About this Article
Written by: Nathan Pandian
Written on: April 4th, 2011
Tags: aerospace engineering, mechanical engineering, power, transportation, energy & sustainability
Thumbnail by: Rob Bulmahn/Wikimedia Commons
About the Author
Nathan Pandian is a mechanical engineering undergraduate at the University of Southern California. In addition, he is the aerodynamics and composites captain of the USC Formula SAE design team.
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Volume XIV Issue I > Drag Reduction: The Pursuit of Better Fuel Economy
Aerodynamics was first used to increase vehicle performance in race cars during the 1970s. Race car engineers realized that air flowing around the vehicle could be used to increase downforce and reduce aerodynamic drag on the car. As fuel economy became a strong factor in road vehicle design, engineers soon realized that the methods of reducing aerodynamic drag on race cars could be transferred to road vehicles in order to improve fuel economy. To decrease the amount of drag created by a vehicle, automobile manufacturers began incorporating vehicle body designs that would allow the vehicle to be more streamlined. Methods of decreasing the drag coefficient of a vehicle include re-shaping the rear end, covering the underside of the vehicles, and reducing the amount of protrusions on the surface of the car.

Aerodynamics and Fuel Economy

Imagine holding a large traffic cone outside of your car window while driving on a freeway at seventy-five miles per hour. You are given two options: either hold the cone so that the pointed end faces the same direction the car is moving, or so the pointed end faces away from the direction the car is moving (Fig. 1). In which scenario would it be easier to hold the cone, given the effects of air resistance? While intuition may cause many to think that holding the pointed end of the cone forward is more effective, the opposite is actually true. This phenomenon is an example of aerodynamic drag, the concept that engineers use to design the car's shape and minimize the engine power needed to push the car forward.
Fornax/Wikimedia Commons
Figure 1: A cone whose pointed end faces away from the direction that the car is moving is actually more aerodynamics than facing the other way.
Optimizing the shape of a vehicle to reduce aerodynamic drag can allow vehicle designers to build cars with increased fuel economy. Little effort is needed to reveal the ever-increasing importance of fuel economy for production vehicles. In 2004, the price for a barrel of crude oil was around $30, yet economists at the time projected this would increase dramatically in the future [1]. Just seven years later, the price for a barrel of crude oil is at $108 and increasing [2]. This 360-percent increase in the price of crude oil affects everyone who drives a car, which is a majority of the population in industrialized countries. In addition to the high price of gasoline, carbon dioxide emissions from the tailpipes of cars have been a concern from the standpoint of sustainability [3]. Better automotive aerodynamics lead to a reduction in fuel consumption, helping drivers save money and lowering carbon dioxide emissions.

Borrowed Technology

One important consideration that modern vehicle engineers take into account while designing a car is aerodynamics. Aerodynamics is the study of both the motion of air and the forces created on an object moving through air. When an automobile is in motion, a large amount of air is displaced and must flow around the vehicle. Although scientists and engineers have studied aerodynamics ever since the 19th century, the importance of aerodynamics on automobiles was only realized in the middle of the 20th century [4]. Race car engineers were among the first to analyze the interaction between the air flow around a vehicle and the resulting effects on vehicle performance; this analysis led to the beginning of the design of aerodynamic features for race cars in the late 1960s [5]. During this time, designers of race cars began to test their cars in wind tunnels to determine effects of the air flow on the vehicles. Engineers soon learned that incorporating aerodynamic elements into race cars would affect the cars by means of two phenomena: downforce and aerodynamic drag [5].
Downforce and aerodynamic drag are two of the forces created by the air that flows around the car. Downforce is the downward force on a vehicle created by a difference in air pressure and it can increase a car’s performance by increasing the force that presses the car to the ground. If there is a larger force pressing the tires of the car to the ground, the amount of friction and traction available to the tires is increased. Just as a wing on an airplane can create a lift force to allow the plane to take off, the same wing can be inverted on a car to create downforce. Although these inverted wings began to appear on many race cars in the 1970s, the downforce came at a cost—the wings increased the aerodynamic drag on the race cars [5]. Aerodynamic drag is the force that opposes the direction of motion of the vehicle. In order to incorporate inverted wings onto race cars without the sacrifice of too much induced drag, engineers devised methods of decreasing drag, thereby decreasing the engine power needed to maintain the vehicle at a certain speed. By reducing the drag on a car, the corresponding fuel economy would increase.