Riding on Air: Activated Carbon Aims at Autos

An old piece of science is bringing new applications across the mobility spectrum.

Carbon Air’s activated carbon is fixed inside the pressurised cavity of the air spring, occupying space that would otherwise be occupied only by the air inside the chamber. (Carbon Air)

Activated carbon may seem an esoteric piece of science in the automotive space, but it is quietly playing a key role in Audi A6 and A7 air-suspension systems. The technology also is promising for new applications in vehicle safety, comfort and acoustics.

Carbon Air, a company spun off from a U.K. university, is developing activated carbon for use in cars, light trucks, heavy commercial vehicles, buses, motorcycles and even mountain bikes. The technology, which is based on activated charcoal, has long been used in other industry sectors including medical, agricultural, water treatment and food production. Its opportunities in mobility appear to be equally broad, the company noted.

Activated carbon’s novelty is not in the material itself, but rather in its effect on the mechanical behavior of the air surrounding it, said John Coakley, Carbon Air’s chief technology officer. In an interview with SAE International, he said the company aims to leverage its Audi success with other partnerships and broader applications “including the benefits of ride-height control, preemptive intelligent autonomous vehicle (AV) chassis subtlety, noise absorption, seat comfort and high-quality infotainment audio.”

Packing in the molecules

Carbon Air CTO John Coakley is confident of activated carbon’s potential in many vehicle applications. (Carbon Air)

Activated carbon is formed by subjecting organic carbon matter (typically coal dust, sawdust and even coconut shells) to super-heated steam and chemicals. This creates a dense network of microscopic pores which can vary in type, each presenting differing adsorption characteristics. These can be tuned to many disparate applications.

Effectively, activated carbon changes how closely air molecules can pack together, increasing the apparent volumetric capacity of a container. As the air inside is compressed and pressurized, activated carbon can adsorb a greater volume of air than the space it occupies. It subsequently contains more molecules. “This means that the stiffness of the pressurized air increases in a more gradual and linear way, not exponentially as it does in a container without activated carbon,” Coakley explained. “As such it is ideal for vehicle air springs.”

The genesis of Carbon Air can be traced back to a Ph.D student at the University of Salford in Manchester, England, who was validating claims made by a loudspeaker company that activated carbon in one of its speaker cones provided a deeper, richer sound. The student found the claims to be valid. The activated carbon was placed inside the rear volume of the loudspeaker, changing its behavior.

“The material’s secrets started to be unlocked,” said Coakley, who at the time was the university’s technology-transfer officer, responsible for commercializing its R&D innovations. Other potential acoustic applications became apparent. An initial patent filed by the university in 2010 led to the founding of Carbon Air (the student was given shares in the company, which now has multiple patents granted and pending). Development for air-suspension applications led to a Tier 1 supplier licensing the technology, which has been installed on certain Audi A6 and A7 models since 2018.

Refining ride quality

Carbon Air is learning the process of working with automotive Tier 1s and OEMs. “It takes a long time to design and integrate an entirely new technology into a mainstream vehicle, particularly in a safety-critical area like the primary load-bearing element of a car’s suspension,” Coakley noted. “In the case of Audi and our Tier 1 licensee, it was around four years.”

Other carmakers have tested prototype Carbon Air air springs. Coakley believes more will follow since the springs’ robustness and technical viability have been proven in Audi series production. Ride quality is the primary benefit for the air spring application. The pressures used in automotive air suspension are largely fixed while the vehicle is on the move, and the load that the spring must carry determines its cross-sectional area. The key variable is volume, which determines spring rate.

“To achieve a certain stiffness for a given spring and suspension geometry, a certain volume is required – unless you have access to Carbon Air technology,” stated Coakley. “But there is a further important benefit. A normal air spring has two spring rates; static and dynamic. The static rate occurs in slow actuations, like pitch and roll; car makers want that to be as firm as possible. The dynamic spring rate occurs in faster actuation – such as when a car rides over bumps – and so needs to be as soft as possible. However, with conventional air springs the opposite occurs – it becomes much stiffer.

“Carbon Air technology greatly reduces this dynamic stiffening under rapid actuation, making for a softer, more refined ride over bumpy surfaces,” he said. “So, the strongest benefit is in dynamic conditions, because these are the ones the car occupants normally feel.”

Gas-charged dampers, which become stiffer as they compress, are another opportunity. “Our technology enables a more constant and predictable damping rate. The benefits of this will be felt in handling and ride,” Coakley said. He expects the technology will enable the development of smaller air springs to do the same job as current conventional springs, while offsetting the added cost of Carbon Air’s technology.

AV seats and audio

The dynamic force and stiffness as an air spring is compressed through its designed travel. Graph at left shows how much softer the Carbon Air spring is for a given pressure and static load. The right-hand graph shows greater consistency of air stiffness with the Carbon Air spring. The data result from dyno tests carried out by Carbon Air on a production spring from a Mercedes GL-class SUV, at 6 bar. (Carbon Air)

Among other things, the company’s sights are set on emerging opportunities with air springs used in car seats, particularly in AVs. Coakley sees its technology being capable of making adjustments in anticipation of cornering, braking and acceleration forces as they are applied. This could contribute to greater comfort and a sense of stability for AV occupants. Employing the technology to help reduce AV occupants’ potential motion sickness could be another course of development.

Air-suspended seats already are commonly used in trucks, buses and off-highway vehicles, but in many cases there is very little space under the driver’s seat; packaging improvements enabled by activated carbon would be welcomed by designers. Coakley injects another potential vehicle application: “Mountain bikes are also an exciting opportunity for us,” he said. “Air is preferred [as a spring] because it is light; suspension actuation is long, performance expectations are high.”

As for carbon activation for acoustics, Coakley sees two potential areas of interest. First, Carbon Air has identified what it terms “high-grade activated carbons” and discovered how to impregnate them into open cell foams in a way that the binder does not mask their pores. The result is a range of materials said by the company to have “extraordinary noise-absorption characteristics.” At the lowest frequencies, the materials can outperform the best conventional acoustic foams of more than twice the thickness, saving weight and space, the company claims.

“This creates opportunities throughout the vehicle, particularly for the newer generation of coupes and sports SUVs that have high floors and low roofs, and where every millimeter of trim saving can be critical,” Coakley said. As active noise control gains favor, “the more that can be achieved passively, the better the performance of the active system,” he added.

The second application is for loudspeaker enhancement, particularly around larger sub-woofer installations. “One luxury car maker explained to us that their top-of-the-range audio systems occupy almost a third of trunk space due to use of an enlarged sub-woofer. Activated carbon, properly specified, can reduce the space required for that by up to a half,” noted Coakley. He believes there is an opportunity for fusion of the two applications. In-vehicle audio is carefully tuned to match the acoustic behavior of the cabin interior and Carbon Air’s modeling and materials capabilities are claimed to allow it to improve both the sound of the speakers and the environment in which the system operates.

“We can halve the size and significantly reduce the weight of the loudspeaker, without a loss in bass performance. And at low frequencies, our materials can be half the thickness of the best acoustic foams,” Coakley asserted. “In pneumatics, there are limits on which cavities you can occupy (activated carbon is largely uncompressible), and the benefit changes according to the pressure under which the component operates. But, very roughly speaking, for each half liter of activated carbon added to an air spring, it behaves as if it is nearly one liter larger.”