a bearing load that is over 1000 psi?  

[carusoam]

3 minutes ago, Pinecone said:

what does 100 psi of oil pressure mean to a bearing load that is over 1000 psi?  

Oooh I think I got this one….   

If 100psi is in the green zone of the OilP gauge….

The right oil… at the right temperature… will be delivered to the bearings as expected…

As long as the geometry of all the oil galleries and passages hasn’t changed…

 

How the pressure floats the bearings… back to the hydro-dynamic discussion above… 

Tremendous pressure can be built between moving parts… as the oil gets squeezed between them….

That pressure in the finite regions pretty much goes unmeasured…

 

I think I would want ICPs if I could get quirky pressure readings for a dollar….   
 

Fortunately, somebody at the engine factory had to figure this out, and simplify it for the users….

 

They even oversimplified some things… like one EGT and one CHT….  They are perfect until something usual goes wrong…

Another place where OilT and OilP data would be nice to have…. In or near the turbo…. So much going on here as things wear in their normal lifetime….

and… we could definitively answer if it makes more sense to idle the engine to let the turbo cool down today….   
 

If I could see the OilT in my old O360…. In the valve guides… I would have easily seen an exhaust valve was going to jam open….  The oil was turning into carbon chunks and blocking the passage….

So….

our single gauge for OilP and OilT are nice… they indicate things are working as expected….

But, they don’t say much about what is going on at all times inside the engine anywhere….

PP thoughts only…

Best regards,

-a-

[Pinecone]

Yeap, you got it.

[M20F]

What happens to oil pressure if say the airplane is on a treadmill?

[EricJ]

FWIW, if you understand basic electricity and Ohm's law, you understand it's analog to fluid pressure systems, which includes pneumatic and hydraulic (e.g., water, oil) systems.   The analogs of Voltage and Pressure, current and flow, and resistance and restriction are consistent.   So in V = iR, the voltage drop across is a resistor is proportional to the current flow through it.   Likewise the pressure drop across a restriction is proportional to the flow through it.   In a typical engine oil system the available pressure comes only from the pump, which is the hydraulic energy source (and it gets its energy from the crankshaft).   So there's only so much to go around, and then the regulator attempts to make that constant (just like a voltage regulator downstream from a generator or alternator) within the range engineered to maintain the proper flow (or current, i) to deal with the expected restrictive load.  

If you understand what a voltage divider is, or two resistors in series, then you can understand how pressure and flow are affected by restrictions.   If the voltage across both resistors is kept constant, or the pressure regulated across restrictions in series, then changing the resistance of the second resistor does change the voltage between the resistors.   If you make the second resistor bigger (increase restriction), then the voltage at the center node goes up, but the current (flow) goes down.  The pressure out of the regulator is unaffected.  

In the oiling system there are ports and galleries that the pressurised oil has to flow through before it gets to any bearing.   Those are restrictions that impede flow prior to the bearing load that are analogous to the first resistor in a voltage divider.   So the model of a voltage divider works well to understand how and where restrictions affect pressure in a typical oiling system.  The pressure due to a restriction increase is prior to the restriction, not after it, and it has the effect of reducing flow (current).

Anyway, it's just a tidbit for people who might understand basic DC electrical behavior that they can apply the same knowledge to hydraulic (or pneumatic systems), and vice versa.    The analogs are quite strong.   The voltage regulator downstream from the generator/alternator does essentially the same job as the pressure regulator after the oil pump.  That's probably obvious to many, but to some it might be a useful 'aha'.

Edited June 10, 2022 by EricJ

[Hank]

Sorry, @EricJ, you lost me with the electric mumbo jumbo. In school, they taught water flow through pipes to explain electricity, but they never explained why the basic equations developed by Maxwell et al c.188?, written to describe the flow of a positive charge through a circuit, have never been fixed since the discovery that electricity is actually a flow of negative particles . . . and no analogy to liquid will explain transistors or inductors.

Oil flows through openings; restrict the opening, the flow rate will be reduced, flow speed through the restriction will increase, and pressure in the restriction will reduce. If you pump a constant volume of oil towards a restriction, if it cannot all go through it will pool up; if there is nowhere to pool because it's in a hose, the pressure upstream of the restriction will increase and the pump will work harder to maintain the flow rate, but the total flow rate is regulated by the restriction. This can easily go really bad if the restriction gets smaller for any reason . . . Can't think of an electrical analogy for that . . . Then again, I can't push electrons around with my hands, either.

[EricJ]

19 minutes ago, Hank said:

Sorry, @EricJ, you lost me with the electric mumbo jumbo. In school, they taught water flow through pipes to explain electricity, but they never explained why the basic equations developed by Maxwell et al c.188?, written to describe the flow of a positive charge through a circuit, have never been fixed since the discovery that electricity is actually a flow of negative particles . . . and no analogy to liquid will explain transistors or inductors.

Yeah, the analogs for basic DC are very strong, but they only go so far beyond that.   Not all the way to Maxwell's equations (or, more commonly, Heaviside's expression of Maxwell's equations).  I'm not aware of a need for a unifying theory for hydraulic pressure fields. 

19 minutes ago, Hank said:

Oil flows through openings; restrict the opening, the flow rate will be reduced, flow speed through the restriction will increase, and pressure in the restriction will reduce. If you pump a constant volume of oil towards a restriction, if it cannot all go through it will pool up; if there is nowhere to pool because it's in a hose, the pressure upstream of the restriction will increase and the pump will work harder to maintain the flow rate, but the total flow rate is regulated by the restriction. This can easily go really bad if the restriction gets smaller for any reason . . . Can't think of an electrical analogy for that . . . Then again, I can't push electrons around with my hands, either.

The electrical analogy still holds even if the restriction stops flow completely.   The pressure at the restriction (plug) is then just the hydrostatic pressure at the last upstream node that still has flow, or if it is a closed system, the source pressure.   If you break an electrical circuit so that there's no current flow, the voltage across the break will be the source voltage if there's only one branch back to the source.

Edited June 10, 2022 by EricJ

[Hank]

A good discussion of hydraulics, but you lost me again at "voltage."

[T. Peterson]

Well, my hat is off to all you fellas! I didn’t follow everything, but sometimes I wondered if you weren’t saying similar things, just from different perspectives. I am envious that I cannot contribute any technical insights but I am not in your league as far understanding these things. However I very much enjoy reading your posts and absorbing as much as I can. 
Now I certainly understand A64’s illustration of the thumb over the garden hose and I am very happy the engineers have designed my engine with some sort of mechanical thumb to make sure oil gets to all the important places!! How’s that for a technical contribution! Lol!

Torrey

[Mooneymite]

As the OP, I am amazed at the incredible collective technical knowledge of Mooneyspace.  You guys are incredible...not just in your technical expertise, but your ability to explain difficult concepts to us, "less than brilliant" pilots.

However, I have been so distracted by the erudite discussion that I have missed the answer to my original question:

"....can anyone explain why Lycoming does not make recommendations for straight weight viscosity based on the average engine oil operating temperature instead of just ambient temperature at startup? "

Can I actually use W80 in Atlanta in the summertime if the ground runup ambient temperature is less than about 90 degrees F?  My biplane oil temp runs about 190F summer, or winter.  Lycoming would have me believe the answer is "Yes, W80 is fine".

[PT20J]

4 hours ago, Mooneymite said:

As the OP, I am amazed at the incredible collective technical knowledge of Mooneyspace.  You guys are incredible...not just in your technical expertise, but your ability to explain difficult concepts to us, "less than brilliant" pilots.

However, I have been so distracted by the erudite discussion that I have missed the answer to my original question:

"....can anyone explain why Lycoming does not make recommendations for straight weight viscosity based on the average engine oil operating temperature instead of just ambient temperature at startup? "

Can I actually use W80 in Atlanta in the summertime if the ground runup ambient temperature is less than about 90 degrees F?  My biplane oil temp runs about 190F summer, or winter.  Lycoming would have me believe the answer is "Yes, W80 is fine".

Startup is a critical time for lubrication. If the oil temps are fine during operation at whatever the ambient temperature then you are fine. Read Lycoming SI1014 in its entirety.