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Pascal Hayek
TOP STORIES 
 

CAR AERODYNAMICS

 

What is aerodynamics? It is the study of moving gases (in our case air) over a body in motion, and how that airflow will affect the body's movement through the flow. In other words it is the study of forces and the resulting motion of objects through the air.

In this article we are concerned about car aerodynamics. In the illustration below, the moving car is being subjected to several forces. The driving force is the one that causes the car to move to the right, while all the other forces are acting against that. The weight of the car, the friction between the tires and the road as well as the air resistance are forces that are responsible for slowing down the movement of the car.

Air resistance (the forces acted upon a moving object by the air -- also defined as drag) has­ a tremendous effect on the way a car accelerates, handles and achieves better fuel consumption. The air resistance should not be underestimated. As an example, have you ever put your hand out the window of a moving car? the higher the speed, the bigger the force of the air being exerted on your hand. Motorcyclists understand better what I mean by air resistance since they can feel its effect especially at high speeds.

About 60% of the power required to cruise at highway speeds is used to overcome air drag, and this increases very quickly at high speed. Therefore, a vehicle with better aerodynamics is the one that has the least effect of air resistance. The less the drag, the higher the speed that can be attained and the better the fuel consumption.

­Essentially, having a car designed with airflow in mind means it has less difficulty accelerating and can achieve better fuel economy numbers because the engine doesn't have to work nearly as hard to push the car through the wall of air.

To give you an example of how important the drag or air resistance is: the force acting against a car in motion by the air is a function of: Cd x Frontal Area x Density of Air x Speed Squared.

 Air resistance W = Cd. A v2 Q/2

Where, Cd = drag coefficient

v=driving speed
A =cross-sectional area (frontal area)
Q =air density

 

Speed clearly is an important part of the equation. At stop-and-go speeds, drag isn't a big deal, but the faster you go, the more it matters. At a speed of 70 km/h, you've got four times the force working against your car that you have at 35 km/h.

 Cd varies for different shapes and designs of cars and is influenced by how the air flows around the car. The lower the Cd the better the fuel consumption and the higher speeds that can be attained. The shape of a tear drop has the lowest coefficient of drag of 0.05.

 In 1921, German inventor Edmund Rumpler created the Rumpler-Tropfenauto, which translates into "tear-drop car." Based on the most aerodynamic shape in nature, the teardrop, this car had a Cd of just .27, but its unique looks never caught on with the public. Only about 100 were made.

The frontal area of the car is an important factor in determining drag. That is why trucks and most SUV’s have higher drag than sports cars. Another factor that designers take into consideration is the CdA.

In aerodynamics, the product of frontal area and the drag coefficient is called drag area (CdA). Average full-size passenger cars have a drag area of roughly (0.790 m2). Reported drag area ranges from the 1999 Honda Insight at (0.474 m2) to the 2003 Hummer H2 at (2.44 m2).

Automakers have been interested in aerodynamics since the early twenties. But the need to improve fuel economy in recent years has pushed aerodynamics toward the top of automakers' priority lists. Automakers rely on computer software and wind tunnels to ensure vehicles meet their aerodynamic targets. The main concerns of automotive aerodynamics beside fuel economy are reducing drag, reducing wind noise, and preventing undesired lift forces at high speeds.

 

Race cars aerodynamics

Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. In racing cars, a designer's aim is to increase down-force, increase grip and allow for greater cornering speeds.

There's more to aerodynamics than just drag -- there are other factors called lift and down-force, too. Lift is the force that opposes the weight of an object and raises it into the air and keeps it there. Down-force is the opposite of lift -- the force that presses an object in the direction of the ground.

It is important to minimize lift, hence increasing down-force, to avoid the car becoming airborne. Increasing the down-force pushes the car down onto the race track—allowing higher cornering speed. It is also important to maximize aerodynamic stability. For best cornering and racing performance, as required in Formula One cars, down-force and stability are crucial and these cars must attempt to maximize down-force and maintain stability while attempting to minimize the overall Cd value.

 

F1 cars are built to generate as much down-force as possible. At the speeds they're traveling, and with their extremely light weight, these cars actually begin to experience lift at some speeds -- physics forces them to take off like an airplane. Obviously, cars aren't intended to fly through the air, and if a car goes airborne it could mean a devastating crash. For this reason, down-force must be maximized to keep the car on the ground at high speeds, and this leads to a high Cd.

For F1 cars the limitations on ground effects, limited size of the wings (requiring use at high angles of attack to create sufficient down-force), and vortices created by open wheels lead to a high aerodynamic drag coefficient (0.7 to 1.1)

 One cannot discuss this topic without mentioning the importance of the spoilers on cars. Most people unfortunately believe that the spoiler is there for aesthetic reasons. Of course in some cases it might improve the looks but the main purpose of it is to help increase the down force at high speeds and hence prevent the car from getting subjected to lift.

How Automakers Improve Aerodynamics

As I have mentioned earlier, that aerodynamicists, designers and engineers use wind tunnels and computer software simulations in order to achieve best aerodynamic results. In essence, a wind tunnel is a massive tube with fans that produce airflow over an object inside. From a room behind the tunnel, engineers study the way the air interacts with the object. The car inside never moves, but the fans create wind at different speeds to simulate real-world conditions. Sometimes a real car won't even be used -- designers often rely on exact scale models of their vehicles to measure wind resistance. As wind moves over the car in the tunnel, computers are used to calculate the drag coefficient (Cd).

Computer simulations are starting to replace wind tunnels as the best way to measure the aerodynamics of a car. In many cases, wind tunnels are mostly just called upon to make sure the computer simulations are accurate.

 

While some shapes are inherently more aerodynamic than others, aerodynamicists and designers subtly shape every vehicle to reduce drag. They look at all areas of the car that come in contact with the air. Upper surface shape, under floor, wheels, bumpers, fenders and even cooling and engine bay. In the end they fine tune the way the air attaches to the vehicle's surface, and the way it leaves the rear end.

To reduce Cd, designers may make the following changes:

  • Round the edges of the front end.
  • Tune the grill and fascia openings.
  • Tune the wheel openings.
  • Place spats (small spoilers) in front of the tires to reduce turbulence.
  • Re -design the size and shape of the outside mirrors and their attachment arms.
  • Re-shape the water channel on the A-pillars.
  • Adjust the front fascia and air dam to reduce drag under the vehicle.
  • Add side skirts.
  • Tune the deck height, length and edge radius.
  • Install a rear spoiler.
  • Adjust the angle of the rear window.
  • Tuck up the exhaust system.
  • Use diffusers to tune air coming off the underside and the rear.
  • Install underbody panels that cover components and smooth airflow.

  

Today's cars have a Cd around 0.25-O.35 and designers are trying to achieve lower figures. Radical concept vehicles have gotten as low as 0.19. A reduction in Cd from 0.25 to 0.24 will result in a 4% (0.01/0.25) reduction in fuel needed to overcome drag. Just by reshaping the car, aerodynamicists can reduce fuel consumption by up to 25% without even modifying any parts of the engine or its management systems.

Refer to the table below for some comparison of coefficient of drag figures on few cars. 

 

Cd↓

Automobile↑

Year↓

0.7 to 1.1

Typical values for a Formula One.

0.57

Hummer H2

2003

0.38

Smart For Two

1998

0.37

Ferrari F50

1996

0.36

Subaru Impreza WRX

2010

0.35

MINI Cooper

2008

0.30

Nissan 370 Z

2009 [

0.29

Peugeot 308

2007

0.27

Nissan GTR

2008

0.25

Toyota Prius

2010

0.195

General Motors EV1

1996

       

Wikipedia.com

 

To conclude this article, we will definitely see in the future more aerodynamically shaped cars since fuel consumption is a leading issue in the automotive field. As for the futuristic sports cars, attaining higher speeds will always be on top of the list for auto designers and engineers, who will strive more to reach lower drag figures.

Written by: Pascal Hayek

 

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