Horizontal Exhaust Valves (or booster shut-off)

Sujet: Horizontal Exhaust Valves (or booster shut-off) Dim 22 Mar 2015 - 0:56

Does it exist an exhaust valve which slides horizontaly vs usual vertical valves? A sort of booster's shutter that'd affect surface vs timing.

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Djamitague a écrit:: Does it exist an exhaust valve which slides horizontaly vs usual vertical valves?

Yes, the Honda Power Port system, used in several CR125 and CR250 models.[Vous devez être inscrit et connecté pour voir cette image]

Quel cheni (What a mess)! Great pics thanks a lot Frits. That means in power valve systems we always influence timing, just widening the surface won't be effective?

Djamitague a écrit:: Quel cheni (What a mess)! Great pics thanks a lot Frits. That means in power valve systems we always influence timing, just widening the surface won't be effective?

Not really. The 'problem' with a two-stroke is caused by the exhaust resonance. At the optimum rpm it sucks spent gases out of the cylinder and fresh mixture out of the crankcase. Some of the fresh mixture is sucked all the way into the exhaust duct, but right after the transfer ports are closed, a reflected pressure pulse from the exhaust pipe pushes this fresh mixture back into the cylinder, and then the piston closes the exhaust port.
But at 2/3 of the optimum rpm the pressure pulse arrives back at the cylinder when the transfer ports are still open. Then the fresh mixture in the exhaust duct and in the cylinder is pushed back into the crankcase with a disastrous effect on engine power.
If you want to improve this situation, you must open the exhaust port later, so the exhaust pulse starts later and comes back later. Narrowing the exhaust port will not change its timing, so it will not do much good for the power band.

Nombre de messages : 351 Localisation : Belgique Date d'inscription : 05/12/2011

Frits Overmars a écrit:: If you want to improve this situation, you must open the exhaust port later, so the exhaust pulse starts later and comes back later. Narrowing the exhaust port will not change its timing, so it will not do much good for the power band.

if only we could do that scratch

Dernière édition par Frits Overmars le Dim 22 Mar 2015 - 14:02, édité 1 fois

JanBros a écrit:: if only we could do that

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This does not mean that I am in favor of a pneumatic operation for the power valve. It is a cheap solution, but not a very good one. It is governed by the mean pressure in the exhaust pipe, which drops during gear shifting, and, even worse, it also drops when the engine power drops due to very high revs. And the last thing you want then is to reduce the exhaust timing.
Reduced exhaust timing at low revs may broaden the power band by weakening the wrongly-timed exhaust resonance, but the best solution (apart from a CVT) is a trombone pipe.

The HPP system was so good that a few years later the RC valve made a triumphant return

Frits Overmars a écrit:

Djamitague a écrit:: Quel cheni (What a mess)! Great pics thanks a lot Frits. That means in power valve systems we always influence timing, just widening the surface won't be effective?

Not really. The 'problem' with a two-stroke is caused by the exhaust resonance. At the optimum rpm it sucks spent gases out of the cylinder and fresh mixture out of the crankcase. Some of the fresh mixture is sucked all the way into the exhaust duct, but right after the transfer ports are closed, a reflected pressure pulse from the exhaust pipe pushes this fresh mixture back into the cylinder, and then the piston closes the exhaust port.
But at 2/3 of the optimum rpm the pressure pulse arrives back at the cylinder when the transfer ports are still open. Then the fresh mixture in the exhaust duct and in the cylinder is pushed back into the crankcase with a disastrous effect on engine power.
If you want to improve this situation, you must open the exhaust port later, so the exhaust pulse starts later and comes back later. Narrowing the exhaust port will not change its timing, so it will not do much good for the power band.

Ok so I guess pulse timing of the pipe does not vary so much regarding rpm. I thought its temperature would increase with rpm, thus wave speed in proportion(also depends on exhaust location and material...)

Djamitague a écrit:: I guess pulse timing of the pipe does not vary so much regarding rpm. I thought its temperature would increase with rpm, thus wave speed in proportion(also depends on exhaust location and material...)

Generally speaking exhaust gas temperature increases with horsepower delivered, but not sufficiently so.
Hence the need for a variable ignition: later = hotter.

Nombre de messages : 36 Localisation : Denmark Date d'inscription : 26/02/2013

Frits Overmars a écrit:: Reduced exhaust timing at low revs may broaden the power band by weakening the wrongly-timed exhaust resonance, but the best solution (apart from a CVT) is a trombone pipe.

A pair or four exhaust valves VVT in head migth be more reliable than trombone?

Is TROMBONE a moving pipe? Like the one I had on my MBK 51 in 1987 Very Happy

Niels Abildgaard a écrit:: A pair or four exhaust valves VVT in head migth be more reliable than trombone?

Fourstroke-type valves in a two-stroke engine get too hot and open too slowly. They are only good for low power and low revs. But then why would you want the complication?

Djamitague a écrit:: Is TROMBONE a moving pipe? Like the one I had on my MBK 51 in 1987

Yep: [Vous devez être inscrit et connecté pour voir ce lien]

N'allez pas trop vite, j'ai du mal à traduire !

Quelqu'un pourrait-il me dire ce que veut dire "engine", ça m'aiderait.

Horizontal Exhaust Valves (or booster shut-off) 921866

Horizontal Exhaust Valves (or booster shut-off) 921866

Nombre de messages : 36 Localisation : Denmark Date d'inscription : 26/02/2013

Frits Overmars a écrit:

Niels Abildgaard a écrit:: A pair or four exhaust valves VVT in head migth be more reliable than trombone?

Fourstroke-type valves in a two-stroke engine get too hot and open too slowly. They are only good for low power and low revs. But then why would you want the complication?

Djamitague a écrit:: Is TROMBONE a moving pipe? Like the one I had on my MBK 51 in 1987

Yep: [Vous devez être inscrit et connecté pour voir ce lien]

[/quote]

Just dreaming of better racing class criteria where two strokes were back and dominating.
The present racing two stroke was maybe its own worst enemy.
Plus 50 horsepower from something like 50 times 50 mm and an exhausts system so big as the petrol tank has no real relation to people and chainsaws.
Total engine and exhaust volume and mass was maybe more meaningfull.
If exhaust is valved and therefore can be unsymetrical,the main job of exhaust system will be to get the used gas out.
I have dreamed a scheme where total engine volume and mass can maybe compete with the present form where mean effective pressure and piston speed is everything.

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In term of dream, Guy Nègre designed a F1 engine head fit with a rotary valve shaft, supposed to support higher engine rev. But in a two stroke...? That's more or less a power valve

Nombre de messages : 351 Localisation : Belgique Date d'inscription : 05/12/2011

JanBros a écrit:

Frits Overmars a écrit:: If you want to improve this situation, you must open the exhaust port later, so the exhaust pulse starts later and comes back later. Narrowing the exhaust port will not change its timing, so it will not do much good for the power band.

if only we could do that scratch

been thinking about this, but I don't get it. how can the valve provide assymetric open/closing times compared to BDC ?
unless the valve is so responsive it goes up and down with every stroke Horizontal Exhaust Valves (or booster shut-off) 55116

Horizontal Exhaust Valves (or booster shut-off) 55116

I did not say anything about asymmetrical timing, Jan. I just said opening the exhaust port later would help, and we can easily accomplish that with a power valve.

Djamitague a écrit:: Is TROMBONE a moving pipe? Like the one I had on my MBK 51 in 1987

Yep: [Vous devez être inscrit et connecté pour voir ce lien]
[/quote]
Hot stuff!
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Nombre de messages : 351 Localisation : Belgique Date d'inscription : 05/12/2011

Frits Overmars a écrit:: I did not say anything about asymmetrical timing, Jan.

you are right, don't know how come I read it that way scratch

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