Author Topic: Why Does a Saw 4-Stroke?  (Read 161 times)

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Offline Chris-PA

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Why Does a Saw 4-Stroke?
« 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. 

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Offline aclarke

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Re: Why Does a Saw 4-Stroke?
« Reply #1 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?

Offline Al Smith

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Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #3 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. 

Offline aclarke

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Re: Why Does a Saw 4-Stroke?
« Reply #4 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??

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #5 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.

Offline 1manband

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Re: Why Does a Saw 4-Stroke?
« Reply #6 on: December 30, 2016, 04:45:41 pm »
thought it was lack of air?
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Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #7 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?

Offline RoyM

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Re: Why Does a Saw 4-Stroke?
« Reply #8 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.
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Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #9 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?

 

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