Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: Chris-PA on December 29, 2016, 09:55:33 am

Title: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 29, 2016, 09:55:33 am
This is just an engine nerd post on a snowy morning:

I know that the root cause is a too rich mixture from a carb with no air corrector jets, but what I haven't figured out is why it fires every other revolution.  From what I have seen looking at audio spectrum plots from videos it really does fire every other revolution.  Why?  Why not every 3rd or 4th revolution?  I would have expected it to misfire until the rpms (and air velocity through the carb) drop enough for the mixture to come back into proper range.

If the mixture is too rich and it misfires, what happens different so that it fires again next revolution?  At maybe 10,000rpm the engine cannot possibly slow down much from one cycle to the next, and it's still WOT, so wouldn't it pull just as much fuel?  If so, why isn't it still too rich, and why does it fire the next time?

Is it related to the different pressures in the transfers and case due to the missed firing?  This seems like the most likely explanation, although I still can't see the mechanism.  I can see that since it didn't fire there was little cylinder pressure so the incoming charge might blow out the exhaust easier, or perhaps there is more oxygen in the air pulled back in from the exhaust through reversion - but then again all the air moving through the carb is picking up too much fuel, which was the cause of the misfire to begin with. 
Title: Re: Why Does a Saw 4-Stroke?
Post by: aclarke on December 29, 2016, 11:46:23 am
I always thought it was from ineffective scavenging at high rpm with a given mixture ?  Load the engine in a log with the same mixture and  the scavenging is effective enough at the lower rpm to clear the engine and it smooths out?
Title: Re: Why Does a Saw 4-Stroke?
Post by: Al Smith on December 29, 2016, 01:37:51 pm
https://en.wikipedia.org/wiki/Four-stroking
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 30, 2016, 11:16:53 am
https://en.wikipedia.org/wiki/Four-stroking
Thanks for the link, I never thought there'd be a page on the topic!  There is some good info there that may answer my question and I will read it though more thoroughly later. 

There appear to be some things I'll disagree with too, including the statement "Four-stroking is not caused by an over-rich mixture".  Given that you can make it start or stop only by adjusting the mixture, the claim that it is not primarily a function of mixture is dubious at best.

Also, the claim that "Four-stroking begins gradually" does not match experience.  Four-stroking turns on and off like a switch with maybe a 10% change in rpm.

What appears on first quick read to be missing from the article is a good general understanding of how carbs work, and the fundamental difference between all position carbs and all other types - which just happens to create mixtures that get very rich with minor increases in air velocity.  Still, there may be enough clues to answer why it fires every other revolution, which is what I was curious about. 
Title: Re: Why Does a Saw 4-Stroke?
Post by: aclarke on December 30, 2016, 12:45:24 pm
You can also make it start or stop by loading the engine so I would imagine that by loading the engine you would lower the rpm and also increase the EGT hence changing the scavenging in the process and time area needs in the process??
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 30, 2016, 02:22:27 pm
You can also make it start or stop by loading the engine so I would imagine that by loading the engine you would lower the rpm and also increase the EGT hence changing the scavenging in the process and time area needs in the process??

Yeah, sure, and it's a puzzle because so many of those factors are interrelated, which makes it hard to separate out what causes what.  The thing is that I have saws that will switch between 2-stroking and 4-stroking with very small changes in rpm - it's got to be barely 10%.  It's just hard for me to believe that some of these other effects change so drastically with such small rpm changes, while I know that the fuel mixture can.

2-strokes have been used in many applications that did not use all-position carbs, such as many little cars in Europe at one time.  There are also fuel injected 2-strokes in sleds and boats.  Do these things 4-stroke in the same way as saws?  It seems to me that if they don't, then that would mean it's a characteristic of the fuel system, not of the engine (which is what I suspect).

I spent many years studying, modifying and tuning carbs on cars, and given the way these carbs are made this is exactly the behavior I would expect.
Title: Re: Why Does a Saw 4-Stroke?
Post by: 1manband on December 30, 2016, 04:45:41 pm
thought it was lack of air?
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 30, 2016, 07:10:28 pm
From the Wikipedia article:

Two stroke engines rely on effective scavenging in order to operate correctly. This clears out the combustion exhaust gases from the previous cycle and allows refilling with a clean mix of air and fuel. If scavenging falters, the mixture of unburnable exhaust gas with the new mixture may produce an overall charge that fails to ignite correctly. Only when this charge is further diluted, by pumping through a second volume of clean mixture, does it become flammable again. The engine thus begins to 'fire-and-miss' every second cycle (every four strokes), rather than correctly on every cycle.  Four-stroking begins gradually, so the engine first starts to run with an unpredictable mixture of two- and four-stroke cycles. When severe, this may even become six- or eight-stroking.

"If scavenging falters" is not an explanation or statement of cause.  You're running a chainsaw at maybe 10,000rpm, and something associated with a modest increase in rpm causes a missed ignition - the mixture fails to burn.  The first question is why?  There is no explanation here. 

The next revolution takes 6ms, and the rpm cannot appreciably change in that time.   The throttle is still wide open, and the air coming into the cylinder passed through the same carb.  Regardless of what caused the misfire, what has changed now that it will fire this time?  This was the main question I was trying to get to - I'm already pretty convinced of what causes it to begin with, although I know others don't share that belief.  To my mind it would seem that the lack of a firing means there are different pressures in the cylinder, and pushing back down the transfers, and that somehow this changes the mixture and conditions in the cylinder making it more conducive for firing next time.

Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem. Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging.

And yet chainsaws are loop scavenged, and the effect we're discussing happens at high rpm.

Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse.[note 1]

This is simply an assertion without reference or support.  Note 1 merely discusses block air filters. 

Unfortunately there is little substance to this article and I didn't learn anything from it. 

thought it was lack of air?
Hey Joe!  Lack of air into the cylinder?  What would change about air flow between consecutive firings at 10,000rpm?
Title: Re: Why Does a Saw 4-Stroke?
Post by: RoyM on December 30, 2016, 07:22:05 pm
I have always understood it is a function of speed and fuel/air mixture. The engine speeds up to a point where it runs out of fuel and is overly lean, it momentarily drops back then starts revving again. Once it is under load it doesn't rev high enough to run out of breath.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 30, 2016, 09:48:33 pm
I have always understood it is a function of speed and fuel/air mixture. The engine speeds up to a point where it runs out of fuel and is overly lean, it momentarily drops back then starts revving again. Once it is under load it doesn't rev high enough to run out of breath.
But at 10,000rpm a revolution takes 0.006s, and when it's 4-stroking it will fire every other one.  I have a hard time believing it's actually changing speed back and forth that rapidly. 

Also, if it were going lean at higher rpm that caused it, then how would turning it richer cause it to 4-stroke more?
Title: Re: Why Does a Saw 4-Stroke?
Post by: sharkey on December 30, 2016, 10:30:47 pm
Im with Roy.  I think its more related to piston speed and load.  When the engine is unloaded is when it 4 strokes.  In the case of the older carbs with the harmonic spring flooding, they just wouldnt spin any faster more often than 4 stroke, especially when under load.   
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on December 31, 2016, 02:40:32 pm
Setting aside for the moment the question of why it misfired in the first place, my main question was why does it set up in a pattern of fire-misfire-fire-misfire, etc?  One of the things I saw in the literature that caught my eye was that inert gas in the combustion chamber slows down flame propagation speed (this was in reference to EGR). 

With a 2-stroke there will always be some unscavenged exhaust gases left in the cylinder - unless of course it didn't fire last time.  So that means that after whatever happened to cause a condition where the mix did not fire, after that if it does manage to ignite the flame will propagate faster than otherwise. 

Going back to the original cause, my guess is that at higher rpms the time available to burn the mixture is decreasing while the mixture is getting increasingly rich due to the carb characteristics.  At some point it does not burn completely and there is a misfire - but the next time the mixture burns faster due to the lack of inert exhaust gases in the mix, and it completes the burn in time.  And the pattern repeats.

Also, flame propagation speeds generally get faster with richer mixtures, up until a point and then it falls fast.  This also fits well, because leaning it just a bit moves the 4-stroking point to a higher rpm.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Al Smith on January 01, 2017, 07:33:34 am
I'm not so certain if one could  actually determine by sound if in fact it were firing exactly every other cycle .It appears to but that might not be the entire story of what is happening .

A richer mixture of fuel and air burns at a slower rate than a leaner mixture ,a long established fact .Fact it's one of the basics  of tuning by ear. Now perhaps if saws were electronically controlled  with fuel injectors,knock sensors and variable ignition timing like modern automobiles this age old method would not apply.Weather that will ever happen in the future remains to be seen .

Now along comes the diaphragm carb.,basically the standard for the last 50 some years .A mechanical wonderment which is really a hybrid of a carb and a mechanical fuel injector but it's not perfect .It cannot perfectly respond to varied RPMs in relationship to engine loading so tuning of same goes back to the old adage"when in doubt error on the rich side ".
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 01, 2017, 10:30:44 am
Lean mixtures are known to burn much slower, but mixtures richer than stoichiometric burn a bit faster - to a point.  Richer than 11:1 AFR flame speed drops like a rock.  The graph attached shows that.  That kind of threshold effect is really what I'm looking for, as it matches the observed running characteristic where 4-stroking breaks out suddenly.

Richer than about 9.5:1 AFR you get misfire. 

I have sometimes been able to see 1/2rpm lines in audio tracks from saws that are 4-strokking, but sometimes it's just not visible.  I've also been able to see 1/3rpm spikes from saws on a rev limiter - it's clear to me they cut out 2 out of every 3 ignition pulses.  Looking at a video from an old Stihl red light when it was clearly on the limiter it appeared to be only knocking out every other ignition pulse, and it didn't work well.  The saw could blow right past the limit. 

Diaphragm carbs are simply missing the entire air corrector jet, emulsion tube and fuel well that normally goes between the main jet and venturi on other carbs.  They had to get rid of that because the fuel chamber under the diaphragm is held below atmospheric pressure in order to keep the fuel from spilling out, and so they could not have an air bleed feeding into it.  Those parts have a function, and it is to make the AFR constant as air velocity varies.  Actually by adjusting the fuel and air jets you can make the AFR anything you want as air velocity varies.  Without that system the diaphragm carb is stuck with one curve - it tries to deliver a volume of fuel that is the square of the air velocity.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 01, 2017, 10:42:47 am
So here's my scenario: 

1.  You start with the saw tuned so it is firing evenly at some rpm under load

2.  Then you lift a bit, and the rpm goes up

3.  The air velocity through the carb goes up proportionately to the rpm increase

4.  The mixture gets much richer, passing the 11:1 AFR, and flame speed gets much lower

5.  The time available to complete the combustion burn goes down due to the higher rpm

6.  It misfires

7.  Because of the misfire, there isn't any exhaust gas in the next filling, and flame speed goes up again.

8.  It fires
Title: Re: Why Does a Saw 4-Stroke?
Post by: aclarke on January 01, 2017, 11:56:09 am
All things being the same, wonder if a few more degrees of blowdown would allow the lower temperature exhaust gas out and stop the 4 cycling?
Title: Re: Why Does a Saw 4-Stroke?
Post by: mdavlee . on January 01, 2017, 12:03:31 pm
All things being the same, wonder if a few more degrees of blowdown would allow the lower temperature exhaust gas out and stop the 4 cycling?
That can change how high a saw will 4 stroke.
Title: Re: Why Does a Saw 4-Stroke?
Post by: aclarke on January 01, 2017, 12:14:06 pm
Interesting dynamic.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 01, 2017, 01:38:37 pm
That would fit - a shorter blowdown would push more exhaust down the transfers, probably ultimately trapping more in the cylinder and slowing flame speed.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Al Smith on January 01, 2017, 02:22:40 pm
As a point of debate rather than  a point of argument a leaner mixture again to a point burns more completely or as much as possible.However under loaded conditions it's a shorter burn all things considered .Richer burns at a lower temperature burn longer but again with the right air mixture .Again it takes fuel to make power but at the correct air to fuel ratio

During WW2 the British Spitfires and US P 51 Mustangs ran Rolls Royce Merlin engines which were water cooled .Often they got shot up ,lost the coolant ,ran hotter than a fire cracker but ran in spite of that.A Merlin was one tough old engine .A trick they learned to limp across the English channel full of bullet holes  was to slightly pull  the choke out to induce a richer  mixture allowing them to make it back under decreased power blowing black smoke and drinking huge amounts of fuel .That little trick saved the lives of many US  army air corps and RAF pilots .How many ditched in the channel I have no idea nor do I of those the cylinder temperature rose so high it locked up the engines

Keep in mind the glid ratio of a Mustang at 175 MPH  is  15 to one .If they somehow had enough power left in those shot up engines and could only maintain 1000 feet of altitude and it died on the vine they only had about three miles until they splashed down .Not good .

Now of course all this superfluous trivia has nothing to do with "4 cycling "  ;)
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 01, 2017, 03:19:19 pm
There are surely a lot of complex interactions and I know I've been oversimplifing.   It will always be beyond my capabilities to take it beyond a plausible theory, but that's good enough for what I was trying to do. 

There was a reason I posted this here rather than elsewhere - thanks for the useful discussion!
Title: Re: Why Does a Saw 4-Stroke?
Post by: Al Smith on January 01, 2017, 04:21:05 pm
Let me lay on another factor .At 12000 Rpm the engine would be firing 200 a second .The human ear cannot accurately determine with that frequency of change if the saw is firing every other stroke or every 4 or 6 strokes .It can determine something is different though .
 A little background on myself at this time .In my tour of duty in the US Navy in the submarine service I was a sonar tech.At that time we could by nature of sound determine  propeller turns per minute,type of propeller ,type of propulsion system etc all my ear ..However things like diesel engine RPMs ,number of cylinders and numbers of engines,supercharger speeds,steam turbine speeds  we had to rely on instrumentation to slow the revolving noises and band pass filters to block  certain ambient noises to accurately determine high speed sounds

Again more trivia but the bottom line is perception being "4 cycling " would be accurate in the discussion based on what the ear hears .
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 01, 2017, 05:04:14 pm
This is why I've been playing with audio spectrum plots.  I'll have to look for a better recording of 4-stroking to look at (my saws are not tuned to 4-stroke much), but the one I've attached shows some.  The saw is running at 13500rpm (225Hz), but there is a peak at exactly half that (6750rpm or 107Hz) that shows up just as you can hear it break up in the video.  This saw was running no load WOT, so it is extreme. 

I'll look for a better example later, I gotta clean up for dinner now.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 02, 2017, 12:22:36 am
The link below is a short video clip showing 2.5s of audio track spectrum analysis from 4-stroking chainsaw.  Each frame looks at 380ms of data. 

The saw was someone else's Echo 590 with a muffler mod that was pig rich.  It's 4-stroking like crazy until it get to the widest part of the log, then it stops.

Starting at 6.5s the saw is very rich and 4-stroking.  At 9s the saw has reached the widest part of the cut.  As the wood gets wider the load gets higher and the rpm drops, and at 8s the 4-stroking stops.

After it begins firing every stroke the rpm rises, but not enough to initiate 4-stroking again.

In the first frame the saw is running at 8950rpm (149Hz), but there is a smaller spike at 1/2rpm (75Hz) that is the 4-stroking signature.  It disappears as the 4-stroking stops.

https://youtu.be/wFuBevhUP9A (https://youtu.be/wFuBevhUP9A)
Title: Re: Why Does a Saw 4-Stroke?
Post by: mdavlee . on January 02, 2017, 10:39:38 am
Here's a video with pretty good 4 stroking.
https://youtu.be/O6303vYQzQw
Title: Re: Why Does a Saw 4-Stroke?
Post by: mdavlee . on January 02, 2017, 10:40:50 am
Pipe outlet muffler same saw.
https://youtu.be/iIfEQXm4vEA
Title: Re: Why Does a Saw 4-Stroke?
Post by: aclarke on January 02, 2017, 11:39:11 am
Would be interesting to "tune" a muffler with different length outlet tube lengths based on improving scavenging, hence clearing up the 4 stroking at desired rpm.  Could use Chris's Audio meter to detect smaller changes.
Title: Re: Why Does a Saw 4-Stroke?
Post by: mdavlee . on January 02, 2017, 02:12:27 pm
You probably could do that. Never thought about trying that. I know a shorter piece inside to a certain point makes more power.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 02, 2017, 04:59:57 pm
I looked at the first video of the 2171 which has a couple of short 4-stroke burps, and they all have the same characteristics.  The rpms get high enough and a 1/2rpm spike pops up, only to disappear as soon as the rpms drop again.  They kinda ugly because they are so short, but still clear, and they are exactly 1/2rpm, or every other revolution.
Title: Re: Why Does a Saw 4-Stroke?
Post by: mdavlee . on January 02, 2017, 11:38:30 pm
I could dig and find longer videos but they won't have much more 4 stroking to analyze.
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 03, 2017, 08:36:39 am
I could dig and find longer videos but they won't have much more 4 stroking to analyze.
Thanks, but I think that is good and I'm not sure there is much more to see in it.  I have the one I did the video of where it was pig rich and 4-stroking all the time, except it cleaned up just at the greatest load.  Then there was the 2171 of your saw that was running clean except for a couple of brief 4-stroking moments.  Both looked the same in that you can see the 1/2rpm spike disappear as the rpm drops. 
Title: Re: Why Does a Saw 4-Stroke?
Post by: Al Smith on January 04, 2017, 05:10:37 am
It's all some neat stuff.I'm not so certain a high tech method of tuning a low tech item like a chainsaw would be something most people would do though ,interesting just the same

FWIW I deal with "rocket science'" every day .Robotics,PLC's,RFD tags ,ethernet .It's interesting but frustrating.I find it to be way more complicated than need be .On the other hand at work I get paid well to be frustrated ;)
Title: Re: Why Does a Saw 4-Stroke?
Post by: Chris-PA on January 04, 2017, 08:23:30 am
It's all some neat stuff.I'm not so certain a high tech method of tuning a low tech item like a chainsaw would be something most people would do though ,interesting just the same

FWIW I deal with "rocket science'" every day .Robotics,PLC's,RFD tags ,ethernet .It's interesting but frustrating.I find it to be way more complicated than need be .On the other hand at work I get paid well to be frustrated ;)
Yeah, I was not really thinking of it as a practical way to tune, more just trying to understand what might be happening to make it fire every other time as opposed to maybe every 3 or 4 times. 

And yeah, I'm busy as hell rushing a new product to market.  I just released the files for 5 new circuit boards, and we're starting to get delivery on the case parts, labels, etc.  It's really our first all new design using 3D rendering for the mechanical design.  Things get complicated as hell, and there are a couple of us who have been doing it for 30 years, and the damn parts keeps getting smaller as our eyes get worse.  Mostly I just want to use tools I can relate to like an axe - a chunk of iron on a stick is more my speed these days.