Single ring V.S. double ring pistons

[slipknot]

Just curious here...I know some saws have a single ring(dolmy 5100-s) and some saws have double ring pistons(most efco's) Is there a durability issue here with the single ring pistons v.s. the double ringed pistons? Anyone have some good theories? All you saw techs out there.. here is your chance to sound off your thoughts on the matter!

I am still droolin over a dolmar 5100-s..I learned it was a single ring piston and was wondering if They have durability issues(longevity)??? I would really like to have the fastest production saw (50cc under) but not if it's not gonna last!

 

[NWCS]

i have not had any trouble with single ring pistons like the husky 266. those can be made into one h*ll of a saw with a woods port mod. if the compression ever gets low hone the cyl and slap a new ring in. thats easy work. single ring has a bit less drag. run 40:1 fuel mix and tune the carb a tiny bit rich, she will last and take some abuse. i run all my stock & woods port 2cycles at 40:1. my race saws get something a little special

 

[Freakingstang]

Single rings offer less resitance, but in the long run, the ring will wear a tad quicker than the double rings. Take the 372 for example. it is a good runner. in stock form, 99% of the time, the 371 takes the cake if it is the non epa 371. Like ben said, if it loses some compression over the years, slap a new ring in it and be done. I have many saws with lots of hours that still have the orignal single ring. Most got used harder than I will ever use them before I got them.

Both my 365's were single rings, and I;ve had them for 7 years now, both on the orignal ring.

I wouldn't think too much about it.

-Steve

 

[clearance]

Yeah, the 266XP, took a licking and kept on ticking, I had a couple, worked them all day long, spacing or slashing. One ring, so what, revs up faster, should last a weekend warrior forever.

 

[pallis]

I am still droolin over a dolmar 5100-s..I learned it was a single ring piston and was wondering if They have durability issues(longevity)??? I would really like to have the fastest production saw (50cc under) but not if it's not gonna last!

Double or single ring, it seems a number of other things tend to wear out before the rings. I don't have much time on my 5100s, so all I can say is that it will put a smile on your face. My last small Dolmar, a 110, had so much bad luck over the years it was held together with duct tape, but the engine was still like new when I parked it.

 

[Freakingstang]

The 110's were a screaming saw too. They only had one ring. Kinda wish I still had mine

 

[Four Paws]

Well, I have long been a believer in robust designs. 2 seems better than one, right? But, with so many PROVEN saws having 1 ring, I really wouldn't worry about it. It may seem strange, but a well kept piece of equipment will perform for a long time. Neglect can destroy even the most robust machine.

 

[Lakeside53]

So... is one thick ring better than two thin rings? I doubt it... Not many saws made with one ring now, so there must be a reason.

 

[Four Paws]

All my shindaiwas have 2 rings...

 

[SWE#Kipp]

Both my saws have single ring pistons 346xpg and 372xpg and they work fine. My brother have one of the earliest 242:s I'm guessing it's maybe 10-12 years old with allot of use and abuse, it's a single ring piston saw with high rpm (15500) and still works perfect ,,,

 

[grossm]

well sure, but the rings also function to transfer heat to the cylinder wall - so it seems to me that (not considering engine cooling or cylinder material) the 2 ringed pistons would be better....
that being said, I am sure that any respectable saw maker knows this and if they do decide to use 1 ring, they probably do a good deal of research on the heat capacity of the cylinder wall material - and in that manner eliminate any potential problems. It really does have a lot to do with heat transfer; after all, these are 'heat' engines.:monkey:

 

[Steve128]

problem with my thinking!

I too was a big two ring fan. So much so, it would be a big factor in my purchase decision.

Yet, many outstanding two stroke engines I now own have just one ring. I am realizing that maybe the life of an engine has more to do with maintenance, proper oil and good gas.

 

[buck futter]

shooting some holes.

the 357 and 359 also have a single ring along with the 51/55. It seems stihls are 2 ring and huskys are one ring.

heat transfer? so with all of the aluminum surface area the piston has the cooling is accomplished with the small rings made of some type of steel?
I must respectfully disagree.

I am with the camp that singles work fine.

Buck

 

[Lakeside53]

The aluminum surfaces of the piston do little to transfer the heat to the cylinder... very high thermal resistance as they are barely in contact... It is via the rings...

However, as a prior poster said, assume the manufacturer did the calculations and made the right choice, one ring or two (modern saws only).

 

[buck futter]

the piston itself is separated with one barrier of oil gas mix, I am pretty sure that oil is at least a fair conduit for heat transfer. the rings have two gaps, less mass and the aluminum itself is better than the steel at conducting. also the majority of the cooling is acomplished by the vaporized gas on top of the piston.
I guess I kindly disagree andy, can you give a little more info to prove me wrong. My wife is always reminding me that I make frequent mistakes.:stupid:

oh well,

Buck

 

[buck futter]

Right tree co,
The ring itself has the pressure of expanding gasses pushing it down and out.even more heat. I don't know if more cooling is accomplished on the top or bottom of the piston. The top of the piston would be hotter in my mind due to the compression and direct contact to the explosion.

buck

 

[Lakeside53]

the piston itself is separated with one barrier of oil gas mix, I am pretty sure that oil is at least a fair conduit for heat transfer. the rings have two gaps, less mass and the aluminum itself is better than the steel at conducting. also the majority of the cooling is acomplished by the vaporized gas on top of the piston.
I guess I kindly disagree andy, can you give a little more info to prove me wrong. My wife is always reminding me that I make frequent mistakes.

Heat is transfered both by the rings and the oil/contact of the piston skirt, but it's a matter of degree. The heat transfer by the rings is a primary function, and although simple in principle, it's complex...

It's hard to answer something when you just "known it" forever but can't remember the exact sources.. here's a few hits from a quick search", and there are many more... Probably better example but I can't find them right now.



From lycoming:

Always a motivating factor in fixing a problem is knowing what might happen if it is not fixed. Exhaust gases at the EGT probe run about 1,400 degrees F. Inside the cylinder, combustion gases runs closer to 4000 degrees F. The top of the piston is a large heat absorption surface that would quickly melt if it couldn't conduct heat to the cylinder. The piston's heat conduction path is primarily to the piston rings and there to the cylinder barrel, cylinder barrel fins, and into the atmosphere.


Heat flows thru the piston rings and into the cylinder barrel. If the rings don't make good contact with the cylinder wall they:
overheat
stick
break
score the cylinder wall



The heat path requires a good contact surface between the rings and cylinder wall. A good contact surface also makes a good sealing surface where combustion gases can't leak. Blow-by past the rings means they aren't making good contact and can't conduct heat out of the piston. Consequently, the piston gets hot and starts cooking the oil in the piston rings. Next, the rings stick, once stick you have massive blow-by. Stuck rings have a habit of breaking and then scoring your cylinder wall. Everything gets expensive real quick.

http://www.sacskyranch.com/blow.htm




Here's some "light" reading on the subject...mainly 4 smokers, but..

http://www.engr.colostate.edu/~allan/heat_trans/page1/page1.html


Another interesting article -

http://www.babcox.com/editorial/us/us20114.htm

 

[Lakeside53]

Here's a good explanation for a 2 smoker...

As the piston approaches TDC, cylinder pressures are building rapidly, the piston crown is getting very hot, and the ring is doing its best not to allow any of the pressures to blow past it. Then combustion occurs (hopefully at about 11-14 degrees ATDC). The ring now takes on its role as a heat transfer. The crown temps are very high and rising and if these temps do not get reduced the aluminum piston will surely melt. The ring will try and absorb as much of this heat as it can and transfer it to the cylinder walls where it can be dissipated . Ridding the piston of these high temperatures is a very important function of the piston ring.


The rest of the story can be read at http://www.2strokeheads.com/flat top vs crowned pistons.html

 

[sawinredneck]

Nice posts Andy, very informative!!!! Thank you.
Andy

 

[Four Paws]

Right tree co,
The ring itself has the pressure of expanding gasses pushing it down and out.even more heat. I don't know if more cooling is accomplished on the top or bottom of the piston. The top of the piston would be hotter in my mind due to the compression and direct contact to the explosion.

buck

Many brilliant men have theorized and documented their findings on these exact topics. Follow the link for an explanation of thermodynamics and physics at the various stages (intake, compression, combustion, exhaust) of a gasoline, spark ignition engines. Although the airplane engine described does it in 4 strokes and the chainsaws we all run do it in 2 strokes, the theory is the same. The transfer ports just eliminate the 2 additional, energy consuming strokes.

http://www.grc.nasa.gov/WWW/K-12/airplane/otto.html

To answer the specific heat transfer questions regarding combustion temperatures and radiation vs. conduction these may be more helpful.

http://www.engr.colostate.edu/~allan/heat_trans/page1/page1.html

*EDIT* well, as I was composing my post, looks like Andy beat me to it.