A multicylinder internal combustion engine having separate cylinder groups, each of which is associated with a fuel-air mixture supply means for selectively supplying a fuel-air mixture or only air thereto, and a process for supplying of a fuel-air mixture thereto. In accordance with all embodiments, air only is supplied to the second cylinder group when the engine is operating in a warmed-up condition, under partial load operating conditions, a fuel-air mixture being supplied to both cylinder groups, regardless of engine temperature conditions, once the partial load range has been exceeded. A fuel-air mixture is supplied to the first cylinder group in accordance with all embodiments, regardless of engine temperature or load conditions. In accordance with a preferred embodiment of the engine, a single cam disk is utilized to control the air supply through the air intakes for both cylinder groups in cooperation with a vacuum cell, and a temperature controlled venting device for the vacuum cell. In accordance with the process embodiments, the fuel-air supply to the second cylinder group during warm-up is either progressively increased and then decreased to an amount approximating zero, or is held constant throughout the predetermined load range; while, with the engine warmed-up for operation, the supply of air only to the second cylinder group, in the partial load range, is either progressively increased, held constant, and then decreased to an amount approximating zero, or is increased, held constant to the end of the partial load range and then abruptly decreased to the amount approximating zero.