[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

Architito2002 a écrit:

Good morning Frits, good morning everyone, trying to understand the two-stroke exhaust I assimilated this to a helmholtz resonator, but then analyzing the pulsations that occur inside I seemed to understand that in the end the resonance does not occur but is exploited "only" "the reverberation of the exhaust impulse. Is this a correct interpretation in your opinion? Thanks

Io non vengo considerato!! [Vous devez être inscrit et connecté pour voir cette image]

Architito2002 a écrit:

Good morning Frits, good morning everyone, trying to understand the two-stroke exhaust I assimilated this to a helmholtz resonator, but then analyzing the pulsations that occur inside I seemed to understand that in the end the resonance does not occur but is exploited "only" "the reverberation of the exhaust impulse. Is this a correct interpretation in your opinion? Thanks.

Io non vengo considerato!! [Vous devez être inscrit et connecté pour voir cette image]

Potito, I went back all the way to your date of inscription, trying to find the origine of your question. Apparently I missed it the first time; I am sorry about that.
Wave behaviour is not the only effect playing a role in exhaust pulsations; Helmholtz resonance is very important too. That is why the diameters of  the exhaust duct and the header are so critical.

.

Frits Overmars a écrit:

Architito2002 a écrit:

Good morning Frits, good morning everyone, trying to understand the two-stroke exhaust I assimilated this to a helmholtz resonator, but then analyzing the pulsations that occur inside I seemed to understand that in the end the resonance does not occur but is exploited "only" "the reverberation of the exhaust impulse. Is this a correct interpretation in your opinion? Thanks.

Io non vengo considerato!! [Vous devez être inscrit et connecté pour voir cette image]

Potito, I went back all the way to your date of inscription, trying to find the origine of your question. Apparently I missed it the first time; I am sorry about that.
Wave behaviour is not the only effect playing a role in exhaust pulsations; Helmholtz resonance is very important too. That is why the diameters of  the exhaust duct and the header are so critical.

@Frits
Grazie Frits, immaginavo l’avessi solo persa.
Ma quindi il nostro obbiettivo è quello di avere la massima pressione alla chiusura dello scarico che corrisponde alla portata minima vero?
Se la situazione fosse questa allora dobbiamo avere uno scarico la cui frequenza di risonanza è doppia rispetto alla frequenza del motore? Oppure lo scarico ha la stessa frequenza del motore e si sfrutta l’armonica superiore? Tutto questo con lo scarico in risonanza.
Ma questo vorrebbe dire avere anche la massima pressione nel collettore all’apertura dello scarico (OE)[Vous devez être inscrit et connecté pour voir cette image]

Architito2002 a écrit:

...quindi il nostro obbiettivo è quello di avere la massima pressione alla chiusura dello scarico che corrisponde alla portata minima vero?

Vero.

Architito2002 a écrit:

Se la situazione fosse questa allora dobbiamo avere uno scarico la cui frequenza di risonanza è doppia rispetto alla frequenza del motore?

Anche questo è vero.

Architito2002 a écrit:

Ma questo vorrebbe dire avere anche la massima pressione nel collettore all’apertura dello scarico.

È vero di nuovo. And now let us maximize the pleasure of our French hosts by continuing in English (désolé les gars, mon français n'est pas assez bon pour écrire sans fautes).

Some time ago I dropped a remark about the optimum effective exhaust timing being 180°. Naturally someone asked why, so I had to come up with an explanation. This is what I wrote.

When the exhaust port opens, a pressure pulse starts moving through the exhaust pipe. It is reflected at the end cone and it should be back at the cylinder just before the exhaust port closes.
Next, a part of this reflected pulse bounces off the partly-closed exhaust port and a residual pulse starts moving down the exhaust pipe. This residual pulse too is reflected by the end cone and starts moving back to the cylinder. Ideally it will arrive at the exhaust port just when the port opens again. Then the cylinder pressure and the pressure of the residual pulse combine their energy and the resulting pulse will be stronger than the pulse from the previous cycle. And the combined pulse from the next cycle will be stronger still, and so on; we have achieved true resonance.

Some may argue that we want a low pressure in the exhaust pipe when the port opens because then the spent gases will experience less resistance while leaving the cylinder. But that is not true. Gas flow depends on a pressure ratio. But once that ratio reaches 2, the flow velocity will reach Mach 1, the speed of sound. Raising the pressure ratio any further will not raise the flow velocity any further.
The cylinder pressure at exhaust opening can be as high as 12 bar and the pressure of the reflected pulse will then be about 2,5 bar. Thus the pressure ratio is well above 2, so lowering the pressure in the exhaust duct outside the cylinder will not do any good to the flow.

What has the exhaust timing got to do with the 'true resonance' I mentioned above?
The initial pulse starts moving at Exhaust Opening and it has to be back at Exhaust Closing, or a little earlier. This pulse travels with the speed of sound and its journey up and down the exhaust pipe will take t seconds.
The residual pulse starts moving at Exhaust Closing and it has to be back at the next Exhaust Opening. This pulse also travels with the speed of sound and its journey up and down the exhaust pipe will also take t seconds.
So from EO to EC takes t seconds and from EC to EO also takes t seconds. In English: the exhaust port should be open just as long as it should be closed.
Assuming that the crankshaft rotates with a uniform speed, this means that the crank angle during which the exhaust is open must be equal to the crank angle during which the port is closed. So both angles must be 180°.

I developed this line of thought some 45 years ago. When I first published it in 1978 (in the motorcycle magazine Moto73 of which I was the technical editor) everybody called me crazy. Some people still do, but I got used to it .

Above I made a couple of assumptions. The crankshaft does not rotate with a uniform speed, but at high revs the deviation is negligible. In case you really want to know, I did the math for the Aprilia RSA125. At a nominal rpm of 13.000 the minimum rotation speed is 12940 rpm @ 110° after TDC and the maximum value is 13060 rpm @ 351° aTDC. What's more significant: the deviation in crankshaft position from truly uniform rotation is always less than 1°. So that really is negligible.

Second assumption: both the initial pulse and the residual pulse move with the speed of sound,
True, but the speed of sound is not a constant. The pulse pressures in exhaust waves are so high that acoustics rules do not apply any more. We are dealing with gas dynamics here and the stronger a pulse, the higher the speed of sound is for this pulse. Since the residual pulse is weaker than the initial pulse, they move at different speeds. But we will leave this aside for now.

Third assumption: the initial pulse starts moving as soon as the exhaust port starts opening.
More or less true, but we are not interested in the first weak appearance of the pulse; we want to know when the pulse reaches its maximum amplitude. And that requires a certain amount of open exhaust port area. It turns out that for our desired theoretical exhaust timing of 180° we will need a geometrical exhaust timing of about 190°, depending on the shape of the port: does it open gradually or does it open over its full width all at once.

The obvious question will be: why has the Aprilia RSA125 a geometrical exhaust timing of 202°? True, at 190° the maximum torque value would be higher, but the engine would not want to rev because the blowdown time.area would be too small.
The 202° are a compromise: a bit less torque and a bit more revs; as long as the torque decline is smaller than the rpm rise, we gain horsepower.

By the way, if you ever heard a König four-cylinder boxer engine on full song, you will have noticed the distinctly musical sound from its 2-into-1 exhaust systems.

In a normal two-stroke the exhaust system performs two complete wave cycles per revolution of the crankshaft. The first cycle is performed during the open period of the exhaust port.The second cycle is a residual wave with a lower amplitude because now the exhaust port is closed, but with the same frequency as the first wave. So what you hear during a crankshaft revolution, are alternately one strong and one weak wave.

But in the König engine, when one exhaust port is open, the exhaust port of the other cylinder that uses the same exhaust pipe, is closed, and vice versa, so all waves have the same amplitude. This creates the wonderful König sound.

Grazie molto Frits.
Spero che il mio scrivere direttamente in italiano non crei problemi con i traduttori.
Farò sicuramente una ricerca sul motore boxer che hai menzionato per godermi la sinfonia.
Credo di aver compreso tutto ciò che gentilmente hai spiegato e condivido.
Solo ho un dubbio sulla risposta dell’espansione quando la luce di scarico è aperta e quando è chiusa. Nel primo caso se l’espansione ipotizziamo di considerarla come un tubo aperto ad una estremità e chiuso all’altra avrà una frequenza di risonanza differente rispetto alla configurazione successiva del tubo chiudo - chiuso. Se fosse valida questa ipotesi non riusciremmo ad avere la risonanza, sbaglio?
Cosa ne pensi di questa configurazione che permetterebbe di considerare sempre un tubo aperto-chiuso ed ottenere la risonanza a 360*? Grazie infinito.
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🤩

Y’a pas à dire, le post de Frits a gardé tout le charme du continental circus !

Adco a écrit:

Y’a pas à dire, le post de Frits a gardé tout le charme du continental circus !

Vero!!

When the exhaust port opens, the cylinder side of the exhaust system is obviously open. And the other end, at the tail pipe, can be considered closed.
When the positive pressure pulse returns to the cylinder, the exhaust port is still partially open, but the cylinder volume above the exhaust port is so small relative to the volume of the entire exhaust system that the cylinder pressure rises rapidly, making the positive pressure pulse feel as if it is hitting a wall, i.e. a closed end.
The residual pulse will therefore again be reflected as a positive pressure pulse towards the tailpipe.
From there it is again sent back to the cylinder port as a positive pressure pulse, and just as it gets there, the exhaust port opens again, with a much higher pressure than the residual pulse, which again feels like it is hitting a wall.

I'm afraid I don't fully understand your configuration Potito, but I do know that I need a complete wave cycle in the exhaust system, from maximum pressure at the cylinder when the exhaust port opens, to minimum pressure at the cylinder around Bottom Dead Center, to maximum pressure at the cylinder again when the exhaust port closes, so within a time frame of about 180 crankshaft degrees.

I tried to find you some videos with König sound, but I could not find the real thrilling music that I remembered. Nevertheless, here are two links that may give you some impression.

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Frits Overmars a écrit:

When the exhaust port opens, the cylinder side of the exhaust system is obviously open. And the other end, at the tail pipe, can be considered closed.
When the positive pressure pulse returns to the cylinder, the exhaust port is still partially open, but the cylinder volume above the exhaust port is so small relative to the volume of the entire exhaust system that the cylinder pressure rises rapidly, making the positive pressure pulse feel as if it is hitting a wall, i.e. a closed end.
The residual pulse will therefore again be reflected as a positive pressure pulse towards the tailpipe.
From there it is again sent back to the cylinder port as a positive pressure pulse, and just as it gets there, the exhaust port opens again, with a much higher pressure than the residual pulse, which again feels like it is hitting a wall.

I'm afraid I don't fully understand your configuration Potito, but I do know that I need a complete wave cycle in the exhaust system, from maximum pressure at the cylinder when the exhaust port opens, to minimum pressure at the cylinder around Bottom Dead Center, to maximum pressure at the cylinder again when the exhaust port closes, so within a time frame of about 180 crankshaft degrees.

I tried to find you some videos with König sound, but I could not find the real thrilling music that I remembered. Nevertheless, here are two links that may give you some impression.

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Grazie Frits, veramente emozionante il rumore di quei motori. Ascoltandoli mi è venuto in mente che l’unica gara che sono andato a vedere del motomondiale, al Mugello (quando Kevin Schwants ha dato l’addio alle gare), la domenica mattina, dopo aver dormito in tenda nella curva al fondo del rettilineo, son stato svegliato dal rumore celestiale dei sidecar 4 cilindri due tempi…….magari motorizzati con il Kõnig, comunque suono porno.
Per quanto riguarda il mio schizzo provo a postarne un altro, tanto per confondere tutto.
Comunque l’idea sarebbe quella di chiudere completamente l’espansione e utilizzarlo “solo” come un risonatore e aprire subito dopo la luce di scarico del cilindro un altro tubo di scarico.
Nel nuovo schizzo provo ad illustrare le varie fasi.
Il risonatore progettato con una frequenza doppia rispetto a quella del motore, come indichi anche tu.
Questo per permettere alla massa M di continuare ad oscillare con la frequenza propria di risonanza.
Il tubo aggiuntivo permetterebbe  , durante la fase di ascesa del pistone che va dalla chiusura della luce di scarico al punto morto superiore (TDC), alla massa M di ritornare verso il risonatore….. Spero di essere riuscito a spiegarmi meglio.
Certo in questo caso ipotizzo che lo scarico sia sempre aperto in prossimità del cilindro.

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Altra cosa che notavo nei video dei motori da te menzionati è la bassissima fumosità!

To stay with the König, what about this one: [Vous devez être inscrit et connecté pour voir ce lien]

carlovitch1 a écrit:

To stay with the König, what about this one: [Vous devez être inscrit et connecté pour voir ce lien]

I did consider this video too, Carlo, but it does not offer the characteristical high-pitch König scream that I would have liked to share.
It does however give the impression that the König was a very fast bike, seeing how easily it overtook that bunch of four-stroke competitors in the first minutes of the video.
In fact it was very fast when the late Kim Newcombe rode it, but in later years it would not have been a match for a good Yamaha TZ250, let alone for a 250cc Aprilia with 30 hp more than the 78 hp of the 500cc König.

Qui se ne parla
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Two-stroke 3,5 HP to 28 HP in 7 minutes:

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Very nice work ! There's a lot more than 7mn on it...

Frits Overmars a écrit:

Two-stroke 3,5 HP to 28 HP in 7 minutes:

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Sto pensando a come possa funzionare la valvola. Quando questa si chiude, il volume diminuisce e quindi il sistema dovrebbe diventare più rigido e quindi con una risposta accordata ai regimi più alti. Non credo possa essere usata anche nei transitori. O aperta o chiusa. Sbaglio?

About the 7mn video, around 4:17mn I see a tool for cutting the pipe plate that I don't know, it appears to cut quite precisely, does any of you have the reference of this tool?

Une grignoteuse.
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OK, merci.

Frits Overmars a écrit:

...but it does not offer the characteristical high-pitch König scream that I would have liked to share.

May be, there it is.

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What do you guys think of this Engine design With transfer Ports under the Exhaust Port.
My idea is to get a better flow and more gas into the Engine.

Have a look at this paper, it has been tried.

CFD Simulation and Validation of the Scavenging Process in a 125cc 2-Stroke Racing Engine