Aerodynamic coefficient: we reveal the secret of energy efficiency
A lower air resistance implies less electricity consumption and, therefore, more autonomy. We will tell you everything you need to know about these two magic numbers.
The scientific definition of aerodynamic coefficient reads: “This is one of the dimensionless numbers used to study forces and moments suffered by any moving body in the air in aeronautics, the automotive sector and other fields”. In simple terms: this measures an object’s resistance in the wind. In the case of a car, the less force opposed, the lower the energy consumption. Therefore, this figure is extremely important in the car industry, and even more so if we speak of electric mobility.
This is the reason why the first hybrid and electric vehicles, such as for example the Toyota Prius, came about with unusual designs: its bodywork prioritised above all as low an aerodynamic coefficient as possible, and this of course would have an impact on its forms. Today, electric cars have designs increasingly similar to their gasoline and diesel counterparts thanks to resources such as using flat wheel covers, thinner wheels or removing the front grill. All these measures reduce the air resistance.
The aerodynamic coefficient is denoted by the letters Cx. For most production cars today this is between 0.25 (least air resistance) and 0.40 (higher, this is a figure more related to lorries). Some very specific models and prototypes designed come down to 0.20. Unlike what most could imagine (because of their smaller size) the aerodynamic coefficient of motorcycles is higher than in cars and is usually around 0.60. This is the reason why many of the new electric scooters coming out on the market present cleaner and simpler designs, almost as if they were toys. The lesser the cleft, aesthetic components and frills, the lower the air resistance.
The wind tunnel
To measure the coefficient we use scale models that use dimensional analysis techniques, but we can also calculate this in an actual size pre-production model inside a wind tunnel. This device with a name that would not have been out of place in a Jules Verne novel, although it is highly sophisticated, is more than 300 years old. In 1746, the British engineer and mathematician Benjamin Robins presented a kind of tunnel with a rotating arm that moved at a speed of several feet per second (between 4 and 5 kilometres an hour). Little by little, this was perfected and used by illustrious names such as the Wright brothers, who were aviation pioneers.
Today, it is capable of simulating any actual condition, however extreme, to be able to verify a car’s behaviour in the face of winds of different forces, and its air resistance.
The most aerodynamic cars in the world are electric
Ascertaining which vehicles offer less wind resistance is simple: those that have a lower aerodynamic coefficient. Today, the record is not held by a production car but rather the prototype XL1 from Volkswagen, whereby the brand sought to demonstrate to what extent we can save fuel by refining aerodynamics as much as possible: its aerodynamic coefficient is only 0.18 Cx.
But if we speak of cars that do exist and circulate on our roads, electric and hybrid cars come out in pole position. The Tesla Model 3 has an aerodynamic coefficient of just 0.21 Cx, which in part accounts for the fact that it helps the batteries to attain autonomy that reaches 500 kilometres. Such a low air resistance has a great advantage if we consider the size of the car and especially its interior space which is capable of seating five adults and which hinders being able to reduce the height of the bodywork (one of the resources to reduce the Cx).
The aesthetics of the Toyota Prius always generated some controversy since the launch of the first-generation. But what is true is that everyone has different tastes and the Japanese brand has always prioritised maximum efficiency in this model to maximise use of its battery charge. The current version attains a surprising Cx of just 0.24, exactly the same as the Model S, Tesla’s luxury Berliner.
Among the 10 vehicles with the lowest aerodynamic coefficient there is of course a sports hybrid. The BMW i8, able to cover certain distances only in electrical mode, has a Cx of just 0.26. Of course, in this case, it was easier for German engineers because a sports car of these characteristics may be extremely low, almost at ground-level, because the interior space does not even feature among their priorities.