An internal combustion engine cylinder assembly comprising an elongated cylinder having substantially vertically disposed interior walls defining an elongated firing chamber therein; piston means housed within the firing chamber and adapted for vertical reciprocation within the firing chamber; gas inlet channels in the lower portion of the firing chamber; a crankcase housing having a gas compression chamber disposed therein; a reed valve means pivotally secured to the inner walls of the gas compression chamber and adapted for passing fuel/air mixtures into the gas compression chamber upon the depressuring thereof; a cylinder closure means positioned at the upper end of the cylinder defining the upper end of the firing chamber and being provided with exhaust gas valve means, adapted for cyclic opening and closing to alternatively permit the removal of exhaust gases from the firing chamber and the pressuring to the fresh fuel/air mixtures in the firing chamber; fuel ignition means for igniting a compressed fuel/air mixture in the firing chamber; at least a portion of the elongated cylinder inner walls extending downwardly into the gas compression chamber and adapted to house at least a portion of the piston means therein during the full downstroke of the piston means; the upper portion of the crankcase housing having inwardly sloping walls to define a converging gas space in the upper portion of the gas compression chamber annularly about the downwardly extending cylinder portion.
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