bimotion theory

[RodneyPierce]

ok guys, whats everyone's take on the bimotion theory? Good, garbage, decent? Please provide reason.

 

[hydro rob]

Depends, Do you like math and long sleepless nights?

http://www.datafan.com/bimotion/2stroketheory.html

 

[RodneyPierce]

ok, after more reading, im curious as to why we dont use something like below. We tend to use head buttons like:

Why not use buttons like below so that they create a gas gap in the upper corners (above the red line) to decrease the super heated temps that cause early ignition IE: detonation?

 

[Tom Foley]

That "Gas Gap " would more than likely cause detonation (collision of uneven flame fronts not radiating from point of ignition ) .

Did you get a VISE yet ? Just messin ...

 

[RodneyPierce]

haha, no go on the VICE.

The above pictures are from the bimotion article. Was just curious. I know there is alot of talk about flat roof'ed buttons, if you will, in the "head button shape" thread.

 

[Ray Sametz]

That "Gas Gap " would more than likely cause detonation (collision of uneven flame fronts not radiating from point of ignition ) .

Did you get a VISE yet ? Just messin ...

Stone cold Ball Breaker.... :lol:..

 

[Steve Anderson]

.

 

[RodneyPierce]

well Steve, pushing the combustion chamber out like that wouldn't necessarily mean a reduction in compression. just means you would need a shallower bowl to maintain the same ratio. as for the rest of your post, well that is some very good food for thought.

 

[Steve Anderson]

.

 

[dwilfong]

I have tried a new chamber design on my 90RS. The top of the chamber is bowl shaped and the sides are strait. Still tuning on the eng. but the first race it worked good. Eng has lots of power can pull allot of prop. Now I will shorten the pipe and see if it can REV.

I don't think that a high RPM eng will benefit from this design. I feel that flame propagation will be to slow at high RPM. A hemispherical chamber will promote good flame travel and with the strait entrance to the cyl. from the sides being strait at the end. Should direct the charge strait down at increased velocity. Charge velocity will move the piston faster.

It all sound good will see what happens.

David

 

[Andy Brown]

Detonation starts at the O.D. of the squish band. Research shock tubes and you'll understand why.

 

[dwilfong]

Detonation starts at the O.D. of the squish band. Research shock tubes and you'll understand why.

Thanks Andy lots of meat on that bone.

Already started reading good stuff.

David

 

[hydro rob]

Detonation starts at the O.D. of the squish band. Research shock tubes and you'll understand why.

I agree!

 

[RodneyPierce]

Alright Andy. Ive been reading through this shock tube stuff, and nearest I can gather so far, it would start at the outer diameter of the squish band because the high pressure created at the edge of the squish, by the pistons upstroke, compressing the gas charge, would cause the charge to go off early or pre ignite causing the detonation? Am I close, or way off in left field. Man, there is ALOT to chew on there, and some of it im having a hard time trying to compare to 2 stroke engines..

 

[Andy Brown]

That's kinda close but yet way off.

If you have a tube with an open end and a closed end (a shock tube) and you fill it with a combustible gas and you ignite the gas at the open end of the tube, the flame front will steadily burn the gas as the flame front moves toward the closed end. When the flame front reaches the end of the tube the flame will quietly cease to burn.

On the other hand, If you place an ignition device at the closed end of the tube and ignite the gas from that point and immediate shock wave will ensue. This shock wave is Very high in Pressure and temperature. In some cases it can damage (Explode the Shock tube).

I remember hearing my dad talk about making Acetylene cannons back in the day. All that was required was an old drive shaft and an Acetylene cutting torch. (Dangerous thing to do) You get the picture!

In our Engines...

Once the piston reaches TDC the chamber becomes the open end of the tube and the cylinder wall becomes the closed end of the tube.

If the metal in this area (at cylinder wall) gets too hot, it causes the fuel to ignite at the cylinder wall before the flame front that was gently propagating from the glow plug reaches the cylinder wall....and Walla! Big Explosion! That's why a brand new engine will sometimes burn the plug on the first run...because of the heat created by the piston/cylinder friction and the ensuing Explosion (detonation). Many times it's the shock vibration of this event that simply breaks the glow plug wire, but is not intense enough to damage metal parts (pepper head button and piston) . Keeping the metal parts cool in this area is important and of course it's easy to see why detonation creators can often be found near the exhaust port area first.

 

[RodneyPierce]

ah ha!!! Makes perfect sense. See, I just needed someone to lead me to the water I was thinking about it a minute ago, before I came back here, about the sharp edges on a head's combustion chamber, and that area trapping heat, and causing pre ignition. Just wasnt equating it quite right to our little model motors. NOW, how do we keep that area of the head cool enough to NOT pre ignite the fuel?? More squish angle in the button itself, or a small flat around the outer edge of the button, then the angled squish, with a rolled edge? The flat to me would create more heat... but I could be wrong. I know some of your buttons have a very flat roof with straight walls in the chamber, but I havnt actually had one to examine and see if the squish is flat or not.

 

[Andy Brown]

ah ha!!! Makes perfect sense. See, I just needed someone to lead me to the water I was thinking about it a minute ago, before I came back here, about the sharp edges on a head's combustion chamber, and that area trapping heat, and causing pre ignition. Just wasnt equating it quite right to our little model motors. NOW, how do we keep that area of the head cool enough to NOT pre ignite the fuel?? More squish angle in the button itself, or a small flat around the outer edge of the button, then the angled squish, with a rolled edge? The flat to me would create more heat... but I could be wrong. I know some of your buttons have a very flat roof with straight walls in the chamber, but I havnt actually had one to examine and see if the squish is flat or not.

For the area in question...the outer diameter of the piston, head and cylinder wall, it's a matter of adequate cooling. Either with outside sources such as water and air and inside sources such as fuel.

Radiant heat transfer is also a problem. Heat transfers by radiation...just like the sun heating the earth. So once the flames starts at the glow plug it immediately begins heating the end gases and metal in the cylinder.

I have never been able to prove the sharp edge at the ID of the squish band to be a contributor to detonation.

BTW- The squish band is a simple mechanism that acts as a "fuel injector". A "poor mans" fuel injector" if you will.

 

[RodneyPierce]

ok, makes sense. I knew the squish was in charge of helping the fuel mixture get to the bowl for ignition, but never looked at it like a "fuel injector". So in theory, why arent people running larger angle squish, other then the 2-4* norm? Or is there an area, that it just doesnt get any better or faster ignition, no matter how much angle you add?

 

[dwilfong]

Andy There has bin allot of discussion of Maximum Squash Velocity. Where doses this fall in to the equation if at all.

David

 

[RodneyPierce]

ah ha!! MSV, im betting that has to do with the angle of the squish