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.
Rear Diffuser and Sided Skirts Shown
