ESE's works engine tuner

[jfn2]

What is the design importance of the bottom of the exhaust port, ( from BDC to transfer opening)? And if there is not much how do you calculate for the exhaust duct design and pipe openings? As in Wobbly's exhaust port design or port area calc's? And if it is important, what shape would you use, or size? I am speaking of the port opening or face area.

[wobbly]

The floor of the main duct was tested by Jan @ 3mm ABDC and a gain realized.
I also have done this as its the only proven example to follow , but is tempered by the fact it only works if the blowdown STA is fully developed.
The exit area I used was calculated @ 75% assuming the port was at BDC , as using the smaller area gave a Mach number closer to 0.9 and this lost power in the sim.
The higher floor also makes the main duct pinch point smaller where the Aux ducts enter from the side , and this may be a factor in the designs success , as well as the possible reduction in
short circuiting from the A port front wall.

[jfn2]

Wobbly
Thank you for the reply. What I'm trying to ask is about the shape of the bottom of the exhaust. On engines I've seen with a single exhaust port the bottom has a elliptical bottom edge and the edges raise straight or with a slight elliptical shape to the top edge where the widest point is the middle of the vertical lines. In the engines with a winged exhaust port the bottom is not as wide and the sides are slanted towards the widest point, or to a point level with the top of the transfer ports, then flare out into the wing shape. Sort of 'V' shaped. If I were to make a replacement sleeve and were to cut the port openings into it what shape would be the best? The 'V' shape, I think, would help with short circuiting. That opens another question, where does the most short circuiting occur from, the top of the 'A' port or the back side of the 'A' port? With Frits' suggestion about the directional angle of the 'A' port I would guess the back side. What do you think?

[wobbly]

You need to decide one way or another , T port or single port.
Each very different in approach re the floor geometry.
No ellipses involved , compound radi.

Edit , I really dont think the A port rear wall is affected by the depression around BDC as much as the front wall. Firstly its miles away from the Exhaust port and it would need the front wall flow regime to have completely collapsed to allow flow attached to
the rear wall to "bend " around across the face of the A port and out.

[jfn2]

Hello again Wobbly
I think I have my front and back walls for the 'A' transfer port mixed up. Sorry 'bout that. Thank you for your reply.
When I wrote ' back side of the 'A' port I meant the short side closest to the main exhaust window in a 3 port exhaust configuration. What I'm doing is installing a fresh air duct or tube into the ' A' port so that when there is short circuiting occurring there is plain oxygen instead of mixed fuel available in the right area of the 'A'port. No where can I find where, in the 'A' port, the right area is. Just so you know this air is reed valve regulated and it's atmospheric air but controlled by jet orifice. I've tried this on a KTM 85 and it seems to work good, but I'm sure it would work better if I new where to place the opening in the port.

[wobbly]

To be clear I regard the " front " of the cylinder is the Exhaust exit , except in the scenario of the cylinder turned around as a KZ is.
If I understand you correctly you are trying to achieve a version of stratified charge exiting the A port , but I really dont see how this can work effectively as the A port will begin flowing , instantly there is a positive pressure ratio across it.

Having a reed controlled auxiliary duct also into the same port , would take far too long to have flow to be initiated ,of air only.
Are you imagining that the initial A/F mixture is already on its way up the loop scavenging column , and then when maximum depression is seen at the Exhaust port around BDC , there is only air exiting the duct into the cylinder ?

[Haufen]

Hello again Wobbly
I think I have my front and back walls for the 'A' transfer port mixed up. Sorry 'bout that. Thank you for your reply.
When I wrote ' back side of the 'A' port I meant the short side closest to the main exhaust window in a 3 port exhaust configuration. What I'm doing is installing a fresh air duct or tube into the ' A' port so that when there is short circuiting occurring there is plain oxygen instead of mixed fuel available in the right area of the 'A'port. No where can I find where, in the 'A' port, the right area is. Just so you know this air is reed valve regulated and it's atmospheric air but controlled by jet orifice. I've tried this on a KTM 85 and it seems to work good, but I'm sure it would work better if I new where to place the opening in the port.

Stratified scavenging two-stroke engines went from reed controlled air ducts to piston controlled seperate air only inlets. The air in these inlets is then directed via closed pockets in the sides of the piston directly into the transfer outlet windows. The fuel / air mixture goes into the crankcase as usual and enters the transfers from there, whereas the air enters from the opposite side.

So the answer for your question with the reed controlled duct is: pretty close to the transfer exit (closer to the outlet than the transfer inlet at the case). But how close exactly would depend on the volume of the transfers you intend to fill, which would depend on rpm, load, mixing with air/fuel etc. As a ballpark, you could check if EngMod has a value for the scavenging efficiency (of the A ports only) and then dig through the numbers from there and see if you can come up with a compromise.



Or just convert the engine to fully stratified by adding a piston ported second inlet and get rid of the transfer-reeds:
the upper inlet is the air only inlet which feeds into the transfer outlet ends.

https://www.youtube.com/watch?v=3LDN0c2lAsQ

[TZ350]

2Stroke Stuffing goes to Germany and meets with very interesting people and brings home treasure.

[jfn2]

Hello Wobbly
Please understand the tube that's placed into the 'A' port is only .475mm in dia. So the amount of clean air is small. Also the reed cage is mounted as close to the port as possible. No more then an 1" to 1 1/2" (maybe 2") away. As the flow starts moving in the port towards the cyl and passes the mouth of the small tube it creates a low pressure over the tube and clean air moves into the port and into the cyl. It will flow more at low rpm's and less at high rpm's. This clean air is in front of the mixed fuel so therefore should be the first out of the exhaust port as short circuited mixture. So no mixed fuel is loss. I've found that I had to increase the low speed jet size about 1 size and sometimes move the needle clip depending on the size of the tube. Actually I have both connected with tubes into a metering block with one jet regulating the amount or air for both sides. So if I want more air I just increase the jet size. I did this to a KTM 85 some years ago for hill climbing and it worked good , but I did not have a dyno at the time so I have no feed back and I don't know where the bike is now or if it is still running. This should work well with also changing the 'A' port directional angle as Frits' mentioned. The only problem I can see is trying to use it with a parallel twin or triple motor.

Hello Haufen
Thank you for the reply. I assumed that was the place to locate it but I was not sure. What I'm still not sure of is how the short circuiting flow flows? The words short circuiting means, to me, is that as the flow comes out of the port it turns into the exhaust or does it travel around inside the bore mixing with the burnt' gases and then out the exhaust? Inquiring minds want to know!

Thoughts and comments.

[Haufen]

Hello Wobbly
Hello Haufen
Thank you for the reply. I assumed that was the place to locate it but I was not sure. What I'm still not sure of is how the short circuiting flow flows? The words short circuiting means, to me, is that as the flow comes out of the port it turns into the exhaust or does it travel around inside the bore mixing with the burnt' gases and then out the exhaust? Inquiring minds want to know!

Thoughts and comments.

There are four basic loss principles:

1) loop scavenging losses: when the mixture goes all the way it is supposed to, but arrives at the exhaust port window while it is still open, so a portion of the mixture is lost. On a chainsaw etc. engine, this accounts for about 40% of the scavenging losses according to a publication from 2002 by STIHL.

2) mixing losses: when fresh and burnt mixture meet, a part of it will mix and join the exhaust gas route directly out of the exhaust port (instead of just pushing it out of the way).

3) wall losses: especially when fresh mixture hits the cylinder wall (e. g. because there is no ideal symmetry in the ports directions), that mixture can travel along the cylinder wall and out of the exhaust port

4) short circuiting losses: that's what happens when mixture, especially the mixture in the transfer ports which are closest to the exhaust port(s) takes a turn (short circuit) directly out of the exhaust port before even joining the scavenging loop.

[TZ350]

2Stroke Stuffing.

[jfn2]

Haufen, again thank you for your reply .
I have a Kawasaki KX 65 I intend to play with as far as the short circuiting. I will try Frits' directional change first and then add the extra reed valves. I'll post the results. Thank you and Wobbly both for the help.

[Peljhan]

The combination of Mallory slugs and hollow light-alloy caps serves to combine a low total mass of the cranks with a high inertia that made the 250 cc twins more rideable. The same setup was used for the 125 singles, but as Jan Thiel said: 'In seven years of experimenting we have not been able to establish what is best: high or low inertia'.
My approach: when in doubt, choose low; it will be a blessing for the transmission and the rear tire.

Frits, I have a question about crank inertia.
I am wondering, shouldn't higher inertia crank be more blessing for transmission and rear tire as it's rotating speed oscilates less in each revolution?
Or is my theory applicable only when engine is more or less steady loaded and your statment refers to oscilations when gearbox is shifted?

P.S. It was nice to see you on video.
Hope to meet you one day.

[Frits Overmars]

Frits, I have a question about crank inertia. I am wondering, shouldn't higher inertia crank be more blessing for transmission and rear tire as it's rotating speed oscilates less in each revolution? Or is my theory applicable only when engine is more or less steady loaded and your statment refers to oscilations when gearbox is shifted?
P.S. It was nice to see you on video. Hope to meet you one day.

Who knows?

As long as we are accelerating in one gear, a high crank inertia is indeed a blessing for the transmission and the rear tire because it will smoothen the torque spikes, caused by the combustion pressure.
The downside is that when shifting gears, the energy required to change the crankshaft rpm jerks the transmission and the tire. Especially downshifting, when the cankshaft is suddenly forced to rev much higher, can cause a heavy burden, corresponding to a multiple of the power produced by the engine.

By the way, some of you may remember the first outings of the Aprilia RSA125, which was sidelined by a broken inlet disk shaft a couple of times. The shaft calculation was based on the friction between the disk and the inlet cover, caused by the crankcase pressure, but the disk's inertia was neglected.
Jan Thiel warned that an 8 mm Ø shaft would not be strong enough, and he was proven right by one rider in particular with a 'heavy shifting down' foot.
There are some great pictures of the way the rider reacted. They are copyright-protected so I cannot show them. But in case you're curious...
https://www.google.com/search?newwin...w=1175&bih=594

[Peljhan]

I remember this story from Jan, nice insight.

Wouldn't be a transmision disturbed more when upshifting with quickshifter as shift is allmost instantanious and when downshifting, clutch is usually used?
I wonder, maybe not, as crank need to drop revs only few hundreds when upshifting.

Maybe he should go play games, where only one ball is required instead of kicking poor Aprilia