Technological breakthroughs

A study in Germany shows that tire-related emissions depend not only on friction but are also affected by wheel load, tire temperature, and resuspended road dust.

A research team from the Technische Universität Ilmenau in Germany, in collaboration with Audi, has developed a system for directly measuring particles generated by tire and road wear while a vehicle is being driven.

Test results showed that vehicle dynamics, including speed, acceleration, and cornering maneuvers, are associated with particle emissions. However, when the measurement approach was transferred from a test track to real-world roads, this relationship became significantly weaker due to numerous confounding factors, particularly accumulated and resuspended road dust.

A Non-Exhaust Source of Emissions

A typical passenger vehicle uses a set of tires for approximately 40,000–50,000 km. Over this period, up to 30% of the tire's surface mass may be worn away and released into the environment as small particles.

The extent of tire wear depends on numerous factors, ranging from tire type and vehicle configuration to driving behavior, including speed, longitudinal acceleration, and lateral acceleration.

Particles generated by tire and road wear, commonly referred to as TRWP (tire and road wear particles), together with resuspended road dust and brake dust, are major sources of non-exhaust emissions.

According to the study, non-exhaust emissions currently contribute a level of traffic-related PM10 comparable to exhaust emissions. TRWP alone are estimated to account for approximately 5–30% of this amount, with PM10 emission factors of around 3.5–9.0 mg/km.

The shift toward electric vehicles has made the issue more noteworthy. Electric vehicles have no tailpipe emissions and may reduce brake wear through regenerative braking systems, but they are generally heavier than internal combustion engine vehicles. The additional weight can increase the load on tires and consequently affect the amount of wear particles generated.
TRWP are also considered a significant source of anthropogenic microplastics. However, there are currently no specific legal limits governing the level of tire wear.

A Measurement System Positioned Directly Behind the Wheel

To investigate how these particles are generated, the researchers used a front-wheel-drive vehicle weighing 2,119 kg and producing 75 kW of power, fitted with summer tires.

A constant-volume sampling system was positioned behind the right front wheel to collect particles generated in the tire–road contact area. The braking system was enclosed to minimize the intrusion of brake dust into the measurement zone.

The sampling funnel was designed using computational fluid dynamics (CFD) simulations to improve particle collection efficiency. The device was attached to the wheel axle and could rotate horizontally when the vehicle cornered, helping maintain an appropriate sampling orientation relative to the tire surface.

The study focused on particles with diameters below 10 micrometers, measured using a DustTrak II instrument at a frequency of one measurement per second.

The vehicle was also equipped with positioning and inertial measurement systems to record speed, longitudinal acceleration, and lateral acceleration, as well as tire-temperature sensors and suspension travel sensors.

When Friction Helps Explain Emissions

The researchers conducted two types of tests.

One test cycle was carried out on an 87-km public-road route covering urban, suburban, and highway sections. The other was conducted on a closed test track to minimize the influence of road dust and uncontrolled changes in real-world road surfaces.

On the closed test track, strong acceleration events, mainly at speeds below 60 km/h, were accompanied by high particle concentrations. Particle levels also tended to increase as vehicle speed rose, even when acceleration was lower.

To explain this phenomenon, the researchers used the concept of friction power—the energy transferred at the contact area between the tire and road surface.

Part of the frictional energy is converted into heat, increasing tire temperature, while another part is associated with material wear. Based on the sliding velocity between the tire and road surface, together with the vehicle's resistance forces and acceleration, the researchers calculated friction power and compared it with the measured particle concentrations.

On the test track, this model provided a reasonably good description of emission patterns. Friction power increased significantly during braking and also rose when the vehicle negotiated corners, even when longitudinal acceleration was low.

However, an unusual phenomenon emerged when left and right turns were compared.

When the vehicle turned right, the right front wheel experienced a lower load and more slip, resulting in higher friction power. Yet the measured particle concentration was higher when the vehicle traveled in the opposite direction.

This finding indicates that friction is not the only factor determining the amount of particles emitted. The load acting on the wheel may also independently influence the formation of PM10 particles.

Real-World Roads Make the Problem More Complex

When the method was applied on public roads, the relationship between friction power and particle emissions became considerably weaker.

At certain points, friction power increased without a corresponding rise in particle concentration. Conversely, some large emission peaks occurred when friction power was relatively low.

According to the researchers, one possible explanation is road dust that had accumulated beforehand and was resuspended by the tires, rather than particles being generated entirely through wear occurring at the time of measurement.

This finding demonstrates that measuring TRWP emissions on real roads is considerably more challenging than on a closed test track because the measured particles may result from multiple processes occurring simultaneously.

Urban road sections recorded significant levels of PM10 despite lower vehicle speeds. The researchers attributed this partly to the higher frequency of driving maneuvers, including repeated acceleration, deceleration, and changes in direction.

In suburban areas, emission levels were significantly lower. On highways, meanwhile, long straight sections produced only a limited number of emission peaks, mainly when the vehicle accelerated or decelerated.

High Speeds Also Complicate Particle Sampling

Another limitation identified by the researchers was related to the measurement system itself.

Sampling efficiency decreased sharply as vehicle speed increased. The system captured approximately 40% of the particles at speeds below 30 km/h, but the collection efficiency fell below 10% when the vehicle traveled at more than 60 km/h.

This presents a challenge when evaluating emissions under real-world traffic conditions, particularly on highways.

Particle-size analysis also showed that most of the PM10 mass was determined by particles larger than 1 micrometer. Particles smaller than 1 micrometer may occur in large numbers but contribute relatively little to the overall mass.

Notably, particles smaller than 1 micrometer are also difficult to attribute directly to TRWP because they may be mixed with dust particles already present in the ambient air.

Toward an Emission Prediction Model

Based on the test results, the researchers proposed that tire-related emissions should be considered as consisting of at least two components.

The first is resuspended road dust, which depends largely on vehicle speed and the amount of dust accumulated on the road surface.

The second is particles generated by direct friction between the tire and road surface, which are influenced by friction power, wheel load, and tire surface temperature.

This distinction could help researchers develop emission prediction models that more accurately reflect real-world traffic conditions.

However, the researchers note that predicting TRWP emissions under actual driving conditions remains challenging because of the large number of influencing variables. Road-surface characteristics and the load acting on tires are two factors that require further investigation.

Tests under more controlled conditions, such as on a chassis dynamometer, could reduce interference and sampling errors, thereby providing data for further refinement of the model.
The study shows that tire emissions cannot be assessed on the basis of a single variable such as friction. Once a vehicle enters real-world traffic, tires, road surfaces, wheel loads, temperature, accumulated dust, and driving behavior all contribute to the resulting emissions “fingerprint.” This also presents a major challenge for developing future standards to control non-exhaust sources of air pollution.

Source: Feißel, T., Kunze, M., Hesse, D., Ivanov, V., Augsburg, K. & Gramstat, S. On-Road Vehicle Measurement of Tire Wear Particle Emissions and Approach for Emission Prediction. Tire Science and Technology, 52(1), 2–14 (2024).