squish bands...yay.
yes, it needs to be a minimum.
these are not diesel engines, "squish velocity" is just a term thats thrown around, like "cnc machining" and "rapid prototyping"... it doesnt make much difference on a two stroke, just like a cnc machine is commonplace...
yes, the squish band is there to confine the air/fuel charge to the combustion area. yes, having it too large (clearance) reduces power drastically.
but no...they arent 100% essential.
they reduce surface area of the chamber, but only when the gap is correct. any other time, they are a detriment.
hemispherical is best, but then, to retain that shape, and to raise compression ratios, the arc becomes far too wide, the "bulge" too low.
the edges of the piston and combustion chamber, forming acute angles, are places of large contact area, far away from the flame kernel, and simply never ignite. the plug can hit the piston... etc etc
the air fuel charge in close proximity to the cylinder piston or chamber wall DOES NOT IGNITE.
so, with a high compression hemi head, the shape is ideal, but the surface area is NOT. the flame has too travel too far...
any other shape is bad, as it only increases surface area, or lowers compression.
in detonation...random pockets of air fuel charge ignite AFTER the initial flame front is established, due to the sudden pressure increase raising them to the point of self ignition. this can be due to load, advanced timing, lean, chamber designs, etc.
(preignition is ignition BEFORE the spark, caused by a hotspot that may be caused by detonation... but usually chamber shape design. mostly in fourstrokes, but also common in twostrokes with lots of carbon build ups... or overly hot plugs)
high octane fuel reduces this tendency to detonate as it burns slower... firstly, the pressure increase is not so sudden... therfeore theres more time for the flame front to spread and ignite more.
secondly, possibly, the detonation still occurs, but the unwanted ignitions are also slowed down, so there is no drastic, damaging, pressure increase at all...that bits conjecture on my part
the extra flame fronts would also help for good overall combustion i guess...if controlled
so anyways. the cooled bits of airfuel charge at the edges of a (high compression) hemi chamber dont burn. therfore, a loss of power. plus the shape is so narrow and wide it really likes to detonate. (because the time for the pressure to increase to the point of detonation is far less than the time the flame takes to propagate through the charge and ignite it properly.)
so we make the chamber a smaller hemisphere, and attempt to get the edge of the head so low and close to the cylinder that there is no airfuel in there at all... or, that being impractical in reality, we settle for a very small percentage of our cylinder contents, being held so closely confined that they DONT IGNITE. regardless of the pressure...the only way to get self ignition in the squish zone is through drastic temperature increases as well.
get the gap a bit too big, and power suffers becuase of the larger percentage of charge within the squish zone that never ignites in comparison to whats in the chamber.
get it way too big and you get detonation as the partially confined charge is compressed, not all cooled by the walls, and therefore highly flammable... so when the ignition pressure spike comes, it is extremely prone to self ignition...
this is what also happens with any other shape but a hemisphere... pockets of charge get trapped, and go BANG.
thats simply all a squishband is. the only sensible, logical method of getting a hemispherical chamber, with the minimum of surface area meaning more bang for your buck as the smaller chamber allows increases to be made to the compression ratio before causing detonation.
any other geometry simply wont do for high compression ratios.
