A diffuser, in an automotive context, is a shaped section of the car rear 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.

What does a diffuser do on a car?

The diffuser acts as an expansion chamber to manage the air as it exits from underneath the car and reintegrates it with higher-pressure ambient air. Smoothing this transition reduces turbulence and drag in the car's wake and improves airflow under the car.

Why you need a car diffuser?

Diffusing a fresh-smelling blend like Simple Clean in your car is a great way to freshen the air naturally. Diffusers also make it much easier to control how much scent is produced because you can unplug it at any time.