A
diffuser, in an automotive context, is a shaped section of the
car underbody which improves the car's
aerodynamic properties by enhancing the transition between the high-velocity
airflow underneath the car and the much slower
freestream airflow of the ambient
atmosphere. It works by providing a space for the underbody airflow to decelerate and expand (in area, density remains constant at the speeds that cars travel) so that it does not cause excessive
flow separation and
drag, by providing a degree of "
wake infill" or more accurately,
pressure recovery. The diffuser itself accelerates the flow in front of it, which helps generate
downforce.