There is provided an apparatus for regulating a compression ratio of an HCCI rotary engine. The engine has a rotor piston rotatable in an operating direction. The stator has one or more combustion chamber throughways, each having at least first and second interior walls. The apparatus comprises, within each combustion chamber throughway, an expansion gate slidably received within the throughway and slidably engaged with and adjacent to the first interior wall, a combustion gate slidably engaged with and adjacent to the expansion gate in the operating direction, and a compression gate slidably engaged with and adjacent to both the combustion gate and the second interior wall, and disposed adjacent the combustion gate in the operating direction. Each gate is slidable longitudinally within the combustion chamber throughway. Rotation of the rotor piston, to a position where a lobe end of the piston aligns with a combustion gate, forms a combustion chamber.