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Why do we preheat?

Hold On!

Made me dig out my Thermo text, pretty dusty, its been a while,

Consulting the standard psychrometric chart,

100% saturated air at 20F has the same amount of water vapor as 17% relative humidity air at 70F. Both about 18 grains of H2O per lb of dry air.

So, yes, air at the two temperatures can have the same water vapor content, but they behave profoundly differently.

100% saturated air at 70F holds 108 grains of H2O per lb of dry air.

and to add that, the likelihood of 17% RH at 70* is pretty small anywhere but a VERY arid desert. 40-100% is more normal at 70*. Here in the upper midwest, if I let the RH drop below about 30% in winter, I start sparking when touching things in the house. Can't even imagine 17%
 
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Oil

I have read all of the posts, and I am still confused :confused: So please put this in terms that a high school history teacher can understand. I have a -3 with an O-320, and I live in Florida. I have used 100wt for as long as I can remember, with no problems. Engine longevity is a concern to me so which is better 15/50 or 100wt. Even on the colder days it stays around 65 degrees in the hanger, so pre-heating is not an issue (or is it?). I am going to start using cam guard on the next oil change. I love reading these posts, cause it reminds me that there are still smart people out there.
Thank you for your replies.
 
In the context of engine storage, either 15 grain air sample will condense obvious liquid water on metal with a surface temperature of about 19.6F. The dry bulb air temperature (20 or 70) doesn't matter. If Alex keeps his metal parts above 20F or so, neither will condense obvious liquid water on those parts.

Water in vapor phase also contributes to corrosion. Here I definitely need more reading, because it appears there are complicated interactions between three major factors...relative humidity, atmospheric pollution, and temperature.
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If Alex does not purge the engine combustion products (and replace with low ambient DP) from the engine case when still warm, it will remain saturated with water . . . period. The corroding element in the blow--by is sulfur (compounds) and condenses at a lower temp than water. With the condensing water, on the internal components, an unhappy pH will result. It is possible that the water will condense and all fall to the bottom of the sump, but the saturated condition is likely to remain.

Once purged, the crankcase will not appreciably approach the outside water/dry air content. I am sure the crankcase would breathe and eventually approach an average ambient, but it will take many months.
 
... I have a -3 with an O-320, and I live in Florida. I have used 100wt for as long as I can remember, with no problems. Engine longevity is a concern to me so which is better 15/50 or 100wt. Even on the colder days it stays around 65 degrees in the hanger, so pre-heating is not an issue (or is it?). ...
There does not seem to be a downside to 15W50 oil, even in warmer climates, except a possibly slightly higher price. Florida can freeze as anyone who grows citrus knows (and fears) but even if you are at 65 or higher, 15w50 will flow better at the lower temps.
 
There does not seem to be a downside to 15W50 oil, even in warmer climates

What about at higher temps? like operating temps of 200 degrees F or so...

I can get to the "it's better at low temps" but not so sure it's better than a single weight oil at the temps I typically fly in in SoCal. But I can be persuaded. Maybe.
 
What about at higher temps? like operating temps of 200 degrees F or so...

I can get to the "it's better at low temps" but not so sure it's better than a single weight oil at the temps I typically fly in in SoCal. But I can be persuaded. Maybe.

See the previously posted viscosity chart. The Aeroshell W100 and 15W-50 lines cross at 100C, or 212F. Sounds like you're concerned about high temperature operation. Above 212F, the 15W-50 is slightly superior.
 
I have read all of the posts, and I am still confused :confused: So please put this in terms that a high school history teacher can understand. I have a -3 with an O-320, and I live in Florida. I have used 100wt for as long as I can remember, with no problems. Engine longevity is a concern to me so which is better 15/50 or 100wt. Even on the colder days it stays around 65 degrees in the hanger, so pre-heating is not an issue (or is it?). I am going to start using cam guard on the next oil change. I love reading these posts, cause it reminds me that there are still smart people out there.
Thank you for your replies.

"Better" tends to turn into a primer war. If you have confidence in W100 and it rarely gets cold where you fly (or you like preheating), keep it.

In the context of a need to preheat, the multi-grade is superior. In cruise flight, there is no practical difference.
 
See the previously posted viscosity chart. The Aeroshell W100 and 15W-50 lines cross at 100C, or 212F. Sounds like you're concerned about high temperature operation. Above 212F, the 15W-50 is slightly superior.

Years ago talking with a Chevron lubrication engineer (tribologist) who used to race cars in our club, he said that the problem with multi-viscosity oils is that the long-chain polymer viscosity enhancer additive that is used to achieve the high temp viscosity rating is not as durable as a higher viscosity base oil. So with multi-viscosity oil, it is better to change it more often, and if operating at consistently high temperatures, a single-weight oil is better. This was, of course in the context of cars, at a time when customary oil change intervals were 3,000 miles. Back then, typical motor oil was API grade SF.

It may be that the additives have been improved a lot over the years since then. After all, our conventional automotive oil nowadays is API grade SP - several generations of improvement over the oil from 35 years ago. Maybe the viscosity enhancers are more durable. (evidence longer change intervals suggested by car manufactures nowadays). I don't really know.
 
I have read all of the posts, and I am still confused :confused: So please put this in terms that a high school history teacher can understand. I have a -3 with an O-320, and I live in Florida. I have used 100wt for as long as I can remember, with no problems. Engine longevity is a concern to me so which is better 15/50 or 100wt.

No doubt you mean 50 wt. Aeroshell 100 is SAE 50 motor oil.
 
Years ago talking with a Chevron lubrication engineer (tribologist) who used to race cars in our club, he said that the problem with multi-viscosity oils is that the long-chain polymer viscosity enhancer additive that is used to achieve the high temp viscosity rating is not as durable as a higher viscosity base oil. So with multi-viscosity oil, it is better to change it more often, and if operating at consistently high temperatures, a single-weight oil is better. This was, of course in the context of cars, at a time when customary oil change intervals were 3,000 miles. Back then, typical motor oil was API grade SF.

It may be that the additives have been improved a lot over the years since then. After all, our conventional automotive oil nowadays is API grade SP - several generations of improvement over the oil from 35 years ago. Maybe the viscosity enhancers are more durable. (evidence longer change intervals suggested by car manufactures nowadays). I don't really know.
That is consistent with my former career contacts inside the company and the big suppliers. There is shear down of the VI improvers and it tends downward - - once upon a time it was accelerated with fuel dilution and could even restrict filters. I keep an eye on my oil analyses and the viscosity has not suffered, so likely the aviation oil additives have improved. It would be a surprise if they didn't.

Alex, that is a really good oil analysis, good supporting information. My iron is higher, and coming down. It jumped to 17 from 8 after flying to the west coast through a lot of smoke haze (4-6 hrs) 18 months ago. I landed in Montana and the plane was covered in ash. My intake filter must need more attention. A good diesel number is 2-3.
 
See the previously posted viscosity chart.

Thanks, Dan. That is pretty convincing but can you tell me why they are using kinematic viscosity as their measurement instead of dynamic viscosity, or even absolute viscosity? I researched the terms but still a bit unsure. Not that I don't fully trust big corporations like Aeroshell.:)
 
Less torque load and wear on the oil pump & drive

Why I pre heat. Have you ever listened to the howl and gear load when an engine is started at -35? Vs 1-3 hours of oil sump and cylinder heater pre heat.
 
This thread has covered the behavior of oils, metal and water at various temperatures, but I'm surprised it hasn't touched on the fuel.

The engine's only going to kick over and start if the induction manifold contains fuel vapor.

When I had a carbureted engine (with no priming system), and it was hard to start on cold mornings, I theorized about some of the fuel vapor condensing to liquid on the cold inside surfaces of the induction system, so by the time the charge entered the cylinder it was too lean to light-off. And that's why getting a bit of heat into the engine made it so much easier to start.

Any validity to that?

(I care a lot less now that I'm running fuel injected. Cold starts are uniformly simple now)

- mark
 
There does not seem to be a downside to 15W50 oil, even in warmer climates, except a possibly slightly higher price. Florida can freeze as anyone who grows citrus knows (and fears) but even if you are at 65 or higher, 15w50 will flow better at the lower temps.

Mike Busch has a different take. I'm surprised that this thread hasn't gone down the rabbit hole of synthetic vs semi-synthetic vs dino. Savvy labels the Aeroshell 15W50 semi-synthetic as "particularly problematic" and recommends Phillips X/C 20W50 (dino) for those airplanes that need a multi-viscosity oil for starting in colder climates.

https://resources.savvyaviation.com/client-resources/savvy-oil-recommendations/

Oil debates are what make the internet go 'round.....:)
 
This thread has covered the behavior of oils, metal and water at various temperatures, but I'm surprised it hasn't touched on the fuel.
The engine's only going to kick over and start if the induction manifold contains fuel vapor.
When I had a carbureted engine (with no priming system), and it was hard to start on cold mornings, I theorized about some of the fuel vapor condensing to liquid on the cold inside surfaces of the induction system, so by the time the charge entered the cylinder it was too lean to light-off. And that's why getting a bit of heat into the engine made it so much easier to start.
Any validity to that?
(I care a lot less now that I'm running fuel injected. Cold starts are uniformly simple now)
- mark
#1 reason in my opinion.
Av fuel can’t be blended for the seasons like mogas, so vaporization is an issue as you noted.
A little heat in the induction system helps even with injected engines 😎
 
This thread has covered the behavior of oils, metal and water at various temperatures, but I'm surprised it hasn't touched on the fuel.

The engine's only going to kick over and start if the induction manifold contains fuel vapor.

When I had a carbureted engine (with no priming system), and it was hard to start on cold mornings, I theorized about some of the fuel vapor condensing to liquid on the cold inside surfaces of the induction system, so by the time the charge entered the cylinder it was too lean to light-off. And that's why getting a bit of heat into the engine made it so much easier to start.

Any validity to that?

(I care a lot less now that I'm running fuel injected. Cold starts are uniformly simple now)

- mark

Many struggle with starting in cold weather becuase they do not understand the relationship between fuel required and air temp once the ambients get in the neighborhood of 40*. Won't go into details, but the colder the intake air is, the more fuel required to get a start (some of this has to do with the poor atomization at colder temps). It is a logrithmic relationship with a sharp gradient. I did a custom EFII from my older 911 and struggled to get it to start below 40*. Here we are using completely different fuel delivery table during the start phase. Once I got to the area of tripling and quadrupaling the flow rates, I began to get starts at 20-30*. Untill I started researching, I couldn't appreciate the RAPID increase in fuel required for start once you get near freezing and below. If you compare the difference in fuel added to go from 60*->40* and that from 40*->20* it is shocking how different they are.

Carbs without primers are much harder to start than FI in sub 30* wx. This is because you need to very rapidly pump the throttle to get enough fuel in there to get it to kick.
 
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I call shenanigans

Lots of good content here, but this raised my eyebrows:

"One cold start can cause more wear than 500 hours of cruise flight!"

I've seen this claim before, but I've never once seen a source or any data to support it. Can it possibly be true? I think of all those flight school planes that routinely get started cold, do they have 176,000 hours worth of wear between overhauls?

https://resources.savvyaviation.com...ure2021/2021-07-27 0830 To TBO and Beyond.pdf

Starting on page 68. Savvy has more detailed presentations, but this has the high points.
 
Thanks, Dan. That is pretty convincing but can you tell me why they are using kinematic viscosity as their measurement instead of dynamic viscosity, or even absolute viscosity?

Strictly a guess, but J1899 (the approval standard) may specify kinematic.

The Aeroshell Book says all the listed viscosity values are given in mm^2/sec, which is centistokes, i.e. kinematic.

Great reference. Download to your hard drive:

https://www.shell.com/content/shell...da955d588ce5/theaeroshellbook-edition2021.pdf

"One cold start can cause more wear than 500 hours of cruise flight!"

I've seen this claim before, but I've never once seen a source or any data to support it.

That's the precise question I had hoped to explore with this thread. Set aside thick oil, weak ignition, and slow cranking, because this is 2023; we have multi-grade, EI and starting systems with real power. What can't or hasn't changed is metal-to-metal dimension change due to temperature. I don't have a problem with the Lycoming recommendation of preheat at 10F and below; rough numbers say it makes some sense. Other than "I like it" (or the personified "My engine likes it"), is there really any reason, based on wear, to preheat when it's 32F, or 45F?
 
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One thing that hasn't yet been mentioned (I believe) is oil pump cavitation. I've heard the tell-tale growl of pump cavitation on the very few cold starts I've had to make (maybe in the 20's or 30's F, with 15-50 oil). It lasts a few minutes. Cavitation is quite violent and causes microscopic damage to the metal. I do not have a clue how long it would have to occur to be meaningful.

For those curious:
https://material-properties.org/what-is-cavitation-wear-definition/
 
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Yep, my engine has growled in protest with a cold start, too. I assumed it had something to do with the oil pump gears, didn't know exactly what was happening but was pretty sure the engine wasn't happy.

It's just too easy to plug in the sump heater and avoid the moanin' and groanin'..... :)
 
That's the precise question I had hoped to explore with this thread. Set aside thick oil, weak ignition, and slow cranking, because this is 2023; we have multi-grade, EI and starting systems with real power. What can't or hasn't changed is metal-to-metal dimension change due to temperature. I don't have a problem with the Lycoming recommendation of preheat at 10F and below; rough numbers say it makes some sense. Other than "I like it" (or the personified "My engine likes it"), is there really any reason, based on wear, to preheat when it's 32F, or 45F?

This seems difficult to settle theoretically? Take, for example, crankshaft bearings. My understanding that they mostly wear during startup, while oil pressure in the bearing builds up. But it seems difficult to establish how this wear depends on the clearance between the crank and the bearing.
 
One thing that hasn't yet been mentioned (I believe) is oil pump cavitation. I've heard the tell-tale growl of pump cavitation on the very few cold starts I've had to make (maybe in the 20's or 30's F, with 15-50 oil). It lasts a few minutes. Cavitation is quite violent and causes microscopic damage to the metal. I do not have a clue how long it would have to occur to be meaningful.

Do you know it was pump cavitation?

Given 15W-50 has the same kinematic viscosity at 32F as W100 at 70F....
 
Do you know it was pump cavitation?

Given 15W-50 has the same kinematic viscosity at 32F as W100 at 70F....

Not possible to know...

However, it sounds very similar to any hydraulic pump that is operating in the cold or to a pump that is slightly low on fluid (which is perhaps a slightly different thing). My hydrostatic tractor makes this sound when started cold. The list of things in our planes that are similar to a hydraulic pump are about one in number - the oil pump. I suppose the prop governor could also be a source, but I would not suspect any flow through it when it is not active. The growl disappears as the oil warms up.

It may not be cavitation in the classical sense, where the fluid is vaporizing. It may simply be a vacuum that is created and then collapses. But, I believe the mechanics of it would be the same. For something as internal as an oil pump to be growling such that it can be clearly heard in the cockpit over all the other (idle) noise indicates some real violence is going on inside it. How much damage it's doing, if any? No clue.
 
Not possible to know...

It's an interesting observation, given the use of 15W-50.

However, it sounds very similar to any hydraulic pump that is operating in the cold or to a pump that is slightly low on fluid (which is perhaps a slightly different thing).

Perhaps tight accessory gear mesh? Have not done the math, but I imagine temperature change would make a significant difference in mesh...CTE in inches per inch, significant distance between gear centers, small backlash dimension.
 
Thanks, Dan. That is pretty convincing but can you tell me why they are using kinematic viscosity as their measurement instead of dynamic viscosity, or even absolute viscosity? I researched the terms but still a bit unsure. Not that I don't fully trust big corporations like Aeroshell.:)

Not wading into the debate on pre-heat, just offering some input on the viscosity terms. . .

Per my 30+ year old fluid dynamics text, absolute and dynamic viscosity are one and the same, with units of force-time / length^2. Kinematic viscosity is just absolute viscosity divided by fluid density, with units of length^2 / time.

The terms poise and stokes are SI (metric) units for absolute and kinematic viscosity, typically prefaced by "centi" for scaling purposes.

Back to your regular programming. . .
 
can you explain it in a way my 90 yr old grandmother will understand it so she can explain it to me? hahaha..:)

Maybe. Dynamic viscosity is the resistance to fluid "sliding" against itself. If you have a fluid between a couple of plates, and slide the plates, the dynamic viscosity is what resists that sliding. It's basically shear load divided by displacement. Kinematic viscosity divides by fluid density to include the effects of inertia. If you're looking at making a plane fly, you're looking at inertial effects on air.

For lubricity, you need to maintain a gap between solid objects while minimizing the force required to move those objects relative to one another. Lower viscosity makes it easier to move, but increases the risk of losing the gap.
 
N
It may not be cavitation in the classical sense, where the fluid is vaporizing. It may simply be a vacuum that is created and then collapses.

This is the very definition of cavitation. Oil does contain some air. Physical bubbles when running and dissolved after some operation time. Example: Rod bearing supply hole location relative to the pressure region near TDC and BDC - is important to cavitation erosion of bearings.

Hydraulic pumps in small regions of the interior can occur at all temps.

There is not a specific viscosity at which it will occur, so highly likely it is cavitation. The intensity and certainly time will determine the damage or rate of damage occurring. In the Lyc oil system, it could even be ( and is likely) tiny leaks in the suction side. Many pumps have machined covers and no o-rings.

But - -I think cavitation erosion is a distant spur off the trunk of this preheat discussion.

DanH says - is there damage - -well . . . damage -wear - there are several specific conditions that meet that criteria for cold starts. Nothing is perfect in an engine. Stuff happens or they would last forever- it is how often does a damaging event occur, does it advance, and how much does it accumulate before it affects the TBO, is it small enough to be self healing or non-progressive? The aircraft engine is more of a case study in damage collection than most engine applications.

Lets talk sliding surfaces like bearings, gears, bushings - - In detail there is no smooth surface, it has a surface finish, in the micro world there are asperities - tips of the high spots that make up this surface finish. The thinner the oil film, the more the asperities make physical contact. This happens on engine start up. ore if the oil was left hot to drip off the parts. This is the zone of "boundary lubrication" where additives provide the benefit of reducing metal transfer in the process. Then - how long the condition persists, hardness of the materials, the surface finishes, is it a continuous process or intermittent, and whether micro zones of heat occur will determine if the damage is wear, scuffing, or worse. Does it self heal when the condition changes? Many questions -the world of the engine designers and developers.

It is not a black and white situation, it is degree of occurrence, collection of events and statistical probability of a catastrophic failure within the desired life.

Surely you haven't read this far, I'll delete it tomorrow. This could become one of never ending debates.

Colder is worse, warmer is better for wear. More colder starts ( assuming the battery and starter hold up) the more accumulation of damage. If this occurs more than the expected use within the TBO, then it develop in to a measurable deficiency in operation - low oil pressure at idle, or an actual bearing failure. Unless the plane is used every 4 days in Canada or Alaska and started below the recommendations, it will still reach TBO. Fewer parts may be reusable, but it will get there. Pete and Vlad (two known VAFers) are leading the way on running past TBO maybe they will say how their engines are operated.
 
The last two posts are a perfect indication of what a fantastic thread this is and that there are some really smart peeps on this forum! Cool:cool:
 
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