Impact on air quality as a benchmark 05.09.2022

Research study on ›zero-impact vehicles‹ (ZIV) shows how the relevance of vehicle emissions on health and environment is declining

A research project initiated by the FVV shows: If vehicles powered by internal combustion engines comply with very strict exhaust emissions limits, they no longer have a negative impact on air quality. However, the maximum permissible emissions limit for such a vehicle depends on the respective driving and traffic situation.

Text: Richard Backhaus // Petra Tutsch

The regulations for exhaust emissions standards for motor vehicles are to be further tightened in Europe. Against this background, a recently completed research study of the FVV examined a question that is only simple at first glance: How far must the emissions of vehicles powered by combustion engines be brought down before they lose their relevance for health and the environment? To this end, a team of researchers led by Prof. Dr.‑Ing. Stefan Hausberger of Graz University of Technology correlated vehicle emissions for two air pollutants - particulates and nitrogen oxides - with the immissions measured at various locations, i.e. the concentration of pollutants in the air. A relevance criterion common in environmental law was used as a target value: If the emission of a technical installation causes a contribution of less than three per cent to the locally permissible total pollution, no further environmental impact assessment has to be carried out. Hausberger explains: »Using such an approach, we can define the emissions limits that a ›zero-impact AQ vehicle‹ must mandatorily comply with.«

To determine the contribution of vehicle emissions to air quality, the researchers first calibrated a simulation model using data from the ›Neckartor‹ measuring station in Stuttgart. This shows: If the entire vehicle fleet were to be converted to the currently applicable Euro 6d final emissions standards, the air pollutants would drop drastically - for nitrogen oxides, for example, by 93 percent compared to the measured values from 2016. However, the three-percent criterion would not be met.

To ensure that the three-percent criterion can be met for the immission at a specific location in the world, not only the emissions of all vehicles in the immediate vicinity of the measuring point are relevant. The respective traffic situation, the built-up area and even the weather must also be taken into account. Using air quality data from all over Europe, the researchers therefore modelled several extreme scenarios. These included a multi-lane urban access road used by 75,000 vehicles per day, as well as stop-and-go traffic on the motorway, a section of motorway with no speed limit and a mountain pass road with a gradient of more than ten percent. For the alpine scenario, different driving modes were also taken into account.

The determining factor for compliance ambitious ambient air quality standards are not particular extreme situations, but primarily everyday traffic in urban conurbations. Extreme driving situations could therefore be addressed with higher emissions limits.

The results of the simulations carried out for the different locations vary greatly. In dense urban traffic, a zero-impact car should emit an average of 6.7 milligrams of nitrogen oxides per kilometre. In stop-and-go traffic on the motorway, it would even be only 2.0 milligrams per kilometre. Due to the significantly lower number of vehicles, on the other hand, the three-percent criterion would be met when driving on the mountain pass road even if emissions rose to as much as 74 milligrams per kilometre. »European exhaust emissions standards do not yet know driving situation-related limit values,« says Hausberger. »However, our results show that particular extreme situations are not the determining factor for air quality, but above all everyday traffic in urban conurbations. Extreme driving situations could therefore be addressed with higher emissions limits.«

This statement is confirmed by further simulations carried out within the project for urban conurbations. Simulations show that with a fleet consisting of 100 percent zero-impact AQ vehicles, nitrogen oxide emissions at the Neckartor in Stuttgart would be at the level of the existing background pollution only.

Ao. Univ.-Prof. Dr.-Ing. Stefan Hausberger is head of the Emissions Research Department at the Institute of Thermodynamics and Sustainable Propulsion Systems at Graz University of Technology. Together with his team, he was in charge of the study on zero-impact vehicle emissions commissioned by the FVV.

Prof. Dr.-Ing Kurt Kirsten is Head of Advanced Engineering and Innovation at APL Automobil-Prüftechnik Landau GmbH. He was head of the committee that guided and coordinated the FVV study.

FVV Research Project No. 1407 (H1295) | Zero-Impact Vehicle Emissions: Definition and requirements of ›Zero-Impact emission levels‹ based on ambient air quality requirements. Conducted at the Graz University of Technology with the support of the consulting companies FVT, Graz, and Aviso, Aachen. Frankfurt/M. 2022

Forschungsgesellschaft für Verbrennungskraftmaschinen und Thermodynamik (FVT) | Graz, Austria
Head of Research: Univ.-Prof. Dipl.-Ing. Dr. techn. Helmut Eichlseder / Ao. Univ.-Prof. Dipl.-Ing. Dr. Stefan Hausberger

Institute of Thermodynamics and Sustainable Propulsion Systems (ITnA) - Graz University of Technology | Graz, Austria
Head of Research: Univ.-Prof. Dipl.-Ing. Dr. techn. Helmut Eichlseder
Research Associates: Dipl.-Met. Dr. rer. nat. Ulrich Uhrner, DipI.-Ing. Werner Stadlhofer

AVISO GmbH (AVISO) | Aachen, Germany
Head of Research: Dr.-Ing. Christiane Schneider


1. What is a ›zero-impact vehicle‹?

A ›zero-impact vehicle‹, or ›ZIV‹, is a vehicle whose emissions are so low that they do not have a measurable negative impact on the environment or human health. Our ›zero-impact‹ approach allows us to expand our analysis of vehicle emissions. Instead of solely focussing on pollutant emissions from vehicles, the immissions – the measured pollutant concentration in the air when using low-emissions vehicles – form the basis of evaluation for the pollutant level.

2. What is the difference between emissions and immissions?

Emissions are pollutants emitted, for example when operating a vehicle. Immissions, on the other hand, relate to the concentration of pollutants to be measured at a specific location at a specific time. They are caused not only by vehicles, but also by industry and heating units, for example.

3. What influences the immission level at a specific location?

Immission levels depend on a variety of factors. Wind and weather conditions, for example, have a considerable impact. Strong winds promote the exchange of air, thereby reducing immissions, since the substances are dispersed more effectively. There are two dominant influencing factors when it comes to vehicle traffic: the number of vehicles in the vicinity of a measuring point and the way they are driven. Vehicles travelling up steep mountain passes emit a comparably higher amount of pollutants per kilometre due to the high load requirements. However, as relatively few vehicles are on the road there, and the air exchange is very good, the immission level is generally below that of a busy thoroughfare in a large city.

4. How can we define a limit below which absolutely no negative effects from pollutants can occur?

A relevance criterion commonly applied in environmental law was used as the target value: If the emissions emitted by a technical plant contribute less than 3% to the total immissions load permissible in that location, there is no need for any further environmental impact assessment. In our study, this corresponds to the contribution of all cars and commercial vehicles with combustion engines that are on the road in the relevant vicinity of an immissions measuring point.

5. What exactly did the study examine?

We analysed the maximum emission level of an individual vehicle to ensure that its traffic-related immissions remain below the 3% limit at all times, wherever it is. In our investigations, we used a wide range of different sites. The Neckartor in Stuttgart was chosen as a typical high-pollution reference location to serve as the basis for our study – effectively the worst-case scenario in terms of immissions, given the especially high pollutant values regularly measured there in the past. These are the result of both the surrounding buildings and the high traffic volume, including many vehicles with comparably high levels of emissions due to driving in urban conditions with a great deal of stop-and-go traffic. We used this data to calculate the maximum level of emissions a vehicle may emit in the vicinity of the Neckartor so that the overall emissions remain below the 3% limit. We then repeated these investigations using the results from measuring points at other representative locations throughout Germany.

Intraurban traffic

Dense urban

  • 75 000 veh./day
  • 3.4% HDVs
  • 10% cold start
  • Heavy traffic driving conditions

Near shopping centre

  • 20 000 veh./day
  • 3.4% HDVs
  • 35% cold start
  • Normal traffic driving conditions

6. What were the results of your study?

We were able to prove that the demands on a ZIV vary considerably depending on the driving situation and environmental conditions. In locations with high traffic volumes, many vehicles emit pollutants. Accordingly, lower emission levels per vehicle kilometre are required in order to achieve the specifications for the 3% target. However, since these locations usually do not display particularly challenging driving conditions, the emission limits are easier to achieve there. Extreme driving conditions, such as driving at high speeds and acceleration, are only possible when there is little traffic on the road. Because this also means that the number of vehicles travelling at the same time in these situations (and emitting emissions) is lower, much higher emissions per vehicle kilometre are possible without a negative impact on immissions. Dense city and motorway traffic are most critical for achieving the ›zero-impact‹ target.

7. How would the current emission regulations and testing procedures for cars and commercial vehicles have to be amended or adapted in order for immissions to be considered as the benchmark?

What matters most is that as many vehicles as possible in the field meet the current exhaust gas emission limits. As our simulation model showed, the pollutant content in the air would drop dramatically – nitrogen oxides (NOx) by 93% compared to the measured values from 2016 – if the entire vehicle fleet were converted to the current Euro-6d final standard. Even then, mean NO2 immissions above the 3% target may still be measured on days with little air exchange. This problem may, however, be solved with the Euro-7 exhaust gas emissions limits currently defined. Future real driving emissions (RDE) testing procedures should focus on complying with emissions specifications in dense city and motorway traffic and examine these situations with appropriate limits. In contrast, it may be sensible to apply other limits to extreme situations, such as driving up mountains, as the leeway for emissions per vehicle kilometre for these driving situations is much greater than it is for dense city traffic. Current RDE approaches focus strongly on examining extreme situations, such as cold starts and driving under high load. But the truth is that these situations actually have a minimal impact on immissions.

Traffic situation interurban

Max traffic MW

  • 15 800 veh./h
  • (200 000 veh./day)
  • 18% HDVs
  • 1% cold start
  • Heavy traffic driving conditions

Max velocities MW

  • 8 700 veh./h
  • (110 000 veh./day)
  • 13% HDVs
  • 1% cold start
  • No speed limits; vmax > 160 km/h

(Extreme) driving alpine

  • 1,360 vehicles/h
  • (normal cond.)
  • 10% HDVs
  • 5% cold starts
  • Avg 10% gradient;
    "Extreme"=full loaded, aggressive driver

8. Does the engine technology even exist to keep emissions in the relevant driving situations low enough to meet the 3% criterion?

Yes, the basic technologies exist. They are largely already used to comply with the Euro-6 final exhaust gas emission limits and will become the standard with the future Euro-7. When considering the 3% criterion, the principal aim is to use these technologies in such a way that they can develop their full potential. FVV – in which manufacturers, suppliers and development service providers collaborate with one another and with universities and other research institutions on a pre-competitive basis – makes a significant contribution here.

9. How could hydrogen or synthetic fuels influence these efforts as alternative energy sources?

Hydrogen can be combusted in engines with low emissions, making it an ideal energy source for ›zero-impact‹ powertrains. When hydrogen is chemically bonded to carbon as a transition element, it can also be easily transported and stored. FVV has successfully verified the benefits offered by synthetic fuels in multiple studies. As fuel synthesis during the production of synthetic fuels can be controlled precisely, and they do not contain any sulphur, they generate lower emissions than fossil fuels. There is less pressure on the exhaust gas system, because there are fewer pollutants needing aftertreatment, and it can be designed better for extreme operating conditions such as cold starts.

10. Most traffic-related emissions are caused by older vehicles that do not comply with the current Euro-6 emissions guidelines. What percentage of the vehicle fleet would have to meet the ZIV requirements before there is any noticeable effect on immissions?

If 50% of cars in the field that meet Euro-5 exhaust gas limits or below were replaced with cars that meet the current Euro-6d standard, traffic-related immissions would drop by more than 30%. Replacing 50% with zero-immission vehicles would result in a significant decline of 49%. We are therefore calling for fast market penetration with vehicles that meet the current emissions guidelines, and for a Euro-7 guideline that is in line with air quality targets and allows ›zero-impact‹ to be achieved in an efficient and technically feasible way.

11. Did the growing market share of battery-powered electric vehicles – which by definition do not cause any local emissions – influence the approach to your study in any way?

Like the FVV fuel studies, our analyses use 100% scenarios to aid comparability. In the case of the ZIV study, the scenario is the complete use of vehicles powered by combustion engines. We generally pursue a technology-neutral approach in which all drive systems have a legitimate place on the market. Customers should be given the freedom to choose their preferred drive concept, without government policies telling them what to do. However, this also means that every drive concept, taken on its own, must satisfy the 3% criterion.