Explain straight weight oil viscosity recommendations to me, please.

[Mooneymite]

I was checking out the Lycoming SI 1014N on oil and noted that choosing the proper single weight (viscosity) is based on "Average Ambient Air Temperature at Startup".

This seems somewhat strange since the oil temperature in flight may be significantly different.  Here in Atlanta in the summer, ground ambient temperature may be quite high, dictating a higher viscosity, but operating at 10,000' the OAT could be about 20 degrees cooler suggesting that a thinner oil might work as well, or better.  Note B tells us that we must consider a wide variety of variables, but does not change the basic recommendations.

Before people go off on the virtues of multiweight oils, 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?

https://www.lycoming.com/sites/default/files/SI1014N Lubricating Oil Recommendations.pdf

Edited June 2, 2022 by Mooneymite

[A64Pilot]

Average engine oil operating temp shouldn’t vary by a whole lot, meaning if your flying up in Alaska or something you block off your oil cooler to gain temp, even my 1946 C-140 has a winterazation kit. It doesn’t have a cooler to block off, and there is a thermostat to allow bypass in cold wx too on our engine with coolers.

But you have to get an engine started to begin with and thick cold oil doesn’t help to say nothing about the thick oil won’t flow very well and pressures may be so excessive as to blow the oil filter can or crush the filters internals.

https://www.qaa.com/resource-center/tempest-tech-tips/alaska-cold-starts I know your not in AK, but even on a cold day in Atlanta it can be in the teens and a Summer oil may be too thick.

I don’t understand peoples resistance to mult vis oil though, but I’ll resist extolling its virtues.

Edited June 2, 2022 by A64Pilot

[Fly Boomer]

2 hours ago, A64Pilot said:

pressures may be so excessive as to blow the oil filter can or crush the filters internals.

Or this.  Oil cooler is smiling, but it's not a happy occasion

[EricJ]

5 hours ago, Mooneymite said:

I was checking out the Lycoming SI 1014N on oil and noted that choosing the proper single weight (viscosity) is based on "Average Ambient Air Temperature at Startup".

This seems somewhat strange since the oil temperature in flight may be significantly different.  Here in Atlanta in the summer, ground ambient temperature may be quite high, dictating a higher viscosity, but operating at 10,000' the OAT could be about 20 degrees cooler suggesting that a thinner oil might work as well, or better.  Note B tells us that we must consider a wide variety of variables, but does not change the basic recommendations.

Before people go off on the virtues of multiweight oils, 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?

https://www.lycoming.com/sites/default/files/SI1014N Lubricating Oil Recommendations.pdf

There's a pressure regulator and a vernatherm (or equivalent) to control pressure and temperature during normal operation after startup.     Prior to those being able to work properly, i.e., at startup, the only way to have much control is via viscosity selection.

[Ragsf15e]

So basically, they’ll all work at normal operating temp, but only the proper viscosity will work when starting an engine “cold” (whatever “cold” means in your climate).

[Mooneymite]

12 hours ago, Ragsf15e said:

So basically, they’ll all work at normal operating temp, but only the proper viscosity will work when starting an engine “cold” (whatever “cold” means in your climate).

That makes perfect sense to me.  But engine start up is only a small part of an engine's life.  There are no listed criteria for oil selection based on operating oil temp once start-up/warm-up is complete.

If you start your engine on a cold day in Alaska using a low viscosity oil, but 99% of the time your engine operates at (for example) 230° F, is that low viscosity oil really the best choice for long term engine life?

It just seems counter-intuitive that greater consideration isn't given to operating temperature for viscosity selection.

 

 

[Marauder]

That makes perfect sense to me.  But engine start up is only a small part of an engine's life.  There are no listed criteria for oil selection based on operating oil temp once start-up/warm-up is complete.
If you start your engine on a cold day in Alaska using a low viscosity oil, but 99% of the time your engine operates at (for example) 230° F, is that low viscosity oil really the best choice for long term engine life?
It just seems counter-intuitive that greater consideration isn't given to operating temperature for viscosity selection.
 
 

Hence the introduction of the multi-viscosity oil.


Sent from my iPad using Tapatalk Pro

[A64Pilot]

1 hour ago, Mooneymite said:

That makes perfect sense to me.  But engine start up is only a small part of an engine's life.  There are no listed criteria for oil selection based on operating oil temp once start-up/warm-up is complete.

If you start your engine on a cold day in Alaska using a low viscosity oil, but 99% of the time your engine operates at (for example) 230° F, is that low viscosity oil really the best choice for long term engine life?

It just seems counter-intuitive that greater consideration isn't given to operating temperature for viscosity selection.

 

 

If using your example of starting an engine in the cold in Alaska the issue in flight is getting up to normal as in say 160 oil temp, no concern with high oil temp unless the pilot restricted the cooler too much.

But if you had a condition of actually running normal as in say 180 oil temp in cruise, then maybe the thin oil for starting isn’t optimal, if it results in low oil pressure, but so long ac pressure is good, it’s fine.

Actually it’s desirable to run the thinnest oil that will give good pressure, the reason is that it gives the highest flow, oil of course is as much a cooling fluid as it is a lubricant and higher flow generally gives better cooling.

You had asked to not hear about multi vis oil, but it’s one of the few cases you can get your cake and eat it too, sort of. I say sort of because there is no free lunch, the downside of multi vis oil is that it’s viscosity improver is usually the first of the additive packages to break down, so it won’t last as long as a single grade oil, but as neither breaks down in 50 hours it’s not usually relevant.

The Russians in WWII could get their aircraft started in the extreme cold where the German’s couldn’t. They were thinning their oil with gasoline and as the engine heated up, it cooked the fuel off. That was sort of the first multi viscosity oil. Oil dilutes weren’t uncommon on Commercial aircraft after the war.

When we flew to AK when I was a kid, we learned that the bush pilots would often immediately drain the oil after landing, then heat the oil with a fire before the wanted to leave.

When I flew the NWT 15 years or so ago, I learned they now carry a Honda generator and just plug in their pre-heaters

[Shadrach]

2 hours ago, Mooneymite said:

That makes perfect sense to me.  But engine start up is only a small part of an engine's life.  

True, but it’s where most of the wear occurs. 
Metal moving over other metal isn’t protected by oil pressure, it’s protected by flow. We use pressure as a proxy for flow. The ambient temp viscosity of an oil isn’t that important at operating temp, But it’s crucial to establishing adequate flow (proper pressure) to keep metal from touching metal. Both too much or too little pressure can mean inadequate flow.

[Yetti]

Multi weight oil has entered the chat

[EricJ]

2 hours ago, Mooneymite said:

That makes perfect sense to me.  But engine start up is only a small part of an engine's life.  There are no listed criteria for oil selection based on operating oil temp once start-up/warm-up is complete.

If you start your engine on a cold day in Alaska using a low viscosity oil, but 99% of the time your engine operates at (for example) 230° F, is that low viscosity oil really the best choice for long term engine life?

If it is designed with a pump that is adequate to supply pressure (via the pressure regulator) with that viscosity oil, then, potentially, sure.   In other words, if it is designed for it and the manufacturer says, "do this", then it's likely that that can be done without worry.

2 hours ago, Mooneymite said:

It just seems counter-intuitive that greater consideration isn't given to operating temperature for viscosity selection.

FWIW, this is why we have multi-viscosity oils, so that you can have the thin oil at startup and a thicker oil during operation.

[Hank]

8 minutes ago, EricJ said:

FWIW, this is why we have multi-viscosity oils, so that you can have the thin oil at startup and a thicker oil during operation.

This is good,since I've seen OAT at 9-10K range from 60°F to well below freezing, sometimes imes in a span of only a couple of weeks.

[Ragsf15e]

5 minutes ago, Hank said:

This is good,since I've seen OAT at 9-10K range from 60°F to well below freezing, sometimes imes in a span of only a couple of weeks.

Yes, but the oat is really not important at cruise for the viscosity, only your oil temp.  If your vernatherm is working ok, you should see ~185 degrees for most of those temps depending on how you run your engine.  Obviously with a super high oat and warm cylinders, you can make your oil hot, but that seems like an outlier?

[Ragsf15e]

I also use multi viscosity, but it’s because sometimes my summer “cold” start is at 90 degrees and sometimes 40 degrees.  It’s a big deal on startup when it really counts.

[GeeBee]

Once hydro-dynamic lubrication starts, you can run a pretty skinny oil and the higher the velocity, the skinnier oil you can run.

 

[Hank]

24 minutes ago, GeeBee said:

Once hydro-dynamic lubrication starts, you can run a pretty skinny oil and the higher the velocity, the skinnier oil you can run.

 

Well, that rules out my C . . . . .

[Shadrach]

1 hour ago, GeeBee said:

Once hydro-dynamic lubrication starts, you can run a pretty skinny oil and the higher the velocity, the skinnier oil you can run.

 

Indeed that’s true. Which is why modern liquid cooled engines with close tolerances can run 0W20 weight oil and still show no visible signs of wear after thousands of start ups and 200,000 miles. So many people are under the impression that high viscosity provides additional protection. All the engine cares about is getting adequate flow ASAMFP…

as you say, it’s about velocity not viscosity.

[carusoam]

See if we can summarize a few things…

1) Viscosity changes with temperature…. A lot.

2) Start-up viscosity is incredibly important for getting the oil moving through the engine AND oil cooler…

3) As the oil viscosity gets thinner… the ideal flat plate lubrication theory is more at risk of failing, the more watery the viscosity becomes…

4) Our oil pressure gauge is technically our oil viscosity meter…. If the viscosity is too low, the OilP will fall out of the green zone…

Our OilT gauge is a viscosity quality gauge… as the OilT rises out of the green zone, our OilP gauge is going to fall out of its green zone as well…

5) on perfectly smooth flat surfaces… it doesn’t take much viscosity to lubricate the parts… (See GeeBee’s note on hydro-dynamic lubrication above)

6) on round parts things get trickier… the lubricated surface is only a small line, not a great amount of surface area…

7) small parts like cam followers… have small surface areas riding on a thin line of the cam lobe….  
 

8) So…. When the cam follower surface looks like the moon…. It is hard to have the follower’s surface float on a film of oil… when the surface is missing parts… then the cutting mechanism gets started…

 

Essentially….

9) Select an oil that is….

• watery enough to get the engine started….
• viscous enough to stay on the parts….
• doesn’t lose its viscosity at engine temperatures… including the time it spends near exhaust valves and turbo bearings…

10) Don’t forget… oil viscosity breaks down with time and temperature and acids (exhaust by-products)….

11) Selecting a more viscous oil over a less viscous oil…. Can affect your gas mileage…

12) Did you know… that lubrication works… by the oil getting squeezed by the two parts that are trying to get closer together… as the parts get closer… the oil has to squeeze out of the way… as the distance between the parts approaches zero…. The pressure of the oil approaches infinity…  thus the infinite oil pressure, acting on the two surface areas, keeps them nicely separated…

13) Tribology gurus call this the lubrication approximation…  they are good at calculus and enjoy flow studies…. 

 

14) When discussing higher velocity… that is probably a reference to speed of one part sliding past the next?… engine rpm, part geometry, of all Mooney engines is going to be awfully similar…

 

We don’t get too many choices for oil…

No matter what you pick… you won’t be totally wrong… by keeping the gauges in the green…. When it is harder to keep in the green, select the next oil grade…

PP thoughts only, not a tribologist…

Best regards,

-a-

[A64Pilot]

No, modern car engines run extremely tight tolerances, that’s why the car can run the 0W oil, The biggest driver to the 0 oil is fuel consumption. The very tight tolerance allow good oil pressure with thin oil. Fuel consumption has been a design driver for a long time.

Air Cooled Aircraft engines run very sloppy tolerances, they have to as the tolerances change greatly with varying temps they experience and they have to be built so no matter what, they don’t seize, so loose tolerances are required, so they require thicker oil

Pressure is at least as important as flow is, you can only run low viscosity oil if and only if you can maintain good pressure with it. Pressure is what keeps metal to metal contact from occurring in a high compression high power engine, back in the day of splash lubrication it worked as long as the loads were low but it’s what determined max power, that’s the reason why aircraft engines were put in early tanks, because of the pressure lubrication power levels could be higher. Auto engines of the time were splash lubricated.

[Shadrach]

11 hours ago, carusoam said:

See if we can summarize a few things…

1) Viscosity changes with temperature…. A lot.

2) Start-up viscosity is incredibly important for getting the oil moving through the engine AND oil cooler…

3) As the oil viscosity gets thinner… the ideal flat plate lubrication theory is more at risk of failing, the more watery the viscosity becomes…

4) Our oil pressure gauge is technically our oil viscosity meter…. If the viscosity is too low, the OilP will fall out of the green zone…

Our OilT gauge is a viscosity quality gauge… as the OilT rises out of the green zone, our OilP gauge is going to fall out of its green zone as well…

5) on perfectly smooth flat surfaces… it doesn’t take much viscosity to lubricate the parts… (See GeeBee’s note on hydro-dynamic lubrication above)

6) on round parts things get trickier… the lubricated surface is only a small line, not a great amount of surface area…

7) small parts like cam followers… have small surface areas riding on a thin line of the cam lobe….  
 

8) So…. When the cam follower surface looks like the moon…. It is hard to have the follower’s surface float on a film of oil… when the surface is missing parts… then the cutting mechanism gets started…

 

Essentially….

9) Select an oil that is….

• watery enough to get the engine started….
• viscous enough to stay on the parts….
• doesn’t lose its viscosity at engine temperatures… including the time it spends near exhaust valves and turbo bearings…

10) Don’t forget… oil viscosity breaks down with time and temperature and acids (exhaust by-products)….

11) Selecting a more viscous oil over a less viscous oil…. Can affect your gas mileage…

12) Did you know… that lubrication works… by the oil getting squeezed by the two parts that are trying to get closer together… as the parts get closer… the oil has to squeeze out of the way… as the distance between the parts approaches zero…. The pressure of the oil approaches infinity…  thus the infinite oil pressure, acting on the two surface areas, keeps them nicely separated…

13) Tribology gurus call this the lubrication approximation…  they are good at calculus and enjoy flow studies…. 

 

14) When discussing higher velocity… that is probably a reference to speed of one part sliding past the next?… engine rpm, part geometry, of all Mooney engines is going to be awfully similar…

 

We don’t get too many choices for oil…

No matter what you pick… you won’t be totally wrong… by keeping the gauges in the green…. When it is harder to keep in the green, select the next oil grade…

PP thoughts only, not a tribologist…

Best regards,

-a-

Oil pressure is a proxy for velocity, not viscosity though viscosity does affects pressure. 

[M20F]

 

[carusoam]

Funky graphic on that first page from Savvy…

Looks like they are trying to show a slightly rough surface at the atomic level…. But need to make it visible… for the audience…

But, technically the large features are interlocking…. No amount of oil can overcome that, without the parts separating… 

-a-

[carusoam]

To add to the conversation…

+1 for relative speed….

+1 for pressure…

+1 Don’t forget geometry… of the moving parts to each other…  

Constraining the flow of the oil allows the pressure to build locally… enough to support the weight and force of the parts getting pushed together…

A slight angle to each other, that squeezes the oil into the gap, is important for building pressure in the gap…

When the surfaces get rough enough…there can’t be enough oil to lubricate the taller points from coming in contact with the smooth parts… a force over area challenge where the area of points are tiny…

Lubrication clearly gets more complex the deeper we look.   

 

Best regards,

-a-

 

[Pinecone]

On 6/4/2022 at 5:21 PM, A64Pilot said:

No, modern car engines run extremely tight tolerances, that’s why the car can run the 0W oil, The biggest driver to the 0 oil is fuel consumption. The very tight tolerance allow good oil pressure with thin oil. Fuel consumption has been a design driver for a long time.

Air Cooled Aircraft engines run very sloppy tolerances, they have to as the tolerances change greatly with varying temps they experience and they have to be built so no matter what, they don’t seize, so loose tolerances are required, so they require thicker oil

Pressure is at least as important as flow is, you can only run low viscosity oil if and only if you can maintain good pressure with it. Pressure is what keeps metal to metal contact from occurring in a high compression high power engine, back in the day of splash lubrication it worked as long as the loads were low but it’s what determined max power, that’s the reason why aircraft engines were put in early tanks, because of the pressure lubrication power levels could be higher. Auto engines of the time were splash lubricated.

The oil pressure on the gauge is not what is keeping the parts from touching.  How does 40 - 80 psi keep a connecting rod bearing, with several hundred psi combustion pressure, acting on a large piston from touching??

Another post touched on the actual mechanism.  The moving surfaces,, especially rotating ones, create a wedge of oil with the pressure needed to counteract the pressure trying to make the two parts touch.  There was a great article in Scientific American on this in the 70s.

Oil pressure gauge only intimates that we have the proper flow of oil required.

[A64Pilot]

A piston isn’t pressure lubricated, only the bearings etc are. Pistons and rings of course are splash lubricated and do fine because the loads are low, and yes even on the splash lubricated parts the oil film prevents direct metal to metal contact. Splash lubrication works fine so long as the loads are low it’s when the loads increase that pressure is required

If you don’t believe pressure is important, then why do modern engines require a min pressure, and what happens if you run with less than min? As an engine wears the pressure drops, volume of oil flow is the same, but when the pressure drops below min metal to metal contact is made and that can lead to galling and a spun bearing from a lack of pressure, not flow as it is the same, but pressure. This is a hydraulic system and as fluids are incompressible at a given engine RPM the flow is steady 

Engines are run every day with no pressure at all, lawnmower engines as an example the small cheap motors are splash lubricated, all old car motors were too.

Pressure is what drives changes in oil viscosity, it’s temperatures effect on pressure is why temp is used as the indicator for when to change viscosity, viscosity, pressure and flow are interrelated of course.

Higher viscosity oil can provide better protection in cases, our cams for instance which are splash lubricated, the thicker oil can help prevent metal to metal contact.

The “most wear occurs on start up” due to lack of lubrication is sort of a myth and was dispelled decades ago by pre-lubrication systems that established both oil pressure and flow before an engine ever turned. If lack of oil oil pressure and flow on start up was the primary wear mechanism then a pre lubricated engine should have greatly extended life, but unfortunately it didn’t happen. I wish it had because there are several ways to make a pre-lubrication system that’s not complex or expensive, and who wouldn’t want to extend an engines life?

 

 

Edited June 6, 2022 by A64Pilot
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