ESE's works engine tuner

[Frits Overmars]

My theory is that at high power crankcase/piston underside pumping means nothing and exhaust pipes everything.

I agree so far, Niels. But then what happens?
The exhaust pipe that is the first to come, sucks up everything it can get, thereby reducing the crankcase pressure. The second exhaust pipe can then suck up less mixture and the combustion in the second cylinder will produce exhaust gas with less energy. In the next cycle, the second exhaust pipe, with less energy to work with, will therefore suck up even less mixture. And so on...

This phenomenon even occurs in two-stroke boxer engines in which both exhaust pipes simultaneously suck from a common crankcase.
In theory this should not cause inequality, but in practice, even the smallest initial deviation leads to one of the cylinders performing worse and worse. Therefore, even in a boxer engine, it is advisable to provide the crankshaft center disk with a circumferential groove and a stationary 'piston ring' that separates both sides of the crankcase.

[Niels Abildgaard]

I agree so far, Niels. But then what happens?
The exhaust pipe that is the first to come, sucks up everything it can get, thereby reducing the crankcase pressure. The second exhaust pipe can then suck up less mixture and the combustion in the second cylinder will produce exhaust gas with less energy. In the next cycle, the second exhaust pipe, with less energy to work with, will therefore suck up even less mixture. And so on...

This phenomenon even occurs in two-stroke boxer engines in which both exhaust pipes simultaneously suck from a common crankcase.
In theory this should not cause inequality, but in practice, even the smallest initial deviation leads to one of the cylinders performing worse and worse. Therefore, even in a boxer engine, it is advisable to provide the crankshaft center disk with a circumferential groove and a stationary 'piston ring' that separates both sides of the crankcase.

Thank You for explaining why show will stop in racing engines.
In aircraft with a Turbo it is still to early to drop it and long uncooled exhaust pipes is not really wellcome anyway

[ceci]

This phenomenon even occurs in two-stroke boxer engines in which both exhaust pipes simultaneously suck from a common crankcase.
In theory this should not cause inequality, but in practice, even the smallest initial deviation leads to one of the cylinders performing worse and worse. Therefore, even in a boxer engine, it is advisable to provide the crankshaft center disk with a circumferential groove and a stationary 'piston ring' that separates both sides of the crankcase.

What happens?, one cylinder is supercharged and the other is canceled.
The result is how this engine? , it is not boxer, its design is similar

[F5 Dave]

I can see some problems with that engine straight away. The lubrication is going to be parsimonious at best, and the compression ratio will struggle to get over atmospheric.

[Frits Overmars]

What happens? One cylinder is supercharged and the other is canceled. The result is how this engine? it is not boxer, its design is similar

Ceci, it may look like a boxer, and the crankshaft balance approaches that of a boxer, but the engine in your picture, a 50cc Motobécane 99Z from 1978, is a single-cylinder engine with a crankcase-pumping opposed piston, like the pre-war DKW racers.
So it has one exhaust pipe and zero problems with one firing cylinder starving the other, because there is no other.

[ceci]

The exhaust pipe that is the first to come, sucks up everything it can get, thereby reducing the crankcase pressure. The second exhaust pipe can then suck up less mixture and the combustion in the second cylinder will produce exhaust gas with less energy. In the next cycle, the second exhaust pipe, with less energy to work with, will therefore suck up even less mixture. And so on...

.

It can be said that the first cylinder is supercharged, it feeds more than it should, because it takes away from the other

[guyhockley]

Snip...
This phenomenon even occurs in two-stroke boxer engines in which both exhaust pipes simultaneously suck from a common crankcase.
Snip...

There was recently a brief discussion on Michael Moore's Chassis list about Konig engines. I realise the Siamese pipes were probably for packaging reasons, but I always wondered if linking simultaneous firing cylinders together might not have been better than the way they did it.
Pretty much like Kevin Cameron did on the TZ750.
Dick De Jager managed to squeeze 4 separate pipes into his KonigYam, anyway.

[Frits Overmars]

...I realise the Siamese pipes were probably for packaging reasons, but I always wondered if linking simultaneous firing cylinders together might not have been better...

Yes, it would have.
Coupling the exhaust ducts from two cylinders that fire at 180° intervals can be very interesting if the exhaust timing is well over 200°. Then the exhaust opening pulse from cylinder A can shove the washed-through mixture from cylinder B back just before exhaust port B closes. And that primary pulse from cylinder A can easily be three times as strong as what a reflected pulse from a conventional expansion pipe could manage. That's some supercharging...

In the König layout the exhaust opening pulse from cylinder A arrives too late at cylinder B, when exhaust port B has already been closed, so the Y-connection from both cylinders to the single pipe has no useful function. On the contrary: when the exhaust system is doing its thing for cylinder A, the duct from the Y-piece to cylinder B is just a dead-ended side branch that dampens the exhaust resonance, like the ATAC volume on early Hondas. In the Hondas this was used to lessen the disturbance of exhaust pulses returning at the cylinder at the wrong time, below the power band. As soon as the engine ran in the power band, the ATAC volume was closed. But in a König that volume stays always open; no wonder its power left something to be desired.

[wobbly]

And adding to the effect of having very high exhaust duration you can cut the skirts short such that the port is open a few mm when the piston is at TDC.
Wreaks havoc with the jetting , but in the project i tried that we were using big pumper carbs.
Very easy to adjust the fuel curve as was needed - but I doubt a normal carb could be made to work.
This setup on a " stock " 950 SeaDoo gave an added 8 Hp in about 80 , so 10% - enough for the title at Havasu.
On a 1180 twin race motor the bump was 15 Hp in 120 so a little less.

[flyincat]

Thanks, Frits and Wobbly! I've always been interested in the 2 into 1 y-pipe and piston skirt discussions. Both on the same page is fantastic.

Frits, do you have any pictures of a y pipe that uses very high duration exhaust to supercharge the other cylinder? It seems the y pipe would have to have the shortest path possible between exhaust ports to limit how far above 200 deg the ex timing needs to be? Also, does the pipe need to be designed to scavange more than normal so there is extra fresh mixture in the y pipe to shove back in the cylinder?

Wobbly, I looked at the suction at the ex port piston face during tdc in engmod. It's there. Too bad there is no way to actually simulate the cut skirt! Is cutting the skirt something that would be more manageable on an EFI, especially direct'ish injected like Flettner or TZ350? Would this method especially benefit an intake restricted engine?

Thanks again.

[jbiplane]

Yes, it would have.
Coupling the exhaust ducts from two cylinders that fire at 180° intervals can be very interesting if

Is there any good to make common resonator for V90 2-cylinder engine with separate crank volumes?

[wobbly]

Flyncat , yes to both questions.

[Niels Abildgaard]

And adding to the effect of having very high exhaust duration you can cut the skirts short such that the port is open a few mm when the piston is at TDC.
Wreaks havoc with the jetting , but in the project i tried that we were using big pumper carbs.
Very easy to adjust the fuel curve as was needed - but I doubt a normal carb could be made to work.
This setup on a " stock " 950 SeaDoo gave an added 8 Hp in about 80 , so 10% - enough for the title at Havasu.
On a 1180 twin race motor the bump was 15 Hp in 120 so a little less.

McCullough drone engines had no exhust pipes whatsoever

https://www.wy2.org/engines/engines-....php#mcculloch

I have had one once and think it had around 10mm clearance when piston were at tdc

[Frits Overmars]

Frits, do you have any pictures of a y pipe that uses very high duration exhaust to supercharge the other cylinder? It seems the y pipe would have to have the shortest path possible between exhaust ports to limit how far above 200 deg the ex timing needs to be? Also, does the pipe need to be designed to scavange more than normal so there is extra fresh mixture in the y pipe to shove back in the cylinder?

Couldn't find any decent pictures. But you're correct on both accounts. The longer the path from cylinder A to cylinder B via the Y-junction is, the longer it takes for the primary pulse out of cylinder A to reach cylinder B, and the higher the exhaust timing needs to be in order to keep exhaust B open until all the fresh mixture has been pushed back into cylinder B.
But you can make the path from A to B too short, for example when you rotate the cylinders towards each other, so the exhaust ports are directly breathing into each others faces. Then the Y-junction, or what there is left of it, is so short that it cannot contain enough fresh mixture. And it's no use having a really strong pulse pushing back mixture that isn't there.

[Frits Overmars]

Is there any good to make common resonator for V90 2-cylinder engine with separate crank volumes?

You could have a twin with a 90° included angle between the cylinders and a firing order of 180°, and then you could use the above-mentioned advantage of a Y-junction combined with high exhaust ports. In fact you might be able to shape that Y-junction even better than in a parallel-twin.
How you balance a 90° V-twin with a 180° firing order, is another matter....