Capture the brake disc and tire abrasion

The VW Jetta Hybrid on the roller test stand in Empa's engine house already has a long career as a fleet vehicle behind it. Since July 2020, strapped into the test chamber, it has been serving a new research purpose: to generate brake dust strictly along the standardized WLTP driving cycle, which is also used to determine tailpipe emissions.

The interest in brake dust measurements is still relatively new: In June 2016, a division of the UN Economic Commission for Europe (UNECE) called the Particle Measurement Programme Informal Working Group (PMP IWG) decided that it was time to develop a generally usable test method for brake dust that could reliably determine the mass and number of emitted particles. Since then, a number of research institutions, vehicle manufacturers and specialist companies for measuring equipment have been working on the issue. But the problem is not entirely easy to solve.

Distributed in all directions

Unlike an exhaust pipe, which blows reliably in one direction, a rotating brake distributes its particles in all spatial directions. So you first have to capture the particles and then let them fly through a funnel toward the measuring device. As little as possible must be lost in the process: Neither light particles may escape, nor may heavy particles remain in the lines.

There are two further complications: A car's brake is attached to a rotating drive shaft, which must be carefully sealed for the measurement so that no particles escape. And a brake needs cooling. In a moving car, the airstream, together with ventilation louvers between the brake discs, provides a cooling draft. By contrast, a fully enclosed brake on a test rig can quickly heat up - and would then produce completely different particles than in real everyday traffic. Such a measurement would be of little value.

Measure all emissions simultaneously 

The PMP IWG working group of the UNECE solves the problem by simplifying it: the desired brake tests are to be carried out in completely enclosed test rigs. Such test rigs do exist. They resemble large cabinets in which brake discs and brake pads rub against each other. So only one component is tested, not the whole car.

"We are trying a different approach," says Panayotis Dimopoulos Eggenschwiler, who is designing the test setup at Empa. "We want to measure all of a car's emissions simultaneously during a road test on the test bench. That has greater significance than data from an isolated brake test rig, which then has to be converted to real conditions."

An airy special construction

Together with engineer Daniel Schreiber, Dimopoulos Eggenschwiler has developed an Empa variant of the test, the results of which must now stand up to comparison with other, international working groups. At Empa, a whole, real car is on the test bench, the VW Jetta Hybrid mentioned at the beginning. The brake of the right front wheel has been encased in a specially designed metal housing. A compressed air hose conveys large quantities of cooling air into the metal casing from the front of the car; at the same time, the air is the transport medium for the abraded brake particles. These are directed into a tube about one meter long next to the sill of the car and, after a short flight time, land in a 13-stage cascade impactor, a special measuring device that sorts particles according to size. After the test, the particle fractions can be weighed, chemically analyzed and, if required, also examined under an electron microscope, for example for their morphology.

"In preliminary tests, we have already determined what the particles consist of," says Dimopoulos Eggenschwiler. "It's mainly iron oxide, which comes mainly from the brake disc, and a number of elements such as aluminum, magnesium, calcium, potassium and titanium, which come from the brake pads." In addition to large, heavy particles, there are also smaller ones that may well be inhaled and enter the lungs.

Do hybrid cars brake differently?

Now that the measurement procedure is running stably, the VW Jetta will initially be operated in the legally prescribed WLTP cycle, delivering its brake particles to the counting machine. After that, further test series are planned. "We want to find out, for example, whether hybrid cars brake differently than cars with conventional drive systems and thus produce different emissions," explains the project manager. Hybrid cars can also brake with the help of their electric motor and therefore need to use the mechanical brakes less often. "With the measured values, it will be possible to optimize the operating phases of future vehicle generations and control brake dust emissions better than today."