A cleaning system for a combine harvester that includes a cleaning shoe having a sieve, a cleaning fan, an air plenum and a pair of air diverters is provided. The cleaning fan is rotatably driven for generating a flow of air and positioned within the air plenum. The air plenum extends across the entire length of the cleaning fan and includes an inlet for the intake of air, an outlet downstream the inlet for directing the flow of air in a first direction, lateral walls forming a portion of the outlet, and a bottom wall extending between the lateral walls. The pair of air diverters extends inwardly from the lateral walls. Each air diverter is configured substantially as a pyramid having a rounded corner that extends inwardly from the lateral wall and upwardly from the bottom wall.
