The role of a diffuser on a racecar is to accelerate the airflow beneath the car. This creates an area of lower pressure and in turn generates downforce – effectively pushing the car onto the track and therefore increasing the available traction.
Downforce is generated by changing the speed of the airflow over a surface. A simple wing on a racecar accelerates the airflow underneath, as it has further to travel. The airflow above the wing is slow moving and at a high pressure. This difference in pressure causes downforce to be generated:
Downforce and traction
Downforce pushes the car onto the ground and through this increases the traction available and therefore improves cornering speeds. Increasing weight can have the same effect, but it will have a negative effect on the handling and also reduces acceleration. Therefore, aerodynamics is very important to a race car and downforce will significantly improve performance.
A diffuser increases in volume as the air passes through. This means that the air is forced to accelerate in order to fill the volume, creating a lower pressure. The lower pressure relative to the upper surface of the car produces downforce. Most diffusers feature ‘fences’, uprights designed to increase efficiency by allowing only air from the underbody to enter and no air from above from being sucked in.
This is an example of the venturi effect:
Basically, the diffuser gives the air underneath the car the largest possible area to exit causing acceleration of airflow beneath the car and resulting in a low pressure.
The 2009 Formula One regulations limited the height and width of the diffuser but a loophole in the rules, exploited by Brawn, Toyota and Williams, allowed teams to effectively add an extra layer to the diffuser. This design had a greater volume than the single layer diffuser, therefore allowing more air to be drawn from the underbody and this increased the downforce generated.
The design was predicted to have given an advantage of up to half a second per lap, with a 5% increase in overall downforce. However, they were banned in F1 by 2011.