Automotive Aerodynamics

Aerodynamics is a branch of physics that deals with the forces exerted by air or other gases in motion. Aerodynamic is by far the most significant factor affecting fuel consumption and power requirements even at normal cruising speeds. For this reason, there has been much interest in the aerodynamic shaping of all vehicles, including standard production cars and trucks. Although the shape of a vehicle is determined by the designer, the vehicle owner sometimes unknowingly makes modifications that reduce the aerodynamic effectiveness of the design. For example, the addition of a ski rack of the opening of a window or sun roof can significantly reduce fuel economy and general performance. I twill there fore be helpful to both the mechanic and the driver to be familiar with the basic principles involved in aerodynamic design.

Although the relationship that exists between the shape of a vehicle and its performance has been known for a long time, it is only in recent years that these principles have been applied systematically to standard production cars. At one time the shape of the vehicle was determined by automotive stylists who responded to whatever public fashion demanded. While the commercial success of a vehicle still depends a great deal on the appeal, the ever-increasing cost of gasoline is making good fuel economy an ever more decisive factor. Good looks and efficient aerodynamics need not be in conflict with each other. The term "functional styling", as applied to automotive design, is essentially the successful blending of these two aspects, as well as comfort, safety and overall efficiency. In addition to reducing weight (On a medium-sized car the reducing of vehicle weight by 100kg results in a drop in fuel consumption of approximately 5%.) proper aerodynamic shaping, more commonly referred to as "streamlining", has been found to be the most effective and least costly method of increasing fuel economy and performance, especially at higher speeds. For this reason all new vehicle designs first undergo extensive testing wind tunnels before they are placed into production. Because of the very high cost involved in producing a prototype car body, it is customary to begin with a scaled-down model made of clay. This method also makes it much easier to make any modifications that may be required in light of the data obtained from the test. The airstreams, whose velocity can be varied is generated by a powerful, propeller-shaped fan. To study the airflow across the top and sides of the vehicle, as well as its underside, jets of smoke are blown into the airstreams. The smoke can be directed by nozzles into any area that is to be examined in detail. Another method is to attach "steamers" of thread or silk all over the body so that the entire flow pattern can be studied at the same time. (Similar observations can be made by watching the movement of water droplets or snowflakes along the surfaces of the vehicle.) In some cases the whole car body, or a scale model of it, is suspended fro wires. This makes it possible to simultaneously study the airflow all around the vehicle and to measure directly the amount of drag. The purpose of the whole exercise is to arrive at the most advantageous compromise between shape and function. As things turn out, more often than not designs that are superior in aerodynamic terms also look quite pleasing to the eye. For example, most people find the advanced designs of modern aircraft very attractive.

To fully appreciate the importance of aerodynamics as applied to motor vehicles it must be realized that, as driving speeds increase, the power needed to overcome air drag multiplies at a very rapid rate. This increase can be calculated by the formula below. . As an example, let us apply this formula to a car whose engine has to generate 18 HP to overcome the air drag at 80 km/h. (Since the rolling friction of the wheels consumes a relatively small amount of power, we shall ignore it in the present case. Already at 89 km/h, air drag constitutes roughly 65% of the total resistance. At 100 km/h it rises to approximately 75 %.) The power needed to double the speed to 160 km is shown in the following example. This is eight times the power needed to travel at 80 km! However, if it were possible to reduce the drag by 25% we would need only 108 HP and fuel consumption would be about one-quarter less than what it was before. It is now obvious why the proper shaping and streamlining of a car pays very significant dividends at little extra cost. The example above also proves that a light foot on the accelerator pedal is the most effective fuel saver yet devised.

In addition to increasing fuel economy and performance, automotive aerodynamics also plays an important role in other related areas. These include high-speed traction; sensitivity to crosswinds; efficient cooling (engine, drive train, exhaust system, brakes and ties); keeping the front windshield, the windows, the mirrors and the headlights clean; combating windshield wiper lift (by attaching an inverted winglet to the wiper); and last but not least, reducing wind noise to a minimum.

The ideal aerodynamic shape of a vehicle would be that of a fish. The least efficient design is that of a box-shaped transport truck (hence the attempt to cut aerodynamic drag by mounting a spoiler on the roof of the cab , as seen on many trucks used for long-distance hauling.) Obviously, since neither of these two shapes is very practical, the body of a passenger car must be a compromise between these two extremes. The amount of drag that is generated by an object placed in an airstreams is expressed by what is called the coefficient of drag, or simple CD. If the drag of an object equals the force of the airstreams directed against it, the coefficient of drag = 1. The CDs of some typical shapes are shown below. As can be seen from these examples, the DC may actually be higher than 1. The CD of the plate is 1.5 because the vortex flow (swirls or eddies) behind the plate result in additional drag. It is this kind of vortex flow that causes the rear windows of station wagons to get dirty very quickly. By the same token, it also explains why the vertical surface of the tailgate makes for very inefficient aerodynamics. This equally applies to large transport trucks where smooth airflow increases profits by reducing fuel consumption.

Sometimes the drag created by these vehicles can actually be observed, as when leaves and bits of paper are seen trailing behind them. The engine must develop a considerable amount of horsepower to overcome this drag. Needless to say, these additional "horses" prove to be very thirsty indeed. (Race drivers often exploit this vortex flow by driving in the "slip stream" of a competitor who is perhaps too eager to stay ahead. By reducing the strain on the engine and saving an extra pit stop for refueling, many a race has been won with this simple but dangerous trick.)

The CD of the plate in the diagram to the right would approach 1 if we were to taper its trailing side like that of the teardrop-shaped object shown below it. However, there are mainly two reasons that dictate against its use with conventional automobiles: the shape would be impractical in terms of interior space and length and it would also produce insufficient down force to provide good wheel traction at high speeds. In fact, an automobile shaped in this fashion at the top but flat at the bottom would turn into an airplane wing. Since the air traveling over the hood, the top panel, and the trunk has a much longer distance to go than the air forced below the vehicle, a considerable amount of lift would be produced even at normal cruising speeds.


Some faster cars are equipped with an air dam at the front end and a spoiler at the rear as shown to the left. , as well as very low side skirts (extended rocker panels) between the wheel wells. This reduces the air-flow below the vehicle while crating an additional down force at the top. A further advantage of the rear spoiler is that it sends the air upwards before braking off at the vertical tail of the vehicle. The rear spoiler may therefore be viewed as a practical compromise between the teardrop design and the box design, that is, a compromise between minimum drag on the one hand and maximum interior space on the other. The airflow characteristics of the "chopped" rear-end design featuring a spoiler at the trailing edge of the vehicle is sometimes called the Kamm effect after the aerodynamicist who first introduced the device.



As illustrated to the left , the most significant component of drag is the fontal area of the vehicle. Hold the cursor over the 4 Door Sedan, Sub-Compact and Sports Car to see their Frontal Area and Coefficient of Drag (CD). However, because the overall dimensions of a passenger car (height, width and length) are pretty well fixed (given certain basic requirements, such as interior space, engine size and trunk volume), efforts to improve its aerodynamics are largely directed at obtaining the most advantageous shape of its external surfaces. The primary objective is therefore to make the body and the underside of the vehicle as "slippery" as possible. Wherever possible, the automotive designer or aerodynamicist seeks to avoid abrupt changes in the airflow that would result in energy-consuming vortices and drag. The slant of the grill, the opening of the radiator air intake, the slope of the hood and the windshield, the flow pattern over the top panel and especially the relatively sharp drop at the rear window and the deck lid are crucial areas if drag is to be kept low. Even the shape of the corner posts, the drip moulding, the outside mirror, the wheel wells, the bumpers and other seemingly minor features are closely examined for their aerodynamic effects. The air flow patterns below indicate the air flow over the design features of a car.

Studies have shown that even reducing the gaps between adjoining body panels, such as those around the doors, can result in very significant reductions in drag. On the other hand, ski racks, sun roofs and open windows were found to greatly increase the CD. Thus the loss of fuel economy attributed to the installation of an air conditioner could well be much less than that lost by an identical car without an air conditioner that is driven with open windows in order to improve ventilation on a hot day. At highway speeds the air-conditioned vehicle would actually prove to be more economical.

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