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 [2 stroke] Spherical vs Toroïdal head design (UK)

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Djamitague

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

MessageSujet: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 28 Fév 2015 - 1:23

Hello,

As I'll maybe get a better feedback in English... I'm interested to know advantage/drawback of a toroïdal shape vs spherical, if it could be compared. It seems to me that toroïdal design is more rev oriented, with lower trapped ratio due to higher exhaust charge, but this is just a "feeling" - which tend to be step #1 in making mistakes Wink Overall, it's looks like pressure is clearly oriented to the piston (again in the range of "feelings" )

My application is a single 2 stroke 150cc, air cooled, reed case induction, with a reso band that ends @ 9 500-10 000 rpm, so I would write a torque oriented set-up.
Actual head design:

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Djami



Dernière édition par Djamitague le Lun 16 Mar 2015 - 19:41, édité 1 fois
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Bob van der Zijden



Nombre de messages : 94
Localisation : Hollande
Date d'inscription : 10/09/2013

MessageSujet: Tor. vs hemisp. chamber   Mer 11 Mar 2015 - 23:15

Just 1 remark about your hemisph. chamber : if  I see it right you plan a radius from the squish area to  the actual chamber. This is detrimental for the squish function. This transition should be SHARP. For a Pmax @ 9500  I think the difference between the toroïdal job and the hemi one wouldn't make much difference.
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Ven 13 Mar 2015 - 7:42

Hi Bob!

Thanks for your tip Wink

You're right, there's a radius at junction; I will remove it then, gasses like sharp transition :). As my cylinder head will be a two-pieces assembly I will start first with casual hemi design.


Cheers
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seattle smitty



Nombre de messages : 29
Localisation : USA northwest
Date d'inscription : 26/10/2014

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Dim 15 Mar 2015 - 17:24

IF, repeat IF I understand correctly, the "toroidal" combustion chamber appears to work best with flat-top pistons, or pistons that are flat-topped but have an angled squish-band, whereas a semi-spherical chamber seems to work a little better with pistons that have the more usual domed crown. The old "half-a-golfball" shape (as in your photo) has been superceded by a wider diameter, more flattened shape for a given volume, with the squish area having got smaller. Have I got this about right, Bob?
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Lun 16 Mar 2015 - 19:32

So, summary:
- Sharped squish edge
- 53% squish area
- 11,5cc volume (without plug hole)
- Squish gap 1,3mm (a rectified ring will take place under the head)
- hemi design  arc design

Note that 3D allows something between both shapes, but let's stick to conventional one. Special stuff is expensive enough to generate additionnal costs wih seizure.

Another question is typical plug to piston top distance?

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(the blue dome comes from Parmakit)
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JanBros



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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Lun 16 Mar 2015 - 23:27

why such a big squish gap ?
how long is your rod/stroke ?
I thinck 1mm (or even smaller ) would be much better.
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 13:39

Hi Jan,

The conrod is 105mm long and it's a 54mm stroke. I know that squish gap should be as small as possible but it's just an air cooled head, for standard 98 fuel.
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GrahamB

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Age : 56
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Date d'inscription : 19/08/2011

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 15:36

My naive guess is that using a partial sphere with the same volume as a toroidal shape implies that the angle at the edge of the squish is less acute. That would potentially be less effective at generating turbulent mixing, in the same direction as using a radius. It's only a guess, however.

1.3mm squish seems about 2x bigger than what I'd shoose to run...
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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: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 16:12

Djamitague a écrit:
Note that 3D allows something between both shapes, but let's stick to conventional old-fashioned one.
Corrected that for you  Wink .
GrahamB a écrit:
My naive guess is that using a partial sphere with the same volume as a toroidal shape implies that the angle at the edge of the squish is less acute. That would potentially be less effective at generating turbulent mixing, in the same direction as using a radius... 1.3 mm squish seems about 2x bigger than what I'd choose to run...
+1
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 17:43

GrahamB a écrit:
My naive guess is that using a partial sphere with the same volume as a toroidal shape implies that the angle at the edge of the squish is less acute. That would potentially be less effective at generating turbulent mixing, in the same direction as using a radius. It's only a guess, however.

1.3mm squish seems about 2x bigger than what I'd shoose to run...

I was also thinking this way: sudden break generates more turbulence. Anyway my piston as a radius on top, so curved shape.
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 18:05

Frits Overmars a écrit:
Djamitague a écrit:
Note that 3D allows something between both shapes, but let's stick to conventional old-fashioned one.
Corrected that for you  Wink . Very Happy Very Happy


GrahamB a écrit:
My naive guess is that using a partial sphere with the same volume as a toroidal shape implies that the angle at the edge of the squish is less acute. That would potentially be less effective at generating turbulent mixing, in the same direction as using a radius... 1.3 mm squish seems about 2x bigger than what I'd choose to run...
+1

Just tell me if I'm wrong: as you wrote, squish clearance can't be too small (regarless minimal mechanical tolerance), but the lower the squish is, higher the head temperature will be?
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Frits Overmars

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MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 18:44

Djamitague a écrit:
the lower the squish is, higher the head temperature will be?
No. A tight squish will speed up the combustion, so the piston, head and cylinder will be exposed to the flame for a shorter period of time. There will be more expansion after the end of combustion, the spent gases will cool down more before the exhaust opens, and less heat will be transferred to the exhaust duct.
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mar 17 Mar 2015 - 20:22

Frits Overmars a écrit:
Djamitague a écrit:
the lower the squish is, higher the head temperature will be?
No. A tight squish will speed up the combustion, so the piston, head and cylinder will be exposed to the flame for a shorter period of time. There will be more expansion after the end of combustion, the spent gases will cool down more before the exhaust opens, and less heat will be transferred to the exhaust duct.

Thanks a lot Frits! (The only problem is that.... each answer leads to twice other question........... :))
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Jeu 19 Mar 2015 - 18:25

Bob van der Zijden a écrit:
Just 1 remark about your hemisph. chamber : if  I see it right you plan a radius from the squish area to  the actual chamber. This is detrimental for the squish function. This transition should be SHARP. For a Pmax @ 9500  I think the difference between the toroïdal job and the hemi one wouldn't make much difference.
Hi Bob,

Re. the squish transition, I have here one head (125 NX4 97) with a consistant radius at transition, is it linked with the toroïdal design of the cup? Does it make the squish band larger?

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Dernière édition par Djamitague le Jeu 19 Mar 2015 - 21:20, édité 1 fois
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Bob van der Zijden



Nombre de messages : 94
Localisation : Hollande
Date d'inscription : 10/09/2013

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Jeu 19 Mar 2015 - 18:57

I see only one combustion chamber on your picture and it shows clearly a radius, which is not good at all. Frits Overmars explained perfectly well what a proper functioning squish band really does: it's a major contribution to a good combustion.
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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: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 21 Mar 2015 - 13:05

Bob van der Zijden a écrit:
I see only one combustion chamber on your picture and it shows clearly a radius, which is not good at all. Frits Overmars explained  perfectly  well what a proper  functioning squish band really does: it's a major contribution to a good combustion.

In addition, is there a typical, or minimal value for the distance shown below?
The spark plug shown is just a sample, I will use one which doesn't protrude (NGK B9EGP)

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Sanderhoutman



Nombre de messages : 51
Localisation : netherlands
Date d'inscription : 23/12/2012

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 21 Mar 2015 - 13:10

Surface discharge sparkplug doesn't hide your spark from the mixture.
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Frits Overmars

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MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 21 Mar 2015 - 16:17

Sanderhoutman a écrit:
Surface discharge sparkplugs doesn't hide your spark from the mixture.
That is correct, but those plugs require an exceptionally strong ignition system, Sander.
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Sanderhoutman



Nombre de messages : 51
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Date d'inscription : 23/12/2012

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 21 Mar 2015 - 19:33

Isn't this depending on the gap for the spark to cross?
I've heared the brisk is performing ok with a ignitech. Or Pvl.
I had trouble with sparkplugs that had the tip concealed.
Creating a head with the ideal squish an volume is less efficiënt when the spark is arround the corner.

I was not aware that the surface discharge plugs put such load on ignitions that the stock ignition might not suffice
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Bob van der Zijden



Nombre de messages : 94
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Date d'inscription : 10/09/2013

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Sam 21 Mar 2015 - 20:31

If you pick the wrong plug and most probably dont even know what side gapping is you cannot expect good results. Also here the word is : do your homework...
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Djamitague

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

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Mer 25 Mar 2015 - 11:22

Here it is:
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Central vs Original:
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Tim Ey



Nombre de messages : 16
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Date d'inscription : 22/05/2013

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Dim 12 Avr 2015 - 22:04

sorry for the shitty image, but "been there, done that" :-D
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Same engine type as yours - but with an old honda cr125 Cylinder. But unfortunatelly I did not had the time to test it...
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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: [2 stroke] Spherical vs Toroïdal head design (UK)   Ven 24 Avr 2015 - 13:55

Hallo Kollegen!

Pleased to meet you here! I'm also on the GSF Wink
Mine I start it up but the clutch reached its own limit.............

Is it LC or AC cylinder? Your squish seems to be stepped...?
I ran mine on a 60x54, head volume is 11,6cc (without plug hole) and I'm pretty hurry to Dynoïze it Very Happy
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Djamitague

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Date d'inscription : 18/07/2014

MessageSujet: Re: [2 stroke] Spherical vs Toroïdal head design (UK)   Ven 24 Avr 2015 - 14:07

I have on the bench a bigger expansion (belly diam. 133mm), I will run with a higher volume (12,7cc) and smaller squish band (46%). But I need first a consistant clutch.

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