why Cx is so crucial for electric cars
With the advent of the electric car, the concept of Cx is increasingly put forward by manufacturers. For what ? For the simple and good reason that the aerodynamic drag coefficient plays an important role on one, if not the key element of an electric: its autonomy. But what is the Cx concretely? Here are our answers.
At the time of all thermals, the aerodynamic drag coefficient was already an important element, but manufacturers did not necessarily communicate on it. At the time of the electric car, this data is present today in all the technical data sheets, even of the most “basic” models, whereas the Cx, in the world of combustion, is often associated with performance and, therefore, to sportsmanship.
For the electric car, it’s different. In this incessant race for autonomy, which should however calm down with the densification and reliability of the charging network and the end of this “fear of failure”, the aerodynamic drag coefficient is an important element since it is often considered as the second most important factor for a car’s range after the battery capacity (in kWh). But what is the Cx concretely?
Before even starting, the Cx plays a role on several tables and not only on the consumption (and therefore the autonomy) of an electric car or a thermal model. No, the aerodynamics of a car is also a variable that will influence behavior or even comfort.
The study of aerodynamics
The study of airflows is the same as the study of fluids, that is, it is the same mathematical equation. Without going into incomprehensible calculations that only certain engineers could understand, know that air is a fluid like any other and behaves in the same way as water. To simplify as much as possible, it’s as if we humans live in a huge aquarium where water is replaced by air.
But air is not water? Indeed, and even if the resistance of air is about 800 times lower than that of water, it is all the same important enough to have a real influence on the movementsespecially when talking about cars.
It is an altogether natural process, which has been “transformed” into an equation in order to better understand its importance. Fluids are part of the concept of drag coefficient (also called Cx), and it is a coefficient that indicates the resistance of a shape against a headwind. It is generally easier to understand when you watch a video on the wind tunnel work of aerodynamic engineers, where the fluids are materialized by these white streaks in the video below.
As you may have noticed, Cx is measured in numbers. It is located between 0.05, the most aerodynamic shape, and 1.4. The most aerodynamic shape is the drop of water, and it is this shape that car manufacturers want to get closer to, while bringing design, emotion, safety, comfort, dynamic behavior, etc. into the equation. . The least aerodynamic shape, logically, is the flat shape facing the wind, the famous “block” in car jargon.
But the drag coefficient is a more complex concept than that, since there are still four different types of drag:
- Form drag : it is the air which comes to collide frontally with a form, the most “simple” notion to understand;
- Surface drag : this is the friction of the air against the bodywork, which also causes a form of resistance;
- Turbulence Trail : it’s the eddies that form around certain shapes that slow everything down even more, that’s why we see on some cars small aerodynamic appendages in certain specific places;
- Internal drag : it is the resistance related to the air which circulates in the car: air recovered for the ventilation of the passenger compartment for example.
With these four elements, you have probably now understood why, on a spoiler for example, there are sometimes certain small vents or, on certain sports cars, there are aero appendages at the level of the shields. Sometimes it even goes even further, because if you look at even a rear view mirror, the possible slight crease in the bodywork that extends over two centimeters is nothing trivial and surely has an aerodynamic function.
Recently, Volkswagen detailed how its new ID.7 could boast a range of 700 km, explaining the aerodynamic elements that made this good performance possible.
Be careful though, because in the world of the car, nothing is simple. There is still an equation within the equation itself to solve. A car that is too aerodynamic is not a guarantee of excellence either.quite the contrary, and it is not motorsport drivers who will say the opposite.
Indeed, a low Cx indicates that the car may have very average downforce, in other words, it may cause them to lose downforce at high speeds and thus reduce road holding. Well, for sports cars, we have found the solution for a long time, in particular with ever more elaborate fins capable of bringing a “load” to the rear part in order to press the car to the ground.
As for electric cars, they have aerodynamics optimized to the maximum, to prevent their autonomy from being too limited. However, this does not mean that they are soaps on the road, and in any case between the weight they make and the performance, this kind of problem does not exist. And even if it exists one day, we will simply see electric cars with aerodynamic appendages close to current thermal sports cars. Especially since their center of gravity is very low, thanks to their batteries integrated into their base.
What are the differences between Cx and SCx?
If you are used to looking at car data sheets, you have probably noticed that there are two elements to measure the aerodynamics of a car: the Cx and the SCx.
These two elements are obviously linked, the SCx being simply the drag coefficient multiplied by the frontal surface exposed to the air in m2. This allows you to really compare two models unlike the Cx alone.
In other words, the Cx refers to the shape of the car, viewed from the side. While the SCx refers to the entire surface of the car that will be in contact with the air.
Indeed, we sometimes arrive at two identical Cx for two cars which do not however have the same shape, in particular between an SUV and a sedan. But when surface difference enters the equation, it is a game-changer. Thus, an SUV, which nevertheless has the same Cx as a sedan, will have a much higher SCx because of the shape of its bodywork, which will have greater air resistance.
And who says higher SCx necessarily means higher consumption, especially at high speeds. Because at low speed (as in town), it is above all the weight that affects consumption.
What influences the aerodynamics of a car?
As stated above, and this is often an abuse of marketing language on the part of manufacturers, the Cx does not only affect the consumption of electric or thermal cars. Admittedly, this is an important element, but it is not the main one.
Aerodynamics will also influence performance and handling at high speeds. Optimized air flow management increases comfort by reducing air noise as much as possible while improving soundproofing. On some cars, the retractable fins help with fuel consumption, but also to improve the flow of flows in order to reduce turbulence as much as possible, which penalizes Cx.
To come back to consumption, and that, everyone understood, reducing the rate of penetration into the air therefore reduces the energy consumed. If, at low speed, the air resistance seems anecdotal, it will increase when the speed increases. From now on, you must probably make the connection with the famous retractable fins on certain cars which activate automatically according to the speed.
The faster you go, the more the resistance increases, and this is also why there are strong disparities between certain electric models concerning their consumption on the highway at 130 km/h. Two models of electric cars can announce a similar autonomy on the theoretical WLTP cycle, but have a totally different autonomy on the highway, because of their aerodynamics. We have already seen the problem with cars that travel long distances as quickly as possible.
Which electric cars have the best Cx?
As you will have understood, for an electric car, a good Cx partly means that the autonomy is often interesting depending on the capacity of the battery and the segment. Here is a small summary of the most aerodynamic electric cars. And as you can see, these are particularly expensive cars, like the first in the ranking, the famous “solar car” Lightyear 0.
| Cars | Cx | SCx | Frontal area |
|---|---|---|---|
| Lightyear 0 | 0.175 | 0.35 | 2.00 m2 |
| Lucid Air | 0.197 | 0.44 | 2.23 m2 |
| Mercedes EQS 450+ | 0.20 | 0.50 | 2.51 m2 |
| Tesla Model S | 0.208 | 0.48 | 2.35 m2 |
| Nio ET7 | 0.208 | 0.52 | 2.50 m2 |
| Hyundai Ioniq 6 | 0.21 | 0.46 | 2.23 m2 |
| Porsche Taycan | 0.22 | 0.51 | 2.33 m2 |
But there are even better than the cars present in the table above. The best of the bunch is the Aptera, an amazing three-wheeled solar car that’s available to order. All the information about it can be found in our dedicated topic. Just hold that it benefits from a record Cx of only 0.13!
More recently, and also more consensual, there was the Mercedes EQXX prototype, which was intended to emphasize the importance of good aerodynamics for good autonomy. Mission accomplished as the Mercedes EQXX concept traveled 1,202 km across Europe, between Stuttgart and Silverstone, taking the Channel Tunnel. The car has a Cx of only 0.17.
The Mercedes Vision EQXX consumed, on average, 8.3 kWh/100 km for a 1.7 tonne car. A figure to be compared with the Lightyear 0 which had traveled more than 700 km with a consumption of around 8.9 kWh/100 km and an average speed of 85 km/h for a weight of 1.6 tonnes. Admittedly, the battery size is not the same (about 100 kWh on the German, against 60 kWh for the Lightyear), but the consumption remains similar.
It is for this reason in particular that Ford now wants to design cars with ultra-worked aerodynamics, to avoid carrying large batteries, and therefore increasing the selling price of the car and its impact on the planet. Same story on the side of Toyota, which works on the design of its cars with artificial intelligence to improve the famous Cx.
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