Can someone explain piston ring construction/movement?

Kql

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21
I've always wondered how piston rings (compression and oil rings) keep contact with the cylinder walls, do they have springs or something, and from what I've seen they are not solid rings, they have a little gap presumably the only way they can be installed , would this always leave a small gap? I'm probably overlooking the obvious but when someone refers to a "stuck" ring, where and how is it getting stuck?
One thing I'm pretty sure if, the oil rings don't provide compression sealing, that's the top ring job.
 
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1,873
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New England, USA
Non compressed by the cylinder, the rings are slightly larger in diameter than the cylinder and are compressed for installation and present a small amount of tension against the cylinder walls. There is always a small gap, there are specs, and in fact on piston aircraft you will occasionally see a low compression reading if the gaps on all of the rings are temporarily aligned. Most if not all rings rotate in use.

A 'stuck' ring is one that either isn't rotating when it should, or more commonly, when it is stuck in the grove and isn't pressing against the cylinder wall and therefore allowing proper sealing.

Some related reading;Piston Ring Gap Alignment

BTW I'm a finance guy who plays at this so take this info for what it is worth :D
 
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2,304
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Seattle-ish, WA
I'm probably overlooking the obvious but when someone refers to a "stuck" ring, where and how is it getting stuck?

Combustion will cook excess oil in the rings and form coke/carbon that can eventually glue the ring in the groove, vs. letting it act with internal spring tension to seal against the wall.

This happens for various reasons - bad state of tune, poor oil drain design, running oil too long, overall engine design, etc. Lots of cars are susceptible to this (I have one). Keeping it well tuned, good gas, using the narrowest viscosity spread synthetic oil you require, changing it often, etc., all help to mitigate it.
 
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10,631
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Jupiter, Florida
Possibly of interest, piston rings are the single most important, and largest, source of friction in an engine.

With that in mind, simple geometry dictates that, other things being relatively equal, the less piston ring "swept area" and engine has, the more efficient it will be.

Engine designers have known this since the very early days. A large piston+cylinder has less "swept area" than multiple small cylinders of the same displacement, or more correctly, power output.

pratt_whitney_r2800_wwii_radial_Engine_piston_clock_f4u_corsair_northrop_grumman_hellcat_p47_thunderbolt_warbird_airplane_furniture_12.JPG


Conclusion: One main reason old aircraft engines had such large pistons was that the piston ring swept area was as low as possible, to greatly improve fuel efficiency and therefore, aircraft range.

It's interesting to note that the best of these old engines were so efficient, Toyota only recently surpassed the 40% efficiency numbers. Same goes for the "airlines" with regard to "passenger miles per gallon". We've only recently "bettered" the pax MPG of the old piston powered airliners!

Piston ring swept area is one main reason!

Modern engine designers strive to use "low tension" piston rings everywhere. It help efficiency greatly. Unfortunately, sometimes the engineers go too far and the rings fail to seal after short periods of time and seemingly insignificant wear rates. Honda/Acura and the Prius come to mind. This often leads to high oil consumption.
 
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25,789
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MA, Mittelfranken.de
Piston rings are some of the most complex and important components in any piston engine. These articles go into more depth than I can here but are worth taking the time to read.

 
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1,873
Location
New England, USA
Possibly of interest, piston rings are the single most important, and largest, source of friction in an engine.

With that in mind, simple geometry dictates that, other things being relatively equal, the less piston ring "swept area" and engine has, the more efficient it will be.

Engine designers have known this since the very early days. A large piston+cylinder has less "swept area" than multiple small cylinders of the same displacement, or more correctly, power output.

pratt_whitney_r2800_wwii_radial_Engine_piston_clock_f4u_corsair_northrop_grumman_hellcat_p47_thunderbolt_warbird_airplane_furniture_12.JPG


Conclusion: One main reason old aircraft engines had such large pistons was that the piston ring swept area was as low as possible, to greatly improve fuel efficiency and therefore, aircraft range.

It's interesting to note that the best of these old engines were so efficient, Toyota only recently surpassed the 40% efficiency numbers. Same goes for the "airlines" with regard to "passenger miles per gallon". We've only recently "bettered" the pax MPG of the old piston powered airliners!

Piston ring swept area is one main reason!

Modern engine designers strive to use "low tension" piston rings everywhere. It help efficiency greatly. Unfortunately, sometimes the engineers go too far and the rings fail to seal after short periods of time and seemingly insignificant wear rates. Honda/Acura and the Prius come to mind. This often leads to high oil consumption.
Cujet, what is that piston from? Didn't certain Lycomings spec a knurled piston back in the day as an update due to some lubrication issues?
 
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1,873
Location
New England, USA
While we are at it, here are some pics with details of the rings, grooves and oil ports on a piston from a Packard Merlin V-1650 in my friend's P51D. Note the depth of the grooves compared to the rings, especially the oil one, and that the oil control ring has a continuous spring component to it. Also the different color of the rings' deposits vs. their place in the stack; L-R is top to bottom.

IMG_3743[2810].JPG
merp.JPG
merr.jpg
 
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10,631
Location
Jupiter, Florida
The "Total Seal" gapless piston ring, a nested pair. It's a neat idea that works well in some applications. And they will reduce leak down test numbers. Although this does not translate into more HP. In other applications, it fails due to poor heat transfer and excessive expansion. They are not well suited for street engines due to relatively short lifespan, and they are not ideal for ultra high performance engines due to poor piston to cylinder heat transfer.

Keep in mind that the end-gap of a conventional ring set does close-up a bit when hot. Leading to good sealing during use. The reason for the end-gap specificiation is to eliminate the risk of the ring ends touching, expanding via heat, and causing cylinder scoring.
TotalSealProSet-2.jpg
 
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742
Location
MN
Combustion pressure also drives the rings against the cylinder wall, the pressure drives the ring to the bottom (lower edge) of it's groove and tries to drive the ring out (toward the cylinder wall) of it's groove which drastically increases the seal at the cylinder wall. Coked up rings won't properly float in their grooves and you'll have excessive blow by and oil consumption.
 
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234
Location
South
Combustion pressure also drives the rings against the cylinder wall, the pressure drives the ring to the bottom (lower edge) of it's groove and tries to drive the ring out (toward the cylinder wall) of it's groove which drastically increases the seal at the cylinder wall. Coked up rings won't properly float in their grooves and you'll have excessive blow by and oil consumption.
Yep. Drag race engines used to have the pistons drilled for “gas ports” to allow more combustion gases access behind the top ring for better sealing. Not sure that’s even done anymore. I recall a Ford F650 gas V8 (early 70s 361 FE) with three compression rings along with the traditional oil rails. They saw a lot of abuse. Typically these engines would burn valves and suffer cracked heads and exhaust manifolds due to extreme heat. We overhauled several using basic pistons/rings (F150) and they lived just as long. Same bore and stroke as the 360 FE used in light duty pickups.
 
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237
Location
FL
Yep. Drag race engines used to have the pistons drilled for “gas ports” to allow more combustion gases access behind the top ring for better sealing. Not sure that’s even done anymore. I recall a Ford F650 gas V8 (early 70s 361 FE) with three compression rings along with the traditional oil rails. They saw a lot of abuse. Typically these engines would burn valves and suffer cracked heads and exhaust manifolds due to extreme heat. We overhauled several using basic pistons/rings (F150) and they lived just as long. Same bore and stroke as the 360 FE used in light duty pickups.
Yeap gas ports are still a thing , laterally as well.
 
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