Why do we preheat?

[BillL]

Originally Posted by scsmith

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.

[rmarshall234]

Originally Posted by DanH

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.

[F1R]

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.

[newt]

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

[MacCool]

Originally Posted by rv8ch

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/...commendations/

Oil debates are what make the internet go 'round.....

[DGlaeser]

Originally Posted by newt

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 😎

[lr172]

Originally Posted by newt

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.

[Dugaru]

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?

Originally Posted by SonexGuy

https://resources.savvyaviation.com/...d%20Beyond.pdf

Starting on page 68. Savvy has more detailed presentations, but this has the high points.

[DanH]

Originally Posted by rmarshall234

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/...dition2021.pdf

Originally Posted by Dugaru

"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?

[AlexPeterson]

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...ar-definition/