Particle sling car: Since particle filters have been prescribed for the exhaust of vehicles, combustion engines emit less fine dust. But brake disc and tire wear also contribute to the fine dust pollution of the air. This is why researchers have now developed a new method to measure this brake dust, identify the particles and, in the future, be able to compare the dust levels of different car models.
Fine dust consists of tiny particles that do not immediately sink to the ground, but rather float in the air for longer. The smaller they are, the more they damage our health, as they can penetrate further into the respiratory tract. Particles a few micrometers in size can promote asthma, for example, and ultra-fine particles can even get into the bloodstream and the brain. Most of these fine dust particles are produced in road traffic by internal combustion engines. The pollution could be reduced somewhat by means of exhaust filters and driving bans in cities.
Why the dust test is so difficult
But it is not just the engines of vehicles that emit large quantities of fine dust. The particles also get into the air through abrasion from brakes and tires. The problem: Little is known about these particles, as there is still no method with which the mass and number of particles emitted by the brake dust can be measured precisely. The difficulty here is that the rotating brakes – unlike an exhaust pipe – do not distribute the particles in one, but in all spatial directions. So you first have to capture the particles and then let them fly through a funnel towards the measuring device. As little as possible should be lost during the process: neither light particles should escape, nor should heavy particles remain in the lines.
There are two further complications: A car’s brake is on a rotating drive shaft, which must be carefully sealed for the measurement so that no particles disappear. And a brake needs cooling. When the car is moving, the airflow, together with ventilation lamellas between the brake discs, creates a cooling draft. A fully enclosed brake on a test bench, on the other hand, can quickly overheat – and would then produce completely different particles than in real everyday traffic. Such a measurement would be of little value. Some of these problems can be avoided by removing the brakes and then testing them in larger chambers. But that only partially captures the real situation on the car.
Captured and sorted
Researchers working with Panayotis Eggenschwiler from the Eidgenössische Materialprüfungs- und Forschungsanstalt Empa have therefore now developed a new measuring method. “We want to measure all emissions from a car at the same time during a test drive on the test bench,” explains Eggenschwiler. “That is more informative than data from an isolated brake tester, which then has to be converted to real conditions.” To do this, the researchers place a common make of car on a roller measuring stand. Then they strap the vehicle tightly and cover the brake of the right front wheel with a specially constructed metal housing. A compressed air hose conveyed large quantities of cooling air from the front of the car into the sheet metal shell, which also served as a transport medium for the abraded brake particles.
The outflowing air is directed into a tube about one meter long and, after a short flight time, lands in a 13-stage cascade impactor – a special measuring device that automatically sorts the captured particles in the micrometer and nanometer range according to size. The fine dust particles sorted according to size can then be weighed, chemically analyzed and examined for their morphology in an electron microscope. The first preliminary tests with this new process have already been successful and have provided initial insights into the composition of the brake dust: “It is primarily iron oxide, which mainly comes from the brake disc, as well as a number of elements such as aluminum, magnesium, calcium, potassium and titanium that come from the brake pads, ”reports Eggenschwiler. In addition to larger particles, the team also detected smaller dust particles that could get into the lungs when inhaled.
The researchers are now planning further series of tests. “For example, we want to find out whether hybrid cars brake differently than cars with conventional drives and thus also cause different emissions,” explains Eggenschwiler. Because hybrid cars can also brake with the help of their electric motor and therefore have 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 to control brake dust emissions better than today,” the scientist concluded.
Source: Empa – Federal Materials Testing and Research Institute