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piston ring wear at low power setting

KayS

Well Known Member
https://www.lycoming.com/content/tips-extending-tbo

came across this link where lycoming provides some info how to be kind to our engines. you will all agree that reaching TBO or being able to go beyond is a good thing.

the section that adresses descent states: "Descent power settings at greater than 20” Hg manifold pressure should be utilized for the greatest possible time to avoid accelerated piston ring wear."

lately i developed the habit to reduce power at cruising altitude to 15"/2000 RPM or so and to let her sink with around 500 ft/min down to pattern altitude, which can take awhile. here lycoming states that low manifold pressure is not so nice for the piston rings (and this should not only apply for the descent phase i think).

there are a lot of OWT's around but this statement comes from lycoming itself. i would be interested to hear your opinion about that...

Best Regards from rainy germany
Kay
 
Hmmmm....

These tips are directed specifically at TI0-540-J series engines, but they may be applied to other Lycoming engines as well.

Several things in there that seem to conflict with other good advice. Most of us are not running turbos, obviously, so it's not clear which of their tips are turbo-specific. This looks very similar to the "shock cooling" OWT which has been pretty thoroughly debunked, but I'm sure that many people are not yet aware.

I was under the impression that piston ring wear was a result of the piston sliding along the cylinder, and is reduced with good lubrication. When there is combustion, the pressure forces the ring to the outside, against the cylinder. Less pressure would seem to me to be associated with less wear, since the ring is not held tight against the cylinder. My guess is that there are other factors that come into play that are not obvious to a layman like myself. This might be a good question for Mike Busch's team of experts.
 
I think they may be talking about ring flutter. The lack of BMEP can let the ring flutter causing wear on the top and bottom of the ring against the ring lands. I don't know for sure but that is what I assume they may be talking about. we would sometimes see this kind of ring wear of traffic spotter craft that flew lots of low power slow flight in their normal operational mode.
Good Luck,
Mahlon
 
Send a note to Lycoming and ask.

Best guess? The top rings are pushed down and outward by cylinder gas pressure. Low pressure can allow the ring to lift off the lower piston land surface, in particular near the ring gap where pressure is small, a phenomenon called ring flutter. The specific engine noted is a TIO540-J, which is only 7.3 CR. With the throttle closed the low CR equates to less sealing pressure (as compared to typical CRs for NA engines), thus more risk of flutter.

EDIT...looks like Mahlon beat me to it.
 
Send a note to Lycoming and ask.

Best guess? The top rings are pushed down and outward by cylinder gas pressure. Low pressure can allow the ring to lift off the lower piston land surface, in particular near the ring gap where pressure is small, a phenomenon called ring flutter. The specific engine noted is a TIO540-J, which is only 7.3 CR. With the throttle closed the low CR equates to less sealing pressure (as compared to typical CRs for NA engines), thus more risk of flutter.

EDIT...looks like Mahlon beat me to it.

Not to mention acceleration forces add to friction force direction opposing gas forces.

Turbocharged? - does the TIO540-J have keystone rings? They can get pretty loose the the grooves compared to standard rectangular.
 
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Normally combustion pressure is pushing down on the piston AND the rings. In a descent at low MAP, the crankshaft could be pulling the cyl down. This moves the ring to the opposite face of the ring land and also eliminates outward pressure of the ring against the cyl wall. In some cases it can create ring flutter, which can put wear on the ring land faces and allow debris build up.

This is definately a major No No during break in, but never heard it was an issue after that. I have no evidence to think Lyc is wrong here ans suspect they may be right. However, doubt the wear is significant.

I am suspicious, as any manual trans car sees this type of activity on a daily basis. Then again, back in the day when this was commonplace, engines didn't last as long as they do today. An auto trans doesn't do this, as the torque convertor releases.

Larry
 
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