Why so few reed valves these days?

[PES+]

Yes I see that

I do appreciate the leeway I have been given and am heartfelt in my apology. I did not mean to imply I knew more and do like the openess and sharing of information on this site. Heck I simplified in my head the reeds at the intake to shut the case and closed my mind there. I am forced to work on far wider ranges of equipment than I would like to and my mind overloads after a while.

I am reading now and searching the archives so I can get current and am not in anyway an expert though I am and always have been a 2-stoke lover. I don't want them to go away.

Thank You all for being patient I am learning. AGAIN.......that's part of the problem with me I guess.

 

[Lakeside53]

Thank You all for being patient I am learning. AGAIN.......that's part of the problem with me I guess.

If you ever stop learning, time to find a pine box

 

[Tzed250]

Well...

Reeds have there limitations. It takes energy to lift the reed off of its seat. This energy has to come from the pumping action in the crankcase. More reeds/bigger reeds means more energy taken away from the pumping action and less mixture transfer. And if we are going to talk about putting the reeds in the transfers we have to remember to get oil the main and rod bearings, or are we talking about a setup like a 2-stroke deisel where the crankcase and transfer/cumbustion chamber area are kept seperate. In theory there are many ways to scavenge the cylinder, but what we get as consumers are the products that are going to make a company profitable. Sometimes the forward leap has to come from a company or person that says innovation first, earnings second. However, the list of failed enterprises with that attitude is a long one.

 

[drmiller100]

Until the transfers are closed that vacume is filled by the open transfers pulling fuel back down.


Now when the transfers close then the leak is shut off and the crank case begins to build vacume again until the intake is opened. Reeds in the transfers could make the pp engine more efficient. Until it was built to see if the advantages out way the flow restrictions; it is just conjecture.

BTW, do not ASSume that every one knows less than you. You will not make many friends that way.

Hmmmm. I think of it differently. At bdc you have momentum of air from the crankcase running up the transfers and into the chamber. These effects are VERY significant. That is why the shape, tuning, length, and width of the transfers is SOOOO critical. You have 4 or 6 little pipes with a full head of momentum at BDC that is sucking from the crankcase up into the chamber.

Reeds have signficant disadvantages. It takes a differential in air pressure to get air past the reeds. They create significant turbulence. Turbulence is again lost energy, and restricts the air, preventing horsepower.

As an interesting exercise, look at reeds at higher rpms. There is some argument that they rarely close on some engines at WOT, WOT RPM. You get enough momentum, and "Ram" effect to keep the reeds open. For sure they make a HUGE difference at low RPM.

Just because I'm new to this site doesn't mean I know less about 2 stroke theory then you. I'm actually pretty shocked how rudimentary 2 stroke chain saws are. It would be interesting to figure out a way to package an effective pipe in a production saw..

 

[drmiller100]

???

Blowers don't need low end HP.

...

Hmmmm. Lets say a given saw motor makes max horsepower at 10,000 rpm. And say it idles at 2000 rpm.

How much load is on the saw engine at idle?? Exactly none, as the chain isn't even engaged. How much horsepower do we need at 5000 rpm, or just off idle???? Just enough to spin the chain.

If we load the saw by putting it ot the wood at 5000 rpm, what happens? The clutch slips.

Now look at a blower.
When it starts at idle, it is blowing air. At half rpm, it MUST put out 1/4 of its total horsepower at WOT. So, if it makes 4 horsepower WOT, it must make 1 horsepower at half RPM, as it still blows air even at half throttle.

I agree they are differnet engines. a chainsaw is like a formula 1 engine. A blower is like a boat engine.
An enduro style motorcycle is like a farm tractor. Huge, wide power band.

 

[Lakeside53]

The clutches disengage typically around 3000-3,300 rpm. I can run my modern saws in the wood at 5000 and keep them there... pressure and throttle. Not desirable but... and I probably have similar % HP being developed as the blower.

BTW, my 051 is very happy cutting at 5-6k... and it's piston ported. On this type of machine it's all about torque.

I buy the snowmobile/boat suggestion, not the blower. The blower motors work quite similarly to the same size chainsaws motors, but are obviously optimized slightly for their conditions.

 

[klickitatsacket]

Hmmmm.
Reeds have signficant disadvantages. It takes a differential in air pressure to get air past the reeds. They create significant turbulence. Turbulence is again lost energy, and restricts the air, preventing horsepower.


Just because I'm new to this site doesn't mean I know less about 2 stroke theory then you. I'm actually pretty shocked how rudimentary 2 stroke chain saws are. It would be interesting to figure out a way to package an effective pipe in a production saw..

Turbulance does not necesarily mean leass HP. Look at the Chevy vortex heads. Also intakes need to be rough, while the exhaust ports need to be smooth. So while I agree that turbulance can be a problem, I would not right it off completely as there are places where is gives gain.

Now no one said you knew less, just for you to relax a little before you assumed every one else knew less.

 

[drmiller100]

Turbulance does not necesarily mean leass HP. Look at the Chevy vortex heads. Also intakes need to be rough, while the exhaust ports need to be smooth. So while I agree that turbulance can be a problem, I would not right it off completely as there are places where is gives gain.
.

the vortek increases turbulence once the air is inside the combustion chamber. the heads are pretty careful to minimize turbulence in the intake tract.

the intake isn't supposed to be "rough", just not as mirror smooth as the exhaust. the goal is to introduce a boundary layer along the intake tract to reduce turbulence in the intake stream.

by the way, the reason we mirror smooth the exhaust is to reduce heat transfer from gas to aluminum.

 

[klickitatsacket]

reduce heat transfer and carbon build up. As for the intake on a 2 stroke you need to build turbulance to maintain oil suspension. You also get more HP by tipping the exit of the intake down creating turbulance. I completely understand the boundry layer but just look at the rubber intake boot on a stihl chainsaw and you will see what I mean.

 

[drmiller100]

grin!!!

ok, so what is oil suspension????? why does it increase horsepower????

in bigger 2 strokes you tip the tops of the transfers to cool the piston top better. i've got more experience with 83 mm pistons then the little guys, so will take your word that the piston center doesn't need cooling.

Do you guys play much with squish angles????

I took the muffler off my 7900 today. Really interesting what they did using the muffler as the expansion chamber.
I'm going to be curious what they did to restrict exhasut flow. More technology there then i thought!!!!
also, is carb size a common limitation on saws? in other words, are you guys swapping carbs to pick up ponies????

 

[bwalker]

As for the intake on a 2 stroke you need to build turbulance to maintain oil suspension.

As soon as the fuel/oil/air charge enters the crankcase most of the fuel changes state from fine liquid droplets to vapors and as a result the oil drops out of suspension. This oil droping out of suspension is what lubes the motor.
Turbulance in the intake is counter productive, but how much is it a issue given the twists and turns the charge must take before making it to the chamber. Not to mention the crank flailing around at 14,000 rpm!
I might also add that most of the flow through a duct or pipe is through the center and not along the walls. So in relaity surface finish has a minute impact. It is also of not that there is some disagreement on leaving a intake port rough vs. polishing. I dont think there is a conclusive answer on which is best.

 

[Tzed250]

Roughness

Some real world input.

I had a KZ1000 dragbike engine built for me in the mid eighties. The Engine builder was George Bryce who owns Star racing. George has won countless national events, many world championships, and built some of the fastest dragbike engines in the world. He explained to me that the reason for the roughness on the intake port walls was to help create a thicker boundary layer against the port wall. This thicker boundary layer kept the atomized fuel away from the port wall. The more fuel in contact with the wall, the more the droplets came out of suspension. If the fuel sticks to the port wall it slows the gas flow. Turbulence is only good once the mixture is inside the combustion chamber.

 

[Lakeside53]

Some real world input.

I had a KZ1000 dragbike engine built for me in the mid eighties. The Engine builder was George Bryce who owns Star racing. George has won countless national events, many world championships, and built some of the fastest dragbike engines in the world. He explained to me that the reason for the roughness on the intake port walls was to help create a thicker boundary layer against the port wall. This thicker boundary layer kept the atomized fuel away from the port wall. The more fuel in contact with the wall, the more the droplets came out of suspension. If the fuel sticks to the port wall it slows the gas flow. Turbulence is only good once the mixture is inside the combustion chamber.

Stihl changed their intake boots for the same reason (they added a section of litle diamonds), about 1990. The early versions were smooth.

 

[spacemule]

All right, no one assume that you know less than me.

 

[Lakeside53]

10-4 spaceman

 

[Tzed250]

Definition

Assume:

To make an ass out of u and me

 

[drmiller100]

As soon as the fuel/oil/air charge enters the crankcase most of the fuel changes state from fine liquid droplets to vapors and as a result the oil drops out of suspension. This oil droping out of suspension is what lubes the motor.
Turbulance in the intake is counter productive, but how much is it a issue given the twists and turns the charge must take before making it to the chamber. Not to mention the crank flailing around at 14,000 rpm!
I might also add that most of the flow through a duct or pipe is through the center and not along the walls. So in relaity surface finish has a minute impact. It is also of not that there is some disagreement on leaving a intake port rough vs. polishing. I dont think there is a conclusive answer on which is best.

so you didn't answer the question about the oil in suspension hurting horsepower. or, if you did, i didn't understand it.

as for frictional drags along the intake tract, can you tell when you have opened the transfers on an engine? Get the right surface, and you have effectively increased the cross section for freely flowing air up the transfers.

likewise, if you took a good running engine, and welded a washer into the transfers restricting the air flow 20 percent in one little spot, could you feel a difference? My bet would be your engine would fall off the pipe at WOT, and she'd be a an ugly pooch. For this reason, I doubt putting 6 reed cages into your 2 stroke is a very good idea.

Let alone considering the effects of increasing the crankcase volume with all your transfer reed cages.

 

[Tzed250]

Volume

Yes...the crankcase volume is important...but some engines will benefit from increased crankcase volume. Yamaha made long rod kits for the reverse cylinder TZs and early nineties YZ250s. The longer rod allowed a spacer to be placed between the cylinder and the cases, increasing the crankcase volume. Some builders will space the reed blocks back to accomplish the same thing.

 

[klickitatsacket]

drmille, you mixed 2 differant things, so I will stick to one item at a time from now. When talking about boundry layers do not forget to mention surface tension. The higher the RPM the more it robs power.

 

[drmiller100]

As soon as the fuel/oil/air charge enters the crankcase most of the fuel changes state from fine liquid droplets to vapors and as a result the oil drops out of suspension. This oil droping out of suspension is what lubes the motor.
Turbulance in the intake is counter productive, but how much is it a issue given the twists and turns the charge must take before making it to the chamber. Not to mention the crank flailing around at 14,000 rpm!
I might also add that most of the flow through a duct or pipe is through the center and not along the walls. So in relaity surface finish has a minute impact. It is also of not that there is some disagreement on leaving a intake port rough vs. polishing. I dont think there is a conclusive answer on which is best.

so you didn't answer the question about the oil in suspension hurting horsepower. or, if you did, i didn't understand it.

as for frictional drags along the intake tract, can you tell when you have opened the transfers on an engine? Get the right surface, and you have effectively increased the cross section for freely flowing air up the transfers.

likewise, if you took a good running engine, and welded a washer into the transfers restricting the air flow 20 percent in one little spot, could you feel a difference? My bet would be your engine would fall off the pipe at WOT, and she'd be a an ugly pooch. For this reason, I doubt putting 6 reed cages into your 2 stroke is a very good idea.

Let alone considering the effects of increasing the crankcase volume with all your transfer reed cages.