install insulated cavities 112 for adjustment of the cavity volume externally accessible cavity volume adjustment bolts 114 can be used adjustment of volume adjustment bolts 114 can be performed prior to engine operation or during engine operation due to the high compression ratio needed the inlet and outlet valve clearance between cylinder head and piston at tdc is limited therefore exhaust valve 118 above piston may have to delay opening until the piston clearance has increased sufficiently then it may be advantageous to add an additional exhaust valve 116 inside one or more of cavities 112 when the crank case compressed inlet air fuel mixture enters through intake port 138 with piston 132 at bottom dead center cylinder scavenging is greatly improved using the now open exhaust valves 116 and 118 by providing unidirectional scavenging to minimize engine knock the clearance between the flat top piston 130 with rounded edges and the near flat cylinder head 136 should be minimized to minimize engine knock compression ignition is induced to originate on the centerline of piston 130 therefore a nearly hemispherical insulated cavity 124 is installed inside the middle of the nearly flat cylinder head 136 on the bottom of a removable plug 126 which can be used to provide transducer access to the combustible mixture for pressure monitoring gas sampling or for spark ignition if desired upper piston ring 128 is shown with piston 130 in position to just close off cylinder wall cavities 112 the lower piston ring 129 seals the cylinder wall cavity 112 on the opposite side of the cavity opening in relation to the upper piston ring 128 intermediate piston rings may be positioned between the upper piston ring 128 and the lower piston ring.