Drag coefficient (Cx)

The Invisible Key to Performance and Fuel Economy

In the automotive world, we often discuss engine power, torque, weight... But one often underestimated, yet critically important, factor is the drag coefficient (Cx). This seemingly innocuous number is at the heart of manufacturers' concerns and directly influences your vehicle's performance and fuel consumption. At WOT, as engine specialists, we know that optimization isn't limited to the engine; it encompasses all the forces that oppose a car's forward motion.

What is Cx and How is It Measured?

The drag coefficient (Cx) is a dimensionless measure that quantifies the aerodynamic resistance of an object moving through the air. The lower this number, the less air resistance the vehicle encounters. It depends on the vehicle's shape, not its frontal area (which is accounted for in the calculation of the drag force itself: Fd​=0.5×ρ×V2×A×Cx​, where ρ is the air density, V is the velocity, and A is the frontal area).

Measuring Cx is a complex process primarily conducted in a wind tunnel. Vehicles are exposed to a controlled airflow, and sensors measure the resistance force they experience. A car's exterior design, from mirrors to wheels, door handles, and diffusers, is meticulously optimized to reduce this drag.

The Evolving Importance of Cx

Historically, in the early days of automobiles and beyond, Cx was mainly a concern for engineers striving for ever-higher top speeds. On racetracks or for record attempts, minimizing air resistance was crucial for gaining those last few kilometers per hour. Iconic sports cars of the 60s and 70s featured sleek, flowing shapes designed to "cut through the air."

However, from the 1980s onward, and especially with the rise of environmental concerns and fuel economy standards, the interest in a low Cx underwent a radical transformation. It was no longer just about pure speed but about energy efficiency.

Why this change? Air resistance increases with the square of the speed. This means that at 130 km/h, it's four times greater than at 65 km/h. On the highway, aerodynamic drag becomes the dominant force the engine must overcome, far surpassing rolling resistance and internal friction.

For manufacturers, reducing Cx has become a major lever for:

• Decreasing fuel consumption: A more aerodynamic vehicle requires less energy to maintain a given speed, directly translating to reduced consumption, especially at high speeds.

• Meeting emissions standards: Lower consumption naturally leads to a reduction in CO2 emissions, a necessity in the face of increasingly stringent regulations (WLTP, future standards).

• Increasing electric vehicle range: For electric vehicles, where every kilowatt-hour counts, a low Cx is vital for maximizing range and reducing charging anxiety.

Cx Today: An Obsession for Manufacturers

Today, aerodynamic optimization is integrated from the very first sketches of a new model. Manufacturers compete to achieve the lowest Cx figures:

• Examples like the Mercedes EQS (Cx of 0.20) and the Tesla Model S (Cx of 0.208) for mass-produced vehicles demonstrate how far engineering can go.

• Even SUVs, traditionally less aerodynamic, are significantly improving their Cx thanks to active elements (grille shutters, retractable spoilers), underbody paneling, and more fluid designs.

At WOT, our role as engine specialists places us at the crossroads of performance. While we optimize power and torque at the core of the engine control unit, we are fully aware that the engine works in synergy with the entire vehicle. An excellent engine in a car with mediocre aerodynamics will see some of its performance "wasted" on overcoming unnecessary resistance. That's why we closely follow advancements in Cx, as they perfectly complement our goal: to offer maximum efficiency and performance to our customers, while respecting standards and the environment.

A low Cx means less effort for the engine, less fuel burned, and more power available for acceleration and driving pleasure. It's a synergy we value and consider essential for the automotive performance of tomorrow.