bbdropshot
ArboristSite Lurker
So why does a saw 4 stroke, and why does it clear up when you put a load on it.
what exactly is saw doing when it 4 strokes
- Thread starter bbdropshot
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My limited understanding of the 2 stroke engine is that it doesn't actually fire on every ignition event. In other words, there are occasions where the piston is coming up from BDC, there's some amount of fresh charge in the combustion chamber ready to go, and yet for some reason (maybe too much unpurged exhaust) the new fuel/air mix does not ignite.
With the momentum of all the rotating components, the crank will cycle around again and maybe the next time will see a combustion event. As the mix gets richer, this pattern may happen more often, so you hear that familiar bop-bop-bop sound (or is it potato potato..oh wait, Harley Davidson patented that one) when you're opening the H needle too far.
With the momentum of all the rotating components, the crank will cycle around again and maybe the next time will see a combustion event. As the mix gets richer, this pattern may happen more often, so you hear that familiar bop-bop-bop sound (or is it potato potato..oh wait, Harley Davidson patented that one) when you're opening the H needle too far.
komatsuvarna
Arboristsite MVP
Id like to know more bout this too. Bump
porsche965
Addicted to ArboristSite
The fourstroking gives me a sigh of relief when deep in wood and lifting to check how she sounds. Usually means safe, and safe means no repairs!
Four stroking is like a moan from the wife at just the right moment, you typically are doing something right!
Saw safe.
Four stroking is like a moan from the wife at just the right moment, you typically are doing something right!
Saw safe.
komatsuvarna
Arboristsite MVP
The fourstroking gives me a sigh of relief when deep in wood and lifting to check how she sounds. Usually means safe, and safe means no repairs!
Four stroking is like a moan from the wife at just the right moment, you typically are doing something right!
Saw safe.
LMAO!!! yeah we get that, but what exactly is the saw doing. And why if its 4 strokin out of the cut does it clear up in the cut? Sounds like itd be the other way around, but i know it aint.
wigglesworth
Booned
Its burbbling
porsche965
Addicted to ArboristSite
I think post #2 makes allot of sense to me.
Also a tach is a great way to remain within specs too. I use both to check each other, the ear and tach. Always try to remain 500 below MAX RPMS to ensure longevity. Never have lost an engine.
Also a tach is a great way to remain within specs too. I use both to check each other, the ear and tach. Always try to remain 500 below MAX RPMS to ensure longevity. Never have lost an engine.
tdi-rick
Addicted to ArboristSite
We've had some good discussions on this in the past.
This is what TW said in response to a post of mine which I think sums it up pretty well.
"I don't think it is a misfire, that is a much more pronounced event.
What I think, though could well be wrong is going on is that as soon as the engine gains enough RPM for the given mixture the piston actually out runs the expanding charge and there is not the same pop when the port opens. As a result full burn and scavenging is hurt and the saw slows ever so slightly though it would not be a total misfire. The next cycle the incoming charge volume will be slightly lower due to losses in scavenging and RPM leading to a slight leaning of the mixture, and with the lower RPM you get a full burn the next time. The cycle then finds a balance of partial burns on 3rd, 4th, 5th... cycles."
http://www.arboristsite.com/showthread.php?t=104072&highlight=stroking
http://www.arboristsite.com/showthread.php?t=109350&highlight=stroking
This is what TW said in response to a post of mine which I think sums it up pretty well.
"I don't think it is a misfire, that is a much more pronounced event.
What I think, though could well be wrong is going on is that as soon as the engine gains enough RPM for the given mixture the piston actually out runs the expanding charge and there is not the same pop when the port opens. As a result full burn and scavenging is hurt and the saw slows ever so slightly though it would not be a total misfire. The next cycle the incoming charge volume will be slightly lower due to losses in scavenging and RPM leading to a slight leaning of the mixture, and with the lower RPM you get a full burn the next time. The cycle then finds a balance of partial burns on 3rd, 4th, 5th... cycles."
http://www.arboristsite.com/showthread.php?t=104072&highlight=stroking
http://www.arboristsite.com/showthread.php?t=109350&highlight=stroking
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jus2fat
Two Harley Fatboys
It doesn't actually 4-stroke---it cant because it's a 2-stroke.
Just give "the sound" of 4-stroking because...it's running RICH mixture.
That's a good thing as "lean condition" can siege (lock up) the piston.
So off throttle...you're running safely rich mixture.
When on throttle...if carb adjusted correctly...mixture will be just right.
Should be very brief interval...second or so...from "blubbers" to "clean".
If it doesn't at WOT...how you run chainsaws...you're too rich.
Got to find the "happy setting"...but better rich than lean.
At WOT you're opening the air supply to carb throat...so all air is let in.
Also NEVER just gun--release...gun--release the throttle trigger....
what---your saw won't idle?? This is Not a guitar solo...you're just putting extra pressure on your clutch bearing...which only turns while you're idling.
Ease up to clutch engage...then WOT...and then cut wood.
Hit that throttle hard and bearing can't release fast enough...just takes a "whollop".
Not good...just add to throttle to engage clutch...then Wide Open Throttle...and CUT !!!
Hope this link helps to explain 2 cycle running although "4 stroke" sound.
http://www.madsens1.com/saw carb tune.htm
J2F
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bbdropshot
ArboristSite Lurker
Ok I think i get it now, when you richen it up to start 4 stroking it's dumping more fuel than the saw can burn an full throttle, because it doesn't actually fire on every cycle cause the piston is turning to fast, but when u put it in a piece of wood and reduce rpm a little it can keep up with the fuel, so it clears up.
komatsuvarna
Arboristsite MVP
tdi-rick That makes sense. At 13+ thousand rpm the spark plug would have to fire close to the bottom of the stroke in order to meet the piston at TDC with the compressed air-fuel mix. And without some kind of mechanical timing advance(weights inside a distributor) that isnt going to happen. At least I think so anyway!
nanuk
ArboristSite Guru
I think most will agree, the engine is getting ignition on every second cycle.
Why? :dunno:
I think engines run better under load when rich, and better lean when no load.
So in my mind, that is whats happening...
but again... why? :dunno:
I'm hoping some of the racers on here will have some answers.
Why? :dunno:
I think engines run better under load when rich, and better lean when no load.
So in my mind, that is whats happening...
but again... why? :dunno:
I'm hoping some of the racers on here will have some answers.
bbdropshot
ArboristSite Lurker
It doesn't actually 4-stroke---it cant because it's a 2-stroke.
Just give "the sound" of 4-stroking because...it's running RICH mixture.
That's a good thing as "lean condition" can siege (lock up) the piston.
So off throttle...you're running safely rich mixture.
When on throttle...if carb adjusted correctly...mixture will be just right.
Should be very brief interval...second or so...from "blubbers" to "clean".
If it doesn't at WOT...how you run chainsaws...you're too rich.
Got to find the "happy setting"...but better rich than lean.
At WOT you're opening the air supply to carb throat...so all air is let in.
Also NEVER just gun--release...gun--release the throttle trigger....
what---your saw won't idle?? This is Not a guitar solo...you're just putting extra pressure on your clutch bearing...which only turns while you're idling.
Ease up to clutch engage...then WOT...and then cut wood.
Hit that throttle hard and bearing can't release fast enough...just takes a "whollop".
Not good...just add to throttle to engage clutch...then Wide Open Throttle...and CUT !!!
Hope this link helps to explain 2 cycle running although "4 stroke" sound.
http://www.madsens1.com/saw carb tune.htm
J2F
Isn't it suppose to burble at wot and not clean up till you put a load on it?
nothing
komatsuvarna
Arboristsite MVP
I know a 4 stroke engine fires ever other time at TDC. Fires at TDC on compression stroke, piston forced to the bottom by the explosion(power stroke), Then back to the top(exhaust stroke, no fire) then back to the bottom again(intake stroke) then back to the top and fire (compression stroke) Iz under the impression that a 2 stroke fires ever time. But i aint for sure.
komatsuvarna
Arboristsite MVP
Yes
tdi-rick
Addicted to ArboristSite
tdi-rick That makes sense. At 13+ thousand rpm the spark plug would have to fire close to the bottom of the stroke in order to meet the piston at TDC with the compressed air-fuel mix. And without some kind of mechanical timing advance(weights inside a distributor) that isnt going to happen. At least I think so anyway!
Not quite, you don't need that much advance
If you did, karts wouldn't run at their 22,000RPM under load.
What Timberwolf described is a partial burn with no load at a given amount of fuel/revs, not an out and out misfire which is how I've described it previously.
Place the engine under load and the excess fuel is suddenly the right amount for the load and revs in the cut.
tdi-rick
Addicted to ArboristSite
re-read timberwolfs explanation that I posted above, it's the best explanation I've seen yet.
komatsuvarna
Arboristsite MVP
Not quite, you don't need that much advance
If you did, karts wouldn't run at their 22,000RPM under load.
What Timberwolf described is a partial burn with no load at a given amount of fuel/revs, not an out and out misfire which is how I've described it previously.
Place the engine under load and the excess fuel is suddenly the right amount for the load and revs in the cut.
Well i kinda do and dont understand. But i still believe a 2 stroke engine fires ever time its at TDC. Maybe thats where the ''4 stroking'' word come from. Sounds like its hittin ever other time. I know just enough to know i dont really know.
tdi-rick
Addicted to ArboristSite
Yep.
very, very briefly, when a two stroke engine fires the expanding, burning gasses obviously forces the piston back down the bore. At a certain point the exhaust port is uncovered and the high pressure gas starts to leave the chamber.
At the same time the intake charge is being compressed in the crankcase as the inlet port has closed and the inlet charge is being forced through the transfer ports which are uncovered at around the same time as the exhaust.
As the incoming mixture continues entering the cylinder from the transfers it helps purge the remaining exhaust out of the cylinder, the piston reaches BDC and starts it's journey back towards TDC, uncovering the inlet port (piston port engine) and the small negative pressure form the expanding volume in the crankcase allows the inlet charge to flow in to the crankcase.
The transfers and exhaust close, the mixture that had already flowed into the cylinder is trapped, it's compressed and the ignition fires the plug just before TDC and so the cycle continues.
