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

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Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #20 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!

Offline Al Smith

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Re: Why Does a Saw 4-Stroke?
« Reply #21 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 .

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #22 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.

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #23 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

Offline mdavlee .

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Re: Why Does a Saw 4-Stroke?
« Reply #24 on: January 02, 2017, 10:39:38 am »
Here's a video with pretty good 4 stroking.
https://youtu.be/O6303vYQzQw

Offline mdavlee .

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Re: Why Does a Saw 4-Stroke?
« Reply #25 on: January 02, 2017, 10:40:50 am »
Pipe outlet muffler same saw.
https://youtu.be/iIfEQXm4vEA

Offline aclarke

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Re: Why Does a Saw 4-Stroke?
« Reply #26 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.

Offline mdavlee .

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Re: Why Does a Saw 4-Stroke?
« Reply #27 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.

Offline Chris-PA

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Re: Why Does a Saw 4-Stroke?
« Reply #28 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.

Offline mdavlee .

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Re: Why Does a Saw 4-Stroke?
« Reply #29 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.

 

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