More interesting reading by Mike Busch

• Over the past two years, ECi and various other interested parties (including me) have been on a campaign to educate pilots and aircraft owners about the critical importance of controlling CHTs, and of installing modern digital engine monitoring equipment with high-CHT alarms. It is interesting to note that there have been ZERO reported separations involving ECi cylinders in the past 18 months. A Weibull analysis calculated that there should have been a half-dozen separations during this period. This strongly suggests that the ECi separation issue is operational in nature, and that the problem is resolving itself through pilot/owner education without the need for FAA intervention.
• Head separations are very rare, and when they happen the result is generally pretty harmless. The head separates from the barrel by less than an inch, the cylinder goes to zero compression, and the engine continues to run on five cylinders and make roughly 80% power. (FAR Part 33 requires that all certificated piston engines must continue to operate safely with one cylinder shut down.) The engine runs rough, and in a single-engine airplane the pilot makes a safe precautionary landing at the nearest airport (which is exactly what happened in every instance). In a twin, the situation is even more benign—and the lion’s share of reported failures of ECi jugs occurred in Cessna 340s and 414s.
• Something rotten in the state of Texas ? My comment. RA

The shear strength of the threads is far less than the 20,000+ pounds of force placed upon them during the combustion event. Were it not for the interference fit--which remains quite adequate at modest CHTs--the head-barrel fit interface would fatigue and separate. When the CHTs get excessively high the interference fit is lessened. This can result in failure since the threads alone cannot sustain the load.

For this reason, the OEMs recommend controlling CHTs to around 380-400dF as a matter of routine. It's good advice.

There is a combination of proper interference fit during manufacture and controlled CHT during operation which results in success. For this reason, if the need is high power, it can better be accomplished LOP where CHTs are lower, pressures in the combustion chamber are lower and the metal is stronger.

All cylinders are not created equal where the interference fits concerned.

Walter
Are you proposing that the head would unscrew under load without a locking device
Considering that a screw thread is a ramp.
Or is it like a main rotor grip that cracks the threads because the load has exceeded the carrying capacity

well they must put the screw thread on for a reason may be to f up post like this lol.


No lop not the main cause for head failure at the end of the screw thread age is the main reason they fail at that point. Don't see first and second life units cracking .Generally older ones that being used past there expiry date on o/h cause the owners never got any coin. I know one operator never had any problems never lop at all but he change his cylinder's after 2 lifes never had problems. Cold morning and people that don't warm there engine up before take off, we preheated oil and engine in the colder months as I said before we never had much of a problem.

Train wheels don't fall off because both components, the hub and the steel tyre, are made of the same metal.
Expansion rates are identical with both components, so the interference fit is virtually unaffected by temperature extremes.

The problem with these engine cylinders and heads, is that the basic head/cylinder design is faulty from the word go - because the expansion rate of aluminium when heated is considerably more than steel. Thus the basic design is setting up the owners/users for a fail.

A good cylinder and head design would have the interference fit zone and thread area, remaining the same interference fit, at all temperature extremes encountered - or even tightening at higher operating temperatures.

The entire cylinder/head design needs to be seriously reworked to eliminate the major reduction in clamping force in the interference fit zone, when operating at high cylinder head temperatures.

yr right wrote: "Cold morning and some people that don't warm there [sic] engine up before takeoff,"

Over thirty years, this has sadly been an all too often observation for me. But I'm talking about other pilots. And that's quite often followed by a downwind takeoff. Why would you want to do that? Saving fuel? :mad:

So perhaps warm the engine correctly, take off into wind and lean to LOP in the cruise... :ok:

(My apology for the thread drift.)

Why bother when good temperature control solves the problem? Just how hot do you want to run them? A better idea would be to have engineers write the POH instead of leaving it to the marketing department.Mike Busch has over 4,800 hours on 9 of his 12 cylinders simply by having an alarm sound whenever any cylinder passes 390*F and then taking immediate action to prevent the temp going higher.

Mike Busch has over 4,800 hours on 9 of his 12 cylinders


That depending is 3 to 4 lives this from here on in is in the danger zone from my experience. Hi heat is not the only thing that is going on. The metal is changing taking on what has been burnt in the combustion process as well fatigue is the biggest killer of cylinder's that also needs to be addressed.
They don't last forever.


Also there is a lot of difference between a train wheel and a removable cylinder on an aircraft wheel, And occasionally they also do let go as I recall Germanys worst train accident was caused by a wheel failure.
Cheers

Round 2
 

Great thread :ok:

Hi Creampuff


May I respectfully ask if you once worked with CASA? Thanks.

It probably ran over one of those ECI cylinder heads raining from the sky.
Due to the appalling safety record of 1 every 10 million miles all train wheels must now be replaced before the train leaves the next station.:ugh:

NOSTALGIA ALERT!

Those of a certain age, may have memories of an entertainment programme called "wheel tappers and shunters club"
Railways employed men with a hammer to "ding" a railway-wheel...the sound would alert to any that were unsound.

I suspect the screw-thread on the head is primarily to overcome the differential-expansion problem, while the engine temperature stabilises. Not enough attention is given to proper warming and heat-soak.

Peterma,

Creampuff is being entirely candid when he claims to have entered an engineering apprenticeship at age 15.

I was rather fortunate to be a little bit older "Adult Trainee" at the same institution that he was within. I was 17 in 1975 there.

Creampuff may have taken advantage of further education since then, though....

Years 11 and 12 maybe? Or perhaps a law degree with First Class Honours?

But as yr right may say: " What would I know.." :E

The maximum combustion pressure I can find for cyl is under 2000 psi.
Where did the 20000 psi come from?

Me? I’m just an acne-stipled, wheelchair-bound geek from Hicksville USA. Flying and airplanes scare me! ;)

"Using 232 psi as the maximum observed cylinder pressure results in a combustion force of 3,488 lbs."

The above is from the testing of a 1910 engine used by the Wright Bros, I think it was around 20 to 30 HP.

I think you'll find Mr Atkinson did not say the pressure in the cylinder was that high. I think you'll find Mr Atkinson was talking about the pressure on the threads.

I'm guessing that the pressure on the threads has something to do with the surface area of the threads, but I'll leave it to the experts to explain. :ok:

Thats right Creampuff, no where in his post did he say 20000 "psi". Still some people are determined to try and trip him up, good luck with that.

Point is and know has yet said at what life are the new cly failing. I have never seen up to a to a 3 life OEM cly fail. If these cly are failing when new there is a problem. Where is the DATA not on failure rates but when.
BTW an academic dose not mean he is a good engineer.

As an LAME im not allowed to interrupt the law, even though I have to pass AA to get a lic im still not allowed to, even though every day I have to work within it and make decisions on it im still not allowed. I am how ever allowed to have an understanding of it.
Now a lawyer is just a word smith. They know zero about engineering. They look at words they allowed to interrupt the law but they have zero understanding of engineering.
Now Jaba ask me how I beat the dark side in court. Well it all depends on the solicitor on the dark side. But if you know the law and engineering and you know they lying as they do and you know how they trying to trick you up its not hard.
And yes Jaba I smashed them.
How ever I have also help them if its required.
The said thing is that every CASA awi dose not work the same as the person beside them, or in another office even though they may work from the same book its all different.


So even though im a dyslexic F^&K I still know my **** because im not an academic who cares I see the result I may learn in a different way to most but I can still have a say.


Cheers


Off to do more science.

Weheke
This is an engineering discussion. 20000 pounds is not a force it is mass or weight relatively speaking

So, an engineering question from a mere academic. ;)

When a doctor puts one pound per square inch pressure on the thumb plate of a syringe, what is the pressure at the tip of the needle?

When the pressure inside a cylinder of an IO 520 is 800 pounds per square inch, what is the pressure on the surface area of the thread between the head and the barrel?

If I have it right. There is about 900psi evenly loaded on the thread. 30000psi shear for Al and things are pretty good. Given 6in bore, at least ten winds of thread...more winds equals less pressure, and 1/8th thread depth.

Does that sound about right?

If its an 8in bore that goes out to about 1300psi evenly applied.

Turn the pressure on the head into a force (F) in Newtons or Lbs, whichever is your flavour of choice then apply the force to the thread area. Less any opposing forces of course.

Another way is by comparing areas but then you miss the value of knowing the forces. :ok:

Onetrack,
Not quite true, the axles and hubs/wheels tend to be something like EN40B, while the removable tires are an SG iron. If I remember correctly, the expansion co-efficient of the hub/wheel is actually higher than the SG iron, so heating during running actually tightens the interference fit.
Tootle pip!!

Frankly, I’m jiggered if I know!

My understanding of Mr Atkinson’s point is that it doesn’t matter. My understanding of his point is that the ‘squeeze’ caused by the interference fit is what holds the head on, rather than the strength of the threads.

If I get a bolt and clamp it in a vice and tighten the vice as far as I can make it go, it ain’t the threads on the bolt that stop me pulling that bolt out of the vice …..

No Hoper.

I am certainly not an engineer.

I think Walter said roughly 20000 lbs of force is applied to the threads as a result of combustion?

You said, Why did you misquote him?

Correct me again if I am wrong, but I think combustion results in a force being applied to something, be it pistons or whatever?

It's beneficial to understand how the head and barrel are mated. The head is heated to expand and the barrel is cooled to contract. The head is then screwed on to the proper alignment. As the two metals reach temperature equilibrium, the interference fit is attained. The design is to allow for adequate strength absent the threads (which will shear at a much lower pressure) by the interference fit up to the design redline temp. The shear strength of aluminum is fairly low (can't recall the exact value). BTW, the interference fit is NOT at the same place as the threads. It is below the threads. The threads do not hold the head on during operation.

As Yr Right as alluded to but not exactly explained accurately, the cyclical fatigue on the aluminum is the issue that results in failure over time--NOT the time in service. If the cylinder is operated below about 380dF, the time in service will be MUCH longer than if the metal gets hotter. In addition, if the internal cylinder pressures are controlled (and by our best estimate based on a LOT of test data) and kept under about 800 psi peak pressures, the longevity is vastly improved.

Remember, the enemies of metal are heat and pressure. Control those and the design parameters are adequate. Operating at high CHTs (above about 380dF) and with high ICPs and the metal will fatigue and fail much earlier.

ROUGH, back of the napkin calculations:
Diameter of the cylinder approx. 5"
Area of cylinder head approx. 20 sq. in.
ICPs during a various mixtures range from 600psi LOP to 1100 psi at 50dF ROP.
20 x 600 = 12,000 pounds of force down on piston and up on head.
20 x 1100 = 22,000 pounds of force down on the piston and up on the head.

NOTE: These are peak pressures (stress) not mean pressures (HP).
Both of the above stress scenarios can actually produce the same HP!!
The former will have CHTs lower by about 50dF.

In which scenario is the cylinder going to last longer?

AH, but have a think about what would happen if the jaws of the vice had a thread of the same diameter and thread form in them as the bolt, and the bolt was clamped into this thread.


Now assume that the vice is tensioned up to clamp onto the thread so now you have the frictional resistance caused by the pressure of the vice on the thread and also the shearing force required to strip out the thread, to be applied before you can pull the bolt from the vice.


I am neither an engineer nor a LAME but I suspect that, in the case of the cylinder heads under discussion, both the thread and the shrink fit are required to ensure that the cylinder head remains on the cylinder barrel. I always believed that the thread was the primary way that the head was secured with the shrink fit ensuring that the thread both on the barrel and in the head remained very tight so that there was no tendency for the head to fret or move during operation, especially as they are subject to such varying temperatures.


Again see my disclaimer above and I certainly will be happy if I have this wrong and someone with accurate information (data) sets me straight.


The 20000 pounds of force mentioned by Walter is just an approximation of the measure of the surface area of the cylinder in square inches multiplied by the maximum cylinder pressure in PSI and is the force being applied to the cylinder head during each power stroke. This is the force that the head to barrel connection must resist, with the appropriate safety margins to allow for changes in temperatures etc.


Edit: I posted this while Walter was posting. I have just learned some more. Thanks Walter.

Forces on a thread are not distributed evenly across the whole of the thread face. Meaning say the nut at the closest point to the surface it mates against takes more load than at the end where the nyloc is. This is why cly crack at the start of the thread and not at the end

This is very important: This, too, is very important:Thank you Mr Atkinson! :ok:

Fatigue is a matter of time and / or cycles. You may have a high time engine with low hours or a low time engine engine with high cycles it evens out. That's why Qantas aircraft at high hours low cycles. That's why we log starts cycles hours.

Walter,
Thanks for the explanation, so the threads are not part of the equation only the area below the threads

So, let’s assume we have two identical model piston aero engines that were manufactured at the same time, are fitted to the same aircraft type and have exactly the same number of cycles and exactly the same time in service with the same operator.

However, let’s also assume that one engine is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 600 PSI, and the other is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 1100 PSI.

Which of those two engines is more likely to have a cylinder failure first? (Fingers crossed and breath bated for a potential breakthrough…. )

Creampuff
The ones fitted with wagon wheels or train tyres?

As a rule of thumb the first 3 threads of any fastener bear pretty much all the load. This is pretty much fastener design 101.

If a part fails at the beginning of the thread my first assumption would be that it is caused by a stress concentrator resulting from the manufacturing process of the thread forming. I assume that cylinder head threads are cut rather than rolled (bolts are rolled threads about 99.9% of the time). Rolling a thread is a process analogous to forging and produces a stronger thread and doesn't introduce stress concentrators from the thread cutting process.

There are formulae to calculate the thread strength. But, I doubt that its an issue because its worked successfully in aircraft engines for decades. I'd be pretty confident that the AD is the result of a material or manufacturing defect.

Properly torqued threaded fasteners do not come loose. Full stop. But typically we use fasteners tightened below the proper torque. Hence the need for Nylok nuts, lock wires, loctite, etc.

My guess would be that the shrink fit of the cylinder head to barrel is more to do with sealing than it is mechanical strength.

As in life there are no guarantees. If the engine is run as per the book you generally don't have problems. As I said before this is really not a lop issue. The engine is designed around the px in the chambers. The bearings will fail before the head. Rapid cooling is something else which is hard on an engine. Flight schools and meat bomb aircraft suffer the worst cracking. That's why you have reduced cruise power settings.

So which will crack first. If you have an aircraft that is px to a max diff of 8.5 psi and you fly it un pressurized will it crack more at max diff or with no px in the cabin.

I've only seen lock wire work once. It didn't stop the bolt from loosening but it stop it falling out of the hole after the thread failied in a magnesium case.

No, that won’t do at all. Learning difficulties are no excuse for deliberate obfuscation.So, let’s assume we have two identical model piston aero engines that were manufactured at the same time, are fitted to the same aircraft type and have exactly the same number of cycles and exactly the same time in service with the same operator.

However, let’s also assume that one engine is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 600 PSI, and the other is always run in the cruise at a mixture setting that produces internal cylinder pressures of around 1100 PSI.

Which of those two engines is more likely to have a cylinder failure first?

It’s not a hard question, and it won’t hurt you to answer it.

They both may or may not make o/h. If you gong to continue with personal abuse two things will happen you decide. If the engine is run to its book it will and should make o/h. Cly px have not much bearing on head failure period. Detenation which has the most extreme px will damage the bearing first