Accumulation Grooves

[johnny comelately]

accumulation grooves, does anyone know the facts on how they work in regard to sealing?
as an example the model aero pistons that have no rings and just an accumulation groove.
there does not seem to be any leakdown, which is surprising

[gruntguru]

"Between a piston’s top ring groove and second ring groove there is sometimes an additional groove, which in section is triangular rather than square. Since blow-by inevitably leaks past the top ring this empty groove has the effect of reducing its pressure. Known as a pressure seal groove it reflects pressure waves, stopping the second ring receiving the full force of the leaking gas pressure. In turn this reduces the tendency to ring flutter and assists the sealing effect of the combustion gas pressure acting from above, which is trying to push the top ring down and out. Although normally associated with the use of three rings, the pressure seal groove concept can be applied also to two-ring pistons. In a two-ring package the pressure waves such a groove creates can help scavenge oil leaking past the oil control ring. The groove also acts as a physical oil accumulator, collecting oil and encouraging it to flow back down. It also beneficially allows for the expansion of high-pressure gas, the increase in volume reducing its pressure." From page 37 of http://www.ret-monitor.com/articles/wp- ... /rings.pdf

Leakdown in a model aero engine is mostly controlled by the close fit of piston in bore. Oil control is not a concern for 2 strokes with total loss lubrication. Piston/bore fit is self regulating to some extent. the piston crown runs hotter than the bore so it expands to a tighter fit when running. This reduces the clearance so the heat transfer from piston to bore increases - cooling the piston, heating the bore and stabilising the clearance.

[johnny comelately]

"Between a piston’s top ring groove and second ring groove there is sometimes an additional groove, which in section is triangular rather than square. Since blow-by inevitably leaks past the top ring this empty groove has the effect of reducing its pressure. Known as a pressure seal groove it reflects pressure waves, stopping the second ring receiving the full force of the leaking gas pressure. In turn this reduces the tendency to ring flutter and assists the sealing effect of the combustion gas pressure acting from above, which is trying to push the top ring down and out. Although normally associated with the use of three rings, the pressure seal groove concept can be applied also to two-ring pistons. In a two-ring package the pressure waves such a groove creates can help scavenge oil leaking past the oil control ring. The groove also acts as a physical oil accumulator, collecting oil and encouraging it to flow back down. It also beneficially allows for the expansion of high-pressure gas, the increase in volume reducing its pressure." From page 37 of http://www.ret-monitor.com/articles/wp- ... /rings.pdf

Leakdown in a model aero engine is mostly controlled by the close fit of piston in bore. Oil control is not a concern for 2 strokes with total loss lubrication. Piston/bore fit is self regulating to some extent. the piston crown runs hotter than the bore so it expands to a tighter fit when running. This reduces the clearance so the heat transfer from piston to bore increases - cooling the piston, heating the bore and stabilising the clearance.

what does the accumulation groove do then?

[gruntguru]

Are you serious?

Since blow-by inevitably leaks past the top ring this empty groove has the effect of reducing its pressure.

So the empty groove is a small reservoir to satch blowby and reduce the pressure before it hits the second ring.

[johnny comelately]

there are no rings in the aero engine, that is what i was referring to.

and in its application for 4 stroke engines, my understanding is that air being a fluid and captured in the grove forms a dynamic seal effect like an oring from the (maybe) air tumbling generating effect within the groove.

so could you back off on the agressive comments and maybe interpret what is being said in a more expansive way

[Tommy Cookers]

50+ years ago model aircraft engines had 30% oil in the fuel ('diesel') or 20% ? (glowplug)
this had the practical effect of giving enough gas sealing for starting

maybe the mystery groove in such engines today is to enable lower oil % ?
helping gas seal at starting rpm

at 20000 rpm gas sealing was enough without rings ?
(and would/could be eg in F1 at 20000 rpm ?)
eg the rumour mill said that the Honda NR500 originally had no rings
in the early days it needed abnormally high rpm to start (and to keep running)

[johnny comelately]

50+ years ago model aircraft engines had 30% oil in the fuel ('diesel') or 20% ? (glowplug)
this had the practical effect of giving enough gas sealing for starting

maybe the mystery groove in such engines today is to enable lower oil % ?
helping gas seal at starting rpm

at 20000 rpm gas sealing was enough without rings ?
(and would/could be eg in F1 at 20000 rpm ?)
eg the rumour mill said that the Honda NR500 originally had no rings
in the early days it needed abnormally high rpm to start (and to keep running)

the model aero engine i looked at was dry and on turning the propellor by hand it generated seal, would love to know the facts!
interesting about the nr500: i know of a couple of engines (4 strokes) that had no rings and they took 5000rpm to start and couldnt idle much under that

[ChrisDanger]

accumulation grooves, does anyone know the facts on how they work in regard to sealing?

Very long and thorough explanation...

what does the accumulation groove do then?

Are you serious?

Further explanation...

so could you back off on the agressive comments and maybe interpret what is being said in a more expansive way

The "aggressive comment" seemed to me like an appropriate response to your second question, which provided absolutely no context or explanation of the situation you were referring to, clarified in your next post. Just be careful of accusing someone of being aggressive, lest you come across as aggressive yourself.

[ian_s]

a very small model aero engine does not have to cope with the same thermal expansion of components so can run at tighter tolerances compared to a road car engine, and so piston rings become an unnecessary complication.

[johnny comelately]

a very small model aero engine does not have to cope with the same thermal expansion of components so can run at tighter tolerances compared to a road car engine, and so piston rings become an unnecessary complication.

the 4 stroke versions have a ring, maybe the 2 stroke has a higher heat accumulation??

[Brian Coat]

This discussion got me wondering, what is the sealing mechanism of the groove, absent any piston rings?

Like in the model aero engine.

Heres some guessing-out-loud.

I wonder, is it analogous to the mechanism in a labyrinth seal air compressor, which has the same geometry but more grooves.

Crudely, it works on a nozzle-vortex-nozzle-vortex sort of mechanism.

I don't see why this could not apply to a single groove, albeit with reduced effectiveness?

[johnny comelately]

This discussion got me wondering, what is the sealing mechanism of the groove, absent any piston rings?

Like in the model aero engine.

Heres some guessing-out-loud.

I wonder, is it analogous to the mechanism in a labyrinth seal air compressor, which has the same geometry but more grooves.

Crudely, it works on a nozzle-vortex-nozzle-vortex sort of mechanism.

I don't see why this could not apply to a single groove, albeit with reduced effectiveness?

yes Brian, our imagination was that the air (fluid) came down the piston bore gap and as it reached the grove it tumbled forming a votex of sorts.(or maybe it acts like a still solid)
the question is can this happen in a blind end situation?

[gruntguru]

Brian. Agreed - there is someone kicking around a ringless piston design using multiple grooves of specific shape.

In place of the rings, each piston has numerous small, angled grooves, semi-circular at their apex. With the small clearances between them, the movement of the piston creates high-speed eddies -- air pressure working like metal rings to cut leakage and loss during the compression and combustion strokes.

From. http://www.motoring.com.au/news/2013/au ... ings-40773

Not much useful detail unfortunately.

[johnny comelately]

Brian. Agreed - there is someone kicking around a ringless piston design using multiple grooves of specific shape.

In place of the rings, each piston has numerous small, angled grooves, semi-circular at their apex. With the small clearances between them, the movement of the piston creates high-speed eddies -- air pressure working like metal rings to cut leakage and loss during the compression and combustion strokes.

From. http://www.motoring.com.au/news/2013/au ... ings-40773

Not much useful detail unfortunately.

very interesting article.
there may be a risk that as piston bore clearance increased there would be the possibility of torching a channel down the side.
how much reliance is there on the boundary layer and squish band, i wonder.

[johnny comelately]

https://www.youtube.com/watch?v=lOTWx69mghM
this may be of interest re air acting as a fluid