For better inner-city air quality

A few years ago, an intensive social and media debate began about driving bans, air pollution in inner cities and limit value violations. “That was the initial spark for us to launch an ‘Air Quality working group’. For us at the company, it’s not just about meeting the legislative requirements in terms of limit values for our vehicles,” says Ernst Peter Weidmann of Mercedes-Benz External Affairs. “We’ve put plans into motion to dig deep into the topic of air quality, understand the complex interrelationships and to firstly make our vehicles’ impact on the air transparent then reduce it where possible.”

In 2019, Mercedes-Benz came up with a very ambitious goal: to offer a new, carbon-neutral fleet of cars by 2039 – accompanied by a binding statement on air quality. The new car fleet’s impact on nitrogen dioxide (NO₂) levels in urban areas is set to be no longer relevant by 2025. “As part of the automotive industry, we want to actively help improve air quality in cities and live up to our social responsibility. Of course, political and public demands, as well as the expected tightening of limits, are also important drivers for our intensive work on the topic of air quality,” Weidmann explains.

From emission to air quality

Philipp Werner from Engine Development has been putting his heart and soul into these complex tasks ever since. For a good three years now, he has been dealing almost exclusively with the issue of air quality. But he’s not alone. Thomas Fetzer, responsible for air quality topics in Group Environmental Protection, provides support as his tandem partner. Together, they form the core of the Air Quality team, and are in constant exchange with the development divisions. “In 2018, we started working on a detailed model that can map the impact of our vehicles on NO₂ pollution at urban hotspots,” Werner explains. “Since we have Germany’s best-known measuring station on our doorstep, the Neckartor in Stuttgart was our focus for simulating and modelling nitrogen oxide levels.” Then in 2019, the new project from Ambition 2039 was added. The assignment was to find a way to transparently show the new Mercedes-Benz vehicles’ level of influence on NO₂ pollution in urban areas.

Philipp Werner and Thomas Fetzer not only looked at the emissions of various vehicles, but also at the lateral effects at the hotspot. The two engineers had to build up new core expertise for this. It was equally essential to keep the big picture in mind and continuously exchange ideas with the scientific community, universities, professors and institutes that deal with the issue of air pollution control.

Thomas Fetzer gives us a glimpse into the depths of the his modelling work. First, the NO₂ pollution from the current year and the previous years is analysed at the hotspot: “We look at several areas that go into calculating nitrogen oxide emissions (NOₓ) and nitrogen dioxide (NO₂) levels in ambient air.” The experts look at the emissions of the passing vehicle fleet. Which drive systems and Euro levels are on the road at the hotspot? What are their exhaust emissions? A further step is analysis of local traffic. What are the prevalent speed limits? Do people tend to drive fast or slow? Another important area is “dispersion”. Wind statistics, road geometry and other emissions in the urban environment play a role here.

Mercedes-Benz engines are no relevant emission source any more

In order to map the impact of the new Mercedes-Benz vehicles on the Neckartor and to meet scientific requirements in doing so, the two had to delve even deeper into the traffic analysis. One of the crucial parameters is where the vehicles passing the Neckartor are coming from, and the distance they have already travelled before arriving there. Both have a heavy influence on local emission behaviour. “Our aspiration was to replicate that as accurately as possible,” Fetzer says.

The Mercedes-Benz engineers are only too happy to share the forecast results of their modelling. All data indicates that the “NO₂ pollution 2025” milestone has already been reached – meaning that Mercedes-Benz EU6d cars no longer have any relevant influence on the NO₂ levels at the measuring station. “We’ve actually already met our targets and are finding that our vehicles on the strict Euro 6d emissions standard, at their lowest emissions, barely contribute to NO₂ pollution any more. It’s clear the engine developers have done a fantastic job. This is a huge leap forward,” says Werner.

No more influence on NO₂ pollution – can you imagine? Philipp Werner and Thomas Fetzer explain it like this: “The NO₂ levels at the hotspot basically come from two sources: the non-traffic background and car and truck traffic. The example from 2019 shows that the share of passenger cars prevails. If all vehicles that pass the Neckartor every day are replaced by Mercedes-Benz EU6d standard vehicles, the traffic induced NO₂ share would shrink to below 2µg/m³. For comparison, 2019 the traffic induced NO₂ share was around 30µg/m³ based on our analysis.” How will this affect NO₂ pollution in the coming years? “NO₂ levels at the hotspots will continue to fall year on year as more and more EU6d vehicles or battery electric vehicles get on the road through constant fleet renewal,” Werner emphasises.

Of course, the Air Quality team didn’t stop at the example of Stuttgart; they looked at other NO₂ hotspots with different traffic characteristics, such as the busy Landshuter Allee in Munich and its heavy truck traffic (daily traffic volume 120,000 vehicles). Here, too, the same effect can be seen as in Stuttgart.

Next big thing: Fine particulate matter

Once the engine is no longer the big driver of air pollution, there is still fine particulate matter, to which car traffic contributes. The Group also wants to address this issue and actively works to understand the connections and research measures to reduce fine particulate matter. “Fine particulate matter is more multi-layered and significantly more complex than NO₂,” Werner explains.

The hard part is that fine particulate matter isn’t a single pollutant, it’s a mix of different particles from different sources. A large proportion is based on mechanical processes such as tyre and brake wear. To protect human health, limit values for the fine particulate matter fraction PM10 (daily limit: 50 µg/m³) apply throughout Europe. But the even smaller particles PM2.5 (1/40 of the diameter of a human hair) are also increasingly coming into focus. Currently, we can only roughly quantify exactly how much particulate matter can be attributed to which source, as the figures vary greatly and are mostly based on models and assumptions. “There is still a lot of pioneering work to be done here. Research is in its infancy; the measurement methods aren’t fully developed yet. Nevertheless, we in the Group want to go deeper and expand our expertise,” says Werner.

In addition, the plan is to set up a model for fine particulate matter hotspots – analogous to the NO₂ model – to accompany the measurement activities in the Group. Philipp Werner: “The model is meant to help us understand the action mechanisms and classify how, for example, a change we make on a vehicle will affect the actual fine particulate matter pollution at the hotspot.” Thomas Fetzer also emphasises that the modelling of fine particulate matter is becoming more demanding. “For nitrogen oxides, we looked at the impact of exhaust emissions on a yearly average. For fine particulate matter, on the other hand, daily values are relevant. We need more precise input data here, and we also have to take into account other emission sources on the vehicle and their effect on fine particulate matter pollution at the hotspot.”

Brakes on the test bench

As part of the “Brake Fine Particulates” project, Georg Frentz and Anna Benkowitsch are currently dedicating a lot of attention to the emissions that don’t come from the exhaust pipe – also known as non-exhaust emissions. At the test bench, which has been specially modified for measuring brake emissions, the engineers examine the emission behaviour of the brakes of various Mercedes-Benz vehicle types. To be able to do this, the brake is “housed” in a box. The next step involves, finely filtered, clean air flowing in. Then the brakes are applied. “At the other end, in the box’s exhaust pipe, we can then measure the emissions under special conditions. From here, we can determine and extrapolate the amount of fine particulate matter in the air to the WLTP-brake cycle developed specifically for this purpose,” explains Benkowitsch, who moved from chassis simulation to the Brake department at the beginning of 2020. WLTP stands for Worldwide Harmonised Light Vehicles Test Procedure.

Georg Frentz has been working on the measurement procedure since as early as 2016. “Then we really got going in 2018. Since then, we’ve been able to conduct measurements on the modified test bench ourselves. We are continually ramping up the tests year on year. At the beginning, the tests were still initial attempts in cooperation with the Institute of Automotive Engineering at the TU Darmstadt. A PhD student who did research for us is still assisting us with further testing,” says Frentz. So there is a lot to measure before we will find the right concepts, measures and levers that can contribute to reducing emissions of fine particulate matter.

There are still several unanswered questions when it comes to brakes, such as how recuperation of electric cars can be evaluated on the test bench and which particle savings can be expected here. “It’s a hefty amount of work that we want to tackle. However, you could say that with increasing electrification, recuperation increases as a matter of course, and thus the topic of brakes is starting to fall out of focus with regard to the PM10 fine particulate matter fraction,” the Mercedes-Benz engineer explains. Georg Frentz emphasises: “We are well on the way to continually improving our understanding of correlations regarding the issue of fine particulate matter. Going forward, we will be able to develop technical solutions that will help to reduce fine particulate pollution.”