Review Reports - Brake Particle PN and PM Emissions of Battery Electric Vehicles (BEVs): On-Vehicle Chassis Dynamometer Measurements

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper investigates the brake particle emissions, including particle number (PN) and particle mass (PM), of battery electric vehicles (BEVs) under real driving conditions. The experimental design is well-executed, using a chassis dynamometer system that simulates real-world driving conditions. The brake particle emissions of three BEVs were measured and compared with those of traditional internal combustion engine vehicles (ICE) and hybrid electric vehicles (HEV). The results show that BEVs equipped with electric braking systems can significantly reduce brake particle emissions, especially when electric braking energy dominates.

The paper compares BEVs with traditional internal combustion engine (ICE) vehicles and hybrid electric vehicles (HEVs), but does not consider other types of electric vehicles, such as hydrogen fuel cell vehicles. Including a broader range of vehicle types in the comparison would help provide a more comprehensive evaluation of the relative advantages of BEVs in reducing brake particle emissions.
While the paper measures particle number and mass, the analysis of the chemical composition and morphology of the brake particles is lacking. The researchers could consider analyzing the harmful metal components within the particles, which would not only provide more comprehensive emission data but also offer more detailed insights into the potential environmental and health impacts of brake particles.

Comments on the Quality of English Language

The language in the article is generally clear, but some sentence structures could still be optimized, especially when presenting complex experimental results. It is recommended to provide more concise summaries in these sections. Additionally, certain technical details, such as the labeling and explanations of figures and tables, could be further simplified and clarified.

Author Response

The paper compares BEVs with traditional internal combustion engine (ICE) vehicles and hybrid electric vehicles (HEVs), but does not consider other types of electric vehicles, such as hydrogen fuel cell vehicles. Including a broader range of vehicle types in the comparison would help provide a more comprehensive evaluation of the relative advantages of BEVs in reducing brake particle emissions.
While the paper measures particle number and mass, the analysis of the chemical composition and morphology of the brake particles is lacking. The researchers could consider analyzing the harmful metal components within the particles, which would not only provide more comprehensive emission data but also offer more detailed insights into the potential environmental and health impacts of brake particles.

Comments on the Quality of English Language

The language in the article is generally clear, but some sentence structures could still be optimized, especially when presenting complex experimental results. It is recommended to provide more concise summaries in these sections. Additionally, certain technical details, such as the labeling and explanations of figures and tables, could be further simplified and clarified.

We thank the reviewer for his positive judgement and the good remarks. We believe he has well understood the goal of our work; a contribution in addressing brake particle emissions of Battery Electric Vehicles (BEVs) and providing an appropriate answer in the current open discussion: Are BEVs emitting more (because of their greater weight) or less (because of electric braking substituting some of the mechanical breaking) brake particles.

Answer to 1. The reviewer underestimates the effort associated with finding the vehicles. We have to address regular owners and only a very small number of them are willing to give us their vehicle for a couple of weeks (although we give them one of the fleet vehicles of our institution in exchange). After receiving a vehicle, technical inspection should ensure that the condition of the vehicle (especially of the brakes) is original and acceptable. A preparation time follows where all related components for the brake particle measurements have to be fitted for the vehicle in question. Performing the measurements, as described, takes a couple of weeks (mainly due to large number of repetitions). Finding a fuel cell vehicle was impossible, given the very small number of such vehicles in Switzerland.

But even if we had one, I do not think that the behavior (concerning brake particle emissions) would be different in respect to the hybrid vehicle whose brake particle emissions have been published by us in reference 21. The fuel cell in such a vehicle produces electric energy which goes directly to the wheels or is stored in a battery (of comparable size of a hybrid vehicle).

To 2: Based on the reviewer's suggestion we performed morphology analysis of the brake particles of the BEVs with SEM. The results are shown in the new Fig. 13 in the manuscript with related discussion. We did not do chemical analysis of the brake particles. We did an extensive study of the chemical composition in the recent past (2022) where we examined the chemistry of brake particles and compared it to the used and new disc and pads chemistry. This study is reference 13 in the present manuscript.

Comments on the Quality of English Language:

We revised the language, the technical details and the labeling and explanations of figures and tables. Should the reviewers request, we will use the MDPI language improvement service after a final version of the manuscript is fixed. Deleted parts are not visible in the revised version. Also, small language corrections (and improvements) cannot be seen. Major changes and new parts are highlighted in red.

Reviewer 2 Report

Comments and Suggestions for Authors

Attached.

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for his positive judgement and the good remarks! We agree with all of his points, and we have tried to revise the manuscript in an appropriate way.

We were not able to copy the etire remrks of the reviewer from his pdf! So the reveiwer's comments in the follwoing with our answers:

Please use standard scientific notation (e.g., 6.4×10⁹ km¹): We agree and have corrected as requested in the abstract.

The high emissions vehicle was BEV1, we corrected the misunderstanding formulations.

The transition to BEV-specific issues (regenerative braking, weight, etc.) could be more explicitly linked to the research gaps being addressed: We agree and have modified the text in the introduction accordingly.

Section 2.1 & 2.2: The description of the brake housing and emergency wheel setup is clear, but the potential impact of the smaller emergency wheel on brake rotational speed and thus particle emissions is acknowledged yet not quantified. A brief discussion on how this may affect cross-vehicle comparability would be helpful: We agree and have modified the text in the manuscript accordingly.

Test Cycle (Section 2.4): The decision to omit the extra-high-speed segment in some tests is justified, but its impact on emission factors should be explicitly discussed in the results (it is mentioned but not interpreted in depth). We agree. Related discussion is added as well as a better distinction of results with or without the extra-high speed segment. However, we report emission factors always including the extra high part. Measurements without the extra high part are used for some comparisons but not for emission factors. We added some clarifications in the manuscript.

Vehicle Details (Table 1): Useful, but additional details on brake pad material type (e.g., low-steel vs. non-steel) would enhance the interpretability of results, especially for BEV1. We did not include chemical analysis of the brake components or the particles. We did an extensive study of the chemical composition in the recent past where we examined the chemistry of brake particles and compared it to the used and new disc and pads chemistry. This study is reference 13 in the present manuscript.

(1) PN Emissions & Bedding Effect (Section 3.2): We agree with the comment, related discussion added in the manuscript.

PM Emissions (Section 3.3): The comparison between BEV2 and BEV3 in terms of particle size vs. number could be expanded: BEV2 has higher PN but similar PM, suggesting finer particles what are the environmental implications. We agree with the comment, related discussion added in the manuscript. We also added some discussion on the morphology of the brake particles based on SEM microscopy.

Emission Factors (Section 3.4): The calculation of total vehicle emissions from a single brake is clearly explained, but assumptions regarding rear drum brakes (zero emissions) should be justified or at least flagged as a limitation. We agree with the comment, related comments added in the manuscript.

Conclusions: Summarizes key findings well, but could more explicitly state the practical implications for vehicle design and regulation.

Once again, thank you for the positive comment! We have modified the conclusions accordingly.

Reviewer 3 Report

Comments and Suggestions for Authors

The study entitled “Brake Particle PN and PM Emissions of Battery Electric Vehicles (BEVs) measured on a Chassis Dynamometer” with a label atmosphere-4026214, uses a chassis dynamometer to measure brake particle emissions from three light-duty battery electric vehicles under real-world driving conditions, which is unique to this study and previous research by the authors. Various characteristics, such as vehicle modes and battery state of charge, were investigated to determine how they affected brake particle emissions. Similar measurements were made in the same chassis dynamometer with ICE and hybrid electric vehicles, allowing for comparisons across different vehicles and potentially improving ecological behavior. Because particle emissions from vehicle brakes make for a large portion of total particle emissions from transportation, this work could be useful in reducing environmental problems.

- I recommend that this paper not be published in its present form considering it is similar to one already published by two of the authors. Even the title is almost identical to the title of reference 21 given in this study (“Brake Particle PN and PM Emissions of a Hybrid Light Duty Vehicle Measured on the Chassis Dynamometer“).

- In some segments of the paper, particularly in the experimental section and conclusion, the authors write too broadly. In conclusion, the authors should clearly state the findings of their investigation.

- Some figures in both papers are identical or extremely similar; it would suffice to cite reference 21 and state that the original/modified experimental setup was described there. The paper includes sixteen figures and four tables. In my opinion, the figure number should be reduced, or certain figures should be moved to the supplementary file. For example, fig. 4 is unnecessary because it is included into fig. 5 afterward (it also appears in ref. 21).

- The authors have cited reference 21 many times as "can be found in". Please only compare the results of this study with the data from the literature, including reference 21 or if something is explained in experimental part just cite the reference.

- Furthermore, 29 of the 45 references cited in this study are identical to the ones in reference 21 (almost 2/3), and reference 21 was quoted (please see the table below). Three of the 45 references, including reference 21, are self-citation.

This paper	Ref. 21	This paper	Ref. 21	This paper	Ref. 21
1	1	16	20	31
2	2	17	21	32
3		18	22	33
4		19	17	34
5	7	20	28	35	34
6	8	21	This ref.	36	35
7	9	22		37	36
8	10	23	32	38	37
9	11	24	33	39	38
10	12	25	19	40	39
11	13	26		41
12	26	27		42	47
13	27	28	29	43
14	18	29		44
15	30	30		45

Excuse me if I made some omission with this list of references.

Additional comments

- Line 113

“2. Experimental Setup Materials and Methods“ – Please use just one of these two titles – perhaps this one “ Experimental Setup“.

- Some of used abbreviations are not explained.

- Several times, authors used a comma before citing a reference for no particular reason.

Author Response

This paper	Ref. 21	This paper	Ref. 21	This paper	Ref. 21
1	1	16	20	31
2	2	17	21	32
3		18	22	33
4		19	17	34
5	7	20	28	35	34
6	8	21	This ref.	36	35
7	9	22		37	36
8	10	23	32	38	37
9	11	24	33	39	38
10	12	25	19	40	39
11	13	26		41
12	26	27		42	47
13	27	28	29	43
14	18	29		44
15	30	30		45

Excuse me if I made some omission with this list of references.

Additional comments

- Line 113

“2. Experimental Setup Materials and Methods“ – Please use just one of these two titles – perhaps this one “ Experimental Setup“.

- Some of used abbreviations are not explained.

- Several times, authors used a comma before citing a reference for no particular reason.

We thank the reviewer for his work and his comments!

We see our present work as a contribution in addressing brake particle emissions of light duty Battery Electric Vehicles (BEVs) providing an appropriate answer in the current open discussion; are BEVs emitting more (because of their greater weight) or less (because of electric braking substituting some of the mechanical breaking) brake particles? We believe this question is of great importance and is currently discussed in scientific but also regulatory circles. In addition, the present work examines the impact of the different driving modes and the battery State of Charge on the brake particle emissions.

We agree that the present work has some similarities with the reference 21, the one we published 2 years ago, examining the brake particle emissions of a hybrid vehicle. The experimental setup is similar to the one in our previous work (reference 21) and the subject is also similar. The vehicles measured and reported this time are the light duty (BEVs).

To our knowledge so far, there are no other studies published, examining the brake particle emissions of BEVs on chassis dynamometer including the entire vehicle. In addition, in the present study we have been able to quantify the contributions of the electric and the mechanical braking. This is a current discussion item for brake particle measurements on a brake dynamometer and the transfer of these results towards the emissions of a vehicle.

To the further comments:

We tried to improve the writing compactness and clear state the findings of the investigation.
We agree with the reviewer and omit fig. 4. We also omit some parts that can be found in reference 21. We added though a new figure after the request of reviewer 1 and 2 for morphological analysis of the brake particles. Addressing all the reviewers' comments though did not result in a shorter manuscript.
However, we rather believe that the present manuscript should also be describing the work independently from our previous work (ref. 21). It is sometimes tedious for reviewers as well as for readers to be able to read a paper only having the previous papers of the authors at hand.
The later comment can also be extended to the discussion of the references. There is a great number of references in common with 21. This is understandable since both studies are in the same field. By reviewing the findings of the past, it is inevitable to cite identical sources. However, in the present manuscript the discussion in the introduction tries to cover all different measurement setups for brake particles used. In addition, we included the most recent studies and their findings of the years 2023-2025 which were not in 21. Should we have omitted important contributions, may we ask the reviewer to indicate these, and it is good possible that we will integrate them in a further revised version. May we also add that the 3 out of 45 self-citations is certainly no exaggeration.
Comment to line 113: We agree and change according to the reviewer.
We were able to identify only 3 abbreviations not explained in the table. We added those in the list. Please indicate to us more, should we have missed some.
Commas around references have been deleted.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors of revised paper entitled “Brake Particle PN and PM Emissions of Battery Electric Vehicles (BEVs) measured on a Chassis Dynamometer” with a label atmosphere-4026214, made some changes to improve the paper and now it appears better. Still there are some comments to the authors, which are given below:

- According to the author's instructions (https://www.mdpi.com/journal/atmosphere/instructions), the conclusion section is optional but can be added to the manuscript if the discussion is unusually long or complex. The conclusion should clearly summarize the findings of the study, as earlier sections have already provided details.

- The figure number should be reduced, or specific figures should be relocated to the supplementary file.

Additional comments

- Line 240

“....seen in Fig. 4 and 7.“ Two figures are mentioned here – so please put Figs. 4 and 7.

- Line 340

“....3.4 and 3.5 evidence show that the....“. Perhaps the authors intended to use “evidence“ or “show“, but not both.

Author Response

Once again with thank the reviewer for his efforts! We took in account all his last comments and we beleive that the manuscript has now improved! in the manuscript only the modified parts following after review 1 are now completed.

According to the author's instructions (https://www.mdpi.com/journal/atmosphere/instructions), the conclusion section is optional but can be added to the manuscript if the discussion is unusually long or complex. The conclusion should clearly summarize the findings of the study, as earlier sections have already provided details.

We shortened the conclusions substantially, trying to focus on the main findings of our work!

The figure number should be reduced, or specific figures should be relocated to the supplementary file.

Two figures have been moved to the supplementary file. So now the overall number of Figures is 14.

Additional comments

- Line 240

“....seen in Fig. 4 and 7.“ Two figures are mentioned here – so please put Figs. 4 and 7.

These figures are there. We slightly modified the statement in order to make the identification of these figures easier.

- Line 340

“....3.4 and 3.5 evidence show that the....“. Perhaps the authors intended to use “evidence“ or “show“, but not both.

Yes, the reviewer is absolutely right. Sentence corrected.