ESE's works engine tuner

[Frits Overmars]

Build a uniflow - supercharger, ported cylinder for intakes and poppets in the head for exhaust. Best of all worlds

Uniflow: OK.
Supercharger: hmmm, not very efficient.
Ported cylinder for intakes: OK.
Poppets in the head for exhaust: no way.
A blowdown angle.area analysis will tell you why. Poppet valves rest on their seats. They do not move. When they must open, they start moving with an initial velocity of zero. The open area slowly increases and the cylinder pressure slowly drops, generating a mediocre exhaust pulse.
A piston port is opened by a piston that has reached its maximum velocity when it starts opening the port. The exhaust area opens with a bang and the exhaust pulse is far stronger than what a poppet valve could generate.
One more thing: poppet valves are heated from all sides by the passing exhaust gas. They dearly need the contact with the valve seats to get rid of that heat.
But in a two-stroke they are heated twice as often and their resting time between exhaust phases is halved.

[Pursang]

Other options for serious power increases: RPM & FUEL.

Nicely done analysis here (Well...certainly the best I've seen...)

[Flettner]

Uniflow: OK.
Supercharger: hmmm, not very efficient.
Ported cylinder for intakes: OK.
Poppets in the head for exhaust: no way.
A blowdown angle.area analysis will tell you why. Poppet valves rest on their seats. They do not move. When they must open, they start moving with an initial velocity of zero. The open area slowly increases and the cylinder pressure slowly drops, generating a mediocre exhaust pulse.
A piston port is opened by a piston that has reached its maximum velocity when it starts opening the port. The exhaust area opens with a bang and the exhaust pulse is far stronger than what a poppet valve could generate.
One more thing: poppet valves are heated from all sides by the passing exhaust gas. They dearly need the contact with the valve seats to get rid of that heat.
But in a two-stroke they are heated twice as often and their resting time between exhaust phases is halved.

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

[Flettner]

Other options for serious power increases: RPM & FUEL.

Nicely done analysis here (Well...certainly the best I've seen...)

So, key to more power is same BMEP but at higher revs. More blowdown is needed to achive this, yes?

[skako]

Frits,

I'm thinking, if we raise the transfers, behind that we can see how much the STA blowdown fell. With one additional poppet valve in the head, they could compensate for the lost blowdown STA and increase STA to match the increased transfer STA. To begin with, I would take the total open time of the poppet valve = 2 x blowdown time < 90 degrees of the crankshaft. Thus, the poppet valve would have more time to cool down and the cleaning (scawening) of the cylinder would mostly retain its characteristic.
What do you think about that?

[Frits Overmars]

So, key to more power is same BMEP but at higher revs. More blowdown is needed to achive this, yes?

Yes, although... The blowdown time.area is needed to have the cylinder pressure drop to below the transfer pressure by the time the transfers open.
But instead of dropping the cylinder pressure you could increase the transfer pressure. So it's either blowdown or blowjob

[wobbly]

Neil , you are fixating on Blowdown as you believe you have a method to increase this aspect , but as I keep repeating using Hp = Tx RPM/5252 then to maintain
the T at a higher rpm you need more STA for ALL the ports as the time period is reduced - where is the increase for the Transfers coming from if the chordal width is already maximized.

[diesel pig]

Flettner, The more down this rabbit hole you go the more I think Frits idea for a cylinder design has more merit and manipulating the exhaust pipe returning sonic wave would be better for making the power band wider.

[Flettner]

Sooo, consensus says no to added blowdown.
I wont waste my time on it then, haha, what was I thinking.

[jamathi]

Both are equally important , if you do an analysis of the RSA the power capability of the engines Transfer and Blowdown STA are perfectly matched , when the affect of
the Ex port timing edge radius on the Cd during Blowdown is taken into account.
There is no more room for transfer chordal width around the bore , and you cant lift them without reducing the Blowdown STA.
So that engine is maxed out , porting wise , and any further power increases have to come from using pipe and or Exhaust duct trickery.

The engine was completely maxed out, but after my retirement I improved on it using ENGMOD.....

[pete376403]

Uniflow: OK.
Supercharger: hmmm, not very efficient.
Ported cylinder for intakes: OK.
Poppets in the head for exhaust: no way.
A blowdown angle.area analysis will tell you why. Poppet valves rest on their seats. They do not move. When they must open, they start moving with an initial velocity of zero. The open area slowly increases and the cylinder pressure slowly drops, generating a mediocre exhaust pulse.
A piston port is opened by a piston that has reached its maximum velocity when it starts opening the port. The exhaust area opens with a bang and the exhaust pulse is far stronger than what a poppet valve could generate.
One more thing: poppet valves are heated from all sides by the passing exhaust gas. They dearly need the contact with the valve seats to get rid of that heat.
But in a two-stroke they are heated twice as often and their resting time between exhaust phases is halved.

(Not trying to wind you up) ok next best will be opposed piston two stroke (Junkers, Napier, Commer) with inlet and exhaust ports. Also use turbocharger(s) to recover exhaust energy feeding into supercharger. However it's all getting a bit heavy and complicated. Maybe just a good dose of Nitro :-)

[Pursang]

I believe Frits knows a useful bit about model aircraft engines.

Do they have any clever details/tricks that can be scaled up?

The 'peripheral' exhaust port on the OS engine is quite different to motorcycle practice...except perhaps old Villiers engines.
Does it produce a swirl during scavenging or charge reverse pulse?

[Frits Overmars]

if we raise the transfers.... with one additional poppet valve in the head, they could compensate for the lost blowdown STA..... I would take the total open time of the poppet valve ....

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?

Sooo, consensus says no to added blowdown. I wont waste my time on it then, haha, what was I thinking.

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 of 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°.

[Gradella23]

Ok now i got It: It's some sort of symmetric scavenging with a second boost transfer under the main exhaust. Mm i can't decide if It's dumb or interesting, for sure It's out of the ordinary

upping this cause it may have been lost in the meantime, but is somehow related to the topic.. how is that "under-exhaust port" transfer acting in terms of added transfer time.area?
i think the main issue is still matching that (assuming there is any advantage) with enough blowdown area (which i can't see in this layout)

[Flettner]

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°.

Thankyou Frits, I figured there would be some use in more blowdown, somewhere.