How the Diffuser, Exhaust and Rear Wing Creates Downforce In Formula One

Formula One is at the pinnacle of innovating technology. Teams are forever revolutionising their racing cars in an attempt to make them faster than the competition. Dictated by the FIA regulations, team are constantly trying to find ‘loopholes’ in the rules in order to gain extra downforce without creating drag. From the diagram below, it is clear the two main areas a Formula One car gains the majority of its downforce is with the front wing and rear end.

Source: Red Bull Racing F1 Car 2010 (RB6), Haynes Publishing
The front wing creates the most downforce because it is the part of the car which gains a perfectly linear air flow over it. Every other part of the car will have an already ‘dirty’ air flow flowing over it since the front wing has been in impact with the linear air flow at the front. This means, for Formula One teams to create the best amount of downforce, they will need to continue the linear flow of air over/under the car to the rear wing. Turbulence creates drag and will slow the car down and is inefficient in creating downforce. This is why the rear end of the car can be seen to hold the most aerodynamic innovations on an F1 car: it has managed to create downforce even though it is at the rear of the car where the flow of air will be the most ‘dirty’.


The way the rear end creates downforce can be broken down into three areas being the diffuser, rear wing and exhaust. Ultimately, engineers will want to link these three areas together to create the best amount of downforce with least drag.



Diffuser


The diffuser doesn't directly produce downforce to the rear end of the car but, in essence, produces downforce along the whole of the car. The job of the diffuser is to diffuse the high velocity air flow running under the car to the rear of the car so that the air flow moves closer to that of ambient pressure. With the diffuser working, an F1 car turns into a venturi tube which is the most efficient aerodynamic package on an F1 car to create downforce. Most people will place a stereotype under aerodynamics that it is the first contact with air flow is the most significant in creating downforce. However, considering the diffuser has the potential to give an F1 car 30-40% its total downforce, the exit of the used air flow can sometimes be deemed much more significant.


Venturi Tube Effect is used in F1



The objective of the diffuser is to give the used air flow from the underbelly of the car as much possible space to exit from the rear end. This ultimately means that if the air can escape more easily from under the car, more air at faster velocities can flow under the underbelly of the car creating a lower pressure and therefore higher downforce. This can be seen most clearly through a diagram:


Digram to Show the Aerodynamics Under the Car
In the diagram I mention Bernoulli's law which is a fundamental equation for fluid dynamics:
Ps + 1/2ρV² = constant
Where Ps = static pressure, and 1/2ρV² = dynamic pressure. If the velocity of the fluid increases, the pressure of the fluid will decrease and vice versa. This equation can be seen to have come from the most fundamental of equations: the conservation of energy:
PE + KE = Total Energy
The ‘Total Energy’ will stay constant due to the conservation of energy. The potential energy in Bernoulli’s law is the static pressure with the kinetic energy being the dynamic pressure. You can see that dynamic pressure has derived from 1/2mV² - for a unit volume of air, the mass will be the same as the air density. However, this does not take into account the area the air is flowing through unlike this equation:
ρV(1)A(1) = ρV(2)A(2) = ρV(3)A(3)... (where ρ [air density] is constant)
Therefore, we can apply these equations to the diffuser diagram:


Using Aerodynamic Equations With F1
F1 engineers are always looking for ways to make the pressure higher or velocity lower at the back of the car because this will make the F1 car more into a venturi tube. Unfortunately, due to FIA regulations, F1 cars will find it difficult to create a perfect venturi tube seeing that the diffuser is limited to being very short (with a maximum height of 175mm above the reference plane. However, with ideas such as the ‘double diffuser’ creating two channels for the air to diffuse into instead of one, F1 teams are finding loopholes around the rules to gain downforce for ‘free’.

Rear Wing

The rear wing is a simpler aerodynamic part of the car because its job is to create downforce by changing the direction of airflow over the car. Using Newton’s third law, every force has an equal and opposite force. Therefore, if a the wing is causing the air particles to move more vertically, then it will cause the car to become squeezed against the road surface.
Another type of wing used in F1 is similar to that used to make planes fly i.e. an aerofoil. By essentially ‘flipping’ the wing design from a plane, low pressure is created closest to the road surface creating downforce.


The Inverted Foil Shape is used in F1 to Create Downforce
A benefit of this wing is that it is the most streamline type of wing creating negligible wake behind it. However, the problem with wings is that they create vortices at the tips of the wings which cause drag. This is why planes travelling at high velocities, such as the Concorde, have wing-tips designed to reduce this turbulence because it forces the airflow to move in one direction rather than multiple which can be seen below.
Source: www.shell.com. The vortices start later on the wing with end plates because the airflow is forced to move in one direction of motion and can only start whirling after the rear wing.
Source: ScarbsF1.com. The vortices can be seen clearly here on the McLaren F1 car of Lewis Hamilton due to pressure differences which creates huge drag.
The main problem F1 aerodynamics face is the turbulence after the rear wing (or the ‘wake’). To make sure wake is reduced to a minimal, the air exiting the car cannot be turbulent vortices. However, due to the venturi tube effect under the car and the wing creating downforce above the car, the two exiting ‘dirty’ air flows meet at the back of the car which can create serious vortices affecting the aerodynamics of the car significantly by creating drag. Therefore, it is always an aim to make the two air flows meet as linearly as possible with minimal drag.

Exhaust

The exhaust is a key aspect to the rear end of a Formula 1 car because it has the potential to create much more downforce for ‘free’. F1 cars are pumping out extremely high and dense air at high velocities from the exhaust. Therefore, engineers are always looking for ways to use the exhaust gases to their advantage. What they came up with was the blown diffuser.
The blown diffuser is the term given to when the exhaust fumes are directed to the rear end of the car to the diffuser and around the tyres (to reduce the drag, wake and degradation of tyres). Therefore, the air will be ‘turbocharged’ by the exhaust fumes causing them to move faster decreasing pressure under the car and reducing wake and drag from the tyres. The problem with this is that the exhaust fumes are solely dependent on the engine’s RPM. At high RPM, more and faster gas will exit the exhaust creating a much higher downforce. The problem is that F1 cars do not use high RPM going around corners (which is where downforce needs to be at its highest). If a car oversteers, the driver will naturally look to correct it by regaining grip through lifting off the throttle. However, with a blown diffuser, lifting off the throttle will cause the downforce to decrease dramatically causing the oversteer to become worse. For this reason, F1 teams implemented a new software into their engines so that when a F1 car is going around a corner and the driver lifts off, the revs of the engine are maintained at being high in order to achieve high downforce throughout the whole of the corner. You will be able to recognise this when watching an F1 car go around a corner where you will hear the engine sound like a ‘bag of nails’. This is the engine maintaining high revolutions during the corner pumping hot energised gases from the exhaust to the diffuser and around the tyres.
Ultimately, the rear end’s objective is to make sure the air exiting the rear of the car does not end up turbulent creating a large wake and therefore drag. Turbulence after the rear of the end causes a significant amount of drag which is clear from modern hatchback cars:
Wake behind a Modern Ford Fiesta due to Lack of Streamline Rear
The huge vortices produced from hatchbacks is the reason they have rear windscreen wipers: to remove the dirt produced from turbulence. Therefore, in F1, it is essential for the car to have a linear (or as close to linear) air flow exiting behind the car to ensure there is no wake behind it. A diffuser is used for this reason along with the end plates on the F1 wing and exiting exhaust gases.