An interesting point about this mismatch is that there is a correlation between the CO2 emissions and the aerodynamics. Actually, aerodynamics is one of the main constraints that the car companies must deal to stay legal. The graph composed by the measured CO2 emissions and CD∙S (Cover Figure) allows to notice that, apart from out of curve points, most of the cars exhibit a clear relationship between the aerodynamics and CO2.

Hence, aerodynamic can be an important tool to solve the problem. The first parameter relative to the aerodynamics is the shape, thus L/h. This is the ratio between the vehicle length and height, it impacts on the vehicle shape, theoretically, thus on the drag coefficient CD. Hence, analyzing the graph that correlates CD with L/h, the situation is a clear trend towards the car shape.

However, there are several overlaps between the categories that makes difficult to compare. Hence, if this correlation accounts CD∙S instead of CD, the situation is absolutely clear, because most of the families stay between 0.55 and 0.85 CDS. SUV and minivans are clearly above this range. These are already part of the emission problem.

Drag coefficient of electric and internal combustion engine cars

When analyzing CD in terms of the powertrain, it is easy to observe that the gap between CD due to internal combustion engine (ICE) and electric motor (EM) powered car is about 10%. The reason behind this is, mainly, the cooling system. The electric car requires much less cooling when compared with an ICE powered one. Hence, the openings of the front grid are smaller, that results in a lower average drag. Another interesting reason is that, electric are more tolerant to functions aspects, thus car stylists can set more engineering features in this kind of car. In other words, the surface features of the body can be more freely improved. The style of an electric car is much more receptive for aerodynamic features relative to a car with ICE. The impact of the aerodynamics in the car development story is illustrated at Figure 5.

It suggests that CD is being reduced about 5% at each 10 years. These number comes from a process that nowadays is completely out of time. For instance, old aerodynamic developments for road cars were basically resumed as wind tunnel (WT) tests at the end of the process. This was a last check in full scale wind tunnel model (WTM) to correct the final design aerodynamic. Hence, in this situation the development of an aerodynamic oriented design was very difficult. Apart from this, calculating averages of all possible cars, now the actual marketing is running towards the scenario observed at Figure 5. The current CD is 0.337. This value is the one for a typical 4.3 m long 2 volume car. Since there are legal homologation rules and electric cars still have to improve the autonomy of their batteries, this CD must be reduced to about 20%, which is a CD about 0.27. The problem is that 0.27 in an ICE powered car is almost impossible. It is very difficult, because this kind of car has style constraint. In other words, to sell a mass production car with CD = 0.27, it is financially impossible. Hence, considering all those obstacles and limitations, this number would be achieved in 40 years, which is not desirable since the green house emissions is a problem that requires a fast response. Therefore, Figure 1 to 5 suggest that the emission problem and its connection with the aerodynamics is more related to the product development.

The motorsport process

The aerodynamic expertise developed by the main motorsport companies can support a lot the road car aerodynamic development. Hence, the cross competences and transverse know-how on the best performance in the possible shortest time. In the motorsport there are no stagnation periods, the timing of the production and any period of stand-by or delay of the process. Hence, the motorsport basically guides the process to obtain the best performance in possible shortest time and this is the logic behind the same automotive market now, because there is such small time that the process must be shaped in the best possible way. An interesting point is, instead of using CD, CDS is adopted, thus the trend of the line observed at Figure 5 changes for the one at Figure 6. The curve became flat. The reason is that, although there had occurred a historic decrease of CD, it also had occurred an increase of the cross section of the vehicle. Hence, what was gained with the optimization of the car body, was lost in the frontal area, which had increased. However, if it would be accepted a reduction of 5-6% of the cross-section, the emission would be limited, since the drag resistance is given by:

D = q∞∙CD∙A

Where q∞, is the dynamic pressure, while CD∙A is the drag coefficient times the vehicle frontal area. Therefore, the process and the personnel behind of it must change, so as the product. There are three important arguments relative to the aerodynamic motorsport process, which is the different relationship between style and function, style and engineering and style and aerodynamic.

When these visions are common between style and aerodynamic, the final result is definitely better and absolutely not comparable with the results obtained in the case of a standard process. The first sketches of the development have the different visions about what was shared and discussed. Hence, from this point on any development step in the car design, this must be shared between style and function. This is essential, mutual awareness between style and function. Stylists are getting use of what is important for the engineers while they are getting use of what is important for the stylists. It is a common grow up. This is basically the story of the current vehicles. Since there is a continuous fight between style and function, there is a need of strong design manager to decide between the functions. After that, the process goes to the operative approach. Instead of using bigger full scale models, the use of smaller scale models as in the motorsport has supported a lot in speeding the process. This is not only important in terms of time, but also in terms of cost, because when project duration is higher. The cost follow this trend. For road cars it is essential that the costs should be kept as low as possible. In addition, this is also important in terms of know-how, because what is learned, it is what is applied on the following projects.

Normally, the level of knowledge is covering much wider map envelope of the car in terms or ride heights (RH). The industrial process of road car aerodynamics used to adopt full scale WTM made on clay, but some conclusions with these kind of WTM are incorrect in the aerodynamic point of view. The reason is, probably, due to the level of car performance, because when this is too much important, the knowledge of the car behavior over the complete aero map is extremely important.

A technical approach to improve aerodynamics is the use of active devices to reduce drag. In addition, it is not something created between the aero department and the style one, because once the car is at running conditions, the configuration is the one that engineering would like to have. On the other hand, when the car is at a parking condition, the configuration is the one that the styling department would like to have. The problem is that the active systems are expensive and heavy. Hence, something that could be a democratic tool, is also one that is present when the level of cost goes far from a specific point. A good reason could be a different scale of values. Normally, in road car production, the cost unit is the cost/kg. For instance, in same car family, the constructor defines its value by price per kg. This is what makes the car competitive in its respective segment. If this can not be guessed, the mix of the cost components must be different. This is something that concerns the constructors and the market, because in the configuration of the car there is a list of options. In some cases, a sort of these options can be refused if the main problem is to match CO2 emission target. For instance, the heating system of a seat could be substituted for an aerodynamic device, but the market culture may not recognize an active aero device and a heating seat system at the same value.

References