Article by: Zheng Wen
Edited by: Zhou Changxian
Recently, a Mercedes VISION EQXX concept car successfully completed a real-world range challenge of 1,202 kilometers on the road, enduring high temperatures and traffic congestion, and arrived at the Silverstone Circuit in the UK. With an average energy consumption of 8.3 kWh per 100 kilometers, it broke its own efficiency record.
This is an interesting achievement. It should be noted that the energy efficiency performance is related to many factors, especially the aerodynamic design of the vehicle. The most important factor is the drag coefficient.
According to Mercedes, the drag coefficient of VISION EQXX concept car has been reduced to 0.17. However, even so, aerodynamic drag still accounts for nearly two-thirds of the energy consumption of the entire vehicle. If the average speed increases, the energy consumption of drag will be even higher, even exceeding 80%.
In theory, when a car’s speed exceeds 80 km/h, the air resistance encountered is already comparable to the rolling resistance of the tires; at a speed of 80 km/h, the air resistance can reach two to three times the rolling resistance. If the speed exceeds 200 km/h, almost 85% of the resistance comes from the air. This is also the reason why cars with low drag coefficients have significantly reduced fuel consumption after driving at high speeds.
Since the birth of the automobile, drag has always been a challenge in the development process, and it has plagued generations of automotive engineers. Today, we can have a glimpse of the mystery of automotive aerodynamics from the aerodynamic optimization done by Mercedes on the VISION EQXX concept car.
Air resistance is the most important external force that a car encounters during its operation. Generally, air resistance can take several forms:
- Form resistance, which is the main air resistance generated when a car is driven at high speeds and can account for around 58% of the total resistance.
- Interference resistance, which is caused by the protruding parts of the car, such as the rearview mirror and door handles, and can account for up to 14% of the total resistance. The development of door handles by car manufacturers demonstrates their efforts in this aspect. For example, some radical manufacturers, such as Tesla, have adopted hidden door handles.
- Internal circulation resistance, which is caused by the flow of air into and out of the vehicle and can account for about 12% of the total resistance. Some pure electric vehicles have even eliminated the grille where air enters.
- Induction resistance, mainly caused by the lift of the air.
- Resistance caused by the impact of airflow on the front of the vehicle.
- Friction resistance, which is the friction generated when air flows over the surface of the car and can be neglected.
The formula for air resistance is as follows:
Where Fd is the frontal air resistance, Cd is the air resistance coefficient, ρ is the air density, V is the speed, and A is the front projection area of the vehicle.
It can be seen that the smaller the air resistance coefficient of a car, the less it is affected by air resistance. The air resistance coefficient of a car can only be measured in a professional wind tunnel or through coast-down tests. Wind tunnel tests can simulate the atmospheric environment, and realize the regulation and balance of airflow. The typical images of wind tunnel tests that we often see show high-speed airflow with smoke added, and the visual effect is very obvious.
As the drag coefficient decreases, the power loss decreases. The most direct benefits for household cars are fuel and electricity savings. In theory, for every 0.01 decrease in the drag coefficient, the cruising range can be increased by 15 to 20 kilometers.
In nature, the drag coefficient of a sphere is around 0.5, while that of a bird ranges from 0.1 to 0.2, and raindrops have a drag coefficient of about 0.05. Penguins have an unbelievable drag coefficient of 0.03 when swimming. However, the drag coefficient of industrial manufactured cars generally ranges from 0.2 to 0.5, while that of airplanes is around 0.08 to 0.1. The average speed of commercial airlines is several times faster than that of cars – 800 km/h. If the drag coefficient is not small enough, it is difficult to navigate the sky freely.
In fact, early on, people did imagine reducing drag by thinking about water droplets. If the shape of a water droplet is too difficult to observe with the naked eye, then Rupert’s Drops would be more intuitive. Molten glass drops into ice water under the influence of gravity to form a “teardrop” shape.
In a foreign country, there once was an energy-saving car competition that only focused on fuel and electricity savings. The following image shows a car that looks like a deformed water droplet. However, it didn’t consider any practicality or aesthetic appeal.
In 1922, Swiss aerodynamics expert Paul Jaray proposed that “the outer shape with the minimum resistance is a body formed by half of the streamlined shape.” In addition, only by eliminating the separation of the tail can drag be reduced.Soon people realized that mastering the absolute truth was not enough, and reducing drag coefficient required attention to detail.
For example, the 1938 Volkswagen Beetle was very smooth and round, and can be said to be the epitome of streamlining. But its drag coefficient was 0.48, much higher than expected. Another example is the Mercedes-Benz B-Class, which is a boxy MPV, but has a drag coefficient of 0.24.
The ideal state for a vehicle when driving is to keep the air in a laminar flow as much as possible, and when the vehicle passes through the air, to keep it as calm as possible without any flow between layers. But this state is fragile and easily disturbed, becoming turbulent (including vortex flow).
In an extreme example, when an iPad moves quickly forward, the air flow hits the surface and is quickly disrupted, causing a large turbulent flow on the back side of the tablet. The flat panel is a good tool to disrupt laminar flow, and the drag coefficient can reach an astonishing 1.2 or so.
The turbulent flow formed at the back of the car also creates negative pressure, producing a rearward force on the moving car, as if there is a fan pulling the car from behind. In addition to the forward thrust, the negative force seems miraculous. The high drag coefficient of the Volkswagen Beetle is also due to the formation of large and small turbulent flows, while the Mercedes-Benz B-Class tries to keep the airflow away from the body and then separate it.
By considering the factors above, we can roughly know the directions to adjust if we want to reduce the drag coefficient of cars. For instance, we can make the chassis as flat as possible and reduce the height difference to avoid the formation of separating vortices caused by various protrusions facing the wind. We can also reduce the generation of vortices in the car body by designing a smooth shape, minimizing the vortex creation of tires, side mirrors and other parts.
So, how to create a car that meets daily driving needs, has super-low drag coefficient, and satisfies design aesthetics at the same time? The aerodynamic optimization of the VISION EQXX concept car is a classic example. Several prominent points are presented below:
VISION EQXX is equipped with a small active intake grille beneath the front bumper that rarely opens. A large cooling plate under the car can achieve cooling in most working conditions, and the active grille only opens when the heat load is too high.
Furthermore, when the intake grille is open, the airflow control system guides the cooling airflow from the high-pressure region of the intake vent under the bumper to the low-pressure area of the outlet vent on the front hood, which greatly reduces the air resistance caused by cooling and cooling wind. According to wind tunnel test results, opening the intake grille will only increase the drag coefficient by about 0.007.
As mentioned earlier, narrowing and elongating the rear end can improve the vehicle’s airflow and reduce air resistance. Therefore, the rear track of the VISION EQXX concept car was tightened by 50mm compared to the front track.
Generally speaking, it is difficult to make a car look good when the rear track is narrower than the front track. However, the wide shoulder design of the VISION EQXX allows for the rear track to not be noticeable when viewed from the back. This is thanks to the active rear diffuser installed on the VISION EQXX, which can extend 200mm downward and backward when the vehicle is traveling at high speeds.
It is well known that diffusers are primarily used in racing to provide downforce and ensure high-speed stability. However, the VISION EQXX’s diffuser has multiple functions.
When the vehicle speed is below approximately 60km/h, the active rear diffuser will automatically retract to improve low-speed maneuverability. When the speed reaches 60km/h, the diffuser can automatically open to optimize airflow, thereby reducing the drag coefficient by approximately 0.01.
In addition to the rounded and smooth front face and streamlined low-drag shape design, the shape of the VISION EQXX concept car’s side mirrors has also been optimized to reduce turbulence. Furthermore, the base of the side mirrors is designed with openings to improve airflow and reduce the frontal area.It is not difficult to imagine that if the VISION EQXX concept car had no rear-view mirrors, its drag coefficient would be even lower, so why design rear-view mirrors? This is because the additional energy consumption generated by external cameras and internal screens may not be lower than the energy consumption generated by increased drag from rear-view mirrors. Even virtual rear-view mirrors consume energy even when in traffic.
The VISION EQXX concept car is designed with air curtains on both sides of the bumper and air guide holes on the side of the front and rear wheels, guiding the air from the front of the car to both sides of the vehicle, optimizing the turbulence generated by the wheels, and reducing the aerodynamic drag it produces. At the same time, the tires and wheel hubs themselves have also been aerodynamically optimized.
It is worth noting that the VISION EQXX concept car does not use a closed rear-wheel design. Rotating wheels at high speeds will generate turbulence around them, and the simplest and most crude way to eliminate this turbulence is to directly enclose the wheels. However, this design completely ignores practical issues such as tire changes and tire cooling, and from a design perspective, this design generally does not look good.
If we look back, as early as before World War II, Mercedes-Benz had begun research on aerodynamics. In 1937, the Mercedes-Benz W125Rekordwagen had a drag coefficient as low as 0.17. However, the W125Rekordwagen looked like this at the time.
Today, the drag coefficient of the VISION EQXX concept car is also 0.17, but this car is very close to mass production, and it is reported that many of its advanced technologies will be gradually implemented in mass-produced cars.The two concept cars, 85 years apart, seem to have the same drag coefficient, but in fact, everything has changed. Carrying these changes is the over hundred years of car manufacturing expertise accumulated by Mercedes-Benz.
Today, in order to achieve better intelligent driving assistance, many models choose to place lidar and cameras on the roof, undoubtedly bringing new challenges to reducing drag coefficient.
Drag, the nemesis of cars.
This article is a translation by ChatGPT of a Chinese report from 42HOW. If you have any questions about it, please email bd@42how.com.