Comparative Analysis of Performance and Emissions of a Two-Stroke Marine Diesel Engine According to CPP Modes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. Figure 4 and its description aren’t closely related to the topic of the manuscript.
2. The conclusions are almost well-known.
3. It’s suggested that the author conducts quantitative research and presents the optimal joint control curve for the engine and propeller.

Author Response

Dear Reviewer 1,

Thank you very much for your careful review and constructive suggestions. We sincerely appreciate your time and effort in helping us improve the manuscript. We have revised the manuscript accordingly, and our responses to your comments are provided below.

Comment 1. “Figure 4 and its description aren’t closely related to the topic of the manuscript.”
Response: Thank you for pointing this out. We have retained Figure 4, but significantly shortened and refined its description to avoid interrupting the technical flow of the manuscript. Our intention in keeping Figure 4 is to provide a concise overview of how measurement and control data are transmitted and collected within the onboard system, which supports the credibility and traceability of the experimental dataset used throughout the study. To ensure better relevance, we minimized the description to only the essential information directly related to data acquisition for this work.

Comment 2. “The conclusions are almost well-known.”
Response: We understand this concern and agree that conclusions should be supported by clear, quantitative evidence. In response, we strengthened the manuscript by adding more quantitative expressions of the results, including additional performance–emission comparisons in brake-specific terms and clearer numerical summaries across load conditions. These revisions aim to present the findings not merely as qualitative tendencies but as measurable outcomes derived from the full-scale sea-operation dataset.

Comment 3. “It’s suggested that the author conducts quantitative research and presents the optimal joint control curve for the engine and propeller.”
Response: We appreciate this valuable suggestion. We have incorporated an additional analysis that introduces a joint-control curve representation to quantitatively describe the operating relationship between engine speed (n) and CPP pitch (θ) under the two CPP operating modes. Furthermore, we added a propulsion-based benchmark interpretation using propeller-law scaling in the relevant operating region (pitch saturation) to support the physical meaning of the observed joint-control characteristics. This addition is intended to provide a clearer and more quantitative basis for discussing how the two operating strategies realize load and how this relates to the resulting performance and emission differences.

Once again, we sincerely thank you for your insightful comments, which have helped us improve both the clarity and the technical depth of the manuscript.

Sincerely,
Jaesung Moon

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

pls find my comments in the attached file.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 2,
Thank you very much for your careful review and constructive comments. I sincerely appreciate the time and effort you devoted to improving the quality and clarity of my manuscript. I have revised the manuscript accordingly, and my point-by-point responses are provided below.

 

Comment 1) Title

Response: Thank you for this suggestion. I agree that the original title could be improved in terms of clarity and readability. Accordingly, I revised the title so that the study subject (engine/model) and the main objective are more clearly conveyed.

 

Comment 2) Abstract

Response: Thank you for your recommendation. I revised the abstract to improve clarity and to better communicate the study objective, experimental conditions, and key findings.

 

Comment 3) Table 6 / How the load points were obtained during sea operation

Response: Thank you for pointing this out. I revised the description related to Table 6 to explain more clearly how the target engine-load points were obtained for each CPP mode during actual sea operation. In particular, I clarified the basis of the engine-load indication (referenced to SMCR), the mode-dependent characteristics of load realization, and the data-processing procedure using stabilized segments (including the averaging window after stabilization).

 

Comment 4) Additional comparative evidence (“model performance”)

Response: Thank you for this valuable suggestion. To improve the persuasiveness of the mode-to-mode comparison, I added additional comparative materials, including (i) an optimal joint control curve and (ii) a comparison framework based on the propeller law. These additions provide a clearer reference baseline and strengthen the interpretation of the observed performance–emission differences between CPP operating modes.

 

Comment 5) Suggested references

Response: Thank you very much for recommending these references. I reviewed both papers carefully; however, I may not have fully understood the intended rationale for suggesting them. From my perspective, the first study addresses PMSM drive control (MTPA/flux-weakening) and the second focuses on deep-learning-based ship detection, which appear to have limited direct relevance to the scope of this manuscript, which is based on full-scale sea-operation measurements and a comparative analysis of the performance and exhaust-emission characteristics of a two-stroke marine diesel engine under different CPP operating modes. Because my study does not involve motor-drive control or computer-vision-based modeling, citing these works would not directly support the experimental design or the interpretation of the results and may weaken the technical focus and coherence of the manuscript. For these reasons, I have chosen not to cite these references, and I kindly ask for your understanding.

 

Once again, I sincerely thank you for your constructive comments, which have helped improve the manuscript substantially.

Sincerely,
Jaesung Moon

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article addresses an important and relevant problem related to improving the efficiency of main marine engines. In particular, it considers the possibility of enhancing the fuel efficiency of low-speed marine diesel engines through the use of a controllable pitch propeller (CPP), which allows flexible adjustment of the engine operating mode depending on the required vessel speed and onboard electrical power demand.

 

The authors correctly note that, in addition to fuel efficiency, compliance with modern regulations on exhaust gas emissions is of significant importance, and they propose a corresponding experimental investigation.

 

The most valuable contribution of the paper is, without doubt, the presentation of experimental data obtained during sea trials directly onboard the vessel. Measurements are provided for four operating points of the main engine under two characteristic modes: constant-speed mode and combinator mode.

 

However, despite the high quality of the experimental data, the manuscript lacks new scientific results and clear scientific novelty. The obtained findings are largely expected and predictable. In its current form, the paper resembles a well-prepared textbook chapter rather than a research article.

 

Additionally, the following comments are offered, addressing which would substantially improve the quality of the manuscript:

 

1. Since the study focuses on the feasibility of using a controllable pitch propeller in combination with a low-speed engine, it would be more logical to compare the propeller law (fixed-pitch propeller mode) with the combinator mode. Operation under constant-speed mode is a priori a less efficient reference. Therefore, it is recommended to include engine parameters corresponding to the propeller law as a reference case.

 

2. When comparing exhaust emissions, the authors make a significant methodological inaccuracy by comparing pollutant concentrations rather than mass-specific emissions per unit of work. Under combinator mode at partial loads, the exhaust gas mass flow rate is considerably lower than under constant-speed mode, as the engine operates at reduced speed. Consequently, for equal pollutant concentrations, the mass-specific emissions (e.g., g/(kW·h)) will be lower in combinator mode. Therefore, the analysis should be based on mass-specific emissions, as this directly affects the final conclusions.

 

3. In Figure 3, it should be explicitly stated that the scales are given in relative values, and the x-axis should be properly labeled (rpm?).

 

4. The phrase “CPP operated at constant-speed mode” should be clarified to indicate that the engine speed is constant, i.e., “CPP operated at engine constant-speed mode”. This clarification should be applied consistently throughout the manuscript whenever constant-speed mode is discussed.

 

5. Page 8: The statement “This approach ensured that the influence of engine speed was minimized, allowing the effects of CPP control mode on engine load response to be isolated” appears unclear. It is not evident which parameters are claimed to be isolated from engine speed effects, especially since engine load is varied.

 

6. In Figure 8b, an error is present: the exhaust gas pressure for constant-speed mode is indicated as zero for all operating points, whereas numerical values are provided in the text.

 

In general, the manuscript contains valuable experimental material, which requires more accurate interpretation, particularly with respect to exhaust emission assessment. In addition, the scientific novelty should be strengthened through post-processing of the experimental data. Possible approaches include analysis of experimental indicator diagrams, comparison with a reference propeller law, or calibration of mathematical models of the engine, propulsion system, or engine subsystems.

Author Response

Dear Reviewer 3,

Thank you very much for your careful reading of our manuscript and for providing insightful and constructive comments. We greatly appreciate your recognition of the value of the sea-trial dataset and your suggestions to strengthen the scientific novelty and methodological rigor. Following your recommendations, we have revised the manuscript to improve the clarity of the methodology, correct figure-related issues, and enhance the interpretation of the emission results, including a work-normalized representation. Our point-by-point responses are provided below.

 

Comment 1. Reference case should be propeller law (fixed-pitch propeller mode) rather than constant-speed mode.

Response:
Thank you for this important suggestion. We understand the Reviewer’s concern that the constant-speed mode may not represent the most physically meaningful baseline when discussing CPP–engine matching. In response, we added an additional reference case based on the propeller law (as a representative fixed-pitch propeller operating characteristic) and included a corresponding figure to serve as a more logical comparison. We also revised the related interpretation in the Results and Discussion to reflect this additional baseline and to clarify the relative operational implications of the CPP modes.

 

Comment 2. Emissions should be compared as mass-specific emissions (g/kW·h), not concentrations (ppm).

Response:
We fully agree that concentration-based comparisons alone can be methodologically limited when engine speed and exhaust throughput differ between modes. However, in our sea-trial campaign, the exhaust mass flow rate was not directly measured, which restricts a strict conversion based on measured exhaust flow.
To address this limitation and your recommendation as closely as possible with the available onboard signals, we additionally estimated work-normalized emissions (g/kW·h) using the measured fuel oil consumption (FOC, kg/h) and shaft power (kW), applying a carbon-balance-based approximation with an assumed fuel carbon mass fraction (w₍C₎). These values are therefore reported explicitly as estimated brake-specific emissions.
For transparency, we retained the original concentration (ppm) results (Fig. 9) to present the directly measured trends, and we additionally included the new g/kW·h-based figure and a quantitative summary table (Fig. 10 and Table 7) to provide a more work-normalized comparison between CPP modes.

 

Comment 3. Figure 3 should clearly state that the scales are relative, and the x-axis should be labeled.

Response:
We appreciate this clarification request. We revised Figure 3 to explicitly indicate that the plotted scales are presented in relative/normalized values, and we corrected the axis labeling to avoid ambiguity (including the x-axis label where applicable). The figure caption and relevant text were updated accordingly.

 

Comment 4. The phrase “CPP operated at constant-speed mode” should be clarified (engine speed is constant).

Response:
We agree that the previous wording could be misleading. We revised the manuscript to clarify that the constant-speed mode refers to engine constant-speed operation, and we applied this correction consistently throughout the manuscript wherever constant-speed mode is discussed.

 

Comment 5. Page 8 statement about “isolating” engine speed effects is unclear.

Response:
Thank you for pointing this out. We agree that the original statement may have implied an overly strong claim. We therefore revised the relevant sentence and surrounding explanation to reflect the experimental intent and limitation more accurately. Specifically, we clarified that the procedure was designed to minimize the influence of engine-speed variation under the constant-speed setting, rather than claiming that speed effects were fully “isolated” while load was varied.

 

Comment 6. Figure 8b shows zero exhaust gas pressure for constant-speed mode, which contradicts the text.

Response:
We appreciate the careful check. We confirmed the inconsistency and corrected Figure 8b by replacing the erroneous zero values with the appropriate dataset values, ensuring consistency with the numerical values reported in the text. We also rechecked the associated descriptions to ensure full consistency between the figure and the manuscript.

 

Once again, we sincerely thank you for your helpful comments and suggestions. We believe that these revisions have significantly improved the manuscript in terms of methodological soundness, clarity, and scientific contribution. We hope that the revised version adequately addresses all of your concerns.

Sincerely,
Jaesung Moon

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This paper presents the results of a comparative study of the effect of CPP operating modes on the performance and emissions of a two-stroke marine diesel engine operating under Tier II fall back  sea operation conditions. The studies were conducted on a real-scale facility, i.e., a research vessel. In my opinion, there is a lack of information about the navigation conditions in which the experiments were carried out, i.e. wind speed and direction, sea state, and the ship's course.

References are sufficient, although in one of the 20 references the author of this article is a co-author. The abstract is well-written.

I recommend standardizing the units to the SI system. The author uses bar, mbar, or MPa interchangeably in many places. Pa or MPa should be used everywhere. For example, Table 2, subsection 3.2, Figure 7, Figure 8, subsection 3.3.

Author Response

Dear Reviewer 4,
Thank you very much for your careful review of our manuscript and for providing valuable comments. We have revised the manuscript accordingly to improve its reproducibility and ensure greater consistency in presentation. Our point-by-point responses are provided below.

 

Comment 1

Because the experiments were conducted under real ship sea-operation conditions, the manuscript lacks information on navigation conditions such as wind speed/direction, sea state, and the ship’s course.

Response 1

Thank you for this helpful suggestion. To improve the interpretability and reproducibility of the full-scale sea-going dataset, we have added a clear description of the navigation and environmental conditions for the analyzed segments. Specifically, we clarified the data-selection criteria in the Methods section by stating that only quasi-steady, straight-course segments were selected, while turning maneuvers and transient periods were excluded. In addition, the ship-log-recorded conditions—wind direction/force, sea-state notation, atmospheric pressure, and course (straight-course confirmation)—have been summarized in Table 7. This addition clarifies the navigation/environmental context under which the results were obtained.

• Revisions made: Added a “Sea-operation and navigation conditions” description in the relevant Methods subsection; created and referenced Table 7.

 

Comment 2

The references are sufficient; however, one of the 20 references includes the author of this manuscript as a co-author.

Response 2

Thank you for pointing this out. To avoid any potential concern regarding self-citation, we have removed Reference [20] from the manuscript. We also revised the associated text so that the research motivation is presented in a more general context without relying on that specific citation.

• Revisions made: Removed Reference [20] and revised the related statement(s).

 

Comment 3

The pressure units should be standardized to the SI system. The author uses bar, mbar, or MPa interchangeably in many places (e.g., Table 2, Section 3.2, Figure 7, Figure 8, Section 3.3). Pa or MPa should be used everywhere.

Response 3

Thank you for this important comment. To ensure unit consistency, we comprehensively reviewed and revised the indicated locations—Table 2, Sections 3.2–3.3, and Figures 7–8—and standardized all pressure-related quantities to SI units, using MPa throughout the manuscript. In particular, the pressure-related figures (e.g., Pmax, scavenge air pressure, and exhaust gas pressure) were updated while preserving the original graphical style; both axis labels and plotted values (including Δ-values) were converted and presented consistently in MPa. As a result, consistent unit usage is now maintained across the main text, tables, and figures.

• Revisions made: Standardized pressure units in Table 2, revised unit/value expressions in Sections 3.2–3.3, and updated Figures 7–8 (axes and captions) to MPa.

 

Thank you for your positive evaluation of the English quality and the abstract. While the wording of these parts was largely maintained, we carefully checked the manuscript during revision to ensure consistent terminology and unit usage across the text, tables, and figures.

 

We sincerely appreciate your constructive comments, which helped us improve the reproducibility and overall presentation quality of the manuscript. We hope that the revised manuscript adequately addresses all of your concerns.

Sincerely,
Jaesung Moon

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It is recommended to refine the conclusion section by removing general statements that reflect well-established consensus in the field; wherever possible, quantitative results should be provided.

Author Response

Dear Reviewer 1,

Thank you for your helpful recommendation to refine the Conclusions by removing general statements and providing quantitative results wherever possible. We have followed this suggestion in the revised manuscript.

In the Conclusions section, we removed broadly stated, well-established statements and revised the text to emphasize the main findings using representative quantitative values drawn from the Results.

Sincerely,
Jaesung Moon

Reviewer 2 Report

Comments and Suggestions for Authors

The authors failed to address comments, and the revised manuscript was submitted with un-English characters which was quite challenge to understand.

Author Response

Dear Reviewer 2,
First of all, I would like to sincerely thank you for your careful review and valuable comments, which have been very helpful in improving the quality and clarity of my manuscript. I have re-checked the manuscript thoroughly and revised it as faithfully as possible according to your suggestions. My point-by-point responses are provided below.

Comment 1) Title

Response:
Following your suggestion, I revised the title to improve readability and to ensure that the study subject and objective are conveyed more clearly.

Comment 2) Abstract

Response:
I reorganized and revised the abstract to improve its overall structure and clarity. The study objective, experimental conditions, CPP operating modes, and the main findings are now presented more clearly and concisely.

Comment 3) Table 6 and explanation of how the load conditions were obtained

Response:
As you pointed out, the previous description may not have been sufficient to explain how the mode-dependent load points were achieved during actual sea operation. Therefore, I reorganized Table 6 and added further explanations describing how the target engine-load conditions were obtained for each CPP mode, so that the condition-setting procedure and data-processing method can be understood more clearly.

Comment 4) Strengthening the persuasiveness of the interpretation (comparative evidence)

Response:
To enhance the persuasiveness of the mode-to-mode comparison, I strengthened the comparative interpretation by incorporating additional analysis frameworks. Specifically, I applied the propeller law as a reference for comparison and used an optimal joint control curve to interpret the mode-dependent operating points and performance differences in a more quantitative and logical manner.

Comment 5) Suggested references

Response:
Thank you very much for recommending the two references. I reviewed them carefully. However, I may not have fully understood the intended rationale for suggesting them. From my perspective, since this study focuses on full-scale sea-operation measurements and a comparative analysis of the performance and emission characteristics of a two-stroke marine diesel engine, the suggested papers appear to have limited direct relevance to the experimental design and interpretation of the results within the scope of this manuscript. For this reason, I decided not to cite these references, and I kindly ask for your understanding.

Once again, I sincerely appreciate your thoughtful comments, which helped me improve the manuscript. I hope the revised version addresses your concerns satisfactorily.

Sincerely,
Jaesung Moon

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

i apreciate you've considered all my points,
the article looks much better,

best regards!

Author Response

Dear Reviewer 3,

Thank you very much for your kind follow-up message.

I sincerely appreciate your time and effort in reviewing our manuscript, as well as your positive assessment of the revised version.

I am pleased that the revisions have addressed your concerns and improved the overall quality of the article.

With gratitude and best regards,
Jaesung Moon

Reviewer 4 Report

Comments and Suggestions for Authors

I am satisfied with the authors' responses to the comments. 

Author Response

Dear Reviewer 4,

Thank you for your follow-up evaluation.

I appreciate your confirmation that you are satisfied with my responses to the previous comments.

Your constructive feedback was very helpful in improving the clarity, consistency, and reproducibility of the manuscript.

Sincerely,
Jaesung Moon

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

1. pls provide a clean version for the purpose of readiibalty and revision
2. the authors were recommeded to conduct a more comprehensive literature review, such as . [1]An end-to-end multilingual framework for intelligent analysis of risk influence factors in ship grounding accidents[2]Intelligent Ship route planning via an A* search model enhanced double-deep Q-Network

Author Response

Comment 1) “Please provide a clean version for the purpose of readability and revision.”

Response:
Thank you for your comment. For readability and ease of review, I prepared and checked a clean version of the manuscript in Round 2. Specifically, I accepted all tracked changes, deleted all comments, and ensured that no non-English/garbled characters remained throughout the main text, tables, figure captions, and figure annotations.

Comment 2) “Conduct a more comprehensive literature review, such as (1) multilingual grounding risk-factor analysis and (2) AI route planning via A* + DDQN.”

Response:
Thank you for recommending these references. I reviewed both papers carefully. However, they primarily address navigation/safety analytics and AI-based route planning, and therefore have limited direct relevance to the core scope of this manuscript, which focuses on full-scale sea-operation measurements and a comparative analysis of the performance and exhaust-emission characteristics of a two-stroke marine diesel engine under different CPP operating modes. Since these studies do not directly support the experimental design or the interpretation of the propulsion/emission results presented here, I decided not to cite them in order to maintain the technical focus and coherence of the manuscript. I kindly ask for your understanding.