spacemule
The Peanut Gallery
bwalker said:
I just calculated the max piston speed of a Husky 3120 with a 1.640625" stroke to be ~58.57 mph at 12,000 rpm.
How would you like to go from 0-60-0 400 times per second?
2 ring piston ?
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I just calculated the max piston speed of a Husky 3120 with a 1.640625" stroke to be ~58.57 mph at 12,000 rpm.
How would you like to go from 0-60-0 400 times per second?
Crofter
Addicted to ArboristSite
Ben, piston speed could become an issue on some of the longer stroke saws when you start to modify them to the higher limits. As you get nearer to the piston speed threshold, maintaining gas pressure is more important in keeping the ring pressed to the bottom of the groove to resist the G forces. Here one is better. I dont think that thicker would be the choice for that scenario. I think your mind is made up that two is better, but you aren't really sure why! Personally I don't know. My mind is still open. I still remember someone adamantly insisting bores were not designed to be round.
Molucule, Like I said earlier dual ring pistons rule the roost.
I agree 100%. the problem is most 1 ring pistons have thicker than normal rings..At least the handfull I have laid eyes on.
Frank, Was that really necessary? Trying to hurt my credibility by dredging up old dirt? Fact is I was wrong in that I misread one of the refferances I had at the time and I freely admitted it on a open forum. I WAS WRONG. So what? I like any one else am not from from error.
Whats ticks me off is when persons will argue for the sake of argument. By your own admission, Frank you dont know what your talking about so why argue?
And FWIW I havent made up my mind which design is better. Both have advantages. I just believe based on what I have gathered that a two ring design is superior for a non race motor and in many cases superior for a race motor.
BTW I dont mean to sound so condesending, but go do some research on your own and see what you find....and I am not talkking about drivil read on a BB..
Crofter
Addicted to ArboristSite
As soon as the ring starts to actually break contact with the lower surface of the groove and start to do the slamming up and down, the game will be over in minutes or maybe sooner. Combustion gas starts to escape past the ring and no longer presses it outward to be cooled by the cylinder wall. I think the figure is something like 120 or 130,000 G's I dont know how many feet persecond or average MPH this equates to. It is hypothetical to be sure for our saws, but you fellows with bikes can push a motor over the red line quite easy if you aren't paying attention.
coefficients of expansion ...
because of their annular construction, rings don't *effectively have a cooefficient of radial expansion. As they heat up, the expansion would be longitudinal-circumferential, around the ring, and not transverse, like across the top-diameter of a piston as it changes temp. The piston would change diameter, as would the cylinder walls, which being a different material, would probably expand at a different rate, have a different coefficient of expansion than the piston. If a ring (or rings) is the only thing providing the seal between these two changing elements, then, up to a point, wouldn't a thicker ring system make a better seal? (On the other foot, it might also be that the thicker the ring, the longer it would take for it to "find its seat" during break-in (there being nothing manufactured perfectly round in the universe). This would mean that a system with 2 thin rings might have a faster break-in period than a system with 1 thick one, though both would provide the same resistance to pressure bypass.)
because of their annular construction, rings don't *effectively have a cooefficient of radial expansion. As they heat up, the expansion would be longitudinal-circumferential, around the ring, and not transverse, like across the top-diameter of a piston as it changes temp. The piston would change diameter, as would the cylinder walls, which being a different material, would probably expand at a different rate, have a different coefficient of expansion than the piston. If a ring (or rings) is the only thing providing the seal between these two changing elements, then, up to a point, wouldn't a thicker ring system make a better seal? (On the other foot, it might also be that the thicker the ring, the longer it would take for it to "find its seat" during break-in (there being nothing manufactured perfectly round in the universe). This would mean that a system with 2 thin rings might have a faster break-in period than a system with 1 thick one, though both would provide the same resistance to pressure bypass.)
Mule,a F1 engine might have a maximun piston speed of over 45 m/s which is almost twice the speed of the numbers you posted for the 3120. keep in mind this is not mean speed, but maximun piston speed. I do not know which way your figured it?
Crofter if flutter becomes a issue in ver high rpm race engines the pistons can be gas ported. Some f-1 teams did this while using copper/ berylium rings. Of course those engines dont last long and gas ported pistons shred rings.
Crofter if flutter becomes a issue in ver high rpm race engines the pistons can be gas ported. Some f-1 teams did this while using copper/ berylium rings. Of course those engines dont last long and gas ported pistons shred rings.
Crofter
Addicted to ArboristSite
"the problem is most 1 ring pistons have thicker than normal rings..At least the handfull I have laid eyes on."
What do you mean you dont wish to be condescending? That is exactly what you mean to be! Lol. Ben I am just getting at you being so darn sure in the face of still conflicting information. That is cute that you are ticked off by people who will argue just for the sake of arguing! Lol! As far as not knowing what I am talking about and thus being unfit to present things for discussion Ben I wouldnt worry about it. I served an apprenticeship and wrote my Motor Vehicle mechanics license in 1967. I am pretty out of date on recent autos but I have no problem discussing or absorbing points of mechanical theory in general. You seem to think no one should disagree with you. The nerve of them eh?
What do you mean you dont wish to be condescending? That is exactly what you mean to be! Lol. Ben I am just getting at you being so darn sure in the face of still conflicting information. That is cute that you are ticked off by people who will argue just for the sake of arguing! Lol! As far as not knowing what I am talking about and thus being unfit to present things for discussion Ben I wouldnt worry about it. I served an apprenticeship and wrote my Motor Vehicle mechanics license in 1967. I am pretty out of date on recent autos but I have no problem discussing or absorbing points of mechanical theory in general. You seem to think no one should disagree with you. The nerve of them eh?
spacemule
The Peanut Gallery
Ok, I'll show you how I calculated and you can point out my error. It's a simple calculation.bwalker said:
1.640625" stroke x pi = 5.1541754473"/revolution.
5.1541754473"/revolution x 12,000 rpm = 61850.1053676"/minute.
61850.1053676"/minute / 12 = 5154.1754473'/minute.
60mph = 5,280'/minute.
5280/60 = 88
5154.17544743/88 = 58.5701755375 mph
Note that this is the angular velocity of the crankpin. The max speed of the piston can never be greater than the angular velocity of the crankpin. I'd have to do an integral to figure average piston speed. As far as calculating the 0 to 60(rounded up from 58) to 0 mph 400 times/second, that's easy too. 2 start/stop cycles per revolution. So, 12,000 x 2 = 24,000 times per minute . 24,000/60 = 400 time/second. Where's my mistake?
Spacmule,I wasnt saying you where wrong one way or the other.
Mean speed is whats often used and the formula is as follows. stroke X 2 X revs per sec
To figure max speed you need the rod length and a few other variables.
Frank, I think its cute your so sure, Im sure....
Mean speed is whats often used and the formula is as follows. stroke X 2 X revs per sec
To figure max speed you need the rod length and a few other variables.
Frank, I think its cute your so sure, Im sure....
Crofter
Addicted to ArboristSite
"Frank, I think its cute your so sure, Im sure...."
You'll have to clarify that for me Ben,
Ben you wouldn't be so much fun to argue with if you didnt get so irate when someone questions your opinion. You call people who hold a different opinion morons and such. You sure deserve points for determination though.
You'll have to clarify that for me Ben,
Ben you wouldn't be so much fun to argue with if you didnt get so irate when someone questions your opinion. You call people who hold a different opinion morons and such. You sure deserve points for determination though.
I called the husky salesman that made this remark a moron " have also been told by a Husky rep two ring is old technology and and a good single ring setup seals just as well." and I stand by it. Off course if huskys where all two ringers they same guy would be spouting off how great they are.
BTW I have never got irate on this board and never will.
BTW I have never got irate on this board and never will.
John Stewart
ArboristSite Guru
ehp said:
Crofter
Addicted to ArboristSite
Ben when I first stasrted seeing one ring pistons it did seem strange. I have changed a lot more rings on diesels and it seems they had about 17 to get the job done! No that is exaggerating. Even that is changing now and there are better ring materials. Really high speed two strokes (where oil control is not a factor) moves the design theory into some whole different areas where different factors altogether control the limits. An issue like this with the number of rings really piques my curiousity. I am the more intrigued if there are contradicting factors to what should be the obvious answer. I like to try to get them all on the scale. I can get testy when I perceive someone is trying to force a conclusion when I feel the evidence is not all in. And Yep! I do like to argue!
OK, Franks lets approach this on a point by point basis then. Does a single ring piston seal as well as a dual ring piston? What say you?
Crofter
Addicted to ArboristSite
without any reference to rpm I would have to say no.
Now which ring system will seat in quicker?
Now which ring system will seat in quicker?
Gypo Logger
Timber Baron
I believe it was Stihl that promoted the two ring design as part of the EPA thing so as to retain as much oil on the cyl.wall inspite of the thin 50:1 mix, however, it was Husky that realized that not only would a single ring do the job better, but would create more power when a 32:1 mix was used.
Yup, that's what it is.
John
Yup, that's what it is.
John
Crofter
Addicted to ArboristSite
John, I could see that as a very good possibility. The area between the two rings would be somewhat shelterd from the combustion and also the wash of the raw mix. I think it would accumulate a good oil film.
EPA dictates? The mixture in the space between the top of the upmost ring and the top of the piston does not burn and contributes to smoggy exhaust. Moving the ring higher reduces this, but puts the ring into a higher operating temperature. That is one tradeoff for epa concerns.
EPA dictates? The mixture in the space between the top of the upmost ring and the top of the piston does not burn and contributes to smoggy exhaust. Moving the ring higher reduces this, but puts the ring into a higher operating temperature. That is one tradeoff for epa concerns.
Ring seating is a function of cylinder material, ring material, load, and the type of hone job done on the cylinder. I dont think the break in time would be much if at all differant, but I dont really know.
In regard to the EPA and ring number and placement. Saws havent got the point emmisions wise where high ring placement is needed. The approach so far by the saw companys to meet the phase 1 specs was to lean the saws out and restrict the muffler like Stihl or to come up with a efficiant cylinder design and lean it out like Husky. the reason you see two rings on Husky coming to the states is to deal with the extra heat created by the lean fuel/air ratios.
Frank, This might happened if a large amount of liquid fuel where flwoing through the engine, but its not...At least not in a engine that is operating properly. The fuel is in gaseous state(or a very large % of) when the engine is warmed up and under load.
design process ?
seems the target bhp and rpm for the entire design might be the first element in the design of a piston top. For a given constraint for maximum available piston stroke and height, the 1-ring might be the only available solution, assuming a minimum separation between wrist pin opening and ring groove. If height is not a constraint, then might not piston mass weigh in as a consideration, where for a given bore, rod strength, etc, a 1-ring piston would have lower inertials than a 2-ring system.
I was wondering about the example of two Dolmars which are dissimilar in design but similar in output: the 120i piston has two rings, running in a 68cc engine (49mm bore x36mm stroke ) cranking 3.6Kw at 12500 rpm. the 6400 piston has one ring, running in a 64cc engine 6% smaller (47mmx37mm, e.g. +1 mm 3% more stroke) cranking 3.5Kw at 13000 rpm. Also, I don't know about motorbikes ... do they run at 10K +/- rpm under load ? ... also, if they operate at a lower rpm, then they have lower inertial considerations and could increase the structural mass of the piston top to add a second ring, and if they have fewer height constraints, that would also "grant" them that 1-mm leeway to add a ring
seems the target bhp and rpm for the entire design might be the first element in the design of a piston top. For a given constraint for maximum available piston stroke and height, the 1-ring might be the only available solution, assuming a minimum separation between wrist pin opening and ring groove. If height is not a constraint, then might not piston mass weigh in as a consideration, where for a given bore, rod strength, etc, a 1-ring piston would have lower inertials than a 2-ring system.
I was wondering about the example of two Dolmars which are dissimilar in design but similar in output: the 120i piston has two rings, running in a 68cc engine (49mm bore x36mm stroke ) cranking 3.6Kw at 12500 rpm. the 6400 piston has one ring, running in a 64cc engine 6% smaller (47mmx37mm, e.g. +1 mm 3% more stroke) cranking 3.5Kw at 13000 rpm. Also, I don't know about motorbikes ... do they run at 10K +/- rpm under load ? ... also, if they operate at a lower rpm, then they have lower inertial considerations and could increase the structural mass of the piston top to add a second ring, and if they have fewer height constraints, that would also "grant" them that 1-mm leeway to add a ring
