FOR more than 150 years, pneumatic braking systems have been continuously refined as technology has evolved to meet the changing demands of railway operations. As operators now increasingly look to reduce costs, energy consumption and environmental impact, Siemens Mobility has developed an air-free braking system based on a fully electronic “brake-by-wire” architecture, eliminating the need for pneumatic brake components and delivering a wide range of technical and operational benefits.
Conventional pneumatic braking systems generally consist of several components, including pressure reservoirs, control panels, valves, pneumatic brake actuators, compressors and pneumatic piping. The air-free system replaces these with electronic brake control units and brake actuators, which are fully controlled by wire (see image). The compact actuator is designed to fit within the existing bogie envelope without requiring additional space. The system itself is certified to meet Safety Integrity Level 4 (SIL4), enabling full independence from pneumatic components while maintaining all relevant safety features.
Operator benefits
The brake-by-wire architecture has several direct benefits for rolling stock operation and maintenance. Eliminating pneumatic brake components reduces the overall number of components, which frees up installation space in the underfloor area and lowers vehicle weight. In the case of the X-Car fleet built by Siemen Mobility for the Vienna metro, the air-free system has reduced the weight of a six-car train by more than 900kg. The reduced component count also simplifies maintenance: the actuator is the only element requiring regular maintenance, and access is made easier as less underfloor equipment needs to be installed.
The fast response times of electronic brake control also improve stopping point accuracy and brake release behaviour, enabling shorter station dwell times. Under ETCS, the decentralised architecture of the air-free system and effective use of available wheel-rail adhesion enables more favourable braking curves, contributing to shorter emergency braking distances and improved headways.
The brake management system and electronic wheel slide protection help to prevent wheels from locking during braking and minimise the associated wear on wheel and rail. The elimination of pneumatic equipment also lowers the overall compressed air demand at train level, resulting in direct energy savings.
Environmental impact
These operational and technical advantages are also the basis of the lower environmental impact of the air-free system when compared with conventional pneumatic braking. To quantify these benefits, Siemens Mobility conducted a comparative lifecycle assessment of the air-free system and a conventional pneumatic system. The study was supervised by Vienna Technical University and carried out in compliance with ISO 14040/44, examining the full product lifecycle from resource extraction and manufacture through operation to end-of-life treatment.
The study found that the air-free system has a 58% lower carbon footprint than a conventional pneumatic braking system, corresponding to a saving of 39 tonnes of CO₂ equivalent over the lifecycle of the train. While such absolute figures depend on the assumed parameters of the study, the result nevertheless provides a clear indication of the sustainability benefits of the air-free system.
With fewer components, the air-free system requires less materials to manufacture, reducing the overall environmental impact of production and maintenance. Lower vehicle weight cuts traction energy demand, particularly in metro applications with frequent stops.
The faster response times made possible by the air-free system deliver a further major reduction in energy consumption. Compared with pneumatic systems, the brake callipers release approximately 1.5 seconds faster when leaving stations. This shortens the period during which traction torque is applied while brake torque is still present, thereby reducing traction energy loss.
While pneumatic braking systems remain a well-established technology in the railway industry, the air-free system has demonstrated its operational viability in passenger service in Vienna and is now being applied in further projects, including the fully automated S-Train fleet that Siemens is supplying to Copenhagen.
*Philipp Loos is sales and project manager at Siemens Brakes in Vienna, Austria.
