Part Three: The Transfer Ports

[RIDE-RED 350r]

How would it pull without it? The escaping gases burn the oxygen and created a very strong vacuum. Something has to fill the void.

Maybe I'm not picking up what you are putting down Jim, but combustion itself, the force that drives the piston downward is rapid expansion, a near instantaneous and massive increase in pressure. That expansion pressure is why we need a certain amount of blowdown, to allow that pressure to be vented out the exhaust enough to allow the transfer charge that is being pushed up from the case by the downward movement of the piston to enter the combustion chamber. That pressure wave is what a tuned expansion chamber pipe utilizes to "stuff" some of the transfer charge that escapes out the exhaust back into the cylinder. I still don't see a vacuum above the piston playing a part, or existing for that matter. In my mind, the only vacuum generated or pulling action that takes place in the entire 2-stroke run cycle is in the crankcase through the intake port. From there it seems to me that everything is pushed, by pressure.

 

[Chainsaw Jim]

Maybe I'm not picking up what you are putting down Jim, but combustion itself, the force that drives the piston downward is rapid expansion, a near instantaneous and massive increase in pressure. That expansion pressure is why we need a certain amount of blowdown, to allow that pressure to be vented out the exhaust enough to allow the transfer charge that is being pushed up from the case by the downward movement of the piston to enter the combustion chamber. That pressure wave is what a tuned expansion chamber pipe utilizes to "stuff" some of the transfer charge that escapes out the exhaust back into the cylinder. I still don't see a vacuum above the piston playing a part, or existing for that matter. In my mind, the only vacuum generated or pulling action that takes place in the entire 2-stroke run cycle is in the crankcase through the intake port. From there it seems to me that everything is pushed, by pressure.

The exhaust is still escaping when the transfers crack open. If the motor relied solely on case compression then you'd only need a little wt carb for an 066.

 

[paragonbuilder]

The exhaust is still escaping when the transfers crack open. If the motor relied solely on case compression then you'd only need a little wt carb for an 066.

I think I get it now...



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[CJ Brown]

I think I get it now...



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I don't...

 

[Mastermind]

He's talking about the scavenging effect. As the exhaust is expelled it's inertia or momentum leaves a low pressure area behind it.......that pulls on the transfer ports.

 

[paragonbuilder]

The crankcase compression only reaches like 10 psi. If you think about the engine running at 50-250 revolutions per second or 3000-15000 rpm, think about how long the transfers are open for each stroke. If you have an air hose pressurized to 10 psi, and could open the gate for 1/50th of a second, and that is a full stroke of the engine at idle, how much air is coming out of that hose?
The only way these engines can pump efficiently is from the energy produced by the burning mix.
At first the expanding gases push the piston down, then as the exhaust opens and the gases hit the muffler can and expand, it created a pulling effect on the rest of the cylinder, vacuum. And a lot of it. So the transfers want to open just as the pressure is changing to a vacuum for the most efficiency.

After pondering this for a bit I see this as the only way to get enough transfer to run the engine. The crankcase compression combined with a vacuum on the other side means the transfer port sees a much greater differential than just 10 psi to atmosphere.

At least this is how I see it in this moment...
[emoji41]


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[paragonbuilder]

He's talking about the scavenging effect. As the exhaust is expelled it's inertia or momentum leaves a low pressure area behind it.......that pulls on the transfer ports.

Well that was said much simpler than my rambling...lol


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[paragonbuilder]

Now I understand why the word scavenging is used...
Never gave it much thought...
[emoji849]


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[RIDE-RED 350r]

Well, that gives me something to ponder, I hadn't thought of it that way.

 

[Hedgerow]

He's talking about the scavenging effect. As the exhaust is expelled it's inertia or momentum leaves a low pressure area behind it.......that pulls on the transfer ports.

I think some mook mentioned that earlier.

SCAVENGING!!!

 

[RIDE-RED 350r]

Now just a rookie speaking here, but I always thought that the scavenging effect on these saws were very little being just a basic can muffler as opposed to a tuned expansion chamber? Am I referring to two different things or am I misunderestimating the scavenging capability of the typical saw exhaust?

 

[Hedgerow]

Now randy, please explain how cross loop scavenging works.

In detail..
200 words or more..

Due tomorrow.

 

[RIDE-RED 350r]

Now randy, please explain how cross loop scavenging works.

Pretty sure Srcarr52 posted about that earlier in this thread... Read it kind of fast on my lunchbreak today. I'll go re-read it and see what I missed.

 

[Hedgerow]

Wonder what Mac had in mind here?
Those guys were too smart for their own good.

 

[RIDE-RED 350r]

What was that??? I can't hear you! LOL!

 

[CJ Brown]

Okay scavenging, but how much scavenging is taking place in a port that is 1.5" long?

 

[Mastermind]

View attachment 68726

Wonder what Mac had in mind here?
Those guys were too smart for their own good.

They were into ear bleeds.

 

[Hedgerow]

Okay scavenging, but how much scavenging is taking place in a port that is 1.5" long?

Sorta depends..

How much "stuff" can ya fit in it?

 

[Mastermind]

Now just a rookie speaking here, but I always thought that the scavenging effect on these saws were very little being just a basic can muffler as opposed to a tuned expansion chamber? Am I referring to two different things or am I misunderestimating the scavenging capability of the typical saw exhaust?

A tuned pipe works in both directions. It pulses. At one point it pulls on the exhaust port......and then it reverses to cram the unburned gasses it just sucked out right back in.

 

[Adirondackstihl]