Aerodynamic drag is a major contributor to the energy consumption of road cars. The electric vehicle (EV) market is on the rise, range per charge is an important measure for EVs; the most aerodynamic car will likely have an edge over its competitors. Recently, car manufacturer Chery was able to achieve a drag coefficient of 0.168, the lowest for a road vehicle.
The drag coefficient is a non-dimensional value that exhibits the aerodynamic performance of a shape. Such a number is obtained by scaling the actual drag force (measured through a wind-tunnel or road test) and is easier for comparative analysis of vehicles, so a dimensional difference would not matter. More on aerodynamic coefficients can be read here.
Before going into the discussion, it is important to understand that the drag coefficient is dependent on vehicle configuration. For example, a four-seater and two-seater, or a sedan and hatchback, cannot be compared. Moreover, drag is proportional to the amount of down-force being generated; therefore, the drag coefficient for a Formula-1 car in both configurations would be different. Suspension height also influences the drag count and active suspension would give a vehicle optimal performance at multiple driving speeds.
Importance of Streamlined Airflow
The drag coefficient has a significant impact on the performance parameters, such as top speed and fuel consumption. Power consumed by the aerodynamic drag force is a cubic function of the speed; it means that a 5% increase in top speed requires a 16% increase in power. This is quite significant as compared to road friction, as shown below:
Ideally, an air-foil-like shape generates a lower drag coefficient because of better management of the adverse pressure gradient. However, practical constraints and aesthetics dictate the final design shape. In the table above, it can be seen that a road car has a lower drag coefficient than a race car, but in terms of overall efficiency, a race car is more efficient (more lift at the expense of drag, i.e., the L/D ratio). Readers can take a look at this article for a better understanding of flow separation and adverse pressure gradients.
Vehicle Drag and Key Contributors
Aerodynamic performance gains can be attained by shaping bodywork, artifacts, engine compartments, wheels, side mirrors, and other minor details. Let’s take a look at the key areas of improvement on a car with respect to drag reduction.
Rear-End Design and Boat-Tailing
Flow separation is a major contributor to the pressure or profile drag. At the rear end of blunt bodies, vortex shedding causes a pressure drop and an increase in drag coefficient. The simplest way is to perform boat-tailing, i.e., extend the rear end with a gradually decreasing cross-section. In the left figure, a sedan has less flow separation as compared to a crossover.
Front Bumper and Wind Shield Angles
A smoother front bumper keeps the flow field attached to the body; otherwise, separation bubbles start to form at the engine hood. Moreover, forward and aft wind shield angles have a significant affect on the drag. A shallower front angle benefits at low Reynold’s numbers and by delaying separation.
Flow Field around Wheels, Side Mirrors and Door Handles
The flow field around the wheels is complex due to rotation, tire treads, and rims. Flow separation can also occur on the sides of the vehicle. Sometimes, air from the front bumper ducts is directed into the wheel well to keep the side flow attached. Similarly, side mirrors generate wake, which is a major source of drag and cabin noise. Retractable door handles are another aerodynamic improvement in present and future cars.
List of Most Aerodynamic Cars
A list of ten top-performing aerodynamic packages is compiled here:
| Car Model | Drag Coefficient |
|---|---|
| Chery Aero | 0.168 |
| Lightyear 0 | 0.175 |
| Lucid Air Touring | 0.197 |
| Mercedes-Benz EQS 580 | 0.200 |
| NIO ET7 | 0.208 |
| Hyundai Ioniq 6 | 0.210 |
| BYD Seal | 0.219 |
| Porsche Taycan | 0.220 |
| Xpeng P5 550 | 0.223 |
| BMW i5 eDrive40 | 0.230 |
Aptera is intentionally not mentioned in the list, although it has a drag coefficient as low as 0.150 due to its unconventional configuration as compared to the other cars on the list. In the end, a pictorial view of the top four cars is shown for readers interest.
