Author Topic: Why Does a Saw 4-Stroke?  (Read 184 times)

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Offline sharkey

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Re: Why Does a Saw 4-Stroke?
« Reply #10 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.   

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #11 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.

Offline Al Smith

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Re: Why Does a Saw 4-Stroke?
« Reply #12 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 ".

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #13 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.

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #14 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

Offline aclarke

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Re: Why Does a Saw 4-Stroke?
« Reply #15 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?

Offline mdavlee .

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Re: Why Does a Saw 4-Stroke?
« Reply #16 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.

Offline aclarke

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Re: Why Does a Saw 4-Stroke?
« Reply #17 on: January 01, 2017, 12:14:06 pm »
Interesting dynamic.

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #18 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.

Offline Al Smith

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Re: Why Does a Saw 4-Stroke?
« Reply #19 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 "  ;)

 

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