what limits RPM

[andrethegiant70]

Andy alluded to heat troubles at the bearing and seal sites... I HAVE to think that metallurgy would play a huge role in how hard you could work a two-stroke. You can only get metal to dissipate heat so fast... once you built up enough heat no amount of design enginuity could keep the saw from melting down. It seems to me that part of the challenge is to engineer the metals to achieve higher melting points.

It's interesting to note that current production models are essentially engineering to lose the piston first. I'm sure this is not an accident and is possibly a fail-safe. If the piston fails, the saw loses its ability to pump air and stops, if the cylinder fails, the saw might just grenade. I wonder what would happen if they nikasil-coated the pistons along with the bores?

 

[timberwolf]

They do coat pistons, moly or teflon type stuff on the sides, and ceramic type stuff on the tops. If piston a bore were both equally as hard they would where on each other and the bore would wear out as quick as the piston.

At the same power output saws run cooler at higher RPM, more airflow and fuel being sent in to quench the cylinder more often. Also the cooling system kicks in more as the fan spins faster. So in that way I don't think it's cooling that limits RPM.

 

[wood4heat]

It's interesting to note that current production models are essentially engineering to lose the piston first. I'm sure this is not an accident and is possibly a fail-safe. If the piston fails, the saw loses its ability to pump air and stops, if the cylinder fails, the saw might just grenade. I wonder what would happen if they nikasil-coated the pistons along with the bores?

Something we used to do in the mx world was to ceramic coat the piston crown. I think the theory was keeping the piston cooler made the engine run cooler.

edit: sorry Timberwolf, you musta got in just ahead of me. Must be one of those great minds things!

 

[timberwolf]

The ceramic does block to some extent heat from going into the piston, this reduces the piston swelling up, reduces preignition from a hot piston crown, and it also reduces heat being transfered into the base and heating the incoming charge reducing charge density and power output. Also the more heat that is kept in the cylinder and sent out the exhaust port the higher the BMEP and the more power the engine makes.

 

[Tzed250]

Ok did a little digging and found a cool site:

http://www.vf750fd.com/Joep_Kortekaas/1966.html

In 1966 the specs were as follows
50cc=21,500 rpm 16hp
125cc=21,500 rpm 38hp
250cc=18,000 rpm 60hp

Sorry if I'm derailing, I'm kinda of a Honda enthusiast/geek

I stood right next to the 250 while it was being warmed up for the crowd at the 2005 MotoGP race at Laguna Seca. Quite impressive it was. One of the loudest bikes ever.

 

[timberwolf]

Here is a 365/372 turning between 17,500 and 18,000 RPM.

http://www.youtube.com/watch?v=9tP61NxYNE8

 

[oldbigred]

To sum up what I have heard thus far, rpm is affected/limited by a multitude of factors from type and quality of construction/materials to the ignition and fuel, so I don't think that it can be stated that one specific thing ultimately controls max. rpm.

I know it might be a bit off subject but to me when you start spinning an engine at 18K plus I think longevity and reliablity suffer. Just gonna throw a number out there: At 20,000 rpm the crank is spinning 333 times per second. Now I'm no engineer but how long can bearings hang together at those speeds, I'm willing to guess not very long, comparatively speaking. It was once said that every turn an engine makes is one less it is going to make.

 

[wood4heat]

Here is a 365/372 turning between 17,500 and 18,000 RPM.

http://www.youtube.com/watch?v=9tP61NxYNE8

The coolest thing about that saw is that it feeds the chips back into the cyl to supplement it's power.

 

[Tzed250]

The bearings in a turbine survive at 100,000RPM + for hundreds of hours

 

[wood4heat]

Bearings are killed by loads, heat, and a lack of lube. The longer the stroke and heavier the piston/rod assy the higher the loads on the crank and bearings at any given rpm.

Sure every revolution an engine makes is one less it will make but not all engines are allotted the same amount of revolutions to begin with.

 

[oldbigred]

"The bearings in a turbine survive at 100,000RPM + for hundreds of hours"

Hundreds? Like 200? Just kidding, however we have a John Deere 4020 diesel with 26K hours and its on its original mains (Lower rpms = longer life in my mind). That and turbines are a different story entirely, they are not subjected to the same environment that they are in an internal combustion engine. (Just my opinions)

 

[timberwolf]

The coolest thing about that saw is that it feeds the chips back into the cyl to supplement it's power.

LOL, I guess my air filtration is a little lacking, at least it stops small children from being suck up in there.

 

[stevethekiwi]

ZZZZzzzz im not even going to bother

 

[rms61moparman]

My word!!!! Youall have given some really good answers and all are correct to some extent or other however, The 2 limiting factors (from countless hours of study on this subject) really are these:
1) As was stated before metallurgy. The ability of the connecting rod and its attachments to withstand the forces of acceleration to maximum speed and immediately decelerate to a complete stop, change direction and accelerate again to maximum speed.
2) And the supreme limiting factor in ANY engine as concerns RPM is when the outside diameter of the rotating mass (in the case of a chainsaw engine, the outside diameter of the crankshaft throw) breaks the sound barrier and the inside diameter of said rotating mass hasn't broken the sound barrier due to its lower rotational speed, the resulting continuous sonic boom WILL grenade the rotating mass. Take that to the bank!

 

[oldbigred]

rms61moparman The guys got a point, i read about the same thing with airplane props. Here's a thought, forged aluminim internals??? (for a lighter weight rotating mass) They do it cars/trucks why not saws? or do they already do that and i am just that far behind the times....

 

[timberwolf]

With the circumfirence of the crank on a larger saw being about 1 foot, speed of sound ~1100 FPS and 60 seconds to the min, that gives us about 66,000 RPM before sonic destruction. So thats not the real world limit/

Another angle; air flowing acts as a non compressible liquid up to about 30% of the speed of sound, beyond that it starts to rapidly loose density, so to go faster ports and intake needs to be bigger and bigger to keep the charge velocity down below this threashold, There are physical limitations to just how big ports can be made, once that is reached the engine cant effectivly breath any more air than what it is taking in and that limits the output. Obviousely though this could be ovecome to a point with external supercharging.

 

[oldbigred]

yeah timberwolf has a point too, should have done the math on the supersonic thing. still wonder about forged aluminum internals though...correct me if i am wrong but they would dissapate heat faster but couldnt handle as much heat as steel, but like some have said maybe use a ceramic capped piston to keep crankcase/ lower end heat down. Then it becomes a cost issue though i would guess.

 

[Crofter]

To sum up what I have heard thus far, rpm is affected/limited by a multitude of factors from type and quality of construction/materials to the ignition and fuel, so I don't think that it can be stated that one specific thing ultimately controls max. rpm.

I know it might be a bit off subject but to me when you start spinning an engine at 18K plus I think longevity and reliablity suffer. Just gonna throw a number out there: At 20,000 rpm the crank is spinning 333 times per second. Now I'm no engineer but how long can bearings hang together at those speeds, I'm willing to guess not very long, comparatively speaking. It was once said that every turn an engine makes is one less it is going to make.

Not a lot to argue with here! Different things are limits to a specific engine. As mentioned a heavily built engine could come apart simply by G forces. Another might be limited by air intake, another by its ignition system, another by ring flutter, another by bearing cage failure or as Lakeside mentions, balls or roller skidding. Eventually if you overcome each limitation (the 45,000 rpm glo engine has no rings, and is not spark ignition etc.) flame speed eventually would be a limitation unless fuel expansion transitions to detonation rather than flame propogation and another barrier is pushed back. Stresses dont increase proportionate to rpm., but rather exponentially. No question wear and metal fatigue are serious limitations to practical life span when you start cranking up the revs on a piston engine. Turbine engines are a different ball game and a lot different bearing technology is used.

 

[rms61moparman]

A turbine engine was where the info on the sonic destruction was derived.
I was only trying to point out that if all of the other limitations were overcome, that would still be the stopper. Sorry if I didn't state myself clearly enough.
Mike

 

[timberwolf]

Turbines have no riciprocating mass or transverse axial loading, so a whole different nearing loading situation also.