Cylinder Glazing, C172 0-320

[aseanaero]

A non aviation friend of mine inherited a flying school and he was perplexed as to why the aircraft were continually requiring cylinder head repairs and replacements due to oil glazed cylinders and high cyl head temps (0-320s). Out of 5 aircraft there is always at least one aircraft in the hangar getting the heads done , they are going through 6 cylinder heads a MONTH !

He asked me to look into this for him and I found that the school has a SOP of max 2,100rpm for cruise (about 90kts) instead of 2,300 to 2,400rpm (110kts).

They are also using autofuel (mogas) but the situation was much the same when using avgas.

Chief instructor is an ex military C-47 pilot and insists that 2,300 to 2,400rpm is too high and won't take any further arguement on the subject.

Can anyone point me to an article that supports higher cruise rpm for engine longevity ? Any other comments or experiences on this issue welcome

[SNS3Guppy]

If indeed glazing is the issue, then the only time operating RPM or power settings will be party to the issue is during break-in. Far too little information is provided here to address the situation. The cylinders and rings in use, as well as what's being done to recondition them, chrome vs. steel, etc, are all worth consideration.

Presumably when you say the cylinders are glazing, you're indicating that the cylinders have low compression. Is this occurring shortly after installation of new cylinders, or is this an issue with older cylinders at this school?

A lower airspeed tends to lead to higher cylinder temperatures, balanced against the power setting needed to achieve that speed, but operating the airplane at 2100 RPM shouldn't present any problem.

[aseanaero]

Is this occurring shortly after installation of new cylinders ?

Yes , within a few hundred hours of a cylinder change, low compression and high oil consumption. I'll get further details on the cylinders and rings

[slatch]

I have operated O-320's in my C172's and have never had a cylinder not make TBO or beyond. From what you have said so far I would say the 2100 RPM at cruise is the problem. 2100 RPM on an O-320 at 1000 feet density altitude is roughly 50 percent power and at 5000 feet density altitude around 40 percent. Normal thought is it takes 65 percent of power to maintain enough ring pressure to scrap oil off the cylinder walls to prevent glazing. Continued operation at lower power settings will lead to cylinder glazing. I believe Lycombing recommends 65 percent as a minimum cruise setting. I use the following in my C172K
Density Altitude RPM
1000 2350
2000 2350
3000 2400
4000 2400
5000 2450
6000 2450
7000 2500 if it will make it
8000 and above " "

Remember we are talking power not RPM There is no reason not to fly full throttle at higher air densities when the engine is making 65 percent power or less.

Other issues are you using Aviation oil of the proper grade? Car oil will not work as it has the wrong additives. What are oil temps at cruise? 210 or below is good, anything over 220 is a concern. You might find the oil runs cooler at the higher rpm due to increased airflow and reduced piston ring blow by adding hot air to the crankcase.

[Fark'n'ell]

Sounds like your cheif instructor needs re educating on engine handling of the 0320.

Initial run in=full throttle for about 5 hours to ensure the rings bed in properly.
Normal running,nothing less than 2400 rpm after run in.
Have seen many 0320 engines needing top end o/h after 1200/1500hrs because of babying the engine in the mistaken belief of looking after it.Run them hard,won't hurt them.

[slatch]

Check out these for more information:
Megginson Technologies Ltd. - Aircraft Power Settings

http://www.lycoming.textron.com/supp...Operations.pdf

[aseanaero]

Thanks for the good responses

[ab33t]

I can confirm all the above we tried to nurse our Lycoming and had to do heads at a low hour . We the ran them top end of the green rpm and low and behold the enginge is still going

[SNS3Guppy]

There's a difference between running at low power settings during break-in, and running at low power settings after break-in.

The PDF attachment cited above specifically refers to engine break-in, not to normal post break-in operations.

The other link in the same post shows yellow settings which at first blush would suggest a caution range; this is not the case. The yellow, as indicated in the link, represents lower airspeeds...and does not denote an unfavorable operating range.

Again, more information is required.

[Teddy Robinson]

check the oil grade ...

The engine should be on straight oil for a specified number of hoursafter a rebuild to allow carbon deposits to build up and seal the engine. A detergent oil will not allow the process to take place.

[SNS3Guppy]

This is another myth. Lycoming calls for straight non-detergent mineral oil for turbocharged airplanes for the break-in, but this isn't a requirement for non-turbocharged airplanes. In fact, Lycoming specifically states that for the 0-320 and O-360 powerplants, either straight non-AD oil, or Ad oil may be used.

Remember that ashless dispersant oil IS mineral oil. An old myth is still very prevalent that mineral oil is different from AD oil...when AD oil is mineral oil. The same myth teaches that an engine may glaze if straight non-ad mineral oil isn't used...this is also untrue.

Again, insufficient information is provided regarding the problem being experienced by this operator. If the problem is occuring as the result of an improper break-in, then that should be revealed. Simply running the engine at a lower power setting is not going to cause the cylinders to glaze.

Aviation Maintenance Magazine :: The Mineral Oil Myth

[ferrydude]

Absolutely correct. in fact, ECI recommends Phillips AD oil for break in of its cylinders

[kevinsky18]

The issue as mentioned above is most likely due to his run in technique.

He needs to run full power or at the very least 75% or greater for the first few hours. This is for the pressure as mentioned before.

Also full rich, this is partly for cooling and dentonation but most importantly for the break in the extra fuel helps wash the cylinder wall of oil "preventing glazing"

Im betting he doesn't want to run full rich, it's hard to watch all that fuel being dumped but it's a necessary part of the break in process.

[SNS3Guppy]

The issue as mentioned above is most likely due to his run in technique.

That's a possibility, in fact a strong one, but again, not enough information has been provided.

I should correct a previous statement; I said that Lycoming turbocharged engines must be broken-in on straight non-AD mineral oil...it's the other way around. Lycoming requires that their turbocharged engines be broken in on ashless dispersant oil. As previously mentioned, the O-320 and O-360 motors may be broken in on either straight mineral or AD oil...it's also worth noting that if the engine is delivered or filled with AD oil, it shouldn't be changed back to mineral.

Power settings must be in excess of 65% during the break-in process, which, if done properly, will be the first 50 hours of operation. Running rich during this period does not enhance the break-in process, and depending on the altitude at which the break-in is conducted, may not permit adequate power to be developed during the break-in process.

[blackhand]

Assuming that the correct break-in proceedure has been carried out, perhaps one might consider excessive running at low RPM as a cause for glazing of the cylinders. i.e too long at idle RPM.


BH

[SNS3Guppy]

Running at idle isn't going to glaze the cylinders...assuming that a proper break-in has already been conducted.

[blackhand]

The glaze is just unburned fuel and oil deposits burned onto the cylinder walls by the higher combustion heat generated at low R.P.M.s when the cylinder is scavenging exhaust gases less efficiently due to lower port velocity.The glaze fills in the cross hatch pattern on the cylinders from when they were honed during production and the piston rings no longer seal.

And the worst is from excessive time at idle RPM.



BH

[SNS3Guppy]

Again,

Running at idle isn't going to glaze the cylinders, if the engine has been properly broken in.

Idle operating temperatures do not cause glazing, and glazing does not occur based on "less efficient scavenging" at idle. Combustion gasses do not bake onto cylinder walls at idle and glaze cylinders.

Plug glazing can take place at idle settings if the idle mixture is incorrect, and excessively rich, but this is a function of lead fouling, not "glazing" as it's commonly taken to be, and won't cause cylinder glazing.

An automotive term which is sometimes used, particularly for diesel engines, is "wet stacking," referring to an increase in oil in the exhaust of diesel engines during prolonged idle. This is commonly given to mean that lower combustion pressures cause insufficient oil control ring expansion, leading to increased oil deposits on the cylinder walls and eventually an increase in oil in the exhaust. This isn't a problem on horizontally opposed air cooled aircraft piston engines...again applicable if proper break-in has occured following engine overhaul or cylinder replacement.

What I do often see are owners or operators who replace a single cylinder and then operate the engine as though nothing has changed. The assumption is that the engine is already broken in, so no special procedures are required. In truth, the engine should be operated as though it's new again...because that one cylinder still requires break-in. During this initial break-in period, which can last up to 100 hours, depending on the engine and cylinder (and break-in techniques and practices), excessive idle can cause glazing, as can most other improper operation. After break-in, idle isn't an issue so far as "glazing" goes.

With respect to the original poster, while apparently no additional information is forthcoming, the limited information given sounds very much like improper break-in has been occuring. If all the cylinder changes are taking place within 300 hours of operation, and 100 hours may be a typical break-in period, one may reasonably point to failure to break-in the engines or cylinders in the first place, as a likely culprit.

[aseanaero]

Hi Guppy,

I've been out of the country for Christmas , I'll be able to give further info next week when I talk with the operator's engineers again.

Thanks for all the comments so far.

[clunckdriver]

First a bit of background, owned five flight schools with a total fleet of thirty aircraft, only ever glazed one engine, was on vacation and a renter was not briefed on how to break in a new engine, this is how you do it! {1} Run straight mineral oil till consumption drops to normal for that engine, then another ten hours to be sure, in extreame cold weather one can use a multi grade mineral for this period.{2} Keep run ups and ground running to an absolute minimum.{3} Run at the highest power you can for the whole break in period, use the aircraft on cross countries rather than local flying, I reapeat, run the piss out of the engines, DO NOT baby them!{4}We used to placard the aircraft and log books so everybody got the word.{5} Use power on apch to keep temps up, we used to slip to make it "draggy," keep taxi time back to the ramp to absolute min, use engine blankets to keep the temps up, thus avoiding thermal shock. There are some very good procedures put out by many overhaul shops on this subject. We had an "on condition" program, most of our engines went at least one thousand over TBO using these procedures.{6} Fire any stupid SOB who "knows better"!