A Review of the Use of Hydrogen in Compression Ignition Engines with Dual-Fuel Technology and Techniques for Reducing NO_x Emissions

Round 1

Reviewer 1 Report

This paper reviewed the application of dual fuel technology in compression ignition engines using hydrogen and diesel fuel. A detailed introduction of recent work on the addition of H2 to CIE was provided. There are still some questions in this paper that are not clear:

1. What was the source of the basic data shown in Table 1, it is recommended to indicate the source clearly, especially for the minimum ignition energy, whose results are usually quite different obtained by different studies.

2. In the review of hydrogen addition effect on NOx emissions, NH3/H2 fuel will be a special case, since the production pathway in NH3 mixtures is no longer just thermal-NOx type, but more fuel-NOx type. Relevant research [Fuel, 2022, 327: 125176] [Int J Hydrogen Energy, 2024, 49: 1336-45] should be cited and listed.

3. The width and position of the Tables in this paper were not uniform, it was suggested to optimize appropriately.

4. It was noticed that the researches in the last three years accounted for a relatively small proportion of the references, it was suggested adding some latest research results in this review.

The English is easy to read.

Author Response

This paper reviewed the application of dual fuel technology in compression ignition engines using hydrogen and diesel fuel. A detailed introduction of recent work on the addition of H2 to CIE was provided. There are still some questions in this paper that are not clear:

1. What was the source of the basic data shown in Table 1, it is recommended to indicate the source clearly, especially for the minimum ignition energy, whose results are usually quite different obtained by different studies.

Authors. According to reviewer comment, data source has been added to Table 1. 

2. In the review of hydrogen addition effect on NOx emissions, NH3/H2 fuel will be a special case, since the production pathway in NH3 mixtures is no longer just thermal-NOx type, but more fuel-NOx type. Relevant research [Fuel, 2022, 327: 125176] [Int J Hydrogen Energy, 2024, 49: 1336-45] should be cited and listed.

Authors. According to reviewer comment, these new references have been added (page 3, line 109, references 24 and 25).

3. The width and position of the Tables in this paper were not uniform, it was suggested to optimize appropriately.

Authors. According to reviewer comment, tables have been optimized.

4. It was noticed that the researches in the last three years accounted for a relatively small proportion of the references, it was suggested adding some latest research results in this review.

Authors. According to reviewer comment, references have been updated, as may be seen from Figure 1, which now includes references up to 2023.

Reviewer 2 Report

Review of sustainability-2942047: “A review of the use of hydrogen in compression ignition engines with dual-fuel technology and techniques for reducing NOx emissions" This review can be considered mostly for single-cylinder CIE applications and cannot be generalized as in the title. Also, the discussion of the effect of adding H₂ on CIE performance needs more generalized work and depth of discussion in a clear way for the effect of controlling and operating parameters.

• Remove the full stop from the title. 
• Why did you not add LHV and HHV as the main combustion characteristics in Table 2?
• In the effect of hydrogen as a dual fuel on combustion performance characteristics, you ignored some important characteristic parameters such as sfc., mean effective pressure (mep), engine speed, engine load, fuel conversion efficiency, etc. why?
• We are talking about real combustion. Why did you not use combustion efficiency in Eq. 2, where the combustion efficiency for both fuels is different, and provide the studies to investigate that?
• What happens to BET if EGR, PVC, catalytic converters, etc. are used? I did not see that.
• Write the name of the parameter, not the abbreviation, in the subtitle as VE, MTE, etc.
• It is illogic at all to use only a single cylinder, which is limited to so small applications or for lab testing for discussing the effect of H₂ addition on the volumetric efficiency and ignition delay (most of the references), because this is considered a weak point in the review. This parameter is so important in engine performance evaluation, as are brake power, brake thermal efficiency, brake-specific fuel consumption, combustion characteristics, and so on.
• Fuel properties and composition are responsible for the physical delay; what about injection system characteristics and their effect?
• Page 10: For commercial engines that can be used in conventional vehicles, research has shown that using H2 can cause ID. Ambiguous statement.
• The crank angle degree symbol needs to be superscripted, not like in the paper.
• Why do the authors depend on the theoretical definitions of ID and CD and not on the practical cases?
• The discussion of hydrogen addition at maximum in-cylinder pressure is very superficial and in-depth.
• Page 15: Results on CO2, CO, HC, soot, and NOx were as follows: Rewrite in logical and expressive form.
• You did not explore the effect of H₂ addition on whether the engine is naturally aspirated, supercharged, or turbocharged.
• What do you mean by the symbol K (m^-1) in Table 11? Also, for particulate matter, define.
• Add a graph for different techniques of adding water and hydrogen to one graph and relate them.
• The authors were concerned about the addition of water (WI) as one of the ways of reducing NOx formation, but they ignored at the same time their effect on engine performance. 

minor revision

Author Response

Review of sustainability-2942047: “A review of the use of hydrogen in compression ignition engines with dual-fuel technology and techniques for reducing NOx emissions" This review can be considered mostly for single-cylinder CIE applications and cannot be generalized as in the title. Also, the discussion of the effect of adding H2 on CIE performance needs more generalized work and depth of discussion in a clear way for the effect of controlling and operating parameters.

    Remove the full stop from the title. 

Authors. According to reviewer comment, the full stop from the title has been removed.

    Why did you not add LHV and HHV as the main combustion characteristics in Table 2?

Authors. According to reviewer comment, energy density has been modified to lower heating value (LHV) in Table 1. HHV has not been included, as both parameters are closely related; LHV is calculated subtracting the water heat of vaporization from the HHV.

    In the effect of hydrogen as a dual fuel on combustion performance characteristics, you ignored some important characteristic parameters such as sfc., mean effective pressure (mep), engine speed, engine load, fuel conversion efficiency, etc. why?

Authors. According to reviewer comment, engine performance was analyzed through HES, engine load, engine speed, CR, maximum in-cylinder pressure, ID, CD, etc., as may be seen from Tables 3-15 and along the text. The addition of new parameters would have significantly increased the length of the manuscript. Please, notice that the main objective of this work is the study of NOx emissions and strategies to reduce them.

    We are talking about real combustion. Why did you not use combustion efficiency in Eq. 2, where the combustion efficiency for both fuels is different, and provide the studies to investigate that?

Authors. According to reviewer comment, though we believe this is an important parameter, there is an important lack of papers including this information. For this reason, this parameter was not considered.

    What happens to BET if EGR, PVC, catalytic converters, etc. are used? I did not see that.

Authors. According to reviewer comment, the influence of EGR on BTE has been studied in-deep (pages 25-27, section 3.1 and Table 13). We would like to point out that the strategies analyzed in this work affect the combustion process, since each of these strategies affect in one way or another the temperature in the combustion chamber, thus affecting NOx emissions. Strategies such as catalytic converters are considered for exhaust gas post-processing, which is outside the scope of this review.

    Write the name of the parameter, not the abbreviation, in the subtitle as VE, MTE, etc.

Authors. According to reviewer comment, abbreviations have been removed from titles (page 3, line 123; page 7, line 282; page 9, line 319; page 10, line 337; page 12, line 377; page 15, line 471; page 17, line 493; page 19, line 517; page 23, line 565; page 25, line 620; page 27, line 682; page 30, line 753).

    It is illogic at all to use only a single cylinder, which is limited to so small applications or for lab testing for discussing the effect of H2 addition on the volumetric efficiency and ignition delay (most of the references), because this is considered a weak point in the review. This parameter is so important in engine performance evaluation, as are brake power, brake thermal efficiency, brake-specific fuel consumption, combustion characteristics, and so on.

Authors. According to reviewer comment, we have only found studies including VE in single-cylinder engines, as may be seen in page 10, line 331. This sentence has been rewritten to make it clearer to the reader. Considering ID, both types of engines have been included, as may be seen in Table 5. We would like to stress that there were only few studies concerning multi-cylinder engines

    Fuel properties and composition are responsible for the physical delay; what about injection system characteristics and their effect?

Authors. According to reviewer comment, injection characteristic effect has been included in pages 5-6 (lines 183-235).

    Page 10: For commercial engines that can be used in conventional vehicles, research has shown that using H2 can cause ID. Ambiguous statement.

Authors. According to reviewer comment, this may be due to the lesser quantity of pilot fuel injected under the dual-fuel mode, replaced by H2. However, other authors, keeping constant the amount of diesel fuel during the dual-fuel mode, found the opposite trend. This fact has been explained in page 12, lines 366-376.

    The crank angle degree symbol needs to be superscripted, not like in the paper.

Authors. According to reviewer comment, the crank angle degree symbol has been corrected and highlighted in the text. 

    Why do the authors depend on the theoretical definitions of ID and CD and not on the practical cases?

Authors. According to reviewer comment, ID and CD have been theoretically defined. However, results from practical cases have been included in the manuscript (sections 2.1.3 and 2.1.4, and Tables 5 and 6). 

    The discussion of hydrogen addition at maximum in-cylinder pressure is very superficial and in-depth.

Authors. According to reviewer comment, an in-depth discussion has been included (page 15, lines 452-458)

    Page 15: Results on CO2, CO, HC, soot, and NOx were as follows: Rewrite in logical and expressive form.

Authors. According to reviewer comment, the sentence has been rewritten to make it clear to the reader (page 15, line 470).

    You did not explore the effect of H2 addition on whether the engine is naturally aspirated, supercharged, or turbocharged.

Authors. According to reviewer comment, the engine type has been defined in each table (naturally aspirated or turbocharged) and each study has been commented, including all the characteristics. However, a specific section about the influence of the type of engine has not been included for the sake of brevity, as the length of the review manuscript is already significant.

    What do you mean by the symbol K (m-1) in Table 11? Also, for particulate matter, define.

Authors. According to reviewer comment, K means opacity. This meaning, including that of PM, has been added as footnote to Table 11. 

    Add a graph for different techniques of adding water and hydrogen to one graph and relate them.

Authors. According to reviewer comment, Figure 2 has been modified.

    The authors were concerned about the addition of water (WI) as one of the ways of reducing NOx formation, but they ignored at the same time their effect on engine performance.

Authors. According to reviewer comment, a paragraph with this information may be found in page 29, lines 733-741. 

Reviewer 3 Report

Authors have analyzed many scientific papers which deal with hydrogen application for combustion engine. I have following remarks and questions: on the line 257 there should be volumetric flow instead of mass flow because the unit is l/min, respectively it is not specified at what conditions (temperature, pressure) this volumetric flow occours. Line 373 - CD. Faster fuel burn in addition to mentioned benefits also results in rough engine operation (higher pressure rise rate for one degree of rotation of the crankshaft). Line 551 - numbering of the subchapter NOx is the same as in the previous one line 534. There is no list of abbreviations for better orientation. It would be better if titles of the subchapters were not abbreviations but full words. In the text there is no mention about hydrogen embrittlement and also about higher emulsion formation (oil and watter), which are also significant while utilising hydrogen in the combustion engine. Submitted paper deeply analyses hydrogen impacts on combustion engine.

Author Response

Authors have analyzed many scientific papers which deal with hydrogen application for combustion engine. I have following remarks and questions:

on the line 257 there should be volumetric flow instead of mass flow because the unit is l/min, respectively it is not specified at what conditions (temperature, pressure) this volumetric flow occours.

Authors. According to reviewer comment, mass flow now appears as volumetric flow (page 6, old line 257, new line 264).

Line 373 - CD. Faster fuel burn in addition to mentioned benefits also results in rough engine operation (higher pressure rise rate for one degree of rotation of the crankshaft).

Authors. According to reviewer comment, this information has been added (page 12, lines 383-385).

Line 551 - numbering of the subchapter NOx is the same as in the previous one line 534.

Authors. According to reviewer comment, the numbering has been corrected (page 23, old line 551, new line 565). 

There is no list of abbreviations for better orientation.

Authors. According to reviewer comment, a list of abbreviations has been added on page 33.

It would be better if titles of the subchapters were not abbreviations but full words.

Authors. According to reviewer comment, abbreviations have been removed from titles (page 3, line 123; page 7, line 282; page 9, line 319; page 10, line 337; page 12, line 377; page 15, line 471, page 17, line 493; page 19, line 517; page 23, line 565; page 25, line 620; page 27, line 682; page 30, line 753).

In the text there is no mention about hydrogen embrittlement and also about higher emulsion formation (oil and watter), which are also significant while utilising hydrogen in the combustion engine. Submitted paper deeply analyses hydrogen impacts on combustion engine.

Authors. According to reviewer comment, information about hydrogen embrittlement is included in page 4, lines 141-144. Emulsion formation has not been included for the sake of brevity, as the many information included has significantly increased the length of the review manuscript.

Reviewer 4 Report

The paper is an interesting one and the subject is of maximum topicality.

The content of the paper is comprehensive. Numerous results obtained with hydrogen fueled engines and NOx reduction methods are presented.

I consider that the paper can be published in Sustainability, but before that some information should be added.

I understood that the authors included the most relevant emission control strategies, that were not extensively analyzed in other reviews, but I think that the gas treatment technologies (Selective Catalytic Reduction - SCR) should also be mentioned. Can AdBlue be used to reduce NOx emissions in hydrogen fueled engines? Does the use of hydrogen affect the efficiency of this technology? Are improvements/modifications needed?

Another recommendation is that graphs containing the data from Tables 3-15 should be added.

Author Response

The paper is an interesting one and the subject is of maximum topicality.

The content of the paper is comprehensive. Numerous results obtained with hydrogen fueled engines and NOx reduction methods are presented.

I consider that the paper can be published in Sustainability, but before that some information should be added.

I understood that the authors included the most relevant emission control strategies, that were not extensively analyzed in other reviews, but I think that the gas treatment technologies (Selective Catalytic Reduction - SCR) should also be mentioned. Can AdBlue be used to reduce NOx emissions in hydrogen fueled engines? Does the use of hydrogen affect the efficiency of this technology? Are improvements/modifications needed?

Authors. According to reviewer comment, the strategies analyzed in our work affect the combustion process, since they influence the temperature in the combustion chamber, thus affecting NOx emissions. Strategies such as SCR and AdBlue are considered for exhaust gas post-processing, which is outside the scope of this review.

Another recommendation is that graphs containing the data from Tables 3-15 should be added.

Authors. According to reviewer comment, publisher rules prevent using the same data for both tables and figures. Otherwise, the information is considered redundant. Moreover, data from different studies are provided using different units, different axis, etc. thus making hard the figure structure. For the sake of clarity, tables have been preferred to figures.

Round 2

Reviewer 2 Report

The authors answer of comments are fair. The modified version of the revised paper becomes better and can be accepted for publication.