Cold start wear and shock cooling - myth or fear?

[n86121]

Oh boy will this cause a stir...

Over decades of owning Potomac Airfield
I have seen lots of new owners who start out terrified of 'shock cooling' and 'cold starts.'

My two cents, fully inviting others to toss pennies in reply.

1. Shock cooling

A very wise IA once pointed out to me years ago that if shock cooling were 'a thing,'
your engine would crack every time you flew into a rain shower.

Water has a LOT better thermal conductivity than air.
Yes, yes, rain is on the outside of the engine,
but that would seem to create an even greater themal gradient through the material.

The engine block is up around what, 300-400F.
Heated by internal combustion over 1,500F

How can a 'small' (relative) temp change really make much of a difference?

Similarly, compare:

Aircraft engines run at near full power continuously.
When you take your foot off the pedal in your car, you are dropping power rapidly.
Does rapidly reducing power in your car 'shock' its (much more fragile?) engine?

ALL (reasonable) aircraft engine throttle changes are VERY gentle.

Sure, gradual changes reduce thermal stress, but do not live in fear!


2. Startup wear / Cold starts

Hard Starting:
Am I correct that carbureted aircraft engines do not have chokes, so can be hard to start?
And yes, cold fuel will atomize a bit less, so COULD be hard to start.
And yes, thick oil means the engine turns more slowly.

Hard starting issues aside...

QUESTION: Other than vague assertions of being 'better' or 'worse,' has anyone ever documented ACTUAL wear or impact on TBO of how cold starts reduce engine life or performance?

The only SAE article I could find (circa 1985) mentions how ETHANOL FUEL at extreme rich cold-start fuel settings MIGHT wash oil film off cylinders during cold starts.

One SAE paper cites using radioactive cylinder rings to trace metals movement.
Clever!

Another SAE article (lost the links, look under SAE site) described how lubricated surfaces have two basic states. Sliding on a filmy surface, and fully suspended by a pressure wave within the oil once moving.

The aforementioned AI also commented that roughly half the lubrication in an engine was from oil pumped to bearings, the other half splashed around.

So, putting that all together...

When (cold) starting,
unless the airplane hasn't operated in a VERY long time,
rotating and lubed surfaces probably still have a film of oil, AND
you are unlikely to EVER be 'dry'-starting

Once rotating, parts are immediately riding on pressure waves within the oil.

Cold oil moves slower, so keep RPM and power low until warm, ...which we do anyway.

'Splashing' lube might be more gradual, but that's about it.

Has anyone ever documented how pre-heating accomplishes anything other than 'being better for your engine?'

EMPIRICAL OIL TEST

An old grizzled IA / A&P / Temko=Vought aerospace engineer did an empirical experiment on this very topic at Potomac Airfield. He took a plate of regular steel (that WOULD rapidly corrode) and brushed various aircraft oils onto it and mounted it vertically outdoors ...in the sun.

Many months later, the straight oil still left a nice coating.

As I recall, the thinner oils tended to bleed off thinner to gone.

A pretty good argument for using single weight to keep surface lubricated !!!

And that's my blather for the day,..

Merry Christmas and a happy new year!

[wslade2]

https://www.avweb.com/ownership/shoc...kill-the-myth/

Avweb agrees
I like the point that the ultimate shock cooling is engine shut down.

Lycoming says:

https://www.lycoming.com/content/how...ng-your-engine

No more than 50 degrees per minute

And about cold starts:

https://www.avweb.com/ownership/cold...ing-it-slowly/

Looks like the definition for cold start is below 20 degrees Fahrenheit. All kinds of machinery (and people) have problems in that temperature (as experienced with the record setting cold snap across 1/2 the US this week).