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
what does the accumulation groove do then?gruntguru wrote:"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.
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!Tommy Cookers wrote: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 wrote:accumulation grooves, does anyone know the facts on how they work in regard to sealing?
gruntguru wrote:Very long and thorough explanation...
johnny comelately wrote:what does the accumulation groove do then?
gruntguru wrote:Are you serious?
Further explanation...
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.johnny comelately wrote:so could you back off on the agressive comments and maybe interpret what is being said in a more expansive way
the 4 stroke versions have a ring, maybe the 2 stroke has a higher heat accumulation??ian_s wrote: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.
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)Brian Coat wrote: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?
very interesting article.gruntguru wrote: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.