An internal combustion engine comprising: a frame structure, a pair of piston and cylinder assemblies (14,18, 20) mounted on said frame structure including two side by side cylinders and pistons movably mounted in said cylinders for simultaneous movements through repetitive cycles, each including simultaneous compression strokes and immediately following simultaneous power drive strokes, an output shaft connected with said pistons so as to be moved by said pistons through a predetermined number of rotational movements during each cycle of movement of said pistons, a fuel injection and charge ignition system (42, 44) including an injector operatively associated with one of said piston and cylinder assemblies and another injector operatively associated with the other of said piston and cylinder assemblies, said fuel injection and charge ignition system being constructed and arranged in one mode of operation to establish at the beginning of the simultaneous power drive strokes of the pistons of both cylinders a charge of ignitable compressed air fuel mixture in one of said cylinders and a charge of unignitable compressed air in the other of said cylinders, characterized by a passage between said side-by-side cylinders constructed and arranged to communicate the high pressure conditions created by the ignition of the charge of ignitable air-fuel mixture in said one of said cylinders with the charge of compressed air to raise the pressure in the other of said cylinders during said one mode to move the number of said pistons associated therewith through the simultaneous drive stroke thereof; said fuel injection and charge ignition system being constructed and arranged to operate in a second mode of operation to establish at the beginning of the simultaneous power drive strokes a charge of ignitable compressed air-fuel mixture in both cylinders so that the ignition of both ignitable charges moves the pistons of both assemblies together through the simultaneous power drive strokes thereof; and a controller for selecting between the first and second modes of operation for the fuel injection and charge ignition system. a frame structure, a pair of piston and cylinder assemblies (14,18, 20) mounted on said frame structure including two side by side cylinders and pistons movably mounted in said cylinders for simultaneous movements through repetitive cycles, each including simultaneous compression strokes and immediately following simultaneous power drive strokes, an output shaft connected with said pistons so as to be moved by said pistons through a predetermined number of rotational movements during each cycle of movement of said pistons, a fuel injection and charge ignition system (42, 44) including an injector operatively associated with one of said piston and cylinder assemblies and another injector operatively associated with the other of said piston and cylinder assemblies, said fuel injection and charge ignition system being constructed and arranged in one mode of operation to establish at the beginning of the simultaneous power drive strokes of the pistons of both cylinders a charge of ignitable compressed air fuel mixture in one of said cylinders and a charge of unignitable compressed air in the other of said cylinders, characterized by a passage between said side-by-side cylinders constructed and arranged to communicate the high pressure conditions created by the ignition of the charge of ignitable air-fuel mixture in said one of said cylinders with the charge of compressed air to raise the pressure in the other of said cylinders during said one mode to move the number of said pistons associated therewith through the simultaneous drive stroke thereof; said fuel injection and charge ignition system being constructed and arranged to operate in a second mode of operation to establish at the beginning of the simultaneous power drive strokes a charge of ignitable compressed air-fuel mixture in both cylinders so that the ignition of both ignitable charges moves the pistons of both assemblies together through the simultaneous power drive strokes thereof; and a controller for selecting between the first and second modes of operation for the fuel injection and charge ignition system.