Diffuser (automotive)

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A diffuser is an aerodynamic device primarily used on automobiles.

Bottom: Bottom-view. Top: Side-cut-view, the red circles mark the front and rear diffuser respectively

1 Rear
2 Front
3 See also
4 External links

[edit] Rear

A diffuser, in an automotive context, is usually a shaped section of the car underbody which improves the car's automotive aerodynamics 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 so that the boundary between the car's airflow and "external" airflow is less turbulent, and it also provides a degree of "wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause pressure drag).

As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and 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 and increasing downforce.

The aft part of a car underbody can be a diffuser. It tries to connect the underbody to the back without producing turbulences so that Bernoulli's principle applies and the pressure increases while the velocity decreases. The side and the roof end in a sharp edge, so that their pressure does not increase. Because the pressure in the back tends to equilibrate, the pressure below the car is lower than on the side and the roof of the car.

Rear diffuser, showing that the area covered by the exhaust even for a supercar is small compared to the area of the diffuser seen from behind. It needs improvement to work: [1].	Muffler as Rear diffuser	The muffler and exhausts make way for the diffuser	Rear diffuser with two massive fins
Rear diffuser with fins with too weak roots	Clean diffuser	Clean diffuser	Ferrari F430 with fins and exhaust giving way to the diffuser

[edit] Front

Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. A splitter separates the flow into the upper path, which will increase pressure in the diffuser and the lower path which stays as low pressure or even reduce pressure as the combination of an up looking splitter and the street acts as a Venturi nozzle. The height of the splitter is as low as possible without hitting small bumps, for street legal cars there is a minimum of about 100 mm. But a diffuser reduces the pressure in front of the car and thereby the amount of air pressed below the car. Sometimes a diffuser is used to let the wheel-casing also pump down the pressure below the car. Many small engine-exhausts in the back also help pumping. The exhaust from the engine compartment in the underbody can blows the air outwards by means of convergent nozzles, which reduce the pressure from the high pressure in the compartment to the low pressure below the car (the nozzle is the inverse of the diffuser).

The single radiator behind the grill inherits its square shape from the square frontal area and the circular fan. Wing below splitter for no apparent reason	Air which entered the diffuser exits on top	Sharp splitter below diffuser	Air which entered the diffuser exits on the sides. The triangular front and wing below the splitter would only make sense for a real formula 1 car
Wings below splitter in front of wheels for no apparent reason	Wing below splitter for no apparent reason	Wing in front of diffuser, may prevent flow separation. Discrete diffuser for breathing air	Sharp splitter and two diffusers for two radiators in front of the wheels
Two diffusers for two radiators in front of the wheels	Discrete brake cooling channels, show how small these inlets only need to be.	Front of Renault Twingo has only an opening for the radiator	Front of Volkswagen Passat has only an opening for the radiator and a discrete breathing intake in the VW sign not really helping to suck air away from the ground, but shows how small such an inlet only needs to be.
MINI (BMW) with discrete diffuser for the intercooler not really helping to suck air away from below the car	Sharp splitter front diffuser and discrete diffuser for breathing on top of cabin	Sharp splitter. The light shining through the grill shows the large volume inside the diffuser.	Low lying diffuser, with wing in front of diffuser
Low lying diffuser, showing that even a small block only needs a small inlet into the diffuser.	The large inlet in the middle spreads to the side under the car and the floor goes down for Venturi, more than allowed in a racing car	Sporting the same concepts as the Enzo in a weaker fashion.	Shows how small such an inlet only needs to be. No separate openings for horn, engine ,or inter-cooler.