stroke combustion chambers with their overhead valves layouts went through many design changes over the years.
The four stroke combustion chamber layouts evolved through the plain cylindrical form with the required clearance volume, the bath tub type, the wedged shape type, and the hemispherical cross flow type. The hemispherical combustion chamber or hemi-head provides room to accommodate larger valves increasing volumetric efficiency and permits centrally located spark plug which contribute to more efficient combustion, better heat dissipation and higher thermal efficiency.
The concept of a portion of the combustion chamber at close proximities to the piston crown at TDC came to be known as "squish" area or "squish" band earlier referred to as quenched area. In principle, the trapped charge between the piston crown and the squish area nearing TDCmachined smooth. The squish area could be a band or a tapered area or two bands on opposite sides. The squish area are either flat or angled depending on the profile of the piston crown. They are machined smooth to a high degree of finish and set up in design with a close tolerance between combustion chamber and piston at TDC preventing contact.
In principle, the piston on the upward stroke causes the compression to progressively increase. Nearing TDC, the gases around the squish band and the piston crown are pushed towards the center scoop causing Turbulence which in turn improve flame propagation as ignition has occurred before TDC and greatly reduces Pinging and Detonation. Thus, present day two stroke combustion chambers are hemispherical or the "top hat" type