ESE's works engine tuner

[peewee]

hey wobbly how many things do you and frits disagree on ? besides if the A or B transfer should open first

[wobbly]

Nothing that i can remember , except a long time ago about the values of peak cylinder pressure - but I am also sure Frits agrees that reverse stagger scavenging favors top end and gives room for bigger Aux ( along with less A port short circuiting ) , plus the higher B,C are wider thus naturally gives more transfer STA to match high Blowdown.
Normal stagger favors front side power , and is ideal if there is no PV - and or for naturally more mid for MX etc..

[peewee]

maybe it wasnt which port should open first but if the one to open first flows first. im sure frits will set me straight

[wobbly]

Thats easy to logic thru. When you see the pressure traces in EngMod ( or even last century in Dynamation ) , at TPO there is more residual Blowdown pressure above the port , than in the case , below it.
Thus this positive P delta, causing reverse flow back into the duct , affects the first opening port the most , this gave rise to the " first to open , last to flow " premise.
It is in fact how port stagger scavenging regimes actually work at all.
Also explains why power drops with excess Blowdown STA numbers.

[husaberg]

Nothing that i can remember ,.

Toroidal vs Bathtub?

[F5 Dave]

What about licorice?

[peewee]

Thats easy to logic thru. When you see the pressure traces in EngMod ( or even last century in Dynamation ) , at TPO there is more residual Blowdown pressure above the port , than in the case , below it.
Thus this positive P delta, causing reverse flow back into the duct , affects the first opening port the most , this gave rise to the " first to open , last to flow " premise.
It is in fact how port stagger scavenging regimes actually work at all.
Also explains why power drops with excess Blowdown STA numbers.

im sure jan explained it hundreds of pages back. was his reason for leaving the A roof 1* lower so the aux exh could be slightly lower or the last to open first to flow like you say ?
im in the early grinding stages on my yami barels and not sure yet how i will go. front side power i dont think i need it as ive got the 2-5 clutchless shifting trans and the ratios seem to be gapped close on the banshee

[wobbly]

The Aprilia /Rotax had reverse stagger before Jan arrived .
And the Banshee , like its old LC cousin with no PV , benefits hugely from normal port stagger.
Its 3-4 shift drop is huge , this is easily fixed with Nova semi race gears - you dont need the full set if drag racing it.
But also the Banshee , like the RZ/YPVS has the B port pointing at the plug , completely at odds with SOTA scavenging principles - reversing the axial up angles on the A/B ports needs epoxy and roof welding and
its twice as effective as anything stagger can achieve.

[Frits Overmars]

hey wobbly how many things do you and frits disagree on ? besides if the A or B transfer should open first

Nothing that i can remember , except a long time ago about the values of peak cylinder pressure - but I am also sure Frits agrees that reverse stagger scavenging favors top end and gives room for bigger Aux ( along with less A port short circuiting ), plus the higher B,C are wider thus naturally gives more transfer STA to match high Blowdown. Normal stagger favors front side power and is ideal if there is no PV - and or for naturally more mid for MX etc..

I agree with your every word Wob, including that pressure issue, where once upon a time a factor of 2 had mysteriously disappeared from my calculations.

maybe it wasn't which port should open first but if the one to open first flows first. im sure frits will set me straight

I could try, but I've tried to do it before so I'm not going to hold my breath.
It's not rocket science, it goes like this: if you open a port over which there is a pressure difference, a flow will start. If you open a second port one millisecond or one crankshaft degree later, a flow through that second port will start one millisecond or one crankshaft degree later.
In short: the port that opens first, flows first, though not always in the desired direction; that depends on the pressure difference.
I know of course what Wobbly means. I'm not arguing for the sake of contradicting him but rather for the sake of pulling his leg.

Toroidal vs Bathtub?

I'm not contradicting Wobbly, I'll get to that in a minute, but first I need to have a word with you, Husa. I have a strong suspicion that your 'bathtub' is in fact a hemispherical combustion chamber. I'm not familiar with your sanitary facilities, but over here a bathtub looks like this:


And since we're splitting hair anyway, I've seen combustion chamber shapes called Top Hat, but that is about the worst shape you can have in a two-stroke (side-valve foul-strokes are even worse). Luckily most people who say Top Hat really mean Bowler Hat, which is a lot better.


OK, over to the toroids, better known as donuts in some circles. The blue drawing shows the RSA combustion chamber dimensions.
From point 2 to point 3 we see a quarter circle and the characteristic of a toroid is that the center of this quarter circle is not on the central vertical axis of the chamber, but off center.

It used to be thought that a hemispherical chamber was best for a rapid burn, but we know now that the rate of burn mainly depends on turbulence, and a toroid is the best shape to exploit the squish effect, as seen on the blue+yellow drawing.


We can extend the circular arc between point 2 and point 3 beyond point 3 and then we get a combustion chamber with an intruding spark plug, like on the picture below.


The radially inward flow due to the squish is symmetrical, so an intruding spark plug that brings the spark to the center of the action would seem to make sense. But the scavenging column that must expel the spent gas from the combustion chamber before the squish can become effective, is not symmetrical. That column moves upwards along the cylinder wall oposite the exhaust, and then performs a looping as it follows the outer contour of the combustion chamber. An intrusion hanging from the chamber roof may throw a scavenging shadow and so hamper the scavenging.

While Wobbly has determined that the intruding toroid is the best shape when combined with a flat-top piston, Jan Thiel has determined that due to the Coanda effect a domed piston is better with regard to transfer flow and piston cooling. I am not sure whether Jan, after his comparison tests of flat-top and domed pistons, has tested the combination of a flat-top piston and an intruding toroid.


Today's final picture is for Wobbly. Happy Birthday Wob !

[wobbly]

Good thing Frits is that if you pull my leg , I dont fall over - and Rocket Science is easy compared to 2T technology..
The Chief Engineer at Yamaha , nicknamed " Suzan " many years ago told me that viewing the squish action in a super high speed camera of some sort , his assertion was that the turbulent stream exiting the squishband
remained attached to the chamber wall - thus rotated in the opposite direction. Maybe he was pulling my leg , I always wondered.
The Toroid testing I did was many years ago , now I cant use one in what has become the focus of my work - KZ kart engines.
But it has recently been proven that a semi flat top ( a conic with 50% area flat ) is superior with a bath tub chamber as well , I just need the time to do an optimum timing edge radius test to finish of that area of development.
And thanks for the spark plugs today as well - what more could a man want , apart from liqorice.

[husaberg]

I'm not contradicting Wobbly, I'll get to that in a minute, but first I need to have a word with you, Husa. I have a strong suspicion that your 'bathtub' is in fact a hemispherical combustion chamber. I'm not familiar with your sanitary facilities, but over here a bathtub looks like this:


And since we're splitting hair anyway, I've seen combustion chamber shapes called Top Hat, but that is about the worst shape you can have in a two-stroke (side-valve foul-strokes are even worse). Luckily most people who say Top Hat really mean Bowler Hat, which is a lot better.


OK, over to the toroids, better known as donuts in some circles. The blue drawing shows the RSA combustion chamber dimensions.
From point 2 to point 3 we see a quarter circle and the characteristic of a toroid is that the center of this quarter circle is not on the central vertical axis of the chamber, but off center.

It used to be thought that a hemispherical chamber was best for a rapid burn, but we know now that the rate of burn mainly depends on turbulence, and a toroid is the best shape to exploit the squish effect, as seen on the blue+yellow drawing.


We can extend the circular arc between point 2 and point 3 beyond point 3 and then we get a combustion chamber with an intruding spark plug, like on the picture below.


The radially inward flow due to the squish is symmetrical, so an intruding spark plug that brings the spark to the center of the action would seem to make sense. But the scavenging column that must expel the spent gas from the combustion chamber before the squish can become effective, is not symmetrical. That column moves upwards along the cylinder wall facing away from the exhaust, and then performs a looping as it follows the outer contour of the combustion chamber. An intrusion hanging from the chamber roof may throw a scavenging shadow and so hamper the scavenging.

While Wobbly has determined that the intruding toroid is the best shape when combined with a flat-top piston, Jan Thiel has determined that due to the Coanda effect a domed piston is better with regard to transfer flow and piston cooling. I am not sure whether Jan, after his comparison tests of flat-top and domed pistons, has tested the combination of a flat-top piston and an intruding toroid.


Today's final picture is for Wobbly. Happy Birthday Wob!

A picture paints a thousand words and a thousand words paints a picture thanks for the explanation.

My description was clumsy, Yes your first pic is the std 4t like version of a bathtub.
much like a modified dutch ABSA Goldstar or a million modified harleys.
i cant find a pic of the Dutch BSA version i have posted it before but it looks like this hardly Davidson aftermarket one below.



What i was meaning was this cross section of a bathtub


I never asked if the Dutch ABSA had tulip valves. but on either of the bathtub head the plug is at the top. is the traditional usage.
The worst head shape i have seen was open Villiers. The top hat at least had a squish band.


okay i found this Villiers one i had not seen.

you would think the wider end would be near the plug not further away.

Funny enough villers was the first time i seen a toridal term used

It was stolen from a 1960's diesel.
I don't think it was ever manufactured in this from. well I hope it wasn't

[Frits Overmars]

Funny enough villers was the first time i seen a toridal term used

It was stolen from a 1960's diesel. I don't think it was ever manufactured in this from. well I hope it wasn't

You can abandon all hope, Husa. Villiers reported problems with their toroidal cylinder head, and you don't get those unless you actually manufacture and run it.

One consolation: the toroidal head never made it to serial production.

[oldjohnno]

Apologies if this has been discussed previously. I've been wondering about the evolution of 2Ts from the 70's to today, and a couple of the things that stand out are the understanding of blowdown and the dramatic increases in transfer area. Back then it was thought that too much transfer T/A was detrimental and people like Jennings warned against it. Now, we basically ring the cylinder with transfers and make much more power. What has enabled the use of so much more transfer area when previously it reduced output?

[F5 Dave]

I'll take a stab at this. The quantum change was the shift in reliance on using the bottom end as a pump, to the pipe creating increasingly stronger forces. Everything else then needed to evolve to keep in step with that and transfer control became critical as did more and more blow down area.

As usual you have to change everything to gain proper advantage with less detrimental effects that warned off early experiments (Jennings working with limited resources).

[Flettner]

You can abandon all hope, Husa. Villiers reported problems with their toroidal cylinder head, and you don't get those unless you actually manufacture and run it.

One consolation: the toroidal head never made it to serial production.

I wonder, was that because they didnt have a decient ignition to compliment it?