NO Formation in Combustion Engines Fuelled by Mixtures of Hydrogen and Methane

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, NO formation in combustion engines is studied based on the extended chemical kinetic mechanisms of Zeldovivh.  The effect of four variables on the NO formation is numerically investigated and concluded.  Some minor questions are listed below:

1. The four main assumptions are mentioned in the abstract in line 16. However, they are not mentioned in the main text. It is recommended that these assumptions be summarized before they are made in the numerical methods section.

2. Can the authors find relevant experiments from other literature to verify the numerical results?

3. In line 277, the statements describing time-step independence could be improved. 'the third decimal place' should refer to 0.001kmol⋅m-3.

 

4. The structure of the paper may be subject to revision. The five parts of the manuscript are Introduction, Internal Combustion Engines and H2, Generation of NOx, Mathematical model, and Results. However, the first three parts of the manuscript are essentially introductions and should be considered for merging.

Author Response

Comments 1: The four main assumptions are mentioned in the abstract in line 16. However, they are not mentioned in the main text. It is recommended that these assumptions be summarized before they are made in the numerical methods section.

Response 1: Thank you for your comment. We inserted these assumptions in section “Program Developed”  

Comments 2: Can the authors find relevant experiments from other literature to verify the numerical results?

Response 2: Thank you for your comment. Combustion in engines is turbulent and coupled with the fluid velocity. Turbulence introduces questions concerning how the nonlinear kinetic equations should be time-smoothed. The precise model for reactive turbulent flows is still speculative.  However, applications of kinetics have played an important role in understanding mechanisms for the formation of emissions. Particularly, NOx emission control by combustion modifications has benefited from kinetic modeling, even though the ability to predict NO levels is limited to about a factor of 2 in gasoline engines, for example. In short, there is a scarcity of relevant experimental results concerning reactant mixture with predominantly H₂, and when they exist, they are not meaningful de per se. At this stage, when modifying this or that factor (equivalence ratio, reactants composition, compression rate, etc.) only trends should be analyzed.   

Comments 3: In line 277, the statements describing time-step independence could be improved. 'the third decimal place' should refer to 0.001kmol⋅m-3.

Response 3: Thank you for your comment. We improved according with your suggestion.    

Comments 4: The structure of the paper may be subject to revision. The five parts of the manuscript are Introduction, Internal Combustion Engines and H2, Generation of NOx, Mathematical model, and Results. However, the first three parts of the manuscript are essentially introductions and should be considered for merging

Response 4: Thank you for your comment. We implemented your suggestion    

Reviewer 2 Report

Comments and Suggestions for Authors

The article presents detailed results highlighting the effect of initial temperature and low hydrogen concentration in reducing NOx production. These findings are well known to the combustion community. The modelling adopted is a 'global' approach and the detailed distribution of temperature and fuel is not addressed. My suggestion is to discuss the effect of the heterogeneity of the conclusions and how the model developed can be implemented in a CFD approach. 

Comments on the Quality of English Language

The article is well written with a good english quality

Author Response

Comments 1: The article presents detailed results highlighting the effect of initial temperature and low hydrogen concentration in reducing NOx production. These findings are well known to the combustion community. The modelling adopted is a 'global' approach and the detailed distribution of temperature and fuel is not addressed. My suggestion is to discuss the effect of the heterogeneity of the conclusions and how the model developed can be implemented in a CFD approach.

Response 1: Thank you for your comment. Combustion in engines is turbulent and coupled with the fluid velocity. Turbulence introduces questions concerning how the nonlinear kinetic equations should be time-smoothed. The precise model for reactive turbulent flows is still speculative.  However, applications of kinetics have played an important role in understanding mechanisms for the formation of emissions. At this stage it is not possible to take into account chemical composition heterogeneities within the cylinder. It would require the coupling of models for mass transfer, moment transfer, energy transfer and chemical kinetics which would render the solution impossible, at this stage of human knowledge.  Besides, in the article the effect of compression ratio, reactant mixture composition and equivalence ratio are studied.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have presented a well-structured study on the relevant topic of NOx formation, which is important for reducing emissions and developing cleaner combustion technologies. The mathematical model developed using the extended Zeldovich mechanism and the insights gained regarding factors influencing NO formation, such as lowering the equivalence ratio and increasing the compression ratio, are valuable contributions.

I have a few suggestions that could enhance the study further:

The authors could consider validating their model predictions with experimental literature data or mention the need for future experimental work to validate the model as a scope for future research. Obtaining experimental data would strengthen the credibility of the model and its findings.

The authors may want to explore whether including other nitrogen oxides (e.g., NO2) and their formation mechanisms is within the scope of this study or could be considered for future work. A more comprehensive understanding of various nitrogen oxide emissions would be beneficial.

Furthermore, a more detailed discussion on the practical implications of the results for engine design, emission control strategies, and potential trade-offs between NO reduction and other performance parameters would highlight the study's applicability and relevance to industry and policymakers.

Exploring the effects of other engine parameters, such as injection timing, ignition timing, and exhaust gas recirculation (EGR), on NO formation in hydrogen-methane mixtures could provide additional insights and broaden the study's scope.

Author Response

Comments 1: The authors could consider validating their model predictions with experimental literature data or mention the need for future experimental work to validate the model as a scope for future research. Obtaining experimental data would strengthen the credibility of the model and its findings.

Response 1: Thank you for your comment. Combustion in engines is turbulent and coupled with the fluid velocity. Turbulence introduces questions concerning how the nonlinear kinetic equations should be time-smoothed. The precise model for reactive turbulent flows is still speculative.  However, applications of kinetics have played an important role in understanding mechanisms for the formation of emissions. Particularly, NOx emission control by combustion modifications has benefited from kinetic modeling, even though the ability to predict NO levels is limited to about a factor of 2 in gasoline engines, for example. In short, there is a scarcity of relevant experimental results concerning reactant mixture with predominantly H₂, and when they exist, they are not meaningful de per se. At this stage, when modifying this or that factor (equivalence ratio, reactants composition, compression rate, etc.) only trends should be analyzed. 

Comments 2: The authors may want to explore whether including other nitrogen oxides (e.g., NO2) and their formation mechanisms is within the scope of this study or could be considered for future work. A more comprehensive understanding of various nitrogen oxide emissions would be beneficial.

Response 2: Thank you for your comment. The NO generated within the cylinders of an ICE oxidizes only in the atmosphere, after the tailpipe, in the presence of ultraviolet sunlight, producing NO_2. There is in fact a kinetic mechanism to produce NO₂, but it is nor relevant in the current manuscript, since it deals with the nitrogen oxides produce inside the cylinders.

Comments 3: Furthermore, a more detailed discussion on the practical implications of the results for engine design, emission control strategies, and potential trade-offs between NO reduction and other performance parameters would highlight the study's applicability and relevance to industry and policymakers.

Response 3: Thank you for your comment. The main conclusion of the paper, as stated in the “Conclusions” and Shown in the “Results” is that the most efficient way to lower the NO emissions is to lower the temperature of the gases after the explosion, which is already achieved through EGR, for example. 

Comments 4: Exploring the effects of other engine parameters, such as injection timing, ignition timing, and exhaust gas recirculation (EGR), on NO formation in hydrogen-methane mixtures could provide additional insights and broaden the study's scope.

Response 4: Thank you for your comment. The EGR analysis is implied in low equivalence ratios situations: all in all, it is the effect of low temperature which is assessed. In the case of injection timing and ignition timing, both factors may be important regarding combustion, but not so important in the post-flame reactions which produce NO. The NO production becomes relevant after the combustion process.

Reviewer 4 Report

Comments and Suggestions for Authors

1.Contextualization:

Although the introduction offers a comprehensive overview of the topic, it could benefit from the inclusion of more recent studies. To guide the reader, clearly state the research questions and objectives.

2.Literature Review:

Expand the literature review to incorporate recent studies and identify gaps that your research addresses. This will enhance the contextualization and relevance of your work.

3.Research Design:

Clearly state your research questions and hypotheses. This will provide a clearer focus for your study and make your objectives more explicit.

4.Details on Methods:

While the methods are described, providing additional details, especially about the mathematical model and its validation, will improve the clarity and reproducibility of your work.

5.Presentation of Results:

Although the results are well-presented, make sure to clearly label and explain all tables and figures. This will help readers understand the data better.

6.Critical Analysis:

Enhance the discussion by critically analyzing your results in relation to existing literature. Address any limitations in your study and suggest possible reasons for discrepancies with other research.

7.Linking Findings to Literature:

Compare your findings with those of other studies. Discuss how your findings align with or differ from existing research, as well as their implications.

8.Summary of Findings:

Clearly summarize your study's key findings. Make sure your results directly support your conclusions.

9.Implications and Future Research:

Discuss the broader implications of your findings. Suggest areas for future research based on the gaps and limitations identified in your study.

10.Minor Editing:

Review the paper for minor grammatical errors and awkward phrasing. Ensuring clarity and readability will enhance the overall quality of the manuscript.

11.Consistency:

 

Ensure consistent formatting throughout the document. This includes headings, subheadings, and citation style.

Comments on the Quality of English Language

The overall quality of the English language in the paper is good. The text is mostly clear and comprehensible. There are minor grammatical errors and awkward phrasing throughout the paper. 

Author Response

Comments 1: Contextualization: Although the introduction offers a comprehensive overview of the topic, it could benefit from the inclusion of more recent studies. To guide the reader, clearly state the research questions and objectives.

Response 1: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 2: Literature Review: Expand the literature review to incorporate recent studies and identify gaps that your research addresses. This will enhance the contextualization and relevance of your work

Response 2: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 3: Research Design: Clearly state your research questions and hypotheses. This will provide a clearer focus for your study and make your objectives more explicit.

Response 3: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 4: Details on Methods: While the methods are described, providing additional details, especially about the mathematical model and its validation, will improve the clarity and reproducibility of your work.

Response 4: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 5: Presentation of Results: Although the results are well-presented, make sure to clearly label and explain all tables and figures. This will help readers understand the data better.

Response 5: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 6: Critical Analysis: Enhance the discussion by critically analyzing your results in relation to existing literature. Address any limitations in your study and suggest possible reasons for discrepancies with other research.

Response 6: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 7: Linking Findings to Literature: Compare your findings with those of other studies. Discuss how your findings align with or differ from existing research, as well as their implications.

Response 7: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 8: Summary of Findings: Clearly summarize your study's key findings. Make sure your results directly support your conclusions.

Response 8: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 9: Implications and Future Research: Discuss the broader implications of your findings. Suggest areas for future research based on the gaps and limitations identified in your study.

Response 9: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 10: Minor Editing: Review the paper for minor grammatical errors and awkward phrasing. Ensuring clarity and readability will enhance the overall quality of the manuscript.

Response 10: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Comments 11: Consistency: Ensure consistent formatting throughout the document. This includes headings, subheadings, and citation style

Response 11: Thank you for your comment. Your suggestions were implemented in the present version of the manuscript.

Round 2

Reviewer 4 Report

Comments and Suggestions for Authors

This paper can be accepted in its present form.