Abstract: A selective-cycle engine selectively operable in a 2-cycle mode and a 4-cycle mode, the selective-cycle engine including a cylinder including a head portion and a sidewall defining a cylinder interior, a piston driven in a reciprocating fashion within the cylinder interior, a head intake port and an exhaust port each defined in the head portion, a first sidewall intake port defined in the sidewall, an exhaust valve operable to open and close the exhaust port, a head intake valve operable to open and close the head intake port, and a first sidewall intake valve operable to open and close the first sidewall intake port. The head intake valve is maintained in a closed position to close the head intake port during 2-cycle mode while the first sidewall intake valve is opened and closed to provide intake air to the cylinder interior with opening and closing of the first sidewall intake valve being separately controlled from reciprocal movement of the piston.