I can't follow this "suck back" thing

[troutfisher]

Run your saw up to 14 thou and take your finger off the throttle. The remaining intake opening will be approx 1/100th of a square inch so there will be virtually no crankcase compression or flow of charge up the transfers.

I agree, I think. When the piston reaches bottom dead center and there is no charge in the crankcase to fill up the cylinder, it will draw from the exhaust port to fill the void?

 

[THALL10326]

Ceramic exhaust bearings not only prevent suck back but the also provide for a 7% increase in cut speed, sound "cooler" and make chicks swoon at the mere sight of you.
Now, send me your money.....:greenchainsaw:

7 percent, hmmmmmmmmmmm, cool. Where do I send the check,:hmm3grin2orange: :hmm3grin2orange: :hmm3grin2orange:

 

[ozflea]

Do you think it would prevent suck back if you were to install a reed valve on your muffler ? opcorn:

Mac had these on the 10-10's so your a l;ittle late ole mate.

Mc Bob.

 

[buzz sawyer]

Sorry Tex, I had to run abit ago. Bout the only thing the saw will suck in is air. Prime example is to remove the muffler all together and watch how the saw reacts, its idle goes way up. Same holds true for cars. Remember the good ole days of hot rods and if you cracked a exhaust mainfold you would burn a valve if you didn't get it fixed ASAP. Same with a saw. There is some sucking going on though many won't believe it. Whether its srong enuff to actually suck in some kind of object is beyond me.

Right you are, Champ. In fact some auto exhaust gas recirculation valves were modulated by the negative exhaust pulse so the correct amount of exhaust was recirculated based on engine speed and load. The length of the reverse pulse is so short I doubt it could draw in anything of concern but then again, why take a chance?

JB233, that sythetic blinker fluid is also great for freeing up seized flenniken pins.

 

[bwalker]

When the piston reaches bottom dead center and there is no charge in the crankcase to fill up the cylinder, it will draw from the exhaust port to fill the void?

At or near BDC fresh mix is screaming through the transfers and into the cylinder blowing residual exhaust and fresh mix out the exhaust.

 

[GASoline71]

1/100th of a square inch??? We're splittin' hairs now... I still don't buy it.opcorn: opcorn: opcorn:

Gary

 

[chowdozer]

I don't buy the suck back idea.
An internal combustion engine is nothing more than an air pump.Whatever moves around up top has to move around down below. The transfers connect the two. Even when the throttle is closed suddenly at high revs, there's still a charge coming in that gets burned and expands. If you got something going in the exhaust port, you got a gay saw. That's an exit! :hmm3grin2orange:

 

[GASoline71]

HAHAHAHAHAHAHAHA!!!... I tried to rep ya.... but have to spread some around first!

Gary

 

[RiverRat2]

Rep tha chowdozer!!!!!

I tagged himm,,, and Bwalkers post regarding fuel charge at BDC is correct BTW!!!!


funny ****!!!!

 

[rreidnauer]

I agree, I think. When the piston reaches bottom dead center and there is no charge in the crankcase to fill up the cylinder, it will draw from the exhaust port to fill the void?

You got it right. Air will take the path of least resistance, with the throttle butterfly closed, and the piston decending, there is a vaccuum formed, and since air is restricted at the carb, the only other path is from the (wide open) exhaust. It won't be much, since as mentioned, some air will go back and forth through the transfers too, in addition to the exhaust.

I use to start the engine on my 1/4 scale plane taking this fact into benefit. I could only turn up the motor with the electric starter if I had the carb at it's full cut-off position. Then, there was no compression, it would spin up, and I'd open the carb and off she'd go.

 

[Ax-man]

Put me on suckback team or as I would like to call it the scavenging part of the cycle. B Walker and a few others have explained it best. This scavenging, suckback thing is a very real part of the two stroke cycle and some four strokes due to valve overlap in the timing. There used to be a vey good animation here on the site showing this scavenging part of the cycle.

The piped saws with the expansion chambers send sound waves back to enhance the scavenging part of cycle to get the most out of the fresh gas mix that ecscapes out the exhaust port, the waves help put the mix back into the jug just as the piston closes the port for the compression side of the cycle. Not the most technical explanation, but works something like this if memory serves me right.

That suckback description is so crude

 

[manual]

I have seen a lot of threads that relate to a two strokes sucking foreign particles back into the cylinder while running. I just can't quite buy it. Crofter seems to be a believer. I would challenge him (or anyone else) to put some colored, but non-harmful powder into a two stroke muffler and show how it gets back into the cylinder. I am not talking about piped saws but the normal muffler or even modded saws. No cheating by tipping the saw and having gravity do it (which is why I think this "suck back" thing got started).
I have been wrong before but like to see some real test before jumping on the band wagon. Now having said this, I am truly a believer in "spit back". I can observe the clowd of vapor behind a non-filtered carburetor. Some of the big old 125 Mccullochs even had a type of filter to catch it. The 066 Stihls do it also. Mike

start researching the 575XP. You will soon find your answer.

 

[Monkeyhanger]

Just stuff your muffler full of 3M scotchbite pads. That should take care of any suckback issues.

Hi,

I tried hammering wine bottle corks into the exhaust but It didn't work out as well as I hoped so I'm still working on my anti-suckback solver...

Bye

 

[gdn]

A lot of explanations and hype on Ebay sure are a joke. however, if you dont think there can be reverse flow thru the exhaust port try removing your muffler entirely and see how it affects your saw. See how much you have to richen the carb to get the idle back down from a chain dragging 4 grand idle. Is that because clean air instead of escaping charge is coming back in to mix with the next charge. Now imagine another scenario. Run your saw up to 14 thou and take your finger off the throttle. The remaining intake opening will be approx 1/100th of a square inch so there will be virtually no crankcase compression or flow of charge up the transfers. As the momentum of the crank and flywheel forces the piston up and down it will displace air in and out the exhaust port and up and down the transfers till the rpm drops back down to where the idle circuit is supplying enough mixture to recommence firing. A similar but much longer duration motoring of a two stroke engine occurs in ultralight two stroke aircraft on long power off descents and has caused failures from lack of lube and running lean combined with cold squeeze in the winter. (You know what they recommend? blipping the throttle! Lol, but that is another issue)
I think that under sustained load there is darned little if any flow reversals through the exhaust, but at idle or coast conditions .................?

hold a piece of paper to the exhaust when saw is revving down and see if it wants to stay there, for sure air might want to (and does)pulse in and out between the cylinder and exhaust but i very much doubt negative air flow - if the throttle being closed is causing the problem no way is air going to want to go past it back out the air filter...

 

[Crofter]

First lets get our ideas together if we are going to examine the idea seriously. Dont jump to conclusions. First thing is we are talking about a crankcase scavenged 2 stroke engine 4 stroke is a different scenario. next we are talking about momentary pulsing flow reversals not continuous flow in the opposite direction. I think most likely at idle and decelleration though pershaps other conditions. Closed throttle area of .01 of a square inch is very close. Charge will not be rushing up the trasfers when the engine ios coasting down at high rpm with the throttle closed. It is likely base pressure will be varying negative pressures unless the base can suck some air down the transfers to compress. As the piston rises from the bottom of the exhaust port it is displacing air and the exhaust port is wide open above it. if there is not the usuall high base compression forcin up the transfers some air may be sent back down but most will be sent out the exhaust till the piston rises to cover the exhaust port and cylinder compression will start. If there is suffiently rich (undiluted) charge it will fire but there will be very litte exhaust pressure when the piston uncovers the exhaust port on the power stroke. As the piston descends it is creating vacumn above and the wide open exhaust port is right there. Because there was no charge available to be drawn into the base thru the closed throttle the piston will be only reducing vacum in the base as it descended and not base compression therefore there will not be sufficient flow up the transfers to balance the vacumn created above the descending piston so there will be flow in thru the exhaust port during the last part of exhaust duration. Now this is not the usual sustained load operation but the assumed closed throttle coast down from high rpm and the engine being windmilled by the flywheel. That is the situation where I think the most definite flow reversal is. The animations for the strato charged engines show flow reversals under other conditions I believe.

 

[rreidnauer]

YES! Perfectly described.

 

[ChuckinOhio]

Ya know what Frank

You have made me a believer, your explanations are very well put forth and easy to understand.

I know that over time ingesting fine particles will cause wear on the internals, what size matter do you believe that the saw would be able to ingest? Would the size of said matter be in proportion to the size of the saw?

Maybe those spark arrestors serve a dual purpose?

Do you think that any foreign matter would have to be present inside the muffler housing to be ingested, or would the saw be capable of ingesting it from the outside atmosphere.


Chuck

 

[troutfisher]

I believe the exhaust port does pull air back in, in short pulses. However, When the exhaust is dumped into the muffler, the pressure in the muffler rises. The outlet in the muffler is a restriction, and the pressure in the muffler drops as the exhaust exits the muffler. I think the next exhaust charge is dumped into the muffler before it can completely get rid of the last one, therefore there is always positive pressure in the muffler, and a muffler will not "suck back". The exhaust port in the cylinder will. So, a chainsaw will not suck anything into the muffer, but any foreign object inside the muffler most likely will get sucked into the cylinder.

 

[Crofter]

I believe the exhaust port does pull air back in, in short pulses. However, When the exhaust is dumped into the muffler, the pressure in the muffler rises. The outlet in the muffler is a restriction, and the pressure in the muffler drops as the exhaust exits the muffler. I think the next exhaust charge is dumped into the muffler before it can completely get rid of the last one, therefore there is always positive pressure in the muffler, and a muffler will not "suck back". The exhaust port in the cylinder will.

I agree. The amount of restriction and the volume of the muffler to engine displacement, plus load condition will likely be the determining factor of whether there will be reversal pulses at the muffler outlet. That there is in and out flow at the exhaust port itself I think is more definite. For sure on an operating saw the net flow is out but I suppose if there were dirt swirling around between the log and the muffler front some could get directed back in at the flow lulls. I would be more concerned about stuff getting in like mud daubers or wind blown dirt when the saw wasnt running so I like a screen. I think accepting flow reversal pulses at the exhaust port helps to understand how the exhaust system affects charge composition thru dilution and will be key in deciding how a clogged filter can fry a saw. The short stub of an exhaust with an opening much larger than the exhaust nozzle that timberwolf used in some of his muffler experiments gave more power and better operation than no muffler at all. Unless it was containing and directing some of the escaped fuel charge back in I cant explain why it should be a benefit. Momentary exhaust port flow reversals seem counter intuitive but I feel it happens.

 

[ChuckinOhio]

Once again, that was easy to understand!! Thanks

Chuck