The invention pertains to a combine harvester comprising a cleaning system having a sieve that is moveable by a sieve drive assembly which comprises:- a rotary drive,- an eccentric device, which eccentric device has an eccentricity (e) and a driving point (a),- a transmission , that is connected to the eccentric device and to the sieve, which transmission is adapted to cyclically vary the distance between the driving point of the eccentric device and the sieve coupling during each full rotation of the rotatable output shaft of the rotary drive,wherein, due to the cyclic variation of the distance between the driving point (a) of the eccentric device and the sieve coupling (15), during each reciprocating sieve movement at least one of the accelerating portion of the throw stroke, the decelerating portion of the throw stroke, the accelerating portion of the return stroke and the decelerating portion of the return stroke has a duration that is different from the duration of at least one other of said portions of the reciprocating sieve movement.
