How cold is too cold to start

Checkout_my_Six said:
A good friend of mine was a Lycoming engineer and for grins did a stackup analysis of an old design and discovered a few things. Those "things" never made it to the marketing glossy documents. The first was the "possibility", a remote one but, none the less, possibility of spinning a main bearing due to a cold thermal interference. The parts have to be mixed and matched just right on a "new" engine....but mathematically he was able to have metal to metal contact on the front main journal bearing....enough for it to spin ....at temps around 32 F.
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It's easy enough to calculate the difference in contraction between an aluminum crankcase and a steel crankshaft. But add in the steel backed main bearing and it gets a whole lot more complex. Was this included in the analysis?
 
flyingbrit said:
It's easy enough to calculate the difference in contraction between an aluminum crankcase and a steel crankshaft. But add in the steel backed main bearing and it gets a whole lot more complex. Was this included in the analysis?
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I don't know....they may be copper backed and plated with babbit.
 
James331 said:
I might talk to who ever made that policy if I were you, I don't think any of the engine manufactures recommend that, and making things up as you go isn't a good idea.
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Does any engine manufacturer caution to NOT do this? Our club, formed in 1956, with the same VP Maintenance and A&P who instituted the policy at least 25 years ago, has NEVER had an engine failure, and ALWAYS gets TBO or WELL BEYOND on our engines.
The logic is that heating and maintaining the oil and engine temps with ambient at 40F or below for a nearly constant temp is considerably better than bringing cold oil and metal up to some level of warmth that applying 30 minutes of pre-heat might bring, as well as incomplete purge of produced water vapor as part of a quickie pre-heat. But hey, I'm not the engine guy - I just let our record speak for itself. It's been working well for quite some time.
 
remember that cold air does not hold moisture, it by nature is dry....so that does affect things.
 
gkainz said:
Does any engine manufacturer caution to NOT do this? Our club, formed in 1956, with the same VP Maintenance and A&P who instituted the policy at least 25 years ago, has NEVER had an engine failure, and ALWAYS gets TBO or WELL BEYOND on our engines.
The logic is that heating and maintaining the oil and engine temps with ambient at 40F or below for a nearly constant temp is considerably better than bringing cold oil and metal up to some level of warmth that applying 30 minutes of pre-heat might bring, as well as incomplete purge of produced water vapor as part of a quickie pre-heat. But hey, I'm not the engine guy - I just let our record speak for itself. It's been working well for quite some time.
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This is like the person who always locks and unlocks their deadbolt 7 times before going to bed, but hey they have never had a break in, guess it's because they do some silly routine.

As has been said, you're going to have MORE moisture in that engine by keeping it heated all the time vs letting it go ambient unti it's scheduled to fly.
 
James331 said:
This is like the person who always locks and unlocks their deadbolt 7 times before going to bed, but hey they have never had a break in, guess it's because they do some silly routine.
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Disagree completely. Our engines make TBO and well beyond, and show no issues through our oil analysis and filter inspection processes. So you're saying if we didn't do this, then we could go double (or 7 times) TBO?

James331 said:
As has been said, you're going to have MORE moisture in that engine by keeping it heated all the time vs letting it go ambient unti it's scheduled to fly.
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At what point is there no more moisture to drive out? After 1 hour? 6 hours? 12 hours? And how much moisture is driven out after 30 minutes to 1 hour of preheat vs keeping it warm all winter?
 
gkainz said:
Does any engine manufacturer caution to NOT do this? ...
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YES!



IMG_1744.jpg



Personally I prefer the follow the guidance from the people who designed and built and tested and certified the engine in the first place.
 
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I'm no engineer (but I did stay at a holiday inn express), but I've read numerous articles, albeit discussing automotive engines, that talk about cold starts causing the majority of engine wear.

I'm wondering if preheat would be beneficial at all times / temps when considering wear.

And although I have an oil pan heater, I've also made myself a forced air heater with a tube that I can set on low (750 watts) and leave running for extended periods of time. I would think that drawing in cold air from outside the engine compartment, heating it, and forcing it into the engine compartment would dry out an engine, helping prevent rust, as opposed to a sump heater that would cause evaporation / condensation and increased rust...just my thought anyway.
 
gkainz said:
Disagree completely. Our engines make TBO and well beyond, and show no issues through our oil analysis and filter inspection processes. So you're saying if we didn't do this, then we could go double (or 7 times) TBO?


At what point is there no more moisture to drive out? After 1 hour? 6 hours? 12 hours? And how much moisture is driven out after 30 minutes to 1 hour of preheat vs keeping it warm all winter?
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James331 said:
YES!



IMG_1744.jpg



Personally I prefer the follow the guidance from the people who designed and built and tested and certified the engine in the first place.
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Thanks for that ... will do more digging.
 
Checkout_my_Six said:
I don't know....they may be copper backed and plated with babbit.
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Enlightenment from the C-125/C145/O-300 OH manual:

"The alloy steel, one piece, six-throw crankshaft is supported by four main bearings with semi - circular steel backed precision inserts which are easily replaced."
 
flyingbrit said:
Enlightenment from the C-125/C145/O-300 OH manual:

"The alloy steel, one piece, six-throw crankshaft is supported by four main bearings with semi - circular steel backed precision inserts which are easily replaced."
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nice.....so the crank case is made from what material? ....and what happens to the bearing journals when it shrinks due to thermal contraction?
 
Bearing plating is so thin that it doesn't even enter into any approximate thermal expansion checks. And the bearing shell steel has a trivial contribution also. The crankshaft journals of course are steel and the case is aluminum so there is the possibility of getting zero clearances but not from any cold temperatures we are likely to ever encounter given the worst combinations.

What is more critical is that all of the more expensive stuff (like cylinders and camshaft system) in a crankcase receives only a spray or fog lubrication from journal bearing leakage. It takes a while before this gets established on a cold engine spinning at low rpms. There are no features on the crankshafts I've looked at that would promote a centrifugal spray pattern to especially any overhead camshaft lobes such as found in Lycomings. Also any oil pump flow that is forced over the oil pressure relief valve has essentially zero access to the heat of the engine. That's why the oil temps of a cold engine are so slow to heat up.

Don't forget, they sell the spare parts................

That's the real value of preheat. It thins the oil
 
nrpetersen said:
....The crankshaft journals of course are steel and the case is aluminum so there is the possibility of getting zero clearances but not from any cold temperatures we are likely to ever encounter given the worst combinations.....
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that's debatable...given a worst on worst case analysis....and given the poor lubrication at those temps.;)
 
nrpetersen said:
Bearing plating is so thin that it doesn't even enter into any approximate thermal expansion checks. And the bearing shell steel has a trivial contribution also. The crankshaft journals of course are steel and the case is aluminum so there is the possibility of getting zero clearances but not from any cold temperatures we are likely to ever encounter given the worst combinations.

What is more critical is that all of the more expensive stuff (like cylinders and camshaft system) in a crankcase receives only a spray or fog lubrication from journal bearing leakage. It takes a while before this gets established on a cold engine spinning at low rpms. There are no features on the crankshafts I've looked at that would promote a centrifugal spray pattern to especially any overhead camshaft lobes such as found in Lycomings. Also any oil pump flow that is forced over the oil pressure relief valve has essentially zero access to the heat of the engine. That's why the oil temps of a cold engine are so slow to heat up.

Don't forget, they sell the spare parts................

That's the real value of preheat. It thins the oil
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In 1975 when my Cessna rolled out of the factory it was equipped with oil dilution. It was a valve that allowed the pilot to inject raw fuel into the crankcase prior to shut down so that the next start could have thinned oil. I don't know anyone who still has that system installed. Modern multi viscosity oil was/is a much better solution for cold start lubrication within the temperature ranges the engine makers define.

In a nutshell. Preheating the cylinders allows the engine to fire easier but the oil pressure will lag. That's what forced air heaters will do if you don't leave them on for several hours. On the flip side, sump heaters thin the oil and allow faster cranking speeds but the cold cylinders may not be happy. That can result in poor starting, flooding from over priming, and frosted plugs. This is common with sump heat pads in cold temps. I don't mean for guys who plug in at 40*. I'm talking about real winter temps, like preheating outside at -25*. The best solution for me, after trying lots of different preheaters? Heat the oil and the cylinders. I use Reiff but Tanis is also good. For years I used a Little Buddy warm air heater with a sump pad. It worked but wasn't as efficient as the Reiff.

Preheating poorly isn't any better than not doing it at all.
 
denverpilot said:
That’s half of the story. Now you need to publish specific tolerances inside air cooled gas engines.
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Should these arguments be tolerated? That’s the only tolerance here.
 
I have a sump heater, I plug in when it's below 40 or so... really it's just because it gets hard to start the engine when it's around freezing or below that and especially in a hangar with cowl plugs in some of that heat is getting up to the cylinders.
 
From a Lycoming published Table on main bearing limits, a new engine has a minimum of .0015 clearance on a 2.375 inch diameter steel crank aluminum case main bearing. This requires about minus 40 degF to go to zero clearance. And that's for everything going at the worst dimension. Service limits are about .0045 inches clearance which would require colder than minus 200 degF for zero clearance.
 
nrpetersen said:
And the bearing shell steel has a trivial contribution also.
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Trying to understand this. Please bear in mind I am an EE not an ME like yourself. Steel has modulus of elasticity (i.e. stiffness) about three times that of aluminum. However the steel bearing shell is perhaps 1/16" thick while the aluminum crankcase is several inches thick where it is crushing the bearing shells and so the aluminum grossly "overpowers" the steel. Is that about right?
 
Checkout_my_Six said:
let me leave this right chair....note the differences between Aluminum and steel, major engine components in aircraft engines.
gf_thermal_expansion.jpg
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Your table of expansion coefficients is useful. It shows that 40 degrees is enough to make a one-mil difference in expansion of steel vs aluminum

[Calculation: a temperature change of 40 degrees F, four-inch-diameter aluminum and steel parts, the difference in expanded diameters is:

(12.8-6.7)*4*40*10^-6= 0.001”]
 
But the main bearings are only 2.375 inch diameter and have a minimum clearance of .0015 inch
 
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