Emission Rates for Light-Duty Truck Towing Operations in Real-World Conditions

Comment 1:

General comments:

·        Please check the placement of tables and figures. Some of them do not follow the line of the text, some are cut off.

·        Please check, all references and citation. There are errors and the text is difficult to read, especially in chapter 4.

·        Please check and correct line breaks throughout the text and spaces between figures/tables and the rest of the text.

·        Please check that tables are in the same font as the rest of the text.

Response 1:  

Thank you for this feedback. We completely agree and have made sure to correct all formatting errors throughout the manuscript. All references now appear in the correct format as intended, and the placement and formatting of tables and figures have been corrected. We apologize for the formatting issues in the initial submission.

Comment 2:

Chapter 3 “Materials and Methods” – please correct reference or missing informations:

Specifications on selected trucks are listed in Error! Reference source not found., including vehicle make and model, engine size, engine technology, horsepower, torque, gross vehicle weight rating (GVWR), gross combined weight rating (GCWR), and vehicle weights with and without trailers.”

Each vehicle completed at least 6 test runs with a trailer and at least 5 test runs without a trailer (see Error! Reference source not found.).

1. Data Processing: The final cleaned and time-aligned data include per-second data from the parameters listed in

2. Analytical Approach: An overview of the analysis approach is shown in Error! Reference source not found..

3. Calculating MOVES Operating Modes: a. The coefficients A, B, and C from the on-road coastdowns for the three test vehicles are shown in Error! Reference source not found.Error! Reference source not found..; b. After calculating VSP, MOVES operating modes are determined according to brak-ing, idling, and three speed ranges described in Error! Reference source not found..

4. Comparing Instantaneous Emission Rates: The MOVES emission rates were obtained by extracting base emissions rates (the model does not adjust the rates for local temperature, humidity, and fuels) from the MOVES 4.0.1 model database emissionRateByAge table for the parameters listed in Error! Reference source not found.Error! Reference source not found..

Response 2:

Thank you for highlighting these instances. We appreciate the attention to detail and apologize for these errors. All instances of “Error! Reference source not found” have been properly replaced by the intended reference.

(Note: The Chapter numbers have been updated to meet another reviewer’s request to combine the Intro and Lit Review sections. Thus, “Chapter 3” discussed above now refers to Chapter 2.)

Comment 3:

Chapter 4 “Results and Discussion” – please correct reference or missing informations:

·        The portion of time spent in each opmode bin during the cumulative test runs is shown for the three light-duty trucks (C13, F11, F14) in

·        To provide an overview of how median emission rates changed in towing and non-towing operation, the percent difference in median emission rates when hauling a trailer was calculated for each opmode bin (Error! Reference source not found.Error! Reference source not found.Error! Reference source not found.).

·        This may be related to the decreased engine load observed for C13 and F11 trailer-towing data in opmode 30 (as shown in Error! Reference source not found.).

·        To better understand how light-duty hauling may impact tailpipe emissions, the in-stantaneous emission rates (g/s) for CO2, CO, HC, and NOx are compared for towing vs. non-towing operations in , Error! Reference source not found., Error! Reference source not found., and Error! Reference source not found..

·        CO2 emission rates showed roughly similar trends for the three tested trucks, with higher distributions across nearly all operating modes when hauling a trailer (Error! Reference source not found.).

·        For CO emission rates, the three trucks showed statistically-distinct increases from trailer towing in nearly all cases (Error! Reference source not found.).

·        HC emission rates for towing operations showed higher median distributions than non-towing operations at the higher opmode bins for all three trucks, and towing distributions were higher across all opmode bins for F14 in particular (Error! Reference source not found.Error! Reference source not found.).

·        The research team investigated these near-zero observations second-by-second and found that the low values are not as-sociated with a measurement error and are correctly reported as below-detection obser-vations (HC detection shown at 10-5 grams per second on Figure 7).

·        Lastly, NOx emission rates were typically slightly elevated under towing operation across opmodes, with more apparent towing increases occuring at some of the higher opmode bins for vehicle F11, and also occuring across nearly all opmode bins for vehicle F14 (Error! Reference source not found.Error! Reference source not found.).

·        Median emission rates for the three light-duty trucks (shown with 25th percentile and 75th percentile intervals) were compared to MOVES base rates for CO, HC, and NOx in Error! Reference source not found., Error! Reference source not found., and Error! Reference source not found., respectively.

·        Combined CO emission rates (medians with 25th and 75th percentiles) for the tested trucks were compared to MOVES base rates in Error! Reference source not found. (see also Appendix A.1 for plot with CO means and 95% confidence intervals).

·        NOx emission rates show less-distinct trends com-pared to CO and HC, but still show indication of elevated emissions compared to MOVES at the higher opmodes (bins 35-38) as shown in Error! Reference source not found. (see also Appendix A.3 for plot with NOx means and 95% confidence intervals).

Response 3:

Thank you for taking the time to carefully list out these instances. We apologize for these errors. All instances of “Error! Reference source not found” have been properly replaced by the intended reference.

(Note: The Chapter numbers have been updated to meet another reviewer’s request to combine the Intro and Lit Review sections. Thus, “Chapter 4” discussed above now refers to Chapter 3.)

Comment 4:

Figure 2: Please correct first arrow, text in brackets (opmode). It seems to be a print screen.

Response 4:

Thank you for this comment. After consideration, we concluded that Figure 2 should be removed since the figure was redundant and not a particularly beneficial addition to the text. We thank the Reviewer for bringing this figure to our attention.

Comment 1:

The abstract is very brief and should be extended to cover more details of this study.

Response 1:

Thank you for pointing this out. The authors agree with this comment and have revised the abstract section to include more detail that discusses the comparison with the MOVES default emission rates. The following text has been added (Lines 21 – 24):

“The measured emission rates were compared to the default rates used by MOVES, revealing similar overall trends. However, discrepancies between measured rates and MOVES predictions, especially at high speed and high operating modes, indicate a need for refinement in emissions modeling for LDTs under towing operations.”

Comment 2:  

Section 1.1 should be removed and integrated into the introduction section.

Also, section 2 literature review should consider to combine with the introduction section.

Response 2:

Thank you for this suggestion. The “Scope” (Section 1.1) has now been incorporated into a combined section within the introduction.

We also incorporated the “Literature Review” under the introduction as a sub-section (Section 1.2), rather than it being a separate major section (it was Section 2 in the original manuscript).

The paragraph below combines the end of the original introduction section and the original scope section into a single, combined paragraph (Lines 51 - 63):

“Given this context, the primary goal of this research is to provide a more nuanced examination of the impact of towing load on emission estimations from LDTs. The first objective of this study was to measure and compare LDT emission rates for off-cycle operation, specifically for trailer-towing operation. A secondary objective was to compare these LDT emission rates with those generated by an existing emissions model using the U.S. Environmental Protection Agency’s (EPA’s) MOtor Vehicle Emissions Simulator (MOVES) base emissions rates table (‘emissionratebyage’ table found in the MOVES4 movesdb20240104 database) (3). Emission factors for LDTs are currently developed based on the assumption that the trucks are used similarly to passenger cars, i.e., without consideration of hauling operation. The work presented here takes steps to evaluate this assumption by measuring tailpipe emissions under light-duty towing operation, More specifically, this work examines how light-duty towing operations can affect tailpipe emission rates and investigates a potential need to broaden the application of conventional approaches to light-duty emissions modeling.”

Comment 3:  

In Table 2, the authors shows the Test runs driven per speed route by each truck with and without trailer towing. However, some trucks are run more times than the others. Can the author justify the reason behind this decision?

Response 3:

Thank you for this comment. The Reviewer asks a great question and it was our intention to sample the same number of test runs for each speed route with and without trailer towing. Unfortunately, we ran into weather-related issues and limited contractor time that led to some runs not being completed as planned. The main issue was that it was particularly cold on some days which led the PEMS water outlet freezing. We attempted to wrap the outlet in insulative tape but it did not help with the analyzer units open to the air in the back of the trucks. For these reasons, we were not always able to complete our planned number of tests.

Comment 4:  

One of the major issues of this study is the lack of calibration for the PEMS with EPA equivalent methods. Were the equipments calibrated correctly before conducting the experiment?

Response 4:

Thank you for pointing out that this was not mentioned in the manuscript. Yes, each PEMS unit was calibrated prior to each speed route. We have added the following text to the manuscript discussing this in more detail (Lines 179 – 185):

“All PEMS units were calibrated and zeroed before and after every driving route. Specifically, the PEMS equipment used in this study complies with standard operating procedures and is certified and maintained in accordance with the 40 CFR 1065 Subpart J PEMS protocols [42]. In addition, the PEMS post-process software has built-in QA/QC protocols to ensure proper sample flows, temperatures, and subsystem components were operating properly during the test and within the tolerances specified in accordance with 40 CFR 1065 Subpart J protocols.”

Comment 5:  

The figures are not showing correctly in this work, some of the figures are being cut off.

Response 5:

Thank you for pointing this out. This has now been corrected in the revised draft for all affected figures.

Comment 6:  

The limitation of this study is not thoroughly explored in the conclusion section.

Response 6:

Thank you for your comment. The authors added an additional limitation in the Conclusion section (Lines 509 – 518).

“One limitation of this study is that it explains vehicle emissions through opmode, which is modeled by the vehicle’s speed, acceleration, weight, road-load coefficients, as well as the vehicle’s characteristics and road grade. Opmode specific rates were designed to represent fleet average emissions for the purposes of emissions inventory modeling. Analyzing these individual vehicle rates in terms of opmode allows us to compare the emissions of these vehicles to our best understanding of the national fleet of light-duty trucks. However, opmode does not directly represent the load on the powertrain. Likewise, the method averages transient loads over one second intervals. As such, opmode rates provide less insight into the specific mechanisms that may cause the emissions of these vehicles to deviate from the fleet average.”

Comment 7:  

The novelty of this work is unclear and this work seemed to be written in a technical report. It seemed common sense that after installing the trailer to the pick up truck, the emissions of air pollutants increase due to the increase load of the engine.

Response 7:

Thank you for your comment. It seems intuitive that during towing emissions will increase with due to the increased engine load. This is the case for a pollutant like CO₂ which does generally increase proportionally with load. However, for other pollutants the magnitude of the increase is not easily estimated without experiments like the one presented in this study. The emissions of pollutants like HC CO and NO_x are controlled in the exhaust by the three-way catalyst, which is designed for optimal efficiency under stoichiometric conditions. Under high engine loads, such as those generated during towing operations, gasoline engines use “enrichment” control strategies to control engine temperatures and protect the durability of engine components. Under enrichment conditions the efficiency of the three-way catalyst drops dramatically resulting in extremely non-linear changes of emissions with engine load. The magnitude of these emission changes is not well documented or modeled for real-world vehicle operation. This study selected different engine designs that were produced to meet new fuel economy and GHG standards to better understand the scope of emission responses at high loads in the real world. While this study focused on light-duty trucks with towing loads, the same principles also apply to many light-duty passenger cars when fully loaded with passengers or cargo. The results of this research provide additional insights into how vehicle emissions vary significantly under different real-world use cases and how we can better estimate these high-load emissions with tools like EPA’s MOVES model.

Comment 8:  

The QA and QC process of the monitoring equipment is unclear and must be shown in this work.

Response 8:

Thank you for your comment. The PEMS post process software has built-in QA/QC protocols to ensure proper sample flows, temperatures, and subsystem components were operating properly during the test and within the tolerances specified in accordance to 40 CFR 1065 Subpart J protocols. The post-processor also lists or flags any time or instances that these measurements occurred during a conducted test. Updates to the manuscript that incorporate additional discussion of QA/QC protocols are discussed in the prior response to Comment 4 (see ‘Response 4’ above).

Comment 1:

Not use brackets for the references, please change for [X] without cursive.

Response 1:

Thank you for your careful attention to detail. In this case, we will plan to leave the references in their current format since they were changed by MDPI directly after we submitted the initial draft. Since these format edits were performed directly by the journal, we are hesitant to make changes but we sincerely appreciate this suggestion.

Comment 2:

Please review the manuscript in depth in terms of the format: captions, using the template of the journal, errors in the references to figures, etc.

[In the commented PDF, the Reviewer highlighted multiple instances where the references displayed erroneously as “Error! Reference not found”]

Response 2:

Thank you for highlighting these issues. We agree that these were critical to fix and we have made sure to correct all formatting errors throughout the manuscript. All references now appear in the correct format as intended, and the placement and formatting of tables and figures have been corrected. We apologize for the formatting issues in the initial submission.

Comment 3:

To add a section with the achronyms

Response 3:

Thank you for your comment. In our manuscript we defined all acronyms following the method laid out in the journal’s instructions to authors:

“Acronyms/Abbreviations/Initialisms should be defined the first time they appear in each of three sections: the abstract; the main text; the first figure or table. When defined for the first time, the acronym/abbreviation/initialism should be added in parentheses after the written-out form.”

While we understand that a section of acronyms can be helpful, it does not appear to be a standard practice for articles in this journal.

Comment 4:

To review all the manuscript. There are problems with the captions in a lot of figures.

Please use the format of the journal for the captions. Some figures have different formats.

Response 4:

We agree with the Reviewer’s suggestion and have made these changes. As discussed in our response to Comment 2, these instances have been corrected in the revised draft. We apologize again for this error.