The new air-free brake system from Siemens Brakes

The new air-free brake system is the first fully electrically controlled friction brake to be implemented in rail vehicles (brake-by-wire), therefore it‘s also called electric friction brake system. The brake system in the vehicle operates completely independent of compressed air. Besides various technological benefits, the new brake reduces vehicle weight and the vehicles are faster ready for operation, as Robert Steinfelder and Jens Lichterfeld told us in this interview. Both are significantly responsible for the development and market launch of the innovative Siemens Brake.

Mr. Steinfelder, Siemens Brakes has been successfully implementing pneumatic brake systems in a variety of Siemens Mobility projects for a number of years. With this solution, you’ve now added a new and innovative brake system to the MoComp portfolio. How did it come about?

Robert Steinfelder: Since 2003, Siemens Brakes has been an independent department within Siemens Mobility and works closely with the vehicle engineering and commissioning of Rolling Stock. Starting with the first metro project, the V-Wagen in Vienna, we’ve been the in-house system supplier for all metro projects in the past 18 years.

During this time, we’ve been able to acquire a lot of experience with pneumatic brake systems and we have implemented additional synergies across systems, seeing things from a train manufacturer’s perspective. After years of optimizing the conventional air brake, from our point of view, it was time for something new, a technological leap, a genuine innovation in the field of brake systems for rail vehicles.

And with the technological leap you refer to the new air-free brake system, right? Mr. Lichterfeld, what is this exactly?

Jens Lichterfeld: Unlike the conventional pneumatic brake, control of this brake is fully electrical – “brake-by-wire” – which means that all piping and all pneumatic components for controlling the brake can be eliminated. Brake force is generated right in the brake actuator. We worked with Liebherr, a highly experienced company in the aerospace sector, to develop a compact, closed, electrohydraulic brake actuator including all the components necessary for pressure build-up and release as well as local control. The brake actuator is integrated in the bogie in the same installation space and has the same mechanical interfaces as the conventional pneumatic brake caliper.

We’re extremely proud of the fact that we’ve developed our new, innovative brake system which is more than just an alternative to conventional air brake systems, and that we’ve created a genuine unique selling point for our customers.
Robert Steinfelder, Vice President Siemens Brakes, Siemens Mobility GmbH, Erlangen

The pneumatic brake has been used in rail vehicles for 150 years. Is there any particular reason to get rid of it in the future?

Robert Steinfelder: Basically, there’s no reason to get rid of the pneumatic brake. Pneumatic brake systems have been proven successful for decades, experienced continuous improvements throughout this period and will continue to be used in rail vehicles in the future. Our goal for this technological leap was to generate a unique selling point for Siemens Mobility, because we believe that the pneumatic brake system has reached its limits in terms of optimization.

Why do we need a friction brake at all? Isn’t electrodynamic braking via the traction motor enough?

Jens Lichterfeld: Even when the electrodynamic brake is primarily used in operation, you still need the friction brake to safely stop the train and hold it in standstill. Electrodynamic braking via the traction motor can’t guarantee this. The friction brake also has to hold the train safely in place on all possible gradients in the operator’s network after it has been powered off.


Does the new technology meet the high safety requirements for brakes in rail vehicles?

Robert Steinfelder: Of course! We based our development on the highest safety requirements and Safety Integrity Levels up to SIL4 and also the latest rules and regulations for safety verification. Starting at the train level, we broke down these requirements and applied them to the individual components of the brake system: for example, the brake actuator and brake control unit.

What does using the new brake system mean for the vehicles and for operation?

Jens Lichterfeld: Using the air-free brake results in a number of benefits on car-level, including reduced weight and reduced effort for installation and commissioning of the brake system. In addition, installation space that was previously required for the air brake components can now be used for other subsystems. That’s extremely useful when it comes to vehicle development.

In operation, efforts can be reduced in several areas. This doesn’t just apply to the maintenance for the brake system, it’s also relevant for the train’s start-up time, which is the time period that the operator needs to consider before the train is ready for operation. This means that either a driver can take their place in the train much later to ready it for operation, or that a train in an automated system during rush hour can be ready for operation much faster– which means more flexibility.

The trend towards automation or partial automation of train operation means even more benefits. First of all, in terms of the stopping characteristics and the stopping point accuracy, which is comparable to that of the electrodynamic brake. Secondly, headways can be shortened because the brake can release much more quickly, with the result that the train requires clearly less time to get moving after the doors are closed. With headways between 60 and 90 seconds, savings of at least one second are a significant improvement. This allows more trains to be deployed throughout the day in the actual operation of future projects.

Unlike the traditional air brake, control of this brake is fully electrical – “brake-by-wire” – which means that all piping and all pneumatic components for controlling the brake can be totally eliminated.
Jens Lichterfeld, Senior Expert Brakes, Siemens Mobility GmbH, Erlangen, Germany

With the X-Wagen, Wiener Linien is operating the first series project for the air-free brake. How did you get there, and how will you proceed?

Robert Steinfelder: We developed and thoroughly tested the air-free brake as a system over a period of many years. Before its first serial application, we also tested it in passenger service, meaning we subjected the system to intensive pre-testing and really put it through its paces. All the knowledge gained from many years of testing in rigorous passenger service was incorporated in the final technological solution, allowing us to deliver a fully developed solution for the first serial project.
In October 2020, we started the dynamic commissioning of the first six-car metro train, and we’re expecting the X-Wagen to be ready for passenger service by 2022.

Wiener Linien will then be the first transit authority in the world to benefit from our brake system, both in conventional operation with a driver and later in automated operation on the new U5 Line.



The project in Vienna won’t be the only project. What will happen over the next few years?

Robert Steinfelder: We believe that more customers will be benefitting from the new air-free brake within the next few years.

Of course, in addition to solutions for metros like the X-Wagen in Vienna, we’re also working on other applications like mainline trains.

For this to be possible customers and operators would need to specify functional and technology-neutral requirements in future requests for proposals, instead of the usual practice of specifying a pneumatic brake system.