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 so that the boundary between the car's airflow and "external" airflow is less turbulent. 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 works to bring the low pressure air below the car back to the ambient atmospheric pressure without inducing turbulence. It uses Bernoulli's principle, such that the pressure increases while the velocity decreases. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. Injecting the exhaust into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This helps extract the air more efficiently from the underbody.
Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. Instead, a splitter is commonly used here. The splitter serves to reduce the pressure below the front of the car and thereby increase the amount of downforce in that region. The airstream is brought to stagnation above the splitter, by an air dam, causing an area of high pressure. Below the splitter, air is accelerated, as stated above, which causes the pressure to drop and create downforce. Some race cars, such as the C6-R Corvette actually use a proper diffuser in this area to help create more downforce.[1]
For drag we perform coast
down runs for each setting. Without the above we can
only calculate approximate performance levels of each
element. IE: front adjustable splitter, rear wing and
diffuser. Lap times don’t lie.
Using the formula F= Cl * area we arrive at 3psi for the
diffuser which is a total of 480 lbs down force @ 170mph
ideally if we were to maintain 2.5“ ground clearance on
a smooth track on a standard day. Since the rear ride
height is more than that, we have a + rake situation
which in itself causes down force because of the ’Wedge
effect”. The area and ramp profiles were determined by
the space available and no NACA tables were consulted.
The primary reason for the design was to address the
unsightly mess Corvette has created at the bottom of the
rear valence. Imagine the ZR1 supercar with back up
lamps on a fake plastic diffuser. Shucks, this is what
we expect from the orient for little city cars. What
self respecting V8 has four ugly pipes?
Although it accomplishes the appropriate ’Bling Bling‘
effect, our diffuser also function as designed. If you
have all the other necessary elements in place. IE, A
proper floor feeder system, droop limits on the rear
suspension, ability to adjust total center of pressure,
(splitter and rear wing) you will be further able to
exploit our new product.
We also have a side feed entry as on the Ferrari Enzo,
so we don’t get the full suction peak at entry and the
second peak is located at the rear axle. This is because
the engine heat is managed down the centre of the car
and exits at the opening of the exhaust pipes. This
precludes us having a full ’closed in’ floor which would
improve the efficiencly of the diffuser considerably.
Our rear wing is designed for engine heat exit over the
top of the car as in the factory C6R GT1 cars after
which it was designed. There is never the less diffuser
energy augmentation caused by the end plates on this
design. Since this car is also driven on the street and
is used for our product promotion, we elected not to go
with a full race car package.
To calculate approximate down force at other speeds at
the diffuser we know that lift increases at the square
of the velocity. At 75mph we would have 100lbs down
force.
With this installation for the street on a stock C6, you
will improve your gas mileage at highway speeds because
the diffuser allows the air traveling underneath the car
a place to expand and decelerate back to road speed as
well as providing wake infill. IE: It will reduce total
drag.
