[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

Nombre de messages : 151 Localisation : usa Date d'inscription : 29/09/2013

really i dont know because i wasnt there when it was welded. i assumed tig but who knows. its seems harder to grind than the aluminum thats all i know. recently i did go to the local welding supply shop to look at buying a new welder so i can do my own work wanted - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 3 771973

wanted - [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4) - Page 3 771973

Nombre de messages : 56 Localisation : Austria Date d'inscription : 02/03/2014

I have another question, this time about cylinderhead design.
I'm making a head which needs a very low compression ratio for a good reason.
Now I have 2 options, does anybody has an idea which of these 2 will be best?

Option 1.
I achieve the big head volume by making the radius of the combustion chamber so big that I will not have a toroidal shape anymore, but a perfect (half) sfere.

Option 2.
I can use a decent radiuus (like the one from an RSA) so that I have a toroidal shape, but make a sort of vertical wall of 5.9mm before starting the radius. Like this:
[Vous devez être inscrit et connecté pour voir cette image]

Thanks already!

I like the low compression ratio. Senne. But I do not understand you dilemma.
The half sphere option would indicate that there is sufficient height available. And the second option would indicate that there is sufficient radial space and material available. So why can't you make a proper toroidal shape?

Nombre de messages : 56 Localisation : Austria Date d'inscription : 02/03/2014

If I try to do that I have to make te radius (of the toroidal head) so big that is intersects the spark plug, so that there is no horizontal space (or offset like you called it some pages ago) anymore. Maybe this pictures will make it more clear:

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

(the black thing is the spark plug)

In the last picture, the radius is as big as the radius of the inner combustion chamber (minus the the spark plug), thus leaving no offset, or flat horizontal surface, anymore so this is a hemisphere.

(The sizes are just an example)

Senne s a écrit:: If I try to do that I have to make te radius (of the toroidal head) so big that is intersects the spark plug

That is no problem at all, Senne.

Nombre de messages : 56 Localisation : Austria Date d'inscription : 02/03/2014

I do not think I understand it completely Frits, so the last picture I posted is the best design?

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Senne s a écrit:: I do not think I understand it completely Frits, so the last picture I posted is the best design?

yes because it isn't a hemispere. you still have some offset. offset = half the width of the spark plug.

Nombre de messages : 56 Localisation : Austria Date d'inscription : 02/03/2014

Thanks for the answer JanBros,
But then isn't that's like saying there are no real hemispherical 2stroke heads (with the plug in the middle) because every head needs that offset for a plug?

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Senne s a écrit:: Thanks for the answer JanBros,
But then isn't that's like saying there are no real hemispherical 2stroke heads (with the plug in the middle) because every head needs that offset for a plug?

guess if you use a tiny plug, you could make a "almost entire" hemispere ;-)

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

Sorry if this has been asked before, can't remember.
I'm making an excel for calculating heads, but I can not find the formula for calculating volumes like these (basicly some sort of "solid frisbee" ) , for combustion chambers like the drawing.

Mes excuses si c'était déja demander, je ne me rapelle pas, mais je suis en train de fabriquer un excel pour calculer des culasse, et j'ai besoin du formule pour calculer des volmues comme :

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

Nombre de messages : 5 Localisation : Auckland Date d'inscription : 08/05/2013

Hello my name is Andrew and I have gained much information from these pages for which I thank you all very much.
My question is Can the inlet manfold between the carb and the rotary valve be too short??
I race a PVP 251 motor which has a relatively short manifold. It could however be made 5mm shorter quite easily. Would there be an advantage in doing this? Or should I leave it alone?
I have researched these pages but could not find the answer. I aplogise if this topic has been previously covered.

JanBros a écrit:: I can not find the formula

If the height is h and the radius of the cylindrical part is R (so total radius is R+h), I get

V=2*pi() *h² *(R + h/3),

but you'd best check it, it's been a while :)

In general the volume of a solid rotated around the z-axis with height given by z(x) at distance x from the centre is

V= 2 pi integral x z(x) dx,
where the limits of integration are from 0 to the max radius.

JanBros a écrit:: Sorry if this has been asked before, can't remember.
I'm making an excel for calculating heads, but I can not find the formula for calculating volumes like these (basicly some sort of "solid frisbee" ) , for combustion chambers like the drawing.

Mes excuses si c'était déja demander, je ne me rapelle pas, mais je suis en train de fabriquer un excel pour calculer des culasse, et j'ai besoin du formule pour calculer des volmues comme :

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

JanBros, if you draw your chamber in solidworks there is no need , just use the evaluate tab then select mass properties and the volume of the model is displayed.
Mark

JanBros a écrit:: Sorry if this has been asked before, can't remember.
I'm making an excel for calculating heads, but I can not find the formula for calculating volumes like these (basicly some sort of "solid frisbee" ) , for combustion chambers like the drawing.

Mes excuses si c'était déja demander, je ne me rapelle pas, mais je suis en train de fabriquer un excel pour calculer des culasse, et j'ai besoin du formule pour calculer des volmues comme :

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

V = πR[πR(D-R) + D2 + 4R2 - 4DR]/4

where:
D = Combustion Chamber Major Diameter to corner of Squish Band
R = Radius of the Edge Arcs
π = Supposed to be PI, but looks more like an "N"
D2 and R2 are supposed to be D-squared and R-squared

This assumes that the edge radius R runs through 90 degrees, from vertical to horizontal, hence the chamber height is also R. The formula also doesn't (and cannot) take into account any piston dome intrusion into the combustion chamber.

In Excel, if D is in B2 and R is in C2, then Volume cell is:

=PI()*C2*( (PI()*C2*(B2-C2)) + (B2^2) + 4*(C2^2) - 4*B2*C2)/4

It's rare for a volume formula to include a pi squared...

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

oxracer a écrit:: JanBros, if you draw your chamber in solidworks there is no need , just use the evaluate tab then select mass properties and the volume of the model is displayed.
Mark

I know that, but that isn't usable in an excel file. it is usable though to check the presented formula's

say the radius R at the edge is 10mm and the base dia D is 40mm,

than Solidworks says it has a volume of 10.17cc

GrahamB a écrit:: If the height is h and the radius of the cylindrical part is R (so total radius is R+h), I get

V=2*pi() *h² *(R + h/3),

-> volume of 46.08cc

lodgernz a écrit:: [
V = πR[πR(D-R) + D2 + 4R2 - 4DR]/4

where:
D = Combustion Chamber Major Diameter to corner of Squish Band
R = Radius of the Edge Arcs
π = Supposed to be PI, but looks more like an "N"
D2 and R2 are supposed to be D-squared and R-squared

-> volume of 10.32cc
close, but not close enough Wink

GrahamB a écrit:: It's rare for a volume formula to include a pi squared...

Heh heh. Yes indeed.
Think of the chamber as a central "cake tin" cylinder and a surrounding ring with a curved upper surface.
Now imagine the outer ring is actually square in section, R x R. Its volume can easily be calculated, and there will be pi involved.
In reality, the cross-section is not square, but curved with a radius of R. The area of this real cross-section is pi/4 times that of the notional square section, so the volume of it will be pi/4 times the volume of the notional square-section ring. That's where the extra pi comes in. I had to use simple geometry as I've forgotten my calculus of 50-odd years ago.

I could be totally wrong of course, but my calcs on a couple of my heads come out about right, so maybe not.
I'd like to see a more elegant formula though, so go for it.

JanBros a écrit:

than Solidworks says it has a volume of 10.17cc

lodgernz a écrit:: [
V = πR[πR(D-R) + D2 + 4R2 - 4DR]/4

where:
D = Combustion Chamber Major Diameter to corner of Squish Band
R = Radius of the Edge Arcs
π = Supposed to be PI, but looks more like an "N"
D2 and R2 are supposed to be D-squared and R-squared

-> volume of 10.32cc
close, but not close enough Wink

...Rounding errors in Solidworks :o)

lodgernz a écrit:: so the volume of it will be pi/4 times the volume of the notional square-section ring.

No it won't sorry. Because it's swept around in a circle, parts further from the centre contribute more.

BTW, my formula is wrong but not that wrong, if you check my definition or R and h, they are each 1cm, so it gives 8 pi/3.

The correct formula, re-doing more carefully, is

V= Pi * h *(R + 2 * h²/3 +R *h * Pi /2 )

(So it does indeed contain a Pi² !)

Subbing in R= h =1, V=10.1708... Solidworks is not stupid :)

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

tnx for your efforts guys, but your new one doesn't seem to work to Graham, or I'm doing/interpretting something wrong. I've updated my picture, so we can all use the same variables. Could you rephrase your formula using those Graham ?

Sigh, or you could re-read the definition. R is the diameter of the inner cylindrical part. So if D is the total diameter, D=2R+2h, or R=D/2 - h. Hence for 4cm total diameter and 1cm height, R=1cm.

Note that if R=0, it becomes the formula for the volume of a hemisphere of radius h, as it should.

Nombre de messages : 362 Localisation : Belgique Date d'inscription : 05/12/2011

sorry Graham, I was right. I've found the solution here : [Vous devez être inscrit et connecté pour voir ce lien]

so we need to divide that formula by 2 and ad the volume of the inner cylinder, resulting in : pi*R* (pi*R*(4*R/3/pi+A1)/2 + A1²)

to enter in excel and have cc as outcome :

PI()*B1/1000*(PI()/2*B1*(4*B1/3/PI()+B2)+B2^2)

B1 = height of combustion chamber = radius of "edge"
B2 = radius of inner cylinder

GrahamB a écrit:

lodgernz a écrit:: so the volume of it will be pi/4 times the volume of the notional square-section ring.

No it won't sorry. Because it's swept around in a circle, parts further from the centre contribute more.

BTW, my formula is wrong but not that wrong, if you check my definition or R and h, they are each 1cm, so it gives 8 pi/3.

The correct formula, re-doing more carefully, is

V= Pi * h *(R + 2 * h²/3 +R *h * Pi /2 )

(So it does indeed contain a Pi² !)

Subbing in R= h =1, V=10.1708... Solidworks is not stupid :)

Graham, I think your formula has an error. I believe it should be:

V= Pi * h *(R² + 2 * h²/3 +R *h * Pi /2 )

don't forget the volume inside the plug... Rolling Eyes

romeuh80 a écrit:: don't forget the volume inside the plug...

I seem to remember Frits (or maybe Jan) saying the plug volume could be ignored.
I guess that's because the gas in there stays there and has no influence.
Or possibly because the volume of the bit of plug that sticks out into the chamber approximately equals the space inside the plug.
I'm sure someone will comment on this.

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