Impacts on Brake Particle Emission Testing

Round 1

Reviewer 1 Report

This paper investigated the impacts of running in, brake lining composition (mainly between low steel copper free brake pads and NAO brake pads), inertia variation and loading profile on the particle number concentration. A novel dynanometer was used following a novel worldwide light vehicles test procedure cycle. Temperatures at both the brake pads and discs are concidered. Generally, the test is well designed and the paper is well formulated, which gives an informative introduction and thorough results and discussion.

Only one concern exists that in Figure 9, on the y axis, it is unclear that this is average PNC/test or cumulative PNC/test. In the figure caption, it says PNC but the scale in Figure 9 is apparently higher than in Figure 10. This makes me confused.

Otherwise, I recommend to publish this paper in the special issue of brake emissions. 

Author Response

Only one concern exists that in Figure 9, on the y axis, it is unclear that this is average PNC/test or cumulative PNC/test. In the figure caption, it says PNC but the scale in Figure 9 is apparently higher than in Figure 10. This makes me confused.

Answer: please look at Line 188 in the revised version, “cumulative” is added, as well as for figure caption itself

Thank you very much for your valuable feedback,

Sebastian

Reviewer 2 Report

This article presents the results of an experimental lab-based protocol which involves using a brake dynamometer and various sensors to determine factors that influence particulate emissions from light duty vehicles. The study provides valuable insight into these emissions and makes several conclusions based on friction and wear of brake components under various scenarios.

Evaluating tribological properties is clearly a complex process, and the use of experimental protocols like this provides a general understanding that can be used to develop emissions factors for non-exhaust emissions from vehicles.

As a result of the highly-controlled use of laboratory equipment and the measurement of factors including temperature, such studies are useful as tools to evaluate various properties that can contribute to emissions.

Conclusions about real-world emissions from brake materials can only be truly understood when in-vehicle testing on the road or track is conducted.

To improve this article, I recommend that the authors consider clarification of the following points.

1. It’s important to include a section on how non-exhaust emissions compare overall to exhaust emissions. This is important so as to not overstate the relative significance of non-exhaust emissions. It’s also important so that readers, including perhaps decision-makers, have context for comparing the overall emissions of a vehicle. As more battery electric vehicles come onto the marketplace, and given that many of these higher capacity battery-based vehicles will likely be heavier than the ICE vehicles they replace, this level of detail is essential.
2. More information is needed on the scale of particulates being reviewed in this article and the relative proportion and distribution of samples based on particle size. If the authors have information on particulate chemical composition, it would be helpful to include this information since this is germane in the long run to public health.
3. More details on the role that external factors play would strengthen the analysis. The authors do a sound job explaining how temperature makes a difference but ignore other factors like ambient humidity.
4. Given that conclusions beyond the lab are limited with this kind of research, it would be useful for the authors to discuss this in some detail. For example, what role does the friction of road surfaces play in emissions from vehicles brakes?
5. With respect to electric vehicles, the article seems to ignore the role of regenerative braking in such vehicles and how this may affect brake emissions even with vehicles that weigh more.

Overall, a well-written article. Without more context and detail as per above, the article is likely to have a limited readership. There are many opportunities in this article to include an analysis on implications of this work and how to move it forward.

Author Response

1.It’s important to include a section on how non-exhaust emissions compare overall to exhaust emissions. This is important so as to not overstate the relative significance of non-exhaust emissions. It’s also important so that readers, including perhaps decision-makers, have context for comparing the overall emissions of a vehicle. As more battery electric vehicles come onto the marketplace, and given that many of these higher capacity battery-based vehicles will likely be heavier than the ICE vehicles they replace, this level of detail is essential.

Answer: please find an added section from line 33 on: “However, even if the current EF have to be improved, one example of the fraction of NEE on traffic emissions is illustrated by Figure 1. It shows on one hand the importance of brake particle emissions, on the other hand it is also relevant where to measure since the location obviously owns a certain impact on the emission contributions. As a conclusion, it might be helpful to study the overall road transport emissions, as it is shown in 2. Figure 2 indicates amongst others the fraction of brake particle emissions in UK, which might give a “bigger picture” than measurement results of an intersection. The authors of this diagram state that non-exhaust already exceeds exhaust PM (2018). Furthermore, they expect non-exhaust to be 10% while exhaust is expected to be only 1% with respect to overall PM_2.5 emissions in UK.”

2.More information is needed on the scale of particulates being reviewed in this article and the relative proportion and distribution of samples based on particle size. If the authors have information on particulate chemical composition, it would be helpful to include this information since this is germane in the long run to public health.

Answer: please find some added information from line 93 on: “A very profound and in-depth analysis of the wear products is given in [9]. The authors confirm that brake wear particles include a wide size range from the micro-to nano-scale. Electron microscopic analyses confirm that particles occur mainly as polydisperse, heterogeneous aggregates. In regards of the nanoscale particles it is stated that they are often nearly spherical and occur also as single particles. Furthermore, chemical analyses reveal that the particles' chemical composition is dominated by Fe and in smaller sizes by Ca ± Fe ± Ti.”

3.More details on the role that external factors play would strengthen the analysis. The authors do a sound job explaining how temperature makes a difference but ignore other factors like ambient humidity.

Answer: please find some further information on T and RH in line 145: “Since the role of climatic parameters is very important, higher humidity for instance can facilitate particle agglomerations, the cooling air temperature is (20±2)℃ while the relative humidity is (50±10)% RH during all the presented experiments. Both parameters are oriented on the recommendations of the PMP IWG Non-Exhaust Emissions.”

4.Given that conclusions beyond the lab are limited with this kind of research, it would be useful for the authors to discuss this in some detail. For example, what role does the friction of road surfaces play in emissions from vehicles brakes?

Answer: please find some additional explanation in line 66: “However, the use of a brake dynamometer is recommended by the PMP IWG Non-Exhaust Emissions to characterize brake particle emissions since it represents the best compromise between feasibility and a representative measurement approach. The focus is on the friction couple itself, external factors such as the role of friction of road surfaces or the impact of the powertrain concept are not considered by the PMP IWG Non-Exhaust Emissions.”

5.With respect to electric vehicles, the article seems to ignore the role of regenerative braking in such vehicles and how this may affect brake emissions even with vehicles that weigh more.

Answer: even if we know that regen braking has a huge impact on the emission behavior, I would like to refer to the fact that the PMP IWG Non-Exhaust Emission is not (yet) considering regen braking for the methodology development, mentioned already in point 4.

Thank you very much for your valuable feedback,

Sebastian