Vehicle Emissions and Performance

State of the evidence

The evidence establishes a significant gap between certification-level emissions and real-world performance, particularly for NOx in heavy-duty vehicles 10 and various pollutants in cold temperatures 2. While exhaust emissions are heavily regulated, non-exhaust sources like tire wear particles (TWP) and brake emissions are emerging as dominant contributors to urban particulate matter 6,9. However, the evidence base for TWP is weakened by a reliance on secondary data rather than primary measurements 4.

Key findings

• Cold ambient temperatures significantly increase emissions of hydrocarbons and CO, and impact other pollutants not currently covered by EU cold-start regulations 2. 2
• Non-exhaust emissions (tire and brake wear) can account for up to 80% of total particulate matter (PM) emissions in urban environments 6. 6
• Real-world NOx emissions from heavy-duty diesel vehicles often exceed certification standards, with some vehicles emitting over three times the limit during idling and hot-running 10. 10
• Ammonia (NH3) is a significant alkaline gas emission from gasoline vehicles with three-way catalysts, contributing to secondary particle formation 3. 3
• Tire wear particle (TWP) calculations in most country-level studies are based on existing literature rather than original measurement data 4. 4
• Portable Emissions Measurement Systems (PEMS) are critical for verifying in-use compliance across light-duty and heavy-duty fleets 8. 8

Where the evidence disagrees

• Certification vs. Real-World: Significant divergence exists between laboratory-tested emission levels and actual road performance 10.
• Measurement Gap: There is a contradiction between the high policy interest in tire wear particles and the lack of primary empirical measurement data to support those policies 4.

Open opportunities

What the research supports that no idea in the catalog takes yet.

• Cold-Weather Emission Optimization Suite

  Evidence shows Euro 6 regulations ignore several pollutants at cold temperatures, creating a performance gap for OEMs 2.

• Brake Wear Emission Benchmarking Tool

  With Euro 7 proposing limits for non-exhaust emissions, there is a need for realistic brake emission factors to evaluate new formulations 9.

• Real-World SCR Efficiency Diagnostic

  P9 shows SCR systems often fall below the 90% efficiency threshold even at optimal temperatures, suggesting a need for in-use diagnostic tools.

• Primary TWP Measurement Framework

  P3 highlights a critical lack of primary measurement data for tire wear particles, which currently hinders accurate environmental modeling.

References

1. 1.Air pollution from motor vehiclesAsif Faiz, Christopher S. Weaver, Michael Walsh · 1996 · 255 citations
2. 2.Impact of cold temperature on Euro 6 passenger car emissionsRicardo Suárez‐Bertoa, Covadonga Astorga · 2017 · 209 citations
3. 3.Emission factor of ammonia (NH 3 ) from on-road vehicles in China: tunnel tests in urban GuangzhouTengyu Liu, Xinming Wang, Boguang Wang et al. · 2014 · 118 citations
4. 4.Tire wear particle emissions: Measurement data where are you?David Mennekes, Bernd Nowack · 2022 · 93 citations
5. 5.Particle and gaseous emissions from individual diesel and CNG busesÅsa M. Hallquist, Martin Jerksjö, Henrik Fallgren et al. · 2013 · 85 citations
6. 6.A coupled macroscopic traffic and pollutant emission modelling system for BarcelonaDaniel Rodríguez-Rey, Marc Guevara, Ma. Paz Linares et al. · 2021 · 67 citations
7. 7.Static modelling of the material flows of micro- and nanoplastic particles caused by the use of vehicle tyresStefanie Prenner, Astrid Allesch, Margarethe Staudner et al. · 2021 · 57 citations
8. 8.In-Use Emissions Testing with Portable Emissions Measurement Systems (PEMS) in the Current and Future European Vehicle Emissions Legislation: Overview, Underlying Principles and Expected BenefitsTheodoros Vlachos, Pierre Bonnel, A. Perujo et al. · 2014 · 75 citations
9. 9.Light-Duty Vehicle Brake Emission FactorsBarouch Giechaskiel, Theodorοs Grigoratos, Panagiota Dilara et al. · 2024 · 45 citations
10. 10.On-Board Sensor-Based NOx Emissions from Heavy-Duty Diesel VehiclesYi Tan, Paul Henderick, Seungju Yoon et al. · 2019 · 108 citations