Brake Particle PN and PM Emissions of a Hybrid Light Duty Vehicle Measured on the Chassis Dynamometer
Round 1
Reviewer 1 Report
easy to read and well interpreted.
Author Response
We thank the reviwer for his overall positive commets to our manuscript.
Reviewer 2 Report
Very interesting paper and nice approach for measurements on the chassis dynamometer
Some details which you may consider:
Line 72: please indicate, if the 2.0-8.0 mg/km refer to PM10 or total PM or PM2.5. Furthermore you may add a source for the assumption, that Euro 5/6 diesel cars have ca. the same exhaust PM emission level. I would expect on average less than 2mg/km exhaust PM10 for a diesel car with DPF.
As general remark: please define for all PM emission values which size class you are referring to. This is important for brake wear PM.
Line 91: sentence may be misleading. The WLTC brake cycle has a different speed trajectory than the WLTC emission test cycle. Please adjust the sentence and make clear that you used the WLTC emission cycle in your work.
Line 101: results from a single vehicle shall not be suggested to be representative due to the huge variations of emission levels found between brake models and materials. The method for measurement can be suggested to be repeated elsewhere.
Line 123, Figure 2: it is not clear to me, if (a) shows the housing of the emergency wheel from (b) or if you measured the brake emissions on a normal tire in the housing. Please add a sentence in the text before to clarify.
Line 144: are you sure, that the TX 40 filters collect only particles below 30 nm? The sentence reads like that. I assume that the larger particles are already collected on the former stages. Maybe rephrase “which collects the particles remaining from the former stages which are smaller than 30 nm”
Line 224: please add a short explanation, what the pink areas are indicating in figure (b) also in the legend. Explanation in the text comes rather late.
Line 252: did you start the test with newly mounted brake pads? If not, why running-in? In any case, please specify the status of brake disc and pads at test start (aged for xx km,..)
Line 265 ff: it would be interesting to get the brake work per WLTP period as comparison for the PN emissions shown in Figure 10. Maybe you can add this info in text or figure 10
Line 276, Figure 11: please indicate, if the values refer to the one measured brake or to the entire vehicle in the text in the caption (some readers only look at the figures and these may misinterpret the results). It would be nice, to see the PN emission factors also for each WLTP part (you show the totals in figure 10 but not the per km results in figure 11).
Figure 12: the scale on the y axis is a bit confusing, although usual for PN size distribution graphs. Does this mean, that the measured mass [mg] is divided by the log of the width of the diameter bins? With the very different widths on the x-axis this is quite misleading and not representing the mass contributions correctly. I suggest plotting the mg in each bin as they are.
Line 334: why? Maybe engine brake in overrun overtakes a small part of brake energy in hybrid mode but I assume the control of the electric motor is very fast and thus can operate in all modes very similar. Do you have different information? If yes, source and content should be provided as explanation.
Line 343: replace “WLTC start phase” by “WLTC initial phases (first 285 s)”, this is a clearer description.
Line 356: why is the braking force limit of 500 N related to the vehicle speed? From 70 to 0 km/h in 1 second needs e.g. rather more than 2000 N. Is it a limit of the test stand or tire slip?
Line 359: text and figure are difficult to understand. Maybe split the figure? I would assume that PN flow [#/sec] follow brake power and thus also the velocity is relevant. In the text, I am missing the time to decelerate and the different starting speed to understand the points plotted in figure 359. Why do I see only 3 points for the sum values but 9 values for the peaks? Seems you did not use constant brake forces or constant deceleration rates. Please elaborate a bit in the text. Furthermore, the reduction rate from recuperative braking shall depend on the available maximum power and torque of the electric system (motor+battery) related to the brake power and torque distribution of the cycle. The higher the system power, the more energy of brake events can be recuperated. The information is missing, how much the 20 kW motor can cover from your deceleration tests you performed (add a sentence like x% of the total brake energy happened at brake power levels above 20kW and yy% above 30 kW (in case we assume the 20 kW is the durable max power of the motor)).
Line 397: there is an unexpected dot (“.”) in the text with an incomplete sentence following. Please check.
Author Response
We thank the reviewer for the careful work and very good suggestions. We have taken into account all comments and have revised the manuscript accordingly. We believe that the updated version has gained significantly in quality. Our responses to the comments one-by-one (in black, while the reviewer's comments in blue color) are in the attached file.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The brake particle PN and PM emissions of a hybrid light duty vehicle, as measured on the chassis dynamometer, are examined in this work. The subject of this paper is intriguing and merits further research. Notably, the sampling plenum, a specially created enclosed case, was quite special. I have a few concerns.
1. I wonder if the emergency wheel and this specific wheel case might represent actual driving circumstances. This particular set of wheels and tires may have a completely different road-load coefficient than a typical wheel and tire set.
2. Fig. 6 demonstrated that the experiment received an excessive amount of cooling. The airflow needs to be reduced in order to establish the disc temperatures on the right and left side similarly since the brake temperature has a significant impact on the formation of brake wear particles.
3. How did you choose your brake pads? ECE (low-metallic) or NAO? The brake pads were they new? Why didn't you perform the bedding process prior to the experiment?
4. Too much variation was present in PN and PM emissions. The brake pads that underwent the bedding operation before to the main experiment should be used to calculate EF.
5. It doesn't make sense to compare your EF to that of zum Hagen. Because the driving cycle (WLTC vs. LACT) and particle counter (CPC with >23 nm vs. EEPS with 5~560 nm) are completely different.
6. Line 333~
Can the regenerative braking intensity be adjusted for the e-Mode and Hybrid? How much the regenerative braking could replace the mechanical braking under Hybrid and e-Mode?
I don't see why the regenerative braking approach varies depending on the driving mode because you utilized the same vehicle.
Author Response
We thank the reviewer for the careful work and very good suggestions. We have taken into account all comments and have revised the manuscript accordingly. We believe that the updated version has gained significantly in quality. Our responses to the comments one-by-one (in black, while the reviewer's comments in blue color) are in the attached file.
Author Response File: Author Response.pdf
