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 Horizontal Exhaust Valves (or booster shut-off)

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Djamitague

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Nombre de messages : 34
Localisation : Switzerland
Date d'inscription : 18/07/2014

MessageSujet: 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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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 1:37

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

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Nombre de messages : 34
Localisation : Switzerland
Date d'inscription : 18/07/2014

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 11:20

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?
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 13:13

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.
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JanBros



Nombre de messages : 228
Localisation : Belgique
Date d'inscription : 05/12/2011

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 13:18

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
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 13:37

JanBros a écrit:
if only we could do that scratch
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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.


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



Nombre de messages : 148
Localisation : Australie
Date d'inscription : 15/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 13:59

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

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Nombre de messages : 34
Localisation : Switzerland
Date d'inscription : 18/07/2014

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 19:13

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...)
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Frits Overmars

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Nombre de messages : 2131
Age : 70
Localisation : Raalte, Holland
Date d'inscription : 12/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Dim 22 Mar 2015 - 23:13

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.
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Niels Abildgaard



Nombre de messages : 34
Localisation : Denmark
Date d'inscription : 26/02/2013

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 6:52

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?
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Djamitague

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Nombre de messages : 34
Localisation : Switzerland
Date d'inscription : 18/07/2014

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 7:47

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

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Nombre de messages : 2131
Age : 70
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Date d'inscription : 12/10/2010

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 10:52

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

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Nombre de messages : 164
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Localisation : Chartres
Date d'inscription : 12/09/2011

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 14:00

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.

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Niels Abildgaard



Nombre de messages : 34
Localisation : Denmark
Date d'inscription : 26/02/2013

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 14:25

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 Very Happy
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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Djamitague

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Nombre de messages : 34
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Date d'inscription : 18/07/2014

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Lun 23 Mar 2015 - 15:53

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
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JanBros



Nombre de messages : 228
Localisation : Belgique
Date d'inscription : 05/12/2011

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Mar 24 Mar 2015 - 8:17

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
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Frits Overmars

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MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Mar 24 Mar 2015 - 10:57

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.
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Djamitague

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Date d'inscription : 18/07/2014

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Mar 24 Mar 2015 - 11:25

Djamitague a écrit:
Is TROMBONE a moving pipe? Like the one I had on my MBK 51 in 1987 Very Happy
Yep: [Vous devez être inscrit et connecté pour voir ce lien]
[/quote]
Hot stuff!
[Vous devez être inscrit et connecté pour voir ce lien]

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JanBros



Nombre de messages : 228
Localisation : Belgique
Date d'inscription : 05/12/2011

MessageSujet: Re: Horizontal Exhaust Valves (or booster shut-off)   Mar 24 Mar 2015 - 23:12

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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