An air chamber is arranged so that it can rotate concentrically inside a cylinder intake pipe arranged in a circular shape. Each air chamber has a lateral inlet aperture and an outlet aperture opening into the intake pipe. Intake devices of this type are known in the form in which the effective intake pipe length can be varied by rotation of the air chamber. According to the invention the intake pipe side walls are also displaceably arranged in order to obtain a reduction of the effective intake pipe cross-section whilst simultaneously increasing the effective intake pipe length. To this end the said side walls are carried in helical guide grooves of the air chamber wall. The two side walls of an intake pipe are thereby displaced towards one another as required by rotation of the air chamber.
