Once the detail about road cars aerodynamics are known, it is important to understand their effects on specific vehicle categories. SUV represents the main CO₂ emissions problem due to their size effects on drag. High performance cars represents the effort of the automotive industry to reduce drag. This article proposes a brief comment about these two vehicle categories.

Road cars market share

Figure 1 illustrates the distribution in terms of selling for different car classes respective to the average value of drag, which is given by C_D and C_D∙S. It is possible to notice that the critical situation is seen on two-volume cars followed by SUV class. However, the point is that the selling levels (red line) are moving towards sport utility vehicle (SUV) class. The problem is the worst C_D∙S of this class, which has about 45% of the market share.

Sport utility vehicles

The problem with SUV is not selling volume itself. According to Figure 2, it is possible to notice that the wake volume of SUV and two-volume cars are not so different between them in the qualitative point of view. It is obvious that SUV has a big wake volume which is higher from the ground and these two facts put together generates a wake volume that is higher than two-volume cars ones. An interesting improvement is to provide some surface treatments, but the main point is not the pure C_D, because the behavior seen at Figures 1 and 2 are not so different.

The problem is the dimensions of SUV since the wake volume is bigger and higher from the ground. In fact, in terms of pure C_D, SUV are 3% higher than an usual two-volume car. However, when it is considered C_D∙S, SUV cars can exhibit more than 30% than usual two-volume car.

In addition, it should be accounted the ramp angle, which is much higher than any other car class (Figure 4). This angle is usually in a range of 6° to 12°, but in SUV becomes much higher due to off-road requirements. The problem is that a high ramp angle generates compression, thus more lift on the front and more drag. To avoid these effects it is adopted active suspension systems that adapts the ride height according to the situation.

High performance road cars

Figure 5 illustrates that high performance road cars are in the same range of drag than usual road cars, but generating downforce instead of lift. Normally, the style drives the design, because these cars are products with a high emotional appeal. Hence, the development drives its decisions towards the style.

Another constraint is about the approval ones, which are the front ramp, the rear ramp, the front step, the rear step and the bump (Figure 6). Normally putting together all these cases, it seems that these cars should have some aero devices and must not be lifting, or at least, generate zero lift. Actually, zero lift is the minimum requirement. Hence it is possible to notice that the most of part of the problem is on the front, which balances the rear downforce. This is usually adjusted by the rear wing. At the front, it is used some aero device as front underwings and active systems.

The underfloor is usually fully covered and functional (Figure 7). The difficulties in the application of an underfloor is the constraints that are inherent to the car structure. The front mechanics will impact in the front part of the underfloor. The chassi will impact on the possible exiting of the flow through car sides. The powertrain and the rear mechanics will impact in the rear diffuser volume. The suspension arms may impact on the rear diffuser volume and the closure of it. Therefore, for these cars, the underfloor design is a compromise between the performance requirements and the mechanical constraints of the underbody. Since the underfloor is normally plastic, thus its cost impact is not high for this kind of cars.

In terms of cooling constraints, this is one that increases year by year since the engine outputs are overcoming the range of 1000 bHp. It is quite easy to support a 500 bHp in terms of cooling management, but at certain point, a proper cooling system must be designed. The reason is that an engine that deliver outputs higher 1000 bHp, the cooling management is not effectively working all this power installed. Hence, active systems are required to manage the flow evacuations.

The active aero devices are essential in those cars. For instance, Porsche 918 (Figure 9) is 12 years old design and still is a good example of active aerodynamic system properly used. This car had an extremely widespread polar curve for a road car. The correct use of aero devices provides different operational configurations. For instance, a fuel saving configuration, which the system operates closing the front radiators and diffusers and the rear wing is at its low drag position. The top speed configuration command the actuator of the rear wing to reduce drag and to open the radiators since it is being requested full power from the engine. The high performance configuration focuses on the car handling, thus the actuators are commanded to extract the highest downforce as possible. The front diffuser and radiators are open (Figure 9). The polar curve of the Porsche 918 is similar to a race car, which is very difficult to obtain. The comparison with 997 GT3 4.0 and Carrera GT exhibits that the common situation is to have just some points for these cars, instead of a curve. Normally, the problem of those cars is the generation of front and rear downforce to keep the car properly balanced. Usually the rear downforce is ensured by a rear wing, while in the front is mandatory to have an active aero device. The problem of active aerodynamics is cost and weight.

Drag story

The general trend of high performance cars is oriented in attention to drag. Figure 10 illustrates that historically these cars are loosing 1 point of C_DS per year. Apart this number, Figure 10 also helps to understand that drag is also important for those cars. Some years ago car makers only focused on mechanical performance, but now aerodynamics has a great importance on their development.

High performance road cars comparison

The comparison between road and high performance cars usually lies on the body and the underfloor. The body itself for high performance cars has a better shape than road ones. In term of vertical forces, since the underbody is always producing downforce and the top body generates lift, usually high performance cars have an underbody that attend the minimum requirement of balance the lift that comes from the body. Hence, the car is in a zero lift condition just with the underbody, then whatever is gained in downforce is a plus.

Pressure profile along the body

Figure 12 illustrates a typical distribution of the top and the bottom surfaces of overall high performance cars. As can ber seen, there are zones of suction and compression. These are respective to the curvature of the front and the base of the windscreen, where the airflow is compressed. The top of the windscreen has a strong suction area, then the pressure recovers up to the point where there is some device that generates compression. On the underfloor, there is the leading edge that faces a strong acceleration that generates suctions, then the diffuser will develop a pressure recovery that goes through the underfloor end up to the point of the rear diffuser slope.

References

• This article was based on the lecture notes written by the author during the Industrial Aerodynamics course taken at Dallara Academy.