Review Reports

Round 1

Reviewer 1 Report

In this paper, the authors perform a numerical study of IC engine combustion with subcritical and supercritical fuel injection. The paper is not well-written; a number of important details are omitted which casts major doubts on the simulation methodology and results. Hence, the reviewer cannot judge the manuscript properly and recommends major revision. Some major comments that need to be addressed are as follows:

(i)  The authors didn't provide any description of the ECFM combustion model and didn't include any references. From the last paragraph of section 1, it seems like the authors simulated a compression-ignition engine. However, ECFM model is applicable for premixed combustion. 

(ii) No information is given on whether the simulations were RANS or LES. No information regarding the turbulence model is provided. Are these closed-cycle or open-cycle simulations? How many cycles were simulated? How did the authors prescribe the wall temperature boundary conditions?

(iii) No information on how the thermophysical properties under supercritical conditions were calculated is provided. Did the authors make any effort to validate these properties before using them in the simulations?

(iv) No information on what chemical mechanism was used is provided. Also, what is the fuel surrogate used for the gasoline-diesel mixture?

(v) No information about the spray models is provided. No validation of spray models is included.

(vi) The authors mention the max grid size used in the simulations but do not provide any details about the min grid size. The authors should also show the computational domain. Is it a sector mesh or full engine geometry?

(vii) Matching pressure trace in Fig. 2 is not sufficient. The authors should also compare the apparent heat release rates from experiment and simulation.

Author Response

Dear Editor and Reviewers,

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Numerical Investigation of the Combustion Characteristics of an Internal Combustion Engine with Subcritical and Supercritical Fuel”. These comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as follows:

Responds to the Reviewer(s)' Comments:

Reviewer: 1 

Special thanks to you for your good comments. The responses are as follows:

(1)The authors didn't provide any description of the ECFM combustion model and didn't include any references. From the last paragraph of section 1, it seems like the authors simulated a compression-ignition engine. However, ECFM model is applicable for premixed combustion.

Response: Thank you for your careful review. The combustion type of this simulation is mainly premixed combustion. The purpose of using high compression ratio internal combustion engine is to realize the simulation process of supercritical fuel combustion in internal combustion engine by combining the special physical properties of supercritical fuel and the research progress of supercritical injection and combustion technology. The main role of diesel in dual fuel is to ignite the gasoline in cylinder. In addition to that, ECFM model is fully coupled to the spray model and enables stratified combustion modeling, because this simulation uses the dual fuel model, it is reasonable to use ECMF combustion model.

(2)No information is given on whether the simulations were RANS or LES. No information regarding the turbulence model is provided. Are these closed-cycle or open-cycle simulations? How many cycles were simulated? How did the authors prescribe the wall temperature boundary conditions?

Response: Thank you for your careful review. In this paper, we choose the modified standard k-ε two equation model as the turbulence model to study the supercritical injection process [26]. And this simulation is a single-cycle simulation; According to experience, temperature of the top of the piston, cylinder head wall and cylinder liner wall were set as 570.15K, 570.15K and 470.15K, respectively.

 [26] Numerical simulation of the influence of environmental and jet parameters on supercritical injection.

[30] Feng Wang. Study on the formation mechanism and multi-dimension simulation of polycyclic aromatic hydrocarbons in low temperature combustion of diesel engine. Chongqing: Chongqing University, 2012.

 (3) No information on how the thermophysical properties under supercritical conditions were calculated is provided. Did the authors make any effort to validate these properties before using them in the simulations?

Response: Thank you for your careful review. Prior to this simulation, we have established and verified the supercritical fuel injection model [26], and directly quoted the established supercritical fuel injection model in this paper.

 (4) No information on what chemical mechanism was used is provided. Also, what is the fuel surrogate used for the gasoline-diesel mixture?

Response: Thank you for your careful review. Many researchers use n-heptane instead of diesel in their numerical simulations when the diesel supercritical parameters are unknown[32]，so in this simulation, the alternative fuels for gasoline and diesel were isooctane and n-heptane, respectively. And the chemical mechanism used is the single component chemical mechanism reaction of gasoline provided in the AVL-FIRE software.

(5) No information about the spray models is provided. No validation of spray models is included.

Response: The validation of the spray model is also completed by the research group in the early stage. To validate the model accuracy of the spray model, the supercritical nitrogen environment was studied by numerical simulation. The results shows relative error of the jet length is less than 5%, whereas that of the divergence angle is less than 8%. Both values meet the requirements for the accuracy of the calculation simulation [26].

 (6) The authors mention the max grid size used in the simulations but do not provide any details about the min grid size. The authors should also show the computational domain. Is it a sector mesh or full engine geometry?

Response: The grid automatically generated in AVL-FIRE software is unstructured. The unstructured grid is easy to fit the boundary of the region and is suitable for the calculation of fluid and surface stress concentration. According to the selected engine structure, one eighth (45° sector) of the entire combustion chamber was selected as the calculation area.

 

 

(7) Matching pressure trace in Fig. 2 is not sufficient. The authors should also compare the apparent heat release rates from experiment and simulation.

Response: I'm sorry to tell you that there is no experimental data of heat release rate data in this operating condition, and it is too late to calculate the new operating condition, therefore, the comparison between the spray simulation calculation and experiment conducted by our research group was quoted to prove the accuracy of this simulation. Relevant information can be found in the reference cited in this paper.

 

Reviewer 2 Report

The authors present a study on numerical simulation of the combustion characteristics in an internal combustion engine. Studies of this kind can significantly contribute to the development of more efficient combustion engines. In the face of the climate crisis and the transition to a sustainable product cycle, this is a very relevant and interesting topic. The manuscript is well written, the methods used seem to be sound, and the authors clearly communicate the results which they have obtained. I can therefore recommend the publication of this manuscript in Applied Sciences.

However, I have a few suggestions how the manuscript can be further improved. I do not need to review the manuscript again after the authors have considered my comments.

(*) It would be interesting for the reader to add some details on the numerical simulation method. It is stated that the "extended coherent flamelet model" is applied, but there are no citations, and no further information is provided. It would be good to have a few sentences in the manuscript which describe this model. Is it based on finite elements in space? Is the spatial grid resolution uniform, or does the model feature an adaptive resolution (i.e., finer grid in regions with large turbulence)? Does it work with finite differences in time, i.e. time steps? How large was the chosen simulation time step?

On the next page, the grid resolution is discussed (Figure 1 and text above), but without a description of the type of grid which is used (Euclidic? uniform?), these numbers are not too useful.

(*) Similar arguments hold true for the prediction of the exhaust composition. The authors state that they used the Zeldovich model for NO generation and the Kennedy/Hiroyasu/Magnussen model for soot emission. There should be references for these models, and a short outline should be added to the manuscript, describing in a few sentences the fundamentals of the models.

(*) For the results obtained in this study, it is certainly important which type of fuel was used in the simulation, i.e. which chemical compounds were contained in which mixing ratio. Was it pure n-octane in case of the gasoline fuel? What was the chemical composition of the diesel fuel used? What is the heat of combustion per unit of both fuel types? This information should be added to the manuscript.

(*) Please use an uppercase "K" in the left part of Figure 4 to indicate the unit "Kelvin".

 

Author Response

Dear Editor and Reviewers,

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Numerical Investigation of the Combustion Characteristics of an Internal Combustion Engine with Subcritical and Supercritical Fuel”. These comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as follows:

Responds to the Reviewer(s)' Comments:

(1)It would be interesting for the reader to add some details on the numerical simulation method. It is stated that the "extended coherent flamelet model" is applied, but there are no citations, and no further information is provided. It would be good to have a few sentences in the manuscript which describe this model. Is it based on finite elements in space? Is the spatial grid resolution uniform, or does the model feature an adaptive resolution (i.e., finer grid in regions with large turbulence)? Does it work with finite differences in time, i.e. time steps? How large was the chosen simulation time step?

On the next page, the grid resolution is discussed (Figure 1 and text above), but without a description of the type of grid which is used (Euclidic? uniform?), these numbers are not too useful.

Response: Thank you for your careful review. Additional information is as follows as well as in the revised paper:

ECFM is an extension of the coherent flamelet model. To determine the flame surface density, the ECFM uses the fuel/air equivalence ratio in fresh gases, the composition (including residual gases), and the temperature near the flame. The resulting flame surface density is used to describe large scale burned/unburned stratification. And this model is fully coupled to the spray model and enables stratified combustion modeling including EGR effects and NO formation. The grid automatically generated in AVL-FIRE software is unstructured. The unstructured grid is easy to fit the boundary of the region and is suitable for the calculation of fluid and surface stress concentration. According to the selected engine structure, one eighth (45° sector) of the entire combustion chamber was selected as the calculation area.

(2)Similar arguments hold true for the prediction of the exhaust composition. The authors state that they used the Zeldovich model for NO generation and the Kennedy/Hiroyasu/Magnussen model for soot emission. There should be references for these models, and a short outline should be added to the manuscript, describing in a few sentences the fundamentals of the models.

Response: Thank you for your careful review. Additional information is as follows:

The mechanism of the Zeldovich model is：

And the reference about the soot model has been added in the revised paper:

[29] Nagle, J. and Strickland-Constable, R.F., "Oxidation of Carbon Between 1000-2000C," Proceedings of the Fifth Carbon Conference, Volume 1, Pergammon Press, 1962.

(4) For the results obtained in this study, it is certainly important which type of fuel was used in the simulation, i.e. which chemical compounds were contained in which mixing ratio. Was it pure n-octane in case of the gasoline fuel? What was the chemical composition of the diesel fuel used? What is the heat of combustion per unit of both fuel types? This information should be added to the manuscript.

Response: Thank you for your careful review. In this simulation, the alternative fuel for gasoline is isooctane. Many researchers use n-heptane instead of diesel in their numerical simulations when the diesel supercritical parameters are unknown. The temperature and pressure of n-heptane are 540.3 K and 2.74 MPa [30]. And the heat of combustion per unit of isooctane and n-heptane (-537.9705KJ/Kg and -1536.5545KJ/Kg, respectively) have been added in the revised paper.

(5) Please use an uppercase "K" in the left part of Figure 4 to indicate the unit "Kelvin".

Response: Thank you for your careful review. This minor concern has been corrected in the revised paper.

Reviewer 3 Report

The topic is of general interest, and the article adheres to journal standard, after revision This article contains new aspects, but the authors must underline the major findings of their work and explain the progress comparatively with other published papers. Title reflect the contents of the paper. The Abstract must be rewrite and the authors must present major results. The key words permit found article in the current registers or indexes. Please put in alphabetical order. The Introduction it is OK. The presentation not reflects the present state of knowledge; references from last years aren’t presented. The authors presented 1 reference from 2016, 2017, and 2018, but the manuscript will be published in 2020. The text is relatively easy to understand by scientists in other disciplines. The figures have good quality. Please provide statistical analysis. Please present comparison with other studies, if is applicable. Please in the conclusions are presented some results, comparatively. The authors not present information about software license. Please verify all References, and respect the guide of authors, there are small mistakes. Missing minimum 2 references from Applied Sciences (2018-2019) for demonstrated that article is in journal topics.

Comments for author File: Comments.doc

Author Response

The topic is of general interest, and the article adheres to journal standard, after revision this article contains new aspects, but the authors must underline the major findings of their work and explain the progress comparatively with other published papers. Title reflect the contents of the paper. The Abstract must be rewrite and the authors must present major results. The key words permit found article in the current registers or indexes. Please put in alphabetical order. The Introduction it is OK. The presentation not reflects the present state of knowledge; references from last years aren’t presented. The authors presented 1 reference from 2016, 2017, and 2018, but the manuscript will be published in 2020. The text is relatively easy to understand by scientists in other disciplines. The figures have good quality. Please provide statistical analysis. Please present comparison with other studies, if is applicable. Please in the conclusions are presented some results, comparatively. The authors not present information about software license. Please verify all References, and respect the guide of authors, there are small mistakes. Missing minimum 2 references from Applied Sciences (2018-2019) for demonstrated that article is in journal topics.

Response: Thank you for your careful review.

The major findings of our work has been writing in Featured Application We have rewrote the abstract and major results have presented. The key words have been put in alphabetical order. The conclusions have been presented some results between supercritical fuel and subcritical fuel, comparatively. Reference from last year is presented. All references have been verified. Some key references have been added. The research project involved in manuscript was in cooperation with the state key laboratory of engine (Tianjin university), and the simulation in the manuscript was also done there. The state key laboratory of engine (Tianjin university) can provide the software license of relevant software.

Round 2

Reviewer 1 Report

The authors made the changes to address reviewer comments. 

 

Regarding author response to reviewer comment (7), it should be noted that apparent heat release rate does not need to be measured. It can be calculated from the pressure trace using the formula in Heywood' book. The authors should, therefore, calculate the heat release rates from experiments and simulations, and compare them. 

Author Response

Dear Editor and Reviewers,

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Numerical Investigation of the Combustion Characteristics of an Internal Combustion Engine with Subcritical and Supercritical Fuel”. These comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The main corrections in the paper and the responds to the reviewer’s comments are as follows:

Responds to the Reviewer(2)' Comments:

     Thank you for your valuable suggestions. We have consulted relevant formula and calculated the heat release rate of the experiment and simulation from the pressure trace under this operating condition, and added the comparison figure in the revised paper. Thanks again for your help.

     We tried our best to improve the manuscript and made some changes in it. These changes will not influence the content and framework of the paper.

We appreciate for editors and reviewers’s warm.

Author Response File: Author Response.pdf