ESE's works engine tuner

[beta]

The total open time of the poppet valve won't do much good because, as I said, that valve opens really slowly, so 10° after it has started opening the open area is still minuscule and after another 20° it will be hardly any better. You'd better look somewhere else if you want to increase blowdown time.area.
Next: a big advantage of piston ports over valves is that there are no red-hot valves waiting to initiate detonation. And as I pointed out, exhaust valves in a two-stroke get even hotter than their colleagues in a four-stroke.
Lastly: if smearing the exhaust pulse energy because of the length difference between the main exhaust duct and the auxiliary ducts is undesirable, then how are you going to connect an exhaust port in the lower part of the cylinder with an exhaust valve way up high?

The way I read it, consensus says no to higher transfers because they force you to increase the blowdown time.area one way or another. I see no objection to added blowdown per se, and I am very anxious to learn what you were thinking.
An aspect of higher transfers that has not been mentioned yet, is the torque dip at 2/3 the rpm of maximum torque, caused by the exhaust return pulse entering the cylinder while the transfers are still open. The higher the transfers, the worse this gets. It was one the reasons for my symmetrical scavenging with transfers that are no higher than an accepted 130° and exhaust ports with a very mild (compared to most competition two-strokes) 190°.

All good, and then when you know that most of the KZ european/world championship winners in the last years had 198° exhaust 194° aux and 138° A port transfers and they work great (Wold Champ winners) what you have to think?
At the parc fermè you see all these cylnders with 1.5mm of gaskets underneath

[41juergen]

All good, and then when you know that most of the KZ european/world championship winners in the last years had 198° exhaust 194° aux and 138° A port transfers and they work great (Wold Champ winners) what you have to think?
At the parc fermè you see all these cylnders with 1.5mm of gaskets underneath

Maybe because they need more compromise on the given straight line ignition curve whereas any road racer bike used a programmable ignition curve to get the best out of the whole setup and have no gears to shift through?

[beta]

Maybe because they need more compromise on the given straight line ignition curve whereas any road racer bike used a programmable ignition curve to get the best out of the whole setup and have no gears to shift through?

I don't know but it really surprise me the first time I did measure the port timing of the trasnfers.

[wobbly]

Beta , the reason behind the jacked port timings is that I have been instructed that the homologated ports/pipe have to suit " the majority " of the factories customers.
The Euro/Worlds engines are run to over 15,000 rpm all day , then rebuilt after every ( and often replaced during one ) meeting - completely not suited to 98% of the customer base.
In my perfect world the timings would be left down at 196/132 and I would be allowed to shorten the pipe to get the revs and top end power that is being hamstrung by the straight line ignition.
Its a very niche scenario with no useful crossover to " normal " tuning knowledge.

[Flettner]

So what of an extra transfer under the exhaust, an idea worth pursuing?

[wobbly]

Absolutely , but that involves proving having no Exhaust area below TPO actually works as a concept first.
Build a cylinder with both ideas and we will never know the effect each has on performance.

[F5 Dave]

Surely one cylinder, then jb weld transfer closed for 'after' test. Should last long enough. Then burn out. . . Or put on shelf.

[Flettner]

Absolutely , but that involves proving having no Exhaust area below TPO actually works as a concept first.
Build a cylinder with both ideas and we will never know the effect each has on performance.

Would we make it the same height as the rest of the TP's or should we reduce its timing a little?

[Grumph]

Would we make it the same height as the rest of the TP's or should we reduce its timing a little?

I'd be very tempted to open it first. You've already got the cylinder contents heading in that direction by that point.
If the transfer below the exhaust is used to give a pressure pulse to the area below the piston, something's gonna happen.
Any flow back up that port will of course be exhaust contaminated - but if aimed straight across the piston might not affect the scavenge regime.

Happy to be shot down in a puff of exhaust smoke....

[OopsClunkThud]

In CFD simulations the sharp edge of the exhaust port has very little impact during blowdown but as the pressure ratio drops a larger and larger radius is needed to keep the flow from creating eddies* (mixing the exhaust and fresh charge). From this it would seem that if the exhaust floor was raised then it would be helpful to add as large a radius as possible since it has no impact on timing.

But this would get in the way of an extra transfer below the port.

* The downward 25° angle of the exhaust prevents the eddies forming on the roof of the port.

[Pursang]

So what of an extra transfer under the exhaust, an idea worth pursuing?

Possibly some 'challenges' in keeping it well separated from the exhaust duct heat, with a good entry angle.
Maybe it is the transfer to hold up the 'tower' and the (former) A's need to aim elsewhere??
Wobbly is right about evaluating one change at a time but if it is to work well it will be the interaction of multiple elements.

Still... if anyone can, You can do it!

[Pursang]

Yes, exhaust piston on a OP uniflow gets a thermal hiding, key to a successful design is addressing this ..... as you know Frits.

Now we are talking about transfers under the exhaust port....how about secondary transfers under the exhaust piston exhaust ports? Cooling plus some extra transfer STA.
Might also block charge from the intake end escaping out the exhaust. (good for economy, maybe not for racing).

[Frits Overmars]

...How about secondary transfers under the exhaust piston exhaust ports? Cooling plus some extra transfer STA. Might also block charge from the intake end escaping out the exhaust.

We're talking opposed piston uniflow scavenging, right? I'm sure the exhaust piston would love the extra cooling.
But preventing charge coming from the transfer side from escaping out the exhaust? I don't think mixture coming from the transfer side will be influenced by any charge originating from behind the exhaust ports.
I'd sooner see it happening the other way around: the main transfer charge pushing the charge coming from behind the exhaust ports, into the exhaust before it has had a change to do something useful, like scavenging the spent gases.
​Besides,with transfer streams coming from both sides we couldn't call it Uniflow any more

[Flettner]

Possibly some 'challenges' in keeping it well separated from the exhaust duct heat, with a good entry angle.
Maybe it is the transfer to hold up the 'tower' and the (former) A's need to aim elsewhere??
Wobbly is right about evaluating one change at a time but if it is to work well it will be the interaction of multiple elements.

Having a twin exhaust port does change possibilities.
Water cooling is going to have to be applied very carefully.

[Flettner]

We're talking opposed piston uniflow scavenging, right? I'm sure the exhaust piston would love the extra cooling.
But preventing charge coming from the transfer side from escaping out the exhaust? I don't think mixture coming from the transfer side will be influenced by any charge originating from behind the exhaust ports.
I'd sooner see it happening the other way around: the main transfer charge pushing the charge coming from behind the exhaust ports, into the exhaust before it has had a change to do something useful, like scavenging the spent gases.
​Besides,with transfer streams coming from both sides we couldn't call it Uniflow any more

If we are talking OP uniflow, there are many possibilitys, including differential piston sizes / strokes and certainly piston phasing offsets.
Three main issues though
Exhaust piston thermal control
Combustion chamber shape
Transfer flow....

Exhaust piston is a challenge but Im hanging my hat on a steel aluminium composite. To do this I need make an electrode for spark eroding the piston crown underside .... but my CNC is out of action, for some time, so thats put a stop to that, for now.

Combustion chamber shape for spark ignition does hold both advantages and disadvantages, plenty to sort through to get an acceptable outcome.

Transfer, an issue because all the charge wants to do is rush straight for the exhaust. Swirl will slow this but leaves an unscavenged plume in the middle. So far with my OP engines Ive used a combination of some ports swirl and some aiming at the centre of the piston directly opposite each other to help clear the central plume. Now I have a completely different system .... involving transfer 'tumble'. Best idea Ive come up with so far to solve OP uniflow charge transfer issues.

But this will all have to wait ..... CNC electronics are expensive to fix, unfortunately.