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