ESE's works engine tuner

[Frits Overmars]

I've been kicking this exhaust port shape around for months now and I'm finally going to give it a try. Thinking is that by keeping the top of the main port width to ~50% of bore the roof can be flatter and the top corner radius smaller. This gives more blowdown area before the aux ports open and the width that is lost from the main is picked up by the aux ports so more blowdown after the aux are open as well. Should be nicer on the rings too.
Main port angled down 20° at cylinder wall, Aux ports are 0° for maximum flow in blowdown.

Because of the necessary corner radii a port width of 70% of the bore is the optimum (not the maximum) value if there is just one single exhaust port.
But when auxiliary exhaust ports come into play, things change and narrowing the main port in favour of the auxiliaries makes sense.

Will your oblique bridges between the main and the auxiliaries be nicer to the ring? I need to think about that.

I wonder why you maintained the main port width below the blowdown area and why you put the port floor at BDC, by the looks of it.
It will induce more turbulence in the exhaust duct during the blowdown phase and it increases the risk of mixture short-circuiting from the A-transfers into the exhaust.
Raising the exhaust floor will also reduce the duct volume which helps concentrating the exhaust pipe energy at raising the cylinder pressure.
And finally a raised exhaust floor helps the return flow of washed-through mixture over the piston. Remember, as the transfer ports are about to close, the exhausts ports become transfer ports themselves.

A consistent 2,5° taper in the exhaust duct is fine, provided you use the blowdown area as the initial area.

A 0° downward port angle will give the maximum cross flow area but it won't give maximum flow. 25° is better.
Incidentally that's also the maximum angle you can give the auxiliary duct floors because of the A-transfer duct roofs directly underneath that also have a 25° axial angle.

[Frits Overmars]

This is what the RC guys run:

Horrible main port shape, and it makes you wonder why the bridges are there at all; most RC engines do no even have a piston ring.
We use the exhaust duct below for our 6,5 cc MB40 engine. Over the past 25 years it collected more world titles than all its competitors put together so it can't be too bad.

[philou]

for an engine with this configuration. on which side to place the opening of the segment.
opening on the up or down side of the piston?

[Frits Overmars]

for an engine with this configuration. on which side to place the opening of the segment. opening on the up or down side of the piston?

I must admit it took me some time to discover what you meant by 'an engine with this configuration' until I finally noticed the sideways exhaust port. But that shouldn't make any difference.
The best place for the ring gap is diametrically opposed to the exhaust port, also known as the six o'clock position. And that goes for all two-stroke engines, unless a very wide port at that position makes it undesirable.

[OopsClunkThud]

1st - the smaller blowdown initial wave front exiting the main port will be dumping into a relatively greater duct volume , reducing its amplitude.
And thus the diffusers efficiency
This is the very reason the Aux need to have sufficient height stagger, to allow a steep wave front of high amplitude to initially exit down the duct.
The Aux simply allow greater blowdown flow to occur before TPO.

On the first issue I think I'm missing your point. This concept has a larger initial blowdown area than the 70% port and it dumps into a smaller main port duct volume. This should then give better amplitude and diffuser efficiency, or at least that was my thinking.

Point taken on the second issue. I was including the aux and main port cross sections in my "taper" from port area down to 75% of port area at 1.5 bore distance, and the intent was to make sure there was not an abrupt change, but I need more work where they first come together. 25° down on the port roof will help with that.

I wonder why you maintained the main port width below the blowdown area and why you put the port floor at BDC, by the looks of it.
It will induce more turbulence in the exhaust duct during the blowdown phase and it increases the risk of mixture short-circuiting from the A-transfers into the exhaust.
Raising the exhaust floor will also reduce the duct volume which helps concentrating the exhaust pipe energy at raising the cylinder pressure.
And finally a raised exhaust floor helps the return flow of washed-through mixture over the piston. Remember, as the transfer ports are about to close, the exhausts ports become transfer ports themselves.

The bottom of the main port was widened to bring up the STA of the total exhaust (funny to stick to this convention while casting off others). It looks deceptively wide because the top of the port is only 50% of bore, but the bottom is 60% of bore. I'm working around some cylinder studs on this engine limiting how wide I can go with the aux but it also keeps the A ports well away from the main exhaust. I'll have to mess with raising the floor.

A 0° downward port angle will give the maximum cross flow area but it won't give maximum flow. 25° is better.
Incidentally that's also the maximum angle you can give the auxiliary duct floors because of the A-transfer duct roofs directly underneath that also have a 25° axial angle.

This is an intentional deviation. In my CFD work I found that 0° flowed better at high pressure ratio (blowdown) but 15° or more flowed better (and with less large scale eddies) once the flow fell below sonic. I'm choosing to optimize the aux for the blowdown conditions and letting it choke some later when fresh charge would be more likely to enter.

This is what the RC guys run:

Attachment 351077

I somehow find it reassuring that no idea is new

[wobbly]

i was misinterpreting somewhat the 1st critic of your layout i think , as i would have thought the "new " aux ports should be the shape used with the old 70% configuration. Thus in the your new regime the huge side ducts would create a commensurately huge area increase, where they enter the main duct.
That was my concern.
But a word from the wise ( Jan , not me ) lifting the floor only works if the Aux area , and thus the Blowdown STA is fully optimized. I have not yet tried exceeding the 3mm lift that was tested with success at Aprilia ( nor seen any dyno evidence that higher is better ) so maybe this is your mission should you choose to accept.
Its something I need to convince a customer its worth putting in the effort/time/money. But something that has become a norm in recent years of Sim use is that the full Ex STA appears to be pretty irrelevant when compared to optimizing the Transfer and Blowdown STA numbers.

[TZ350]

On the piston port configuration I had to cut the piston skirt away to get enough inlet timing possible (180deg duration). I suspect the marks in the cylinder may be from the short skirt piston rocking and gouging the cylinder with the ring (maybe??). Now the engine is in rotary valve configuration (220deg duration) I am fitting a full skirt piston and new cylinder, hopefully this will eliminate the cylinder damage. Time will tell.

[F5 Dave]

Wow. I never saw that in 20 years of running those engines. But I never cut the Piston, just lowered the port. had heaps of area with 28mm carb.

[TZ350]

Wow. I never saw that in 20 years of running those engines. But I never cut the Piston, just lowered the port. had heaps of area with 28mm carb.

Lowering the port would work but is hard work. With a lowered port the piston has only just fully opened the port before starting to close it again. Cutting the piston got an easy 180 duration with the port fully open for a considerable time. I have not done a "time/area" analysis between them but your way may have been better. Certainly better mechanically.

[F5 Dave]

It doesn't take long with a air grinder. Especially as you've cut the crazy long inlet manifold off with a grinder so you can tack weld & Devcon an ally plate to fit a conventional rubber inlet manifold to rather than the tube type creating a short but well formed inlet (well as best can be with a hole in the bottom of it for the case reed, but there you go, compromise compromise).

[TZ350]

.
Dyno time ...... vrooom vrooom WOT maxed out in 6th ... back wheel stopped turning around. So much for my last new cylinder .......

[F5 Dave]

Geez, what kind of piston do you run. I never, like never seized an RG on std or KX piston.

[TZ350]

Geez, what kind of piston do you run. I never, like never seized an RG on std or KX piston.

Not sure, some 41mm after market piston. Lower crown height and radius compared to the original RG50. I need to take it apart first for a look. I might get away with boring it for the 43mm KX60 piston.

[peewee]

what kind of welder do you fellas use ? just barely i got this new tig and havent yet tried it. perhaps you know of fronius

[philou]

Fronius

for me it is one of the best brands