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

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" [2 stroke] Spherical vs Toroïdal head design (UK) 809262

[2 stroke] Spherical vs Toroïdal head design (UK) 809262

)

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

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

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!

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

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 The old "half-a-golfball" shape (as in your photo) has been superceded by a wider diameter, more flattened shape for a given volume, with the squish area having got smaller.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?

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)

Nombre de messages : 351 Localisation : Belgique Date d'inscription : 05/12/2011

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

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.

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...

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

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.

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

.

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?

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.

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........... :))

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

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.

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

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.

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

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

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...

Here it is:
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Central vs Original:
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Nombre de messages : 20 Localisation : Allemagne Date d'inscription : 22/05/2013

sorry for the shitty image, but "been there, done that" :-D
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[2 stroke] Spherical vs Toroïdal head design (UK) 980796

[2 stroke] Spherical vs Toroïdal head design (UK) 980796

Same engine type as yours - but with an old honda cr125 Cylinder. But unfortunatelly I did not had the time to test it...

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

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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Nombre de messages : 5 Localisation : Barcelona Date d'inscription : 06/01/2021

Hi!

Is the purpose of combustion chambers where the plug is lower than the combustion roof, to centralize the plug and reduce combustion times?

In a big bore two stroke, would a dual plug head be an advantage? If so, what should the combustion chamber look like? I'm imaging something like the plugs situated at 1/4 of the diameter of the bowl from the edge of the bowl. And a kind of ridge on the center of the bowl?

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