To properly identify a combustion chamber, all its aspects including shape need to be considered. For purposes of this article we will limit the discussion to thosefound on most production engines in America.
Hemispherical or Pentroof - The "Hemi" Combustion Chamber
A chamber of this design is considered to offer the least amount of compromise for theefficiency gained. Effectively, a hemispherical combustion chamber is one half of a sphere cast into the bottom of the cylinder head. The valves are placed at the outside of the bore area and at aspecific angle from the crankshaft centerline. Optimizing this position allows for huge airflow gains, since it moves the valve away from the wall to keep it from creating turbulence.
This creates amore efficient cross-flow movement of the charge during valve overlap and limits thermal transfer from the exhaust valve to the fresh charge. As mentioned previously, this design offers the bestsurface-to-volume ratio and also creates a very short direct exhaust port, essential in limiting heat rejection into the coolant.
Hemispherical chambers generally have a central spark plug, whichoffers excellent octane tolerance. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer.
Used over the yearsby almost every manufacturer, this type of chamber resembles an inclined basin recessed into the deck of the head. Inline valves are normally tilted to accommodate the sloping roof of this design. Thespark plug is located on the thick side of the wedge and is usually positioned midway between the valves.
The relatively steep walls in such chamber design force the air/fuel flow path anddeflect and force it to move in a downward spiral around the cylinder axis. During the compression stroke, the compressed air/fuel area reduces to such an extent that the trapped mixture is violently...