[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 1) (Locked)

Ca va être dur dur de ne plus les admirer, ces fabuleuses 125.

snifff , c'est tellement beau !!!! Pardon c'était .....

pourquoi!!!

Hi everyone , this days I did reedless experiment on Piaggio 50 scooter engine. It has 70 cc and modified crankshaft for low crankcase compression. The reeds were completely removed and engine starts only if you block the back of carburator because of backflow effect.
From idle to low rpms you also need to play with backflow but after that dip engine revs very good without sputtering and backflow.
After that experiment, tonight I made simple 24/7 inlet valve. It is just for testing and it is compatible for Piaggio 50-180 engines. I need to find better plastic rod to insert the reed and activation is manual for now.

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Hello. My name is Marko, I'm a two-stroke enthusiast and exhaust builder.
This is a great topic to read, thanks everyone for sharing.

I have a question for you experienced engineers; what about offset cylinders?
By offset I mean cylinders that have the bore offset from the crankshaft center, so the bore is moved forward or backward or leaned forward or backward. I know some 4T engines implement such solution, but on a two-stroke there are more factors that play a role.
I know that by offsetting the cylinder the effective stroke will increase slightly, the connecting rod angle will change, the opening of the ports becomes asymmetric...perhaps more.
But was there any experimenting done, to confirm or discard, that with such solution any performance could be improved?

Thanks.

hiya Marko,
some engines use pistons with the gudgeon pin moved backwards or forwards for the same reasons,

an offset can allso be used so one side of the piston puts more pressure on the cylinder wall to make a better seal, at the exhaust port for example,

as you say, it can change port timings

i(d like to know more aswell

Bonjour monsieur frits et tous les autre.

J'aurais une question pour les membres.
Quelle serait la longueur maximum par rapport a une course donner pour augmenter les temps de transfert sans augmenter les diagramme de façon a "singer" un moteur carré (en time area) et sans tomber dans l’excès (fragilité/poids).
Le moteur qui m’intéresse est un moteur super carré de 70cc environ de course 40 mm et alésages 47.6 mm la bielle actuelle a un entraxe de 85 mm donc un rapport de 2.125 est une aprilia de GP un rapport de 2.22 je crois, donc je pourrais aller vers une bielle de 90? cela augmenterais mes temps d'ouverture de façon significatif?
IL y a t' il d'autre paramètre a prendre en compte comme la cylindrée (alésage ou course) ou les RPM.

Sur ce au revoir et merci.

Bonjour mr. Solexeur,
I am not sure I completely understood your question.
I tried the Google translator, but as usual that only increased the confusion .

If you change the stroke of the crankshaft, all port timings will always change.
If you change the con rod length, you will have to adjust the cylinder height and once you have done that, the port timings will differ very little from their original values.

I'll see if I can do better than google [Vous devez être inscrit et connecté pour voir cette image]

I think the question is:
To what extent can one achieve the same transfer time-area for an over-square motor as in a square motor, by increasing the length of the conrod, without excessive fragility or weight.

The practical example is a motor with bore x stroke 47.6 x 40, and 85 mm rod, so rod/bore ratio 2.125, compared an Aprilia GP ratio of 2.22. Would it be possible to go to a 90mm rod? Is it necessary also to take into account the capacity (or bore or stroke) or rpm?

Bonjour M. Frits

Je parlais de la cinématique bielle manivelle.
En gros du fait que le piston parcours moins de distance pour la même valeur angulaire au vilebrequin avec une bielle plus longue au abords du PMB est du PMH. Donc augmenter les time area de façon a ce rapprocher des valeur d'un moteur carré.
Ou me dire si je fais fausse piste.
Excuser moi pour la mauvaise explication de la problématique.

cordialement

Lucas

GrahamB a écrit:

I'll see if I can do better than google. I think the question is:
To what extent can one achieve the same transfer time-area for an over-square motor as in a square motor, by increasing the length of the conrod, without excessive fragility or weight.

The practical example is a motor with bore x stroke 47.6 x 40, and 85 mm rod, so rod/bore ratio 2.125, compared an Aprilia GP ratio of 2.22. Would it be possible to go to a 90mm rod? Is it necessary also to take into account the capacity (or bore or stroke) or rpm?

Thanks Graham. You can do better than Google (and I always admire your mastery of French. How on earth did you pick that up?)

The answer to the fist question: it is simply not possible. Assuming the full circumference of the cylinder is utilized for porting in both cases, a short-stroke engine will never approach the angle.areas of a square engine.
Using a longer rod is usually a good idea because it will increase crankcase volume. It will also alter the port timings, but this effect is minor; it is not an effective way to improve the angle.areas.

Frits Overmars a écrit:

How on earth did you pick that up?

Des françaises... [Vous devez être inscrit et connecté pour voir cette image]

lesolexeur a écrit:

Bonjour M. Frits
Je parlais de la cinématique bielle manivelle.
En gros du fait que le piston parcours moins de distance pour la même valeur angulaire au vilebrequin avec une bielle plus longue au abords du PMB est du PMH. Donc augmenter les time area de façon a ce rapprocher des valeur d'un moteur carré.
Ou me dire si je fais fausse piste.
Excuser moi pour la mauvaise explication de la problématique.
cordialement
Lucas

Hi Lucas, now I understand your question. A longer con rod will make the crankshaft behave like it has a longer stroke in the vicinity of Bottom Dead Centre, so it will reduce the port timing. You can easily see it graphically when you draw a port with a height of 1/4 of the stroke (that is about the usual height of a transfer port) and then draw crankshaft positions with a very short and a very long con rod.
But like I wrote above, changing the con rod length will not compensate for the shortcomings (!) of a short-stroke two-stroke.

GrahamB a écrit:

Frits Overmars a écrit:

How on earth did you pick that up?

Des françaises... [Vous devez être inscrit et connecté pour voir cette image]

OK, let me rephrase: how did you pick them up ?

EDIT: Don't answer that. I do not want either of us to get into trouble with our grand Taulier.

Bonjour

Merci a GrahamB pour la traduction et a Frits pour l'explication.

lucas

Hello everyone. This is my first post on here and I find this topic fascinating. I race snowmobiles and I am the manufacturer of hydroformed tuned exhaust pipes. I have been developing a rotary valve engine with a variable inlet system. My dyno work has shown that there is little to gain by opening the valve before 150 degrees but there is gains to be made by closing the plate later (up to 110 degrees atdc) The trouble is the engine will not start with the late closing so I developed a very simple method of varying the closing by up to 25 degrees. i use a second plate mounted ontop of the spinning plate that rotates 25 degrees and is cam actuated and operates with the throttle cable. i use what would be the oil injection cable to operate the cam. At idle the plate closes the valve at 80 degrees. it restricts the size of the port but this is not a problem because the port is always larger area than the carb area under the slide at partial thrpottle. At full throttle the plate opens the port fully and closes at 105 degrees. More than 105 degrees and the engine seems to not like it and loses power.
I will try to post some pictures of how it works.

CDan someone help me with how to upload the pictures .

cant wait to see the picture,
sounds great work,

Howard Gifford a écrit:

CDan someone help me with how to upload the pictures .

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In this picture the throttle position would be at idle

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At 1/2 throttle

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And full throttle you see the ports are now unrestricted

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Here is a picture of an example of my hydroformed pipes I manufacture.