The Mechanics of Four-Stroking

[xwray]

Can anyone describe exactly what is happening in the bowels of a saw when it is four-stroking? I know, whatever it is, that experienced folks can use the phenomena to successfully dial in their saws but what's actually causing it. Is it a case of "it works but don't ask me why"?

Is it just related to the fuel/air mixture and if so, how...must be related to gas flow patterns within the engine, port interaction, "ignitionability", whatever - I have no idea what I'm talking about but there's something going on based on the relationship of mutiple variables...maybe it just can't be explained in simple terms.

Is it just firing every other cycle...doesn't seem like that would be it because that would mean it would be running at half the rpm that it normally would, right? I would think it has to be a more complex mechanism at work, I just can't think of anything that makes sense without raising questions that I can't explain.

My mind's eye is quite blind on this one.

 

[ShoerFast]

It's firing every stroke, but there is not enough O2 in the fuel-air mix to support complete combustion, somewhere in or around 10 : 1 .

What I believe your what we're hearing is the ragged sound of an incomplete exhaust pop, commonly called 'burbling' ?

It would take a very fast pressure/vacuum sensor, but my bet is that the exhaust reading would go very rapidly from an exhaust pressure pulse to a slight negative reading as the exhaust flow was incomplete.

Basically, the fire in the cylinder is going out, before all the fuel is burned.

 

[HittinSteel]

Makes sense. On my modded saws when I hear the 4 stroking, I also see a little smoke

 

[GASoline71]

http://www.madsens1.com/saw carb tune.htm

Gary

 

[tdi-rick]

We had an excellent discussion on this recently on another board with a number of engine builders chiming in.
Shame I can't post a link.

I put forward that I thought it was a partial or full misfire, and this is what Timberwolf replied, which I think answers it better.

"I don't think it is a misfire, that is a much more pronounced event.

What I think, though could well be wrong is going on is that as soon as the engine gains enough RPM for the given mixture the piston actually out runs the expanding charge and there is not the same pop when the port opens. As a result full burn and scavenging is hurt and the saw slows ever so slightly though it would not be a total misfire. The next cycle the incoming charge volume will be slightly lower due to losses in scavenging and RPM leading to a slight leaning of the mixture, and with the lower RPM you get a full burn the next time. The cycle then finds a balance of partial burns on 3rd, 4th, 5th... cycles."

 

[PasoRoblesJimmy]

I never heard the term "Four-Stroking" until after I joined this site.

The burbling sound reminds me of my pre-teen years when I flew model airplanes with 1/2A class gas engines. The little 0.049 c.i. reed-valve engines were controlled by a single high speed needle valve.

After these tiny little engines were started, the needle valve would be closed until the engine speed maxed out and then started to slow down. The needle was then opened back up until it peaked and just began to burble. There were no mufflers on these open-ported engines so the sound was quite loud, distinctive and pronounced.

It took some practice to get the right adjustment and hit the sweet spot.

We also had to be careful not to get our fingers hit by the spinning propeller.

If the adjustment was ok, the engine would speed up and go into full 2-cycle after the model airplane was released into the air. The airplane would then fly with good power and good speed.

If the adjustment was too rich, the engine would continue to burble in flight and the airplane would fly low, slow and sluggish.

If the adjustment was too lean, the engine would sputter and die and the airplane would crash.

The little fuel tanks held only 3-5 minutes of fuel. There was little time to fool around with the high speed needle adjustment.

 

[akennyd]

Another model airplane guy here and I'm confused about why a saw would four stroke before being loaded and then switch into two stroke when in the cut (if I'm understanding the tuning process of saws and I may not be completely).

My understanding with the airplane engines was to tune them a bit more into the rich knowing the engine would pick up rpms in the air as the propeller was able to be turned faster in the air. If you didn't the engine would be running lean in the air.

Seems to this dummy (I really want to understand) that when you load a saw in the cut it would loose rpms and run too rich. Especially if the saw is just into four stroking before the cut.

Kenny
John 3:16

 

[Zombiechopper]

I think the difference is that when you release a plane into the air the load decreases - your hand was holding it back. When you put a saw into wood engine load increases

 

[ShoerFast]

Another model airplane guy here and I'm confused about why a saw would four stroke before being loaded and then switch into two stroke when in the cut (if I'm understanding the tuning process of saws and I may not be completely).

My understanding with the airplane engines was to tune them a bit more into the rich knowing the engine would pick up rpms in the air as the propeller was able to be turned faster in the air. If you didn't the engine would be running lean in the air.

Seems to this dummy (I really want to understand) that when you load a saw in the cut it would loose rpms and run too rich. Especially if the saw is just into four stroking before the cut.

Kenny
John 3:16

Real A/C (4-smokers) are close to the same thing, adjust them to slightly rich during run-up and about rotation speed there right on the money when you have better things to do then readjust the carb.

What is happening to a slight burble as it goes into the cut the engine dose slow down, but the cylinder pressure rises as the piston/crank are held back under a load. Higher cylinder pressures can burn richer air : fuel ratios. And just for grins, with less total timing advance.

 

[Boogieman142]

I think I can sum it all up into 2 words "Rich misfire"

 

[tdi-rick]

I think I can sum it all up into 2 words "Rich misfire"

Re-read post # 5

and PR Jimmy, it was called 4 stroking on this side of the Pacific when I stated karting 22 years ago, and I'm guessing for several decades prior too.

 

[akennyd]

Thanks to zombie and shoer.

I need to re-read and re-think my 2 stroke tuning methods. I just assumed (yes, I know what assume stands for) that when rpms went down the engine richened up, period. I did not take into account the increased pressures.

I need to go home and play with all of my 2 strokes, right now!! I am probably running them a little too lean based on my 2 stroke airplane experience.

Can a stock Stihl be adjusted to run like this with the limit stops on the adjustment screws or will they have to be removed?

Thanks for educating a dummy!!

Kenny
John 3:16