Transfer port back flow

[RocketRoss57]

The way I see it is that we aren't doing any precision measurements when it comes to ring end gap unless fitting AM custom pistons or rings. So who knows what most saws are running for ring gap. Ive never measured one myself anyways.. I was thinking more along the lines of displaced carbon from the initial ignition burn passing the rings and being deposited into the easiest places for it to build up at. Mostly in my mind would settle in places of turbulence, transfers being a prime candidate.

I'm not pushing my theory but just merely explaining where my train of thought is going.

☆Edited for punctuation and spelling.

 

[Nathan lassley]

Stratos put a charge of fresh air into the transfers before the exhaust stroke gets going, that's why strato pistons have those side slots(not windowed pistons) fresh air goes into the transfers then helps push the exhaust gases out before the fuel charge comes into the chamber.

As for carbon in the transfers the same thing happens in every 4 cycle engine intake too. Not every bit of fuel mix gets pulled from the transfers, that layer that is left bakes on the transfer walls. How much depends on a lot of variables like fuel/oil mix ratio, fuel mix quality, engine design=rpm under load= transfer velocity, temperature, fuel mixture at high rpm, among other things.

 

[Nutball]

See what super techniplate (20% castor) oil does. It baked on all over. Now my question is: does the carbon on the piston increase piston life by supplying a shield instead of wearing on the aluminum? Or does the carbon displace precious oil holding ability? Or both? Either way I don't want all that carbon there.

http://opeforum.com/threads/jonsered-2188-in-for-servicing.17172/

 

[Bigmac]

I really think its cooking and type of oil, build up under the piston crown wouldn’t be from sitting oil turning into carbon. Look at Red Bull’s caned fuel test, same operator different fuel was the main variable and the carbon on the crown is evident on some mixes more than others, and that was 2 gallons, even on Mx bikes this happens with certain oils too. As a side note on my methanol bike carbon is completely nonexistent, 20-1 run hard, motor doesn’t get hot like gas thou, so no cooking happens. On the same engine on gas it would build a small circle on the crown and a brown stain on the underside of the crown but nothing in the transfers or anywhere else. Cooking is my explanation and I think it’s happening when running, and is more prevalent with certain oils.

 

[Bjorn]

I really think its cooking and type of oil, build up under the piston crown wouldn’t be from sitting oil turning into carbon. Look at Red Bull’s caned fuel test, same operator different fuel was the main variable and the carbon on the crown is evident on some mixes more than others, and that was 2 gallons, even on Mx bikes this happens with certain oils too. As a side note on my methanol bike carbon is completely nonexistent, 20-1 run hard, motor doesn’t get hot like gas thou, so no cooking happens. On the same engine on gas it would build a small circle on the crown and a brown stain on the underside of the crown but nothing in the transfers or anywhere else. Cooking is my explanation and I think it’s happening when running, and is more prevalent with certain oils.

Hi, whitch oil are you using in the methanol bike?

 

[Nathan lassley]

See what super techniplate (20% castor) oil does. It baked on all over. Now my question is: does the carbon on the piston increase piston life by supplying a shield instead of wearing on the aluminum? Or does the carbon displace precious oil holding ability? Or both? Either way I don't want all that carbon there.

http://opeforum.com/threads/jonsered-2188-in-for-servicing.17172/

If it starts flaking off in fair size amounts it can do damage to the piston so it definitely isn't helping. It does also absorb some oil.

 

[Bigmac]

Hi, whitch oil are you using in the methanol bike?

Redline methonal mix

 

[drf256]

Hi. A strato cylinder and piston put a charge of air first in the chamber to flush the exhaust out, and then a charge of fuel mlx from the crankcase in the chamber. I dont understand what you meen with backflo in stratocylinder can you explain a little more.

Yes, strato engines put a small amount on fresh air into the scavenging loop first so the any over scavenging that’s lost through the exhaust port is fresh air and less mixed charge.

But, the strato ports never open into the swept volume cylinder area or the crankcase. They only open to the piston/intake flange area/upper transfer. Negative case pressure which pulls intake charge in also creates negative pressure in the transfer tunnels. When the strato port is open to the upper, the upper pulls in fresh air from the strato port so that the first part of the stacked column of charge will contain more fresh air than the rest which is at the correct stoichiometric mix.

The column of charge isn’t homogenous, it’s stratified, hence “strato”.

 

[Bjorn]

Yes, strato engines put a small amount on fresh air into the scavenging loop first so the any over scavenging that’s lost through the exhaust port is fresh air and less mixed charge.

But, the strato ports never open into the swept volume cylinder area or the crankcase. They only open to the piston/intake flange area/upper transfer. Negative case pressure which pulls intake charge in also creates negative pressure in the transfer tunnels. When the strato port is open to the upper, the upper pulls in fresh air from the strato port so that the first part of the stacked column of charge will contain more fresh air than the rest which is at the correct stoichiometric mix.

The column of charge isn’t homogenous, it’s stratified, hence “strato”.

ok, I understand.

 

[Al Smith]

If you cut to the chase it's just pressure flow .If it can't get out it can't get in .With a pressure can muffler once the pressure inside the can reaches a point it equals the incoming transfer flow it stops .If the blow down time is not enough to scavenge the preceeding power charge it will still contain unburned fuel which it only has one place to go,back into the treansfers .It only makes sense it would coke up to some degree .
There's big long formulas related to port sizes related to rpm of which I certainly cannot remember .The crux of them says as RPM's increase the port time area open of the transfers have to be increased .

 

[XP_Slinger]

So back to your point about some saws having residue in transfer ports and some don’t, is that a sign of too much heat? Improper tune? Or hard running? Restricted exhaust causing heat build up in cylinder?

Yes...lol!

I think all the things you listed are possible but there’s also imbalances in port design, not enough blow down etc. if it is in fact carbon from combustion. Otherwise I believe it to be coking of the oil over time

 

[Al Smith]

Some timer ago I visited a certain web forum I will not mention nor ever go to again .However there was an older guy by the name of Art Martin I think from California .He had a souped up 090 Stihl that he claimed ran in the cut at about 9,000 RPM .It evidently in day was the fastest west coaster running the hot saw circuit .He didn't got into specific details but did mention the transfers had reed valves in them which is a rather novel idea .

 

[Nutball]

Some timer ago I visited a certain web forum I will not mention nor ever go to again .However there was an older guy by the name of Art Martin I think from California .He had a souped up 090 Stihl that he claimed ran in the cut at about 9,000 RPM .It evidently in day was the fastest west coaster running the hot saw circuit .He didn't got into specific details but did mention the transfers had reed valves in them which is a rather novel idea .

I thought about reeds in the transfers yesterday, but thought, nope, that's a stupid idea.

Anyway, I also thought about making a plexiglass transfer tube cover with little strings inside, so the flow direction could be filmed, but it would take a high speed camera to do that. Sony has a 960fps camera, but even 960 is a bit too slow to be useful. You'd only get 8-15 images per revolution, which is low precision.

 

[Al Smith]

Adam Clark out in California would put the cylinder in the sink in water and used the hose to shoot water through a cylinder to try and determine the flow . I might mention the portion of the cylinder above the intact is the most difficult to get a clean transfer .Meaning a clean purge of the unburned gasses or residue of exhaust . To remedy or improve that problem many methods have been used ,too numerous to mention .

 

[Bigmac]

To me it’s not a reversion issue, it’s heat cooking, and oil type. Even reed valve Mx two stroke with tuned exhaust will get cooking on the transfer tunnels and buildup even on the connecting rod with the wrong oils, but with other oils spotless.
I have a decent example of a higher our 461 cylinder that died from intake debris ingestion, the transfers are pretty clean, But the fan side is cleaner than the non-fan PTO side, there is also more cooking on the cylinder wall side of the transfer tunnel from the bottom then the outside exhaust wall, this cylinder was only run on orange bottle stihl oil its entire life

fan sidepto sidetranfer inlet cylinder side
And cool side inlet

 

[MG porting]

I always thought it was because when the piston on the down stroke if you're rings aren't sealing correctly that would cause a lower case pressure to allow more blow back into the transfers before the exhaust opens granted there's always going to be some Flo back down into the transfers just for the simple fact that the upper transfers open before the exhaust douse but that is my thinking anyone feel like kicking me in the nadds now feel free it's cold out.

 

[Basher]

I always thought it was because when the piston on the down stroke if you're rings aren't sealing correctly that would cause a lower case pressure to allow more blow back into the transfers before the exhaust opens granted there's always going to be some Flo back down into the transfers just for the simple fact that the upper transfers open before the exhaust douse but that is my thinking anyone feel like kicking me in the nadds now feel free it's cold out.

?

 

[Terry Syd]

.The crux of them says as RPM's increase the port time area open of the transfers have to be increased .

Correct. The transfer port reversion takes place at LOW RPMs. At peak torque (maximum delivery ratio), the transfer flow exactly matches the closing time of the transfer ports. Above that RPM there isn't enough TIME/AREA for all the charge in the crankcase to get into the cylinder. However, below that RPM there is too much time/area and the rising piston begins to suck the intake charge back out of the cylinder.

That is one of the reasons that rotary valve and reed valve engines can have a broader powerband than a piston port engine. The rotary valve can be opened a few degrees before the transfers close, that opening in the crankcase helps prevent a low pressure in the crankcase developing from the rising piston. With reeds, they can respond to the developing low pressure and open.

Almost all two-strokes will have a bit of blowback into the transfer ports when they first open due to residual pressure in the cylinder. The more blowdown, the less blowback. As stated about the newer stratos, the blowdown period is very short (11-12 degrees). This does allow more blowback into the long transfer tunnels. That gives the transfer charge THREE components to charge the cylinder with (exhaust, fresh air and the intake charge)

The first is the burnt exhaust that blew into the tunnel. When a transfer port first opens on a conventional two-stroke, there is much mixing of the intake and exhaust gases from turbulence. With the first part of the strato charge being exhaust gases there is no problem with mixing. That bit of exhaust gas gets the 'freight train' of transfer charge moving. Then the fresh air can join the freight train and finally the intake mixture.

 

[Bigmac]

Correct. The transfer port reversion takes place at LOW RPMs. At peak torque (maximum delivery ratio), the transfer flow exactly matches the closing time of the transfer ports. Above that RPM there isn't enough TIME/AREA for all the charge in the crankcase to get into the cylinder. However, below that RPM there is too much time/area and the rising piston begins to suck the intake charge back out of the cylinder.

That is one of the reasons that rotary valve and reed valve engines can have a broader powerband than a piston port engine. The rotary valve can be opened a few degrees before the transfers close, that opening in the crankcase helps prevent a low pressure in the crankcase developing from the rising piston. With reeds, they can respond to the developing low pressure and open.

Almost all two-strokes will have a bit of blowback into the transfer ports when they first open due to residual pressure in the cylinder. The more blowdown, the less blowback. As stated about the newer stratos, the blowdown period is very short (11-12 degrees). This does allow more blowback into the long transfer tunnels. That gives the transfer charge THREE components to charge the cylinder with (exhaust, fresh air and the intake charge)

The first is the burnt exhaust that blew into the tunnel. When a transfer port first opens on a conventional two-stroke, there is much mixing of the intake and exhaust gases from turbulence. With the first part of the strato charge being exhaust gases there is no problem with mixing. That bit of exhaust gas gets the 'freight train' of transfer charge moving. Then the fresh air can join the freight train and finally the intake mixture.

I agree a small amount of reversion, but I don’t think it’s the main cause of carbon in the transfers, or other places in the crankcase. What do you feel? Carbon under the crown?

 

[Terry Syd]

Carbon under the crown is from a hot piston crown. When it starts getting white, you can expect a hole in the crown. Carbon all over the rest of the crankcase could be from multiple reasons. Sometimes you can see from the deposits that they are a form of 'varnish'. In that case I expect the person was running old fuel in the saw.

In my previous post I tried to differentiate between REVERSION caused by the rising piston dropping the pressure in the crankcase and BLOW BACK caused by residual pressure in the cylinder when the transfers open. Those are two different concepts and I use different words to describe them so there is less confusion.