Tunnel vision

Traffic hotspot provides vital insights

Conveniently located within a few minutes’ drive of the Freudenberg Group’s global headquarters, the charmingly named “Saukopftunnel” (sow’s head tunnel) provides our filtration experts with a perfect testing ground.

Named after the hill under which it runs, the tunnel has been used by Freudenberg Filtration Technologies as a dedicated test site continually since it was first opened to traffic in December 1999. At 2,715 meters (1.7 miles) in length, it is the longest single-pipe bidirectional road tunnel in Europe outside the Alpine region. It was originally constructed to relieve traffic stress on the center of the nearby town of Weinheim.

Testing under real-life conditions

With over 20,000 vehicles passing through it every day, the tunnel has high concentrations of soot and other vehicle pollutants, especially during peak traffic periods. This makes it ideal for testing the performance of various filter types under actual working conditions. Thanks to the openness and cooperative spirit of the tunnel’s operators, the experts at Freudenberg Filtration Technologies are permitted to maintain a permanent test rig in the tunnel. As with all similar rigs, it is named after a wind. In this case, the warm and dry “Meltemi” that blows southwards from the Greek mainland, and through the Aegean towards Crete from April to October.

Accelerated testing process

The “Meltemi 1” rig consists of a trolley-like test stand on which up to four filters can be mounted. A fan positioned below each filter draws the polluted air through it. The rig is situated behind a fixed noise-suppression wall, located in a rescue layby right at the heart of the tunnel. These facilities are used for set periods of between 10 and 120 minutes to test for particle retention weight, pressure drop and fractional efficiency. The typical volume flow rate is 300 m3/hour. The high density of pollution in the tunnel means that the normal working load on the filters can be intensified by 3-5 times and accelerates the testing process.


In-house and contract testing

The spectrum of results obtained provides precise indications of how air quality changes with different filters. As well as forming a central part of Freudenberg’s own testing processes, these facilities are regularly in demand by OEM and OES automotive customers, as Senior Application Engineer Steffen Grunz explained: “The majority of testing is conducted for our own purposes. However, I would estimate 15 percent of our work is carried out on behalf of our customers, who are looking for precise data on the performance of specific filter concepts. That is something we are happy to do, as it also supports our own research.”


In-car testing innovation

For the last five years or so, Freudenberg Filtration Technologies has also conducted comprehensive in-car cabin air filter tests with moving vehicles as they pass through the tunnel. This enables rapid and reliable comparisons of filter performance under normal and exceptional pollution conditions. As well as the larger soot particles typical of heavy traffic, both the static and in-car test rigs measure a full particle spectrum. This includes ground-level ozone, nitrogen and sulfur oxides, carbon monoxide and the especially problematic organic compounds and heavy metals found in the PM2,5 particle sizes. So, while the “Saukopftunnel” was originally built with the goal of relieving traffic congestion, today it has a possibly even more important function – as an intensive testing ground for the filters that play an increasingly vital role in protecting human health.