On the influence of sorbents on the static stiffness of Air Springs

Air Springs are deployed in many technical areas today. In the premium segment of the automotive industry in particular, they are being used more and more frequently. The load-independent spring rate results in a better ride comfort. Since the stiffness of the Air Spring is significantly influenced by the enclosed air volume, this usually leads to a conflict of objectives between the available installation space and the stiffness optimum. To reduce the spring stiffness while maintaining the same design space, Coackley and Elliot describe in their patent the use of adsorbents such as activated carbon in the enclosed air volume of the spring. By binding air molecules to the adsorbent, more air molecules fit into the enclosed Air Spring volume, while the size and pressure of the Air Spring remain the same. This paper describes how the so-called “virtual volume” is created by adsorption and how it can be determined by simplified measurements using a gas pycnometer. Furthermore, it is shown how these measurements are related to the static stiffness of an Air Spring, so that a prediction of the static stiffness of an Air Spring filled with sorbent, in this case activated carbon, is possible by simple pycnometric measurements.