Piston Ring Question

[a. palmer jr.]

Some chainsaws have a 2 ring piston while others have a 1 ring piston. What's the difference between a one ring piston and a 2 ring piston with the bottom ring left out? I've never tried this but am curious..

 

[Marshy]

The two ring pistons usually have thinner rings. That's about all I have to offer. I'm sure the experts can give more.

 

[a. palmer jr.]

If I remember correctly some of the Husqvarnas I've taken apart have one thin ring. Not perfectly sure about it but seems I remember that. Some of the older saws had 2 thick rings also..

 

[Derf]

Mastermind ditches the bottom ring when he ports the 372xp because the ring lands interfere with his port grinding.

Generally, 1 ring has less friction so RPMs are higher, but compression is lower. Two rings have more friction but better compression.


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[blsnelling]

I prefer a single ring due to less parasitic loss due to friction. There is a significant difference.

 

[blsnelling]

Generally, 1 ring has less friction so RPMs are higher, but compression is lower. Two rings have more friction but better compression.

The difference in compression is negligible.

 

[Derf]

Listen to Brad, but now I have a question. If you believe the single vs double piston rings negligible change compression, but significantly cause parasitic loss due to friction, why use two at all?

Oh yeah, two rings are going to be more stable, and over engine life you'll have a longer lasting piston as it isn't banging against the jug (especially at the higher speeds attained without parasitic losses due to friction)


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[blsnelling]

Oh yeah, two rings are going to be more stable, and over engine life you'll have a longer lasting piston as it isn't banging against the jug (especially at the higher speeds attained without parasitic losses due to friction)

That sounds plausible in theory, but do we know that to be true? I've not seen evidence of it.

 

[Derf]

Well if you have any other plausible theories on why the engineers designed them to use two rings I'm all ears.


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[blsnelling]

I honestly don't know.

 

[a. palmer jr.]

I honestly don't know.

I didn't know either but I know that some have one and some have two, some have wide rings and some have thin ones. I suppose when you re-ring a saw you only need replace the top one and leave the bottom one alone unless it's damaged, it should have less tension against the cylinder.

 

[Moparmyway]

I prefer a single ring due to less parasitic loss due to friction. There is a significant difference.

On that note ............... If I just remove the bottom ring from any of my 70cc and up stihls, what should I expect to notice ?

 

[kz1000]

What's the difference between a one ring piston and a 2 ring piston with the bottom ring left out? I've never tried this but am curious..

ONE!

I would think that you are just asking for carbon build up and scoring due to the empty groove.

 

[blsnelling]

On that note ............... If I just remove the bottom ring from any of my 70cc and up stihls, what should I expect to notice ?

I do it all the time. You will see a measurable increase in RPMs and power in the cut.

 

[a. palmer jr.]

Kinda glad I asked the question! Thanks, all!

 

[Moparmyway]

I do it all the time. You will see a measurable increase in RPMs and power in the cut.

 

[blsnelling]

Your question caused me to go do some research. IMHO, the answer is very clear. It's simply for longevity of the engine's life. It's not stability and it's not compression. It might have an effect on heat transfer, but that's not the reason either.

This is how it affects engine life. The top ring of a multi ringed engine always wears more quickly. It sees the most compression, which forces that ring more tightly against the cylinder bore, which in turn causes it to wear more quickly than any ring below it. In a multi ringed engine, you have other rings to make a good ring seal after that top ring is significantly worn. In a single ring piston you don't have that second ring to continue making good compression after the top rings begins to see excessive wear.

Below you will see a lot of posts that explain in more detail.

I have tested performance with both one and two rings on a two ring piston. Nothing else was changed. Compression did not hardly change at all, and I saw a significant increase in performance, due to the loss of parasitic loss due to friction of the second ring.

Now here's were it's a matter of opinion. I'm willing to take that loss in engine life for the added performance. I believe the gains out weigh the loss. I do not believe that most users put enough time on their saws to see this loss of engine life. If they do, it takes only a few minutes to replace a ring and/or piston. It really is just that simple. On the other hand, if I'm building a saw that I know is going to be used in a commercial environment, seeing way more hours of use than a typical saw, I would put both rings in that saw.

For the same size and tension, 1 ring will wear faster then 2 rings. The top ring wears way faster then the bottom ring. In a single ring engine, you don't have the second...

The 2 rings Husky's were to pass EPA a few years back. The single ring pistons are still available.

It's common to see a top ring with an end gap 3-4X the second ring.

The following explains why the purpose of multiple rings is not for ring stability.

When you compress a ring you are not forcing it against the piston! you have to apply force to compress the ring to fit it into the bore even without a piston. That force is absorbed within the body of the ring itself.

While it does transfer its compressive force to the bore, it has only sliding contact with the ring lands. Gas forces can act in addition to the rings own expansion forces but the gas pressure is evenly distributed between the ring and the piston and does nothing to stabilize anything any more than the air on all sides of your body helps to keep you standing upright.

To add lateral stability takes lateral force and since the ring is free to slide laterally in the grooves it cannot transfer force in that direction.

Brinkwolf, I referred to ring groove depth being greater than ring. That is not reference to groove width. This is not "slack" but intenionally so since the rings are not designed to absorb the piston thrust forces due to rod angularity.. Rings are not there to stabilize loose pistons!

It's a real problem in chainsaws also, but two verses one doesn't have any material effect. The skirt contacts the bore in either case.

I tend to agree, rumors say the only reason Husky use two rings on some larger models in the US, is a massive campaign by Stihl some time ago, with hype about 2 rings beeing better.

Husky still use one ring on the same saws elsewhere, as far as I know......

That is a more likely explanation......

All the rings in the world won't cure piston slap. Rings seal the bore. Piston slap is from excessive piston to cylinder wall clearance, measured at the skirt. Now if you were talking about blow-by then I would agree. Two rings or one if it's got at least 100psi of compression it will run.:greenchainsaw: .....454

The combustion pressure mainly affects the exposed top ring, and forces it out into contact with the bore. More pressure, more wear.

some karting background

Hi,

I've followed this thread with some interest. I have a background racing karts, specifically 100cc air cooled sprint karts, and roadracing 125cc shifter karts and most recently 250cc Superkarts.

With the 125cc shifter karts, we are using MX derived engines, typically CR-125 and YZ-125 liquid cooled two strokes. The engines are square....54mm bore x 54mm stroke and use nikasil plated cylinders. The hot 125cc setup was a Honda RS-125 cast racing piston using a single Keystone style ring. This is a GP motorcycle racing piston. We spent many hours on the dyno working on the engines, optimizing them for pipes, digital programmable ignitions (with multiple advance curves) and of course the intakes....all used reed valves.

There were some earlier comments from racing MX guys about high rpm stress on engines, etc. Believe me, compared to a high speed roadracing application (kart or GP bike) the MX bike is more like a tractor.....braap brapp braap on and off the throttle constantly to get around the course. If you look at my data acquistion traces, over a typical lap, the karts spend a LOT of time at WOT and high rpm around a racetrack like Road Atlanta, or Mid-Ohio. I would typically see a drop in power output after a 20 or 30 minute race. Keep in mind that these are 125cc engines water cooled using expansion pipes revving to peak power at 12,500 to 13,000 rpm. Typical competitive horsepower for 125cc shifter motors are in the low 40s, 250cc singles are 65 and 250cc GP twins are 90-100 hp. So I would replace the ring on my 125cc kart motor after about 30 minutes.


Single rings are best for high revving motors to reduce friction. Dual rings improve longevity and heat transfer, and would make sense for a consumer product like a chainsaw where engine life is more important than all out performance. If I was building a high performance chainsaw for competition usage, I would opt for a single ring piston, knowing that I would be changing it more frequently. This assumes Nikasil plated cylinders.

thanks,
-erik

Hopefully that helps.

 

[Derf]

tldr : two rings wear out slower than one

Well it's a theory, but I read a couple in there
1) heat transfer
2) to pass EPA regs
3) marketing campaign that more is better
4) reduce blow-by
5) engine longevity (better compression down the road)

Thanks for the research, Brad, but to answer the question I quote Tootsie Roll :
" the world may never know"




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[blsnelling]

As far as I'm concerned, it's very clear why they often go with two rings. Did you read the whole post?

 

[Derf]

I read most of this one, it's got a lot more info
http://www.arboristsite.com/community/threads/one-ring-vs-two-rings-on-2-stroke-chainsaws.87781/


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