The Future of Engine Knock and Fuel Octane Numbers in the Era of Biofuels and Vehicle Electrification

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript provides a comprehensive review of the mechanisms of engine knock and its constraints on modern spark-ignition engine design. Using technology forecasting methods, the paper explores the changing operating range of engines in future hybrid vehicles and examines the impact of Atkinson/Miller cycles on knock tendency. It further compares the molecular composition, octane characteristics, and sensitivity of biofuels and e-fuels, assessing their applicability in future hybrid engines. Specific comments are as follows:

1. Although the study employs technology forecasting and literature review, it lacks empirical validation or systematic modeling and simulation support. It is recommended to provide more quantitative results or conduct sensitivity analyses based on experimental/database data.
2. As the K value is a central metric, the derivations and assumptions are described somewhat briefly. The methodology section should include a more detailed explanation of its physical meaning, applicable boundary conditions, and computational procedure.
3. While series and power-split hybrid configurations are discussed, other architectures such as PHEVs and range-extended hybrids are not examined in depth. It is suggested to expand the discussion on how different hybrid types influence engine operating regions and knock behavior.
4. Although the paper emphasizes sustainability, it lacks an in-depth discussion of lifecycle emissions and the alignment of future fuels with regulatory and policy frameworks. Additional background on policy and economic considerations is recommended.
5. Several key references are relatively dated. More recent studies from 2023–2025, particularly on e-fuels and knock trends in hybrid engines, should be incorporated.
6. The Background section is somewhat lengthy, with an extensive review of well-established concepts. It could be condensed to place greater emphasis on future trends and forecasting results, thereby enhancing the forward-looking nature of the paper.

Author Response

Prior to addressing your comments, we wanted to thank you for your time and expertise in reviewing our paper.  Yor insight is very much appreciated, and we truly feel that it has made our paper much stronger.  Thank you.

 

This manuscript provides a comprehensive review of the mechanisms of engine knock and its constraints on modern spark-ignition engine design. Using technology forecasting methods, the paper explores the changing operating range of engines in future hybrid vehicles and examines the impact of Atkinson/Miller cycles on knock tendency. It further compares the molecular composition, octane characteristics, and sensitivity of biofuels and e-fuels, assessing their applicability in future hybrid engines. Specific comments are as follows:

Comment 1: Although the study employs technology forecasting and literature review, it lacks empirical validation or systematic modeling and simulation support. It is recommended to provide more quantitative results or conduct sensitivity analyses based on experimental/database data.

Response: Excellent point.  With the original submission, we only talked about the modeling results without adequately discussing how they were generated.  We have added substantial more information about detailing the models, including how we built them and how we validated them.  These are in Section 3.2.3.  The validation is through comparing the engine map to those in literature.  We also added in our future works section that there is a need to run thorough engine tests to get more robust data.

Comment 2: As the K value is a central metric, the derivations and assumptions are described somewhat briefly. The methodology section should include a more detailed explanation of its physical meaning, applicable boundary conditions, and computational procedure.

Response: We concur with your assessment, and this was an issue brought up by other reviewers as well. We restructured the background section to provide less information on knock and more information about S and K.  We added discussion about the fundamental reason for fuel sensitivity, and how K effects the anti-knock performance of fuel.

Comment 3: While series and power-split hybrid configurations are discussed, other architectures such as PHEVs and range-extended hybrids are not examined in depth. It is suggested to expand the discussion on how different hybrid types influence engine operating regions and knock behavior.

Response: We added in PHEV and range-extended hybrids into the discussion about hybrid architectures. We also added discussion PHEVs when discussing vehicle trends, given the rising numbers especially in Europe and China.

Comment 4: Although the paper emphasizes sustainability, it lacks an in-depth discussion of lifecycle emissions and the alignment of future fuels with regulatory and policy frameworks. Additional background on policy and economic considerations is recommended.

Response: We have added discussion in both the background, results, and the fuel discussion to better capture lifecycle emissions.

Comment 5: Several key references are relatively dated. More recent studies from 2023–2025, particularly on e-fuels and knock trends in hybrid engines, should be incorporated.

Response: Concur.  We have updated our bibliography, adding a number of more recent references especially as they pertain to e-fuels and knock in hybrid powertrains.

Comment 6: The Background section is somewhat lengthy, with an extensive review of well-established concepts. It could be condensed to place greater emphasis on future trends and forecasting results, thereby enhancing the forward-looking nature of the paper.

Response: Concur.  We have cut down portions of the background section, providing a succinct overview of knock.  We then increased the discussion about K and sensitivity.  

Reviewer 2 Report

Comments and Suggestions for Authors

The study entitled "The Future of Engine Knock and Fuel Octane Numbers in the Era of Biofuels and Vehicle Electrification" was thoroughly examined, and it was determined that some of the shortcomings listed below should be addressed by the authors.

1) If the authors include numerical results of the analyses in the abstract, sharing them would increase the readability of the study.

2) The introduction section could be enriched with a much richer literature. The study's contributions to the literature should be itemized in this section, and its differences from previous studies in the literature should be directly addressed.

3) Information is generally not provided as to whether some figures in the study were derived from one study, another, or a simulation. This requires a more detailed explanation of the figures.

4) In the very first line of the methodology section, the authors include the statement "This paper uses a standard technology forecasting methodology, as shown in Figure 3." Instead, it would be more appropriate to provide a brief explanation of the methodology and then cite the figure illustrating the methodology.

5) The methodology should also be examined in scientific publications. Included and cited in particular, a brief introductory literature review of publications addressing this methodology would enhance the study in this context.

6) Sections 4. Engine Knock Behaviors and 5. Fuel Anti-Knock Behavior should be included within the methodology section, and the name of this section should be changed to Materials and Methodology.

7) Section 6. Engine and Fuel Recommendations for Future Vehicles should be included within the conclusion section. The name of the conclusion section should be changed to Conclusions and Recommendations.

Author Response

Prior to addressing your comments, we wanted to thank you for your time and expertise in reviewing our paper.  Yor insight is very much appreciated, and we truly feel that it has made our paper much stronger.  Thank you.

 

Comment 1: If the authors include numerical results of the analyses in the abstract, sharing them would increase the readability of the study.

Response: The abstract was modified to include information about K values and the change in K values associated with hybridized engines.

Comment 2: The introduction section could be enriched with a much richer literature. The study's contributions to the literature should be itemized in this section, and its differences from previous studies in the literature should be directly addressed.

Response: We added material to the background section to make it a bit richer.  We took out background information about knock, but added more material about fuel sensitivity and K.  We brought in newer references as well.

Comment 3: Information is generally not provided as to whether some figures in the study were derived from one study, another, or a simulation. This requires a more detailed explanation of the figures.

Response: We went through the paper and clarified where different results/figures came from, to ensure our contribution was clearer.

Comment 4: In the very first line of the methodology section, the authors include the statement "This paper uses a standard technology forecasting methodology, as shown in Figure 3." Instead, it would be more appropriate to provide a brief explanation of the methodology and then cite the figure illustrating the methodology.

Response: Thank you for the recommendation. We modified that section accordingly.

Comment 5: The methodology should also be examined in scientific publications. Included and cited in particular, a brief introductory literature review of publications addressing this methodology would enhance the study in this context.

Response: We added discussion and a reference for the methodology that we used for technology forecasting.

Comment 6: Sections 4. Engine Knock Behaviors and 5. Fuel Anti-Knock Behavior should be included within the methodology section, and the name of this section should be changed to Materials and Methodology.

Response: We updated the paper accordingly.

Comment 7: Section 6. Engine and Fuel Recommendations for Future Vehicles should be included within the conclusion section. The name of the conclusion section should be changed to Conclusions and Recommendations.

Response: We updated the paper accordingly.

 

Reviewer 3 Report

Comments and Suggestions for Authors

This article is interesting and innovative, presenting a comprehensive analysis of the knocking combustion phenomenon in the context of future automotive trends, such as the development of hybrid drives and electrification. The authors combine knowledge of engine design and fuel chemistry, using modeling and technological forecasting, to identify directions for the development of low-emission fuels. Of particular value is the emphasis on the importance of low-sensitivity fuels and adapting their composition to the operating conditions of future powertrains. The work stands out for its timeliness, practical relevance, and interdisciplinary approach, contributing to the development of transportation technologies and decarbonization.

While the article is valuable and well-written, several aspects require further refinement. The work relies primarily on modeling and technological forecasts, which limits the possibility of fully validating the results in practice, it would be worthwhile to supplement the research with experiments on real engines in the future. The literature analysis is current but focused primarily on English-language sources and technical publications, which could be expanded to include the latest data from energy policy and the global automotive industry. Furthermore, although the methodology is transparent, providing more technical details of the simulations (e.g., the configuration of the Ricardo WAVE models) would facilitate replication of the results by other researchers. Despite these minor criticisms, the paper makes an important contribution to the discussion about the future of fuels and combustion engines in the era of electrification.

Author Response

Prior to addressing your comments, we wanted to thank you for your time and expertise in reviewing our paper.  Yor insight is very much appreciated, and we truly feel that it has made our paper much stronger.  Thank you.

Comment 1: This article is interesting and innovative, presenting a comprehensive analysis of the knocking combustion phenomenon in the context of future automotive trends, such as the development of hybrid drives and electrification. The authors combine knowledge of engine design and fuel chemistry, using modeling and technological forecasting, to identify directions for the development of low-emission fuels. Of particular value is the emphasis on the importance of low-sensitivity fuels and adapting their composition to the operating conditions of future powertrains. The work stands out for its timeliness, practical relevance, and interdisciplinary approach, contributing to the development of transportation technologies and decarbonization.

Response: Thank you.

Comment 2: While the article is valuable and well-written, several aspects require further refinement. The work relies primarily on modeling and technological forecasts, which limits the possibility of fully validating the results in practice, it would be worthwhile to supplement the research with experiments on real engines in the future.

Response: Thank you for that recommendation.  We added that into our future work discussion. We also added more details about the models and how we validated the results.

Comment 3: The literature analysis is current but focused primarily on English-language sources and technical publications, which could be expanded to include the latest data from energy policy and the global automotive industry.

Response: Another excellent suggestion.  We went through and found a number of other references from different research groups and labs from around the world.  We encompassed these into our background section.  We also specifically discuss Europe and China’s rapid move to PHEVs.

Comment 4: Furthermore, although the methodology is transparent, providing more technical details of the simulations (e.g., the configuration of the Ricardo WAVE models) would facilitate replication of the results by other researchers.

Response: Done!  That is a great suggestion.  We have added that material to the paper.

Comment 5: Despite these minor criticisms, the paper makes an important contribution to the discussion about the future of fuels and combustion engines in the era of electrification.

Response: Thank you

Reviewer 4 Report

Comments and Suggestions for Authors

This paper evaluated knock behavior in projected hybrid engine architectures and examines the chemical composition of emerging fuel blends. However, I have some comments.

1. The novelty of the research is not clearly explained. Please highlight what is new.
2. Some parts of the content are hard to follow, and figures are not explained detail. Please modify.
3. In section 4, the description of the engine knock behaviors is too general. Please shorten it and focus only on how it is applied in this study.
4. More verifications need to be provided to prove the superiority of the study.
5. The quality of the pictures is poor, and these need to be improved.
6. The conclusion should discuss limitations and future work.

Author Response

Prior to addressing your comments, we wanted to thank you for your time and expertise in reviewing our paper.  Yor insight is very much appreciated, and we truly feel that it has made our paper much stronger.  Thank you.

Comment 1: The novelty of the research is not clearly explained. Please highlight what is new.

Response: We added a paragraph at the start of the methodology section to discuss the novelty of this research.

Comment 2: Some parts of the content are hard to follow, and figures are not explained detail. Please modify.

Response: We went through the paper and attempted to clarify all of the analysis and figures better to make it clearer.  We also removed some of the tangential material from the background section to more thoroughly discuss K and sensitivities.  We also attempted to provide more details about our analysis such that it’s easier to follow.

Comment 3: In section 4, the description of the engine knock behaviors is too general. Please shorten it and focus only on how it is applied in this study.

Response: Absolutely!  This was a common comment across all reviewers.   We’ve tailored that section down and switched it to focusing on the necessary information on Octane Numbers, Sensitivities, and K values

Comment 4: More verifications need to be provided to prove the superiority of the study.

Response: We added discussion on how we validated our models. In general, we added a lot more information about how models and how we did our analysis. We also touch on this in additional work (using real engine data to provide more robust data).

Comment 5: The quality of the pictures is poor, and these need to be improved.

Response: We went through the paper and updated the figures to improve the quality.

Comment 6: The conclusion should discuss limitations and future work.

Response: Done!  We added limitations and future work into our conclusion section.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

No further comments. 

Reviewer 2 Report

Comments and Suggestions for Authors

I would like thank to authors for improving the study

Reviewer 4 Report

Comments and Suggestions for Authors

Thanks for your revision.