AOPA article on sticking valves

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Rich
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AOPA article on sticking valves

Post by Rich »

Mike Busch has an interesting article in this month's Pilot regarding valve sticking and shows interesting differences between Lycoming and Continental. Spoiler alert: There's such a thing as running CHTs too low.
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Re: AOPA article on sticking valves

Post by Chris B »

Mike mentions Ed Kollin, but I am disappointed that he does not reference Ed's 3-part paper (https://aslcamguard.com/sticking-exhaust-valves/) which forms the content for Mike's article. Ed's paper includes more detail and pictures to illustrate his points.

Highly recommended!

Direct link to Ed's PDF:
https://themes.around29.com/camguard/wp ... 1-to-3.pdf

Ed is gracious and patient in person if you get a chance to meet him. He usually has a booth at OSH (& presumably Sun 'N Fun).

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Re: AOPA article on sticking valves

Post by Rich »

Sooo... One more thing to obsess over :D For example, I was doing some practice approaches today. In the TERM phase I typically target 90-100 knots, which requires very lower power (~45%) in level flight, and even less once descending. I never paid much attention to this phase of flight, under the theory that lower CHT is better. Since these are typically somewhat extended operations at these lower power settings, I found myself fiddling more with the mixture to keep the CHT's above 350. :tap:
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Re: AOPA article on sticking valves

Post by Chris B »

In a similar thread on BeechTalk (link), Ed Kollin follows-up with additional comments.

Chris

Ed Kollin wrote:Some additional detail, I've had numerous reports of big bore Continentals (Bonanza's, Mooney's, Cirrus's, Cessnas's and Lancair's) with intermittent roughness and exhaust valve sticking. They seem to be associated with running a normally aspirated engine high (low manifold pressure) AND deeply LOP. This sets up the conditions for lead oxybromide deposits formation on the exhaust valve stem and potential valve sticking. This is usually accompanied by low CHTs which Mike rightfully states to avoid. But this is not always the case. Reduced airspeed and high ambient temperatures can show mid 300 degree CHTs while still building deposits.

You can reduce these deposits by periodically running the engine very hard and sufficiently ROP (red box) for a few hours.

You can reduce the deposit formation rate by adjusting the ignition timing to the maximum allowable.

You can see these deposits come and go with a boroscope when the exhaust valve is open. I have looked at these deposits come and go on my IO520, unfortunately I'll have to do it again to get pictures.
follow-up question wrote:Is this done by documenting what you see with a valve open via scope, go out & fly it hard, then look again ... or some manner of actively visualizing?

By timing to the most BTC numerically?
Answer by Ed Kollin wrote:Yes, I flew my 210 with a IO 520 for 10 hours at 55% power and 30 degrees LOP.
The exhaust valve stems had deposits on them. I then flew it at 75% ROP for 10 hours, and the deposits were essentially gone.

Basically, if you run a 65% power or less, run at peak EGT so the scavenger chemistry can function as intended. It is the combustion temperature that is important, determined by manifold pressure and mixture, and CHT is an imperfect approximation of it.

Yes, the more advanced the ignition timing the higher the peak combustion pressure and temperature and the less chance of this deposit formation phenonium. +1/2 a degree can make a substantial difference.
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