ESE's works engine tuner

[Frits Overmars]

What limits the mass flow capability of blowdown making the area so critical? I'd expect it to be the speed of sound.
Speed of sound and available time.
In the spirit of this thread however, here's what I intend to try:
Contracting the exhaust area to mach 1 at distance from the bore.. Starting from contraction increase the area with mild convex shape....
Will need to measure and calculate if I can get the contraction and header diameters to match properly while keeping the contraction small enough.

I think there is still a lot to be found there.

[Jannem]

I think there is still a lot to be found there.

Available time could be extended with transfer reeds like Flettner suggested...

Attachment 331923

How did the ex port area of the cylinder compare to the area of the contraction?

[Frits Overmars]

Available time could be extended with transfer reeds like Flettner suggested...
How did the ex port area of the cylinder compare to the area of the contraction?

Reeds in the transfer ducts would give too much flow resistance, and decreasing the transfer angle.area like Flettner suggested will cost power.
Anyway, he was talking about high-pressure scavenging as a consequence of a small under-piston volume, and we haven't got anything like that.

The exhaust port area in the Malossi cylinder you are talking about, equals Ø 26,3 mm; the contraction in the exhaust duct was Ø 24 mm. But that is hardly relevant because:
A- It's the blowdown area that counts and not the total exhaust port area.
B- The exhaust outflow is only sonic over a part of the total blowdown period.
C- the contraction in the exhaust duct not only plays a part in flow energy recovery; it also influences the Helmholtz frequency of the exhaust system,
so you cannot vary it at will; you'll have to find a compromise.

[Jannem]

Starting from contraction increase the area with mild convex shape. At distance of 0,93*diameter

This area should match the diameter of the header.

It's been long time since I tried to get my head around SS nozzles, but the reason for convex expansion was that it was less sensitive for disruption of ss flow and in this case it also flows better backwards which is needed for pipe action.

Match to header diameter for same reasons.

[Michael Moore]

i think it was jan that once said blowdown can be improved without enlarging the window but rather by making the passage geometry better.

How common is the use of a flow bench among 2T tuners? Scott Clough uses one, and they are a normal part of 4T cylinder head porting, but I've rarely seen anyone mention using them to optimize 2T port shapes.

cheers,
Michael

[Frits Overmars]

How common is the use of a flow bench among 2T tuners? Scott Clough uses one, and they are a normal part of 4T cylinder head porting, but I've rarely seen anyone mention using them to optimize 2T port shapes.

Maybe because they do not want to alert their competitors? Joking apart, I know several good tuners without a flow bench. But the best in the business swear by it.
Jan Thiel even used it to flow cooling systems.

[breezy]

...........

[Norman]

A normal twostroke goes flat as it runs out of blow down time area. So if you have an engine that will apparently bypass this requirement, how?
More exhaust time area can be had if you blocked up the transfers. The blow down phase could continue all the way to BDC, and what a lot of time area we would have there. Now pressure is low enough, open the transfers.
If the crank case is a low volume high pressure unit, transfer will shoot up.into the cylinder, inertia might be such that transfer will continue well past the normal time transfer stops right up until when the transfers shut. Normally wasted stroke as I understand transfer is usually all over by 30 degrees ABDC. We are shifting the phasing around.
How do we shut the transfers off until the pressure equalizes? One way valves (reeds) in the transfers, that's how. We are not dealing with a soft inlet pules like a standard large capacity crank case, the inlet will be much sharper (and powerful) like a fourstroke. Fourstroke don't have extra cylinder capacity to shove the extra fuel air into like a crank case but yet they over fill their cylinders somewhat by the powerful induction inertia.
A short rod will help this.
This was suggested early but for what ever reason didn't stick. With what we know now its more relevant than ever, in my opinion.

What if everything sort of takes place at an higher pressure/pressure diff level? If we have a high compressed crankcase just before TPO and the pressure in the cylinder is a higher (not too far off the transfer) due to the comparably small exhaust port. Cylinder pressure (and some exhaust) will be pushed into the transfers, but as the volume there is much smaller than on a conventional engine, the pressure in the transfers will increase quick and also delay the inflow of exhaust into the transfer but also delay the outflow of the fuel/air mix. Next, we will have a bounce back in the other direction. The exhaust (first part) in the transfers will to a large extent go out together with the cylinder exhaust "left over". The mix in the transfers, behind the exhaust part of the gas ther, will very strongly be pushed into the cylinder as the pressure in the transfer decreases. At the same time the exhaust port opening is narrowing which means that a relatively limited amount of the fresh mix escapes. I have learnt here that not that much more mix than the volume from the transfers goes into the cylinder. But in this case this load has a pretty high pressure and at a fairly low temperature. The quick inflow will swirl for good mixing between fuel and air. What do you think?

[shnaggs]

Lucf, How is the Ryger on reed longevity??? If all our assumptions are correct, the Ryger must eat reeds for breakfast, brunch, elevenses, lunch, afternoon tea and dinner

[Haufen]

What if everything sort of takes place at an higher pressure/pressure diff level? If we have a high compressed crankcase just before TPO and the pressure in the cylinder is a higher (not too far off the transfer) due to the comparably small exhaust port. Cylinder pressure (and some exhaust) will be pushed into the transfers, but as the volume there is much smaller than on a conventional engine, the pressure in the transfers will increase quick and also delay the inflow of exhaust into the transfer but also delay the outflow of the fuel/air mix. Next, we will have a bounce back in the other direction. The exhaust (first part) in the transfers will to a large extent go out together with the cylinder exhaust "left over". The mix in the transfers, behind the exhaust part of the gas ther, will very strongly be pushed into the cylinder as the pressure in the transfer decreases. At the same time the exhaust port opening is narrowing which means that a relatively limited amount of the fresh mix escapes. I have learnt here that not that much more mix than the volume from the transfers goes into the cylinder. But in this case this load has a pretty high pressure and at a fairly low temperature. The quick inflow will swirl for good mixing between fuel and air. What do you think?

The more recent types of gardening and forrest tools work like this. Some even use stratified scavenging, so that they scavenge exhaust gas, then air and then mixture of air and fuel. Trapping efficiencies of above 90% of the fuel are not uncommon with these engines, whereas 80% (on the pipe) would be very good for a conventional 2-stroke.
Unfortunately this gives hot and diluted mixture for the combustion process. Not a big problem for the lower specific output of these engines, but for high power you'd want cold and pure mixture.

[Larry Wiechman]

How common is the use of a flow bench among 2T tuners? Scott Clough uses one, and they are a normal part of 4T cylinder head porting, but I've rarely seen anyone mention using them to optimize 2T port shapes.

cheers,
Michael

Do any of the modern simulation software packages allow flow data as an input?
These electronic manometers coupled with a rotary encoder look very tempting.


http://cavanaughracing.com/cdproflow.html

[2T Institute]

How common is the use of a flow bench among 2T tuners? Scott Clough uses one, and they are a normal part of 4T cylinder head porting, but I've rarely seen anyone mention using them to optimize 2T port shapes.

cheers,
Michael

Say hello to Scott next time you see him and tell him we have chicken soup in Australia

This ain't suck/squeeze/bang/blow world in a two stroke at least 2 things are happening at any 1 point in the cycle a gain in one area usually leads to a loss in a opposing area. Lots of work for no net gain. Next one is as Frits enlightened many of us years ago, it isn't just the raw number of the flow out of the duct to look at it's what happens after those streams exit the ducts and interact with one another.
Next we don't need to optimise our shapes, just look at a RSA cylinder and that is as optimised as you can get.
The dyno is as good as any, if it doesn't make more power decide if its worth pursuing. 3D CFD is far more enlightening its not that cut and dried as more flow more power

[jamathi]

How common is the use of a flow bench among 2T tuners? Scott Clough uses one, and they are a normal part of 4T cylinder head porting, but I've rarely seen anyone mention using them to optimize 2T port shapes.

cheers,
Michael

The first flow bench I got was at Garelli, in 1982, I learned a lot from it.
When I started work at Aprilia I was amazed there was no flow bench.
But one was bought very quickly....
Very good, above all, for exhaust duct development!
A smaller duct with more flow always improved power!
For scavenging it does not make much sense....
As the various flows must slow down each other.
But we did it anyway, to check eventual differences.
It was also helpful for improving the cooling system.
As the water circulation must be as quick as possible, to prevent hot spots...
No cylinder ever left Aprilia without having been flowed!

The exhaust duct was flowed with the piston in different positions:
1mm open, then 2 and 3mm, the blowdown, fully open.
And the auxiliaries and the central ducts separately.

The transfers were flowed with the piston in 4 positions
25% open, 50%, 75% and fully open

[TZ350]

And he was correct. In a regular exhaust, where the area in the pipe side of the contraction is not shaped like a supersonic nozzle, you will get shockwaves in the contraction when flow speed reaches mach 1, choking the flow to the speed of sound.

If you have the area behind the contraction shaped correctly, the flow speed will break the sound barrier and accelerate to supersonic speeds taking energy from the heat of the fluid, effectively cooling it.

The whole fun in this is that it's all is achievable from a piece of scrap metal turned in a lathe to fit your exhaust pipe into the cylinder. Can there be any simpler and non-intrusive modification to be tested with better potential?

Sounds great, Jannem if you get to make and test one, please post the results.

[Tim Ey]

It was also helpful for improving the cooling system.
As the water circulation must be as quick as possible, to prevent hot spots...
No cylinder ever left Aprilia without having been flowed!

Jan, I am curious. How much needed the cooling circuit of the cylinder to flow on a bench to be stated a good cylinder?
What were your boundary conditions?
Regards
Tim