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#Post#: 75125--------------------------------------------------
Why Does a Saw 4-Stroke?
DIR By: Chris-PA
Date: December 29, 2016, 8:55 am
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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.
#Post#: 75126--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: aclarke
Date: December 29, 2016, 10:46 am
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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?
#Post#: 75129--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: Al Smith
Date: December 29, 2016, 12:37 pm
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HTML https://en.wikipedia.org/wiki/Four-stroking
#Post#: 75140--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: Chris-PA
Date: December 30, 2016, 10:16 am
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--- Quote from: Al Smith link ---
>
>
HTML https://en.wikipedia.org/wiki/Four-stroking
>
--- End Quote ---
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.
#Post#: 75145--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: aclarke
Date: December 30, 2016, 11:45 am
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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??
#Post#: 75148--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: Chris-PA
Date: December 30, 2016, 1:22 pm
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--- Quote from: aclarke link ---
>
> 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??
>
>
--- End Quote ---
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.
#Post#: 75154--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: 1manband
Date: December 30, 2016, 3:45 pm
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thought it was lack of air?
#Post#: 75160--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: Chris-PA
Date: December 30, 2016, 6:10 pm
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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.
--- Quote from: 1manband link ---
>
> thought it was lack of air?
--- End Quote ---
Hey Joe! Lack of air into the cylinder? What would change
about air flow between consecutive firings at 10,000rpm?
#Post#: 75161--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: RoyM
Date: December 30, 2016, 6:22 pm
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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.
#Post#: 75164--------------------------------------------------
Re: Why Does a Saw 4-Stroke?
DIR By: Chris-PA
Date: December 30, 2016, 8:48 pm
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--- Quote from: RoyM link ---
>
> 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.
>
--- End Quote ---
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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