The Comprex(R) is a pressure-wave supercharger (PWS) for passenger car diesel engines. It has many features which ideally suit it for the continually increasing demands on driveability, fuel economy and reduction of exhaust pollutants. To counter the disadvantages of the previously required belt drive, a free-running machine was developed. It is self driven by the kinetic energy of the exhaust gas; made possible by employing a rotor having reduced inertia. In addition to the well known Comprex features, this advanced development offers advantages such as rapid response, high efficiency, compactness and freedom in placement. The paper discusses the design of the free-running PWS, its construction, supercharging characteristics and preliminary experience.