dual ring pistons vs single ring piston

[crowe]

Hello,
I have recently taken chainsaws a little more serious here as of late and was noticing that my stihl 066 and 046 both have dual ring pistons. I was working on a couple of Huskys a 394 and 365 and noticed thay had only one ring. What are the benefit and drawbacks of having dual ring or single ring pistons. Thanks Wade Huggins

 

[eyolf]

Engineers will say that the top ring contains 99% of the expanding gases, why spend the money for two. Some single-ring proponents even say that their set-up exhibits a slight edge on high-speed, high output engines. Long-time users will maintain that the 2-ring setups last longer between re-ring jobs.

I don't know the answer; some series' saws have been available with both 2-ring and single ring setups...For example Jonsereds supposedly made some 2-ring 630's, which is a hot-rod, high-performance version of the 625, (which is an identical twin sister of the the Husky 61) both of which had (mostly?) single-ring pistons. Doesn't that seem to discredit the performance claim?

Some saws have such short pistons or large cut-aways for mixture transfer that they have no room for two rings.

I imagine the answer may be a little different for every model and make of saw.

 

[rahtreelimbs]

Eyolf is on the mark. I was told double rings seal better and dissipate heat better. It could also be a cost cutting move. One ring being cheaper than two. As far as longevity, I believe it comes right down to how well the saw is maintained. Rich.

 

[Tony Snyder]

Some pretty reliable and high performance saws have had 1 ring at one time or another. If the one ring is well designed it shouldn't be a primary consideration in a chain saw. A second ring means more friction and therefore more heat.

 

[bwalker]

In a two stroke the piston the rings are the main mechanism for heat transfer from the piston the the cylinder walls. A single ring piston will run hotter and not be as durable as a double. Sealing is about the same.

 

[DaveCA]

I'm with BW. Seems all the intelligent books I can remember reading claim the #2 ring is for heat transfer, helps take heat out of the piston.
In my opinion, if a manufacturer thought they could acheive <'their'> goals by eliminating a part ($$$), they would.

 

[tony marks]

an akward question ,but are all huskies single ring.,even the 3120. .
if so just seems to be a difference of opinion among saw makers. im more familar w stihl and its my choice. but i dont hear a lot about these big saws failing in that area. of course i dont keep my ear to the grnd much..

 

[chainsawworld]

before i give an answer i'll just add more theory. one ring=less drag on the cylinder wall=less heat,higher rpms=more power? not if there is more blow-by(won't happen because the burning vapor will send the piston back a you instead of down for power).
two rings=more drag,more heat,higher compression=more power.
i'll start there. marty

 

[bwalker]

an akward question ,but are all huskies single ring.,even the 3120

No, I believe Husky and Stihl only have one or two saws that still use one ring. At one time I think(could be wrong) that husky had quit a few one ring models.

 

[sedanman]

My two cents on the heat issue; The piston gets hotter than just about any other component of the saw. Heat is transferred in two ways for the purpose of our discussion, convection-heat transfer though air, works just not very well. And by conduction- heat transfer from direct contact, works real good just stick you hand in an oven to prove this 350 degree air in the oven won't burn you hand right away but touch the rack and it's a different story. Two rings provide more surface area to conduct heat to the cylinder where the fins and fan can get rid of it. I think the small aditional drag of two rings is offset by the better heat control and tighter sealing of the combustion gases. This is my opinion and I could be wrong.

 

[John in MA]

My Jonsereds 111S came with just one ring (drunk Swede?) when there should be two. Ran OK with the one ring, just not much power. I moved the top ring down and put a new ring in top. Now lots of power and compression, but timing's dangerously off. Fixed those and it's fine.

Just my ring story...

 

[don]

piston ring friction

I never designed a piston ring but in viewing them and some light reading on the internal combustion engine has revealed something. Each ring is designed for a specific task and they can change the tension on two rings to rub "not as hard" on the cyl wall as one ring. Also, they design the two rings to function a different way. Most rings on a piston have a gap to fit over the piston into the grove. Gases leak past this gap. The second ring minimizes this leakage since the gaps in the rings are offset. It should get more torque on a multiringed cyl over a single ring since less gases leak into the crank. Also, the second ring may remove excessive lubrication from the cyl wall (don't know this is the case for an engine with no oil sump) Don't know if (or how) a 2 cyl has a vaccumm in the crank area but it is nescessary for a large (auto) engine.

 

[John in MA]

With the few saws I've ringed, I haven't seen differences like that. They usually have the same part number. OTOH, most four strokes I've seen have compression rings, oil rings, etc.

 

[sedanman]

Don, The crankcase on a 2-cycle engine (as applies to a chainsaw) has a "pulse" it goes from pressure to vaccuum. This pulse is what makes the fuel pump (carb diaphragm) work. With no seperate oils sump, scraping oil off the cylinder walls would serve no purpose as there would be no place to put the oil but out the exhaust where it's going anyway.

 

[don]

vaccumm "pulse"

sedanman, how does a chainsaw create a vaccumm in the crank?
Is is done by chambers cut in the cyl wall?
Or is is done by a vortex created by expansionary gases leaving through the exhaust?

 

[John in MA]

The vacuum is created when the piston travels upward. That causes the fuel/air mix to be sucked in through the reed valves from the carb, or when the intake port is uncovered by the piston. Pressure is created on the downstroke and that blows the mix in the crankcase into the cylinder through the transfer ports/slots on the inside of the cyl. That alternating high/low pressure also causes the fuel pump diaphragm to flap in and out. That's what gets the gas from the tank to the carb.

Kind of a wacky design if you think about it.

 

[tony marks]

just thot id mention. i was talkin to an old timer who said he has seen as many as 6 rings on a piston. he said it kept the piston from wollowing. he was talkin about it, as id ask him what the second ring does. hes a heavy equipment mechanic that doesnt like to work on chain saws. they had him sharpening chains as he was the only one that could do a decent job. and he definitly wasnt happy about it.

 

[WRW]

Marty,
Am I understanding you correctly on your blow-by theory? That the ignited fuel in the crank will send the piston back up the cylinder with similar force to a proper ignition in the cylinder?

 

[chainsawworld]

the purpose of the expanding burning vapors in the combustion chamber is to drive the piston down to create power. the piston is the path of less resistance. (like the barrell of a gun, the breach holds the casing together as the powder burns, the bullet is the path of less resistance so it gets pushed out of the muzzle end of the barrell.) if ignition could take place in the crank, the burning vapors will expand following the path of less resistance pushing the piston up.

 

[WRW]

Marty,
That's a good analogy. What's got me is the similar force. The ignition timing in the blow-by scenario would be pretty much uncontrolled, as opposed to the timed ignition in the cylinder.
The compression should be reduced considerably also. I think the pressure in the crank case is around 2 bar. Surely that's not enough compression to keep the motor turning?