The present disclosure provides a powertrain including an engine housing to which a cylinder head and cam carrier are mounted, a camshaft mounted to the cam carrier, first and second cam portions on which cams are formed, in which the camshaft is inserted, of which relative phase angle with respect to the camshaft is variable and disposed corresponding to a first cylinder and second cylinder respectively, first and second inner brackets transmitting rotation of the camshaft to the first and second cam portions respectively, a slider housing in which the first and second inner brackets are rotatably inserted and of which a position with respect to the cam shaft is variable, a lifter control portion controlling the position of the slider housing, a motor assembly engaged with the engine housing and in which a flywheel connected with a crankshaft is rotatably disposed, and a transmission connected with the motor assembly.