A Homogeneous Charge Compression Ignition (HCCI) rotary engine having a rotor piston within a stator that contains a piston chamber, the rotor piston rotatable about a stator axis within the piston chamber in an operating direction and having one or more lobe ends in slidable contact with an interior stator surface, the stator having defined therein one or more combustion chamber throughways equally spaced about the stator and communicating the piston chamber with an environment exterior to the stator, each combustion chamber throughway having interior throughway walls that include at least first and second walls, the HCCI rotary engine comprising an apparatus for regulating a compression ratio by varying a combustion chamber volume, characterised in that the apparatus comprises: within each of the one or more combustion chamber throughways, an expansion gate slidably received within the respective throughway and slidably engaged with and adjacent to said first interior throughway wall, a combustion gate slidably engaged with and adjacent to the expansion gate, the combustion gate disposed adjacent the expansion gate in the operating direction and having a piston-facing surface oriented toward the piston chamber, and a compression gate slidably engaged with and adjacent to both the combustion gate and said second interior throughway wall, the compression gate disposed adjacent the combustion gate in the operating direction, each of said expansion, combustion and compression gates slidable longitudinally within the respective combustion chamber throughway, wherein rotation of the rotor piston, to a position where a lobe end aligns with the piston-facing surface of a respective one of said one or more combustion gates, forms a combustion chamber defined at least by a portion of interior walls of said respective expansion and compression gates, the piston-facing surface of said respective combustion gate, a portion of the throughway walls of said respective combustion chamber throughway, and said lobe end. within each of the one or more combustion chamber throughways, an expansion gate slidably received within the respective throughway and slidably engaged with and adjacent to said first interior throughway wall, a combustion gate slidably engaged with and adjacent to the expansion gate, the combustion gate disposed adjacent the expansion gate in the operating direction and having a piston-facing surface oriented toward the piston chamber, and a compression gate slidably engaged with and adjacent to both the combustion gate and said second interior throughway wall, the compression gate disposed adjacent the combustion gate in the operating direction, each of said expansion, combustion and compression gates slidable longitudinally within the respective combustion chamber throughway, an expansion gate slidably received within the respective throughway and slidably engaged with and adjacent to said first interior throughway wall, a combustion gate slidably engaged with and adjacent to the expansion gate, the combustion gate disposed adjacent the expansion gate in the operating direction and having a piston-facing surface oriented toward the piston chamber, and a compression gate slidably engaged with and adjacent to both the combustion gate and said second interior throughway wall, the compression gate disposed adjacent the combustion gate in the operating direction, each of said expansion, combustion and compression gates slidable longitudinally within the respective combustion chamber throughway, wherein rotation of the rotor piston, to a position where a lobe end aligns with the piston-facing surface of a respective one of said one or more combustion gates, forms a combustion chamber defined at least by a portion of interior walls of said respective expansion and compression gates, the piston-facing surface of said respective combustion gate, a portion of the throughway walls of said respective combustion chamber throughway, and said lobe end.