Diffuser (automotive)

A diffuser on an automobile is a shaped section of the car’s rear which improves its 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 volume, as density is assumed to be 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. This is achieved by creating a change in velocity of the air flowing under the diffuser by giving it a rake angle which in turn generates a change in pressure and hence increases downforce.

Overview

When a diffuser is used, the air flows into the underbody from the front of the car, accelerates and reduces pressure. There is a suction peak at the transition of the flat bottom and diffuser. This transition is where the lowest pressure generally is located and is called the throat of the diffuser. The diffuser then eases this high velocity air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car. The diffuser also imparts upward momentum to the air which further increases downforce.

The trailing or leading edge of a diffuser may receive a nolder—a precise small lip, protuberance or wing to enhance its performance.