Worn rings, end gap and blowby

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Circlotron
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Worn rings, end gap and blowby

Post by Circlotron »

Just thinking, when rings get a bit worn and blowby increases, is it because of the ring surface that contacts the bore becoming degraded, or is it because of the end gap opening up? Or if both, is one more so than the other?
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Re: Worn rings, end gap and blowby

Post by Roundybout »

Other than dirty oil, fuel wash is/was the biggest culprit in ring wear. I’d see a definite decrease in radial thickness and the gap would open up a bit. The wear on the ring (and bore) had a lot more to do with oil consumption and blow by issues than the ring gap thickness did in my opinion.

Now anytime you get into a situation where the oil isn’t doing it’s job due to whatever reason such as heat, detonation, oil breakdown ect. you’re gonna have abnormal wear and shorten the life. Less wear in today’s modern engines can traced right back to fuel injection and synthetic oils.

It takes abuse or a whole lot of miles to the point the rings are worn out.
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Re: Worn rings, end gap and blowby

Post by PackardV8 »

Roundybout wrote: Tue Sep 29, 2020 10:12 am Other than dirty oil, fuel wash is/was the biggest culprit in ring wear. I’d see a definite decrease in radial thickness and the gap would open up a bit. The wear on the ring (and bore) had a lot more to do with oil consumption and blow by issues than the ring gap thickness did in my opinion.

Now anytime you get into a situation where the oil isn’t doing it’s job due to whatever reason such as heat, detonation, oil breakdown ect. you’re gonna have abnormal wear and shorten the life. Less wear in today’s modern engines can traced right back to fuel injection and synthetic oils.

It takes abuse or a whole lot of miles to the point the rings are worn out.
X2 on this.

We tear down older carbureted engines from 100,000 mile cars and they're sludged up, rings worn out, cylinders with a .005" or greater ridge at the top. A current LS engine with an occasional oil change can run 200,000 miles and still see the cross-hatch in the cylinders.

As to the original question, the bad oils, poor crankcase ventilation, poor air filtration; they all combined to wear the rings and cylinders. Once the rings are being pushed in and out on every stroke, wear accelerates even faster and rings can't seal against a tapered bore.
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Re: Worn rings, end gap and blowby

Post by ptuomov »

We looked into this. If the ring gap wears from an initial 0.025” to say 0.035”, the engine will have a lot more blowby than a fresh engine with 0.035” ring gap. The leakdown is usually only slightly higher but blowby flow much higher, in our experience. So this is a case where leakdown is only useful for tracking the wear of the one specific engine (and detecting broken parts).

Why? The bore doesn’t wear round. Instead, it usually wears to a clover leaf shape. In a live running engine, rings seal great on a round bore, ok on an oval bore, but terribly on a clover leaf shaped bore.

If the engine would wear the bore perfectly equally to a round shape, we’d just be replacing the rings with longer rings every one million miles.
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Re: Worn rings, end gap and blowby

Post by BLSTIC »

I've always thought it was mainly about reduced ring tension as the ring and bore wear.

Not that the gap makes blow by itself, but it represents a ring that has now worn to spread out more (like if a valve spring seat was a wear surface the spring would get marginally shorter free length and marginally longer installed height, both mean less tension) and because it's now thinner has a double whammy of less force per mm of compression.
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Re: Worn rings, end gap and blowby

Post by modok »

You forget a very important thing; how the ring fits in the groove.

The ring lands of the pistons become wider and tapered and rounded.

The more wear.... the more the ring has to TWIST and bend to seat between beat out groove and tapered/ovaled cylinders, ultimately causing the ring surface to be rounded off, scuffed up, or break up into pieces trying.
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Re: Worn rings, end gap and blowby

Post by Krooser »

I've noticed most engines wind up running with the ring gaps lined up after many miles... What can't be good.
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Re: Worn rings, end gap and blowby

Post by David Redszus »

Blow-by
Blow-by is considered the phenomenon where combustion gases flow from the combustion
chamber past the ring pack to the crankcase. The combustion gases flow past the piston ring
at various locations: (a) at the piston ring gap, (b) past the front side of the piston ring at
starved lubrication conditions, (c) or past the backside of the piston ring when the ring is not
in contact with either of the ring-groove walls. The hot blow-by combustion gases cause the
piston and piston rings to overheat. The blow-by disturbs the piston and ring lubrication by
affecting the oil film: combustion gases contaminate the lubricant and cause the oil to entrain
in them. When the combustion gas reaches the crankcase it pollutes the lubrication oil.

Blow-by cannot be totally prevented as long as the rings have gaps and move in their grooves.
This means that some blow-by will always have to be allowed. The blow-by affects directly
or indirectly the engine power (fuel) efficiency: the blow-by consumes some of the combustion
power and increases the friction as a result of less favourable lubrication conditions.

The gap between the piston and liner wall is greater on the anti-thrust side of the piston
than on the thrust side. This requires that the gap between the back-side of the ring and the
ring groove is quite large and thus has a large gas-flow area. Measurements have shown that
the twist of the piston rings affects the amount of blowby past the ring pack. A negative twist
on the second ring can cause instability of the ring, which results in an increase in the blow-by.
A positive twist on the second ring can, in turn, cause high land pressure, which may result in
radial collapse or axial movement of the ring.

Ring wear
The most intensive wear of the piston ring pack and the cylinder liner normally occurs in the
running-in, or break-in, stage of the engine, during which the most predominant surface profile
peaks are worn off by the counter surface, and the surfaces eventually obtain improved
conformity. The abrasive wear of the piston rings decreases when the cylinder liner surface
irregularities become smaller, and the wear of the cylinder liner decreases when the piston
rings become smoother by running in. Consequently, under favourable conditions this
self-stabilising process leads to a decrease in the wear rate of both the piston rings and the
cylinder liner. According to experimental work presented by Henein and co-workers, the wear
rate of a cylinder liner is approximately 12 times higher during the first hour of operation than
during the subsequent two hours. Wear tests with neutron bombarded compression rings in fired
engine tests including a gamma ray spectrometer have shown that the piston ring wear rate
during the start-up period was up to 45 times the steady-state wear rate, and that approximately
84 % of the ring wear occurred during the first approximately 22 minutes of operation .

Ring groove wear
Wear of the parallel surfaces in piston ring grooves, commonly called ring-groove wear,
occurs mainly in the top ring groove. The main reason for the wear is the combined effect
of gas forces and radial motion of the ring, and the wear process is accelerated by poor
lubrication and a high temperature. The reasons for the radial motion of the ring are
the cylinder distortion, the secondary movement of the piston and piston tilt allowed by
the piston/cylinder clearance. Mass forces, friction forces, axial ring movement and ring
rotation increase the ring groove wear. In stationary gas pressure and gas blow-by may cause
radial vibrations in the ring, which accelerates the ring groove wear at the ring-groove contact
areas. As the result of the wear, the lower surface of the ring groove becomes rough and
rounded towards its edge, and the upper surface becomes rough. In addition to the deformation
of the groove arising from wear, the width of the groove increases, and the side clearance
between ring and groove increases. Alternatively to wear, the ring-groove tribosystem
may suffer from ring welding, as a consequence of overheating of the top ring or partial
seizure of the piston owing to poor lubrication.
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Re: Worn rings, end gap and blowby

Post by ptuomov »

Krooser wrote: Tue Sep 29, 2020 1:14 pm I've noticed most engines wind up running with the ring gaps lined up after many miles... What can't be good.
This is the case with bores that have been worn to a cloverleaf shape.
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Re: Worn rings, end gap and blowby

Post by Krooser »

ptuomov wrote: Tue Sep 29, 2020 2:00 pm
Krooser wrote: Tue Sep 29, 2020 1:14 pm I've noticed most engines wind up running with the ring gaps lined up after many miles... What can't be good.
This is the case with bores that have been worn to a cloverleaf shape.
The last three SBM's and two SBC's I pulled aparts all had this going on.

The two Dodge Magnums did not... and the bores seemed to be good, too. FI may be the saving grace.
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Re: Worn rings, end gap and blowby

Post by hoodeng »

One salient point David gave is 'micro welding' of rings in the base of the groove having a detrimental effect throughout the life of the ring, this phenomena occurs very early in the startup phase of running in an engine.

As for ring rotation in a groove, Mahle found this to be a naturally occurring phenomena in a IC engine...

The story goes that Mahle were doing analog pressure testing at different piston locations in the bore the during combustion event [in the 60's], what they found was every now and then there was a spike in the pressure readings at a low in bore test point, then the readings would settle down again for an extended period of time, then there would be a spike again for a short period of time then settle again.

What they deduced from this was that the rings were rotating in the piston groove and the gap was lining up with the test point for a period of time, i would have to dig deep in the "Mahle Piston Manual" to find the article in support of this.

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Re: Worn rings, end gap and blowby

Post by ptuomov »

hoodeng wrote: Tue Sep 29, 2020 9:49 pm
The story goes that Mahle were doing analog pressure testing at different piston locations in the bore the during combustion event [in the 60's], what they found was every now and then there was a spike in the pressure readings at a low in bore test point, then the readings would settle down again for an extended period of time, then there would be a spike again for a short period of time then settle again.

What they deduced from this was that the rings were rotating in the piston groove and the gap was lining up with the test point for a period of time, i would have to dig deep in the "Mahle Piston Manual" to find the article in support of this.
I’ve read some research on this and I’ve satisfied myself of the following: not only does a clover leaf bore shape of the worn bore fail to seal normally, it also impacts the ring rotation such that the ring end gaps of 1st and 2nd rings line up much more frequently than with a round bore.
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Re: Worn rings, end gap and blowby

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ptuomov wrote: Tue Sep 29, 2020 10:44 pm
I’ve read some research on this and I’ve satisfied myself of the following: not only does a clover leaf bore shape of the worn bore fail to seal normally, it also impacts the ring rotation such that the ring end gaps of 1st and 2nd rings line up much more frequently than with a round bore.
Exactly right. If the bore shape is clover leaf, only portions of the ring will make contact with the wall surface, allowing blowby areas and causing severe ring hot spots. Now the very hot ring grows in size, increasing pressure against the wall and closing the end gap.

Ring end gaps measured when cold, within reason, are not an issue since hot ring expansion will reduce the end gap. What is needed is a way to compute ring end gap size when the ring (and bore) at at operating temperature.

What does matter is the ring flow area on the non-thrust side of the piston, which is a function of proper piston sizing and ring wear.

And, over time, ring groove wear becomes an increasingly important factor.

A surface profilometer reading of the burnished surfaces of the bore is an excellent tool to measure bore wear at an early stage.
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Re: Worn rings, end gap and blowby

Post by ptuomov »

David Redszus wrote: Wed Sep 30, 2020 10:29 am
ptuomov wrote: Tue Sep 29, 2020 10:44 pm
I’ve read some research on this and I’ve satisfied myself of the following: not only does a clover leaf bore shape of the worn bore fail to seal normally, it also impacts the ring rotation such that the ring end gaps of 1st and 2nd rings line up much more frequently than with a round bore.
Exactly right. If the bore shape is clover leaf, only portions of the ring will make contact with the wall surface, allowing blowby areas and causing severe ring hot spots. Now the very hot ring grows in size, increasing pressure against the wall and closing the end gap.
The additional point is that cloverleaf bore shape aligns the ring gaps between the first and second ring with relatively high probability.

If the bore is worn to cloverleaf shape, the piston cooling is really compromised, too, as you say — unless we have piston oil cooling jet. One more reason to have piston oil cooling jets.
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Re: Worn rings, end gap and blowby

Post by David Redszus »

ptuomov wrote: Wed Sep 30, 2020 10:36 am
David Redszus wrote: Wed Sep 30, 2020 10:29 am
ptuomov wrote: Tue Sep 29, 2020 10:44 pm
I’ve read some research on this and I’ve satisfied myself of the following: not only does a clover leaf bore shape of the worn bore fail to seal normally, it also impacts the ring rotation such that the ring end gaps of 1st and 2nd rings line up much more frequently than with a round bore.
Exactly right. If the bore shape is clover leaf, only portions of the ring will make contact with the wall surface, allowing blowby areas and causing severe ring hot spots. Now the very hot ring grows in size, increasing pressure against the wall and closing the end gap.
The additional point is that cloverleaf bore shape aligns the ring gaps between the first and second ring with relatively high probability.

If the bore is worn to cloverleaf shape, the piston cooling is really compromised, too, as you say — unless we have piston oil cooling jet. One more reason to have piston oil cooling jets.
Piston oil cooling jets will lower piston temperature by about 27F deg, which is not much when applied to a 450f deg piston temperature. But better than nuttin.
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