I can't follow this "suck back" thing

Treestuff
shop deals on amazon
Shop deals on ebay
Wesspur
Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
crofter:
definition (in double negative):no negative air flow means no suck back.

to be practical lets say unless motor can eat 1/15th of air in exhaust it's neglible.
i'm guessing now but i very much doubt it's over 1/50th
lot of air on way from exhaust exit to cylinder port, no way dust goes from outside thru exhaust in, dust need to be very close to exhaust port when u start motor

back pressure necesary to make two stroke run good, 4 strokes need some too i think

i have seen dust vibrate up a slope before, so if you very very very unlucky maybe a (heavier than dust) particle could ride the pressure pulses into your cylinder, maybe.

no vacuum suck back though.

when you talking about the airplanes maybe the engine cool too quick air gets denser doesn't get enough fuel? doesn't rotor spin engine too? vacuum from outside air pull charge out exhaust? i think more things at play than a (relatively) stationary engine

that all being said you have a good knowledge of how engines work i'm sure i just told you NOTHING, but i think the term "suck back" should hereby be confined for use in VACUUM CLEANER forums.

or perhaps XXX sites, i dunno,
:pumpkin2:
 
I have been lurking on this post and even though I'm sure a long way from being a rocket scientist, I thought I'd toss something out. There is a principal called Boyles law that says pressure and gas volume are inverse at a constant factor, at a constant temperature.

But what happens when the temperature goes way up at the moment of combustion in a 2-cycle engine?

When the piston is TDC and the air/gas charge fires, doesn't the internal pressure in the combustion chamber go way, way, way up? As the piston descends isn't this very high combustion chamber pressure enought to force the burned exhaust out the exhaust port rather rapidly? Does this really high combustion chamber pressure really drop all the way into the negative range allowing air to flow back in through the exhaust port and muffler (which seems to me is currently very busy handling the removal of some very hot gases under very high pressures)? Then doesn't the rapidly moving piston compress the mixture in the crank case "pushing" and "forcing" it under high pressure through the transfer port back into the combustion chamber? I'm thinking the new charge is pushed into the combustion chamber not pulled up into it by a vacuum in it.

I'm having a hard time trying to figure out how in the world the combustion chamber has a "vacuum" in it with all this high (hot) pressurized gas racing through it so that it would pull air back into it through the exhaust ports/system.

I understand the sound pulsing wave part acting to slow the exhaust. Many 2-stroke motorcyles, go-carts, etc. have specially built "tuned" mufflers for just that purpose.

http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html

http://en.wikipedia.org/wiki/Boyle's_law

I'd sure like to hear from the Stihl, Husky, Yamaha, engine designers about what is really happening on the exhaust side and how they regulate and design for it.
 
Last edited:
DGG said:
I have been lurking on this post and even though I'm not a rocket scientist I thought I'd toss something out. There is a principal called Boyles law that says pressure and temperature are inverse. When the piston is TDC and the charge fires doesn't the internal pressure in the combustion chamber so way way way up? As the piston descends isn't this high pressure enought to force the burned exhaust out the exhaust port rather rapidly? Does this high pressure really drop all the way into the negative range allowing air to flow back in the exhaust (which is handling very hot gases under high compression)? Then doesn't the rapidly moving piston pic up a new charge from the transfer port. I'm having a hard time trying to figure out how the pressure gets so low in the combustion chamber that it would pull air back into it. I understand the sound pulsing wave part acting to slow the exhaust. 2-stroke motorcyles and others have specially built mufflers for just that purpose.

http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html

http://en.wikipedia.org/wiki/Boyle's_law
Click to expand...

DGG; I dont think that what we are looking at is any contradiction to the above conditions. They seem to be indicating operation at fairly high load conditions. Just to assess whether there is ever flow in thru the exhaust port I am looking at the conditions that occur after throttle is closed at high rpm conditions and the flywheel is motoring the saw while the rpm drops to what the idle mixture will maintain. It is questionable whether there is enough charge ingested to even fire every stroke. In any case piston velocity is so high in relation to charge expansion that pressures and temperatures during this time will give much less pressure at exhaust opening than would be the case at full load conditions. Blowdown will be very quick and at the same time a very small charge will be coming up the transfers compared to what would be happening at full load.
Idle conditions also appear to cause a brief period of flow reversal also.

Obviously someone has suggested there is a great gulping suck at the muffler exit on every stroke. I guess that has caught the fancy of someone that has made the very notion of flow reversals anywhere in the system to be a great joke. By dismissing the whole idea I think it throws out some usefull means of understanding what is going on inside a 2 stroke engine.

It has been mentioned that "who gives a shat". Well people that design to comply with EPA and make the best compromises with usability are concerned. People who modify saws for much higher output certainly have to deal with the consequences of the changes made to the design: whether they understand exactly why they have to make the changes they do to jetting, exhaust and intake tuning etc., they still have to deal with them. I still dont see and answer to the question of whether a saw goes rich or lean at full load with a plugged filter. I see a possible connection with exhaust flow reversals and subsequent dilution of charge.

In lots of ways it is a trivial pursuit but I like to understand why. When I screw up I like to know why too. Occasionally it applies to parallel situations.
 
Crofter - I understand what you are saying, it is more a change in rpm's (a decrease) that causes the condition. Isn't the carb still pumping in fuel even though there may be less air mixed with it because the throttle is closed (running rich)? Does the pressure in the crank case really drop down when you cut the throttle or am I getting this wrong?

Trout - Great link. Thanks!
 
Last edited:
Well, I am not sure if this whole suckback thing is even worth discussing. But lets say that it is. Wouldn't any foreign material that was in danger of getting sucked into the exhaust port be blown clear by the exhaust. That force is far greater than the so called "suckback" force, right?:dizzy:
 
Rb in Va, I think you have to worry a lot more about lightening strikes than foreign material ingestion when you are out with your saw. Flow reversal has other more important implications in my mind, but I admit it is cluttered! Lol!:laugh:

DGG Average crankcase pressure will drop. Only the low speed jet will supply and the charge is only what would be necessary to maintain 3000 rpm idle; in the meanwhile the saw might be running 10,000 or more at times on coast down so exhaust pressure will be many times less. Possible not any combustion at all for a short period till rpm drops. Products of combustion likely less even than idling.
 
Confessions of a "SUCK BACK" Victim !

Wading through this whole 5 pages of "Suck Back" has been painful for me. Painful because of the memories it has brought back. I have been a victem of "Suck Back" having suffered through both the Positive and Negative versions of it. The POSITIVE came in the late 50's as I was warming up my fuel burning C-dragster ( 296 ci DeSoto 9sec 162mph). It had 38 in. individual pipes and I could see the pressure pulses moving back and forth as it idled. As a curiously demented boy scientist I explored the possibilites by placing a pebble into the pluse at the end of the pipe and smiled as I watched it move in and out of the pipe in sequence to the idle pulses. I lost the smile when the engine coughed and the pebble didn't come back out. (positive suck back). I still have painful memories of my crews reaction to the experiment!
The Negative "SUCK BACK" occured in the late 70's while hill climbing with my expansion chambered Husky 400. I dropped it near the top of the hill and when I remounted to decend I quickly decovered that a small rock had mysteriously been drawn through the pipe and into the cylinder. Once again my day had been foiled by...............You Guessed It.....SUCK BACK !!!!!!!
Painfull Confessions..........Bob
 
Yeah U rite!!!!

Max said:
:biggrinbounce2: The tale of Sappy lives, for those who remember one of the greatest threads ever!!! :laugh:

Best laugh I had here in months!!!
Click to expand...

We should be able to come here and laugh and blow off a little steam!!!!!!

You can see some funny stuff here!!!!!!!!!!! :monkey: :monkey: :monkey:
 
Last edited:

Similar threads

O
Poulan 3400 muffler mod
Replies
3
Views
375
RedneckChainsawRepair
RedneckChainsawRepair
ace4059
Possible new bad gas. What should I do?
6 7 8
Replies
154
Views
5K
Khntr85
Khntr85
V
Homelite super 1050 back in my hands permanently.
Replies
0
Views
543
vintage_ridin
V
R
Fun story: Stihl 026 won't run with wrong carb
Replies
4
Views
302
Chevboy0167
Chevboy0167
Keith Lee
MS 250 no start.
Replies
14
Views
1K
SteveSr
S

Latest posts

Back
Top