Review Reports

Round 1

Reviewer 1 Report

Interesting paper, but there are some shortcomings that should be corrected before publication.

 Abstract:  “The effective thermal efficiency reaches 41.04% after optimization” what was before, 41.03% or 41.05%? Are you sure that the accuracy of this result is as big? I do not thing so. If not it should be properly rounded.

English seems to be good but the are some minor doubts. Some correction in English should be made. E.g. the last by one sentence of the Introduction is not clear.

All acronyms should be explained at the first appearance. Additionally, a list of acronyms would also be helpful. You should also avoid acronyms in Conclusions, if they are not absolutely obvious. On the other hand, I suggest to use acronyms  in the first line of tab. 5.

The terms: optimization and verification used many times in the manuscript eg. “the intake port of the SI NG engine is optimized” seem to be unjustified. The comparison of the two versions cannot be called optimization. The same with verification. The paper does not present any  comparison of the results of simulations and measurements. Thus anything was verified. It should be corrected through out of the text or supplemented.

Tab. 1: should be „rod” not „nod”.

“Based on bench test data, Fig. 4 compares SIP and MFIP in cycle variation and the effective thermal efficiency under different operating conditions. It can be found that MFIP helps to improve the stability of the engine.” Practically the same content is above this paragraph.

SR, TR, engine stability, cycle variation should be defined. There are some figures but it should be given how they were calculated.

“the tumble flow direction is orthogonal to the spark plug axis” and other descriptions of flow (something is pulled, folded, squished and so on) sometimes are not clear.  A drawing or drawings would be helpful.

There are 2 operating points: 1600 Nm at 1300 rpm and 2300 Nm at 1300 rpm. 1) Is it ok? Because max engine torque is 2000 Nm (see tab. 1). 2) 1600 Nm probably out of 2000 Nm, and even out of 2300 Nm can hardly be called low load (see 2 lines above Fig. 4).  

What is the accuracy of determined fuel consumption (should be specific fuel consumption)  and efficiencies given in Table 5? Is there a significant difference between them for SIP and MFIP?

Fig. 7: it should be clearly marked which data are for SIP and which for MFIP.

“It has been verified by bench tests that the thermal efficiency of MFIP reaches 41.04%, which is 0.9%~1.2% more elevated than SIP.” 1) What was verified? Any model, simulations? 2) 41.04% accuracy? 3) Compare this with values of thermal efficiencies given in Tab. 5. 4) the thermal efficiency … is more elevated than SIP? I don’t think this is correct English. If I am right, such mistake should be corrected in many places in the manuscript.

 Generally, all terms should be precisely used and data correctly given.  

Conclusions rather should not contain statements which were not presented/discussed in the text, e.g.  “ the flame front is more extensive, which expands the contact area between the burned and unburned mixture and improves the combustion speed”.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Notes in the file

Comments for author File: Comments.docx

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The studies presented in the publication related to the influences of intake port structure on the performance of a Spark-Ignited Natural Gas Engine. The topic is relevant and fully corresponds to the subject matter of the Energies journal.
The material of the article is presented in a logical sequence; in general, adhering to the recommendations of the journal, the research results are highlighted clearly and reasonably. At the same time, some separate structural additions and a number of clarifications of technological aspects of the article material should be made:
- The abstract section is too long and it should be revised reporting only the main key messages avoiding repetition or some information that can be found easily in the test methodology section. It should be about 150-250 words with concise text in a single paragraph. Answer the questions: What problem did you study and why is it important? What methods did you use? What were your main results? And what conclusions can you draw from your results?
- Please check all the manuscripts in particular when showing numbers, please use the correct dimension unit for example "thermal efficiency reaches 41.0%" instead of 41.04%.
- Most of the ideas written were already described in many kinds of literature. The Authors tried to compile it but lack of the enhancement of the interrelation analysis between the references. It is advised that the authors give a deeper analysis of how these ideas become more applicative strategies so that they can contribute to the next step of implementation.
- Novelty: The authors partly stated what motivated the idea portrayed in this study, what then does this study offer beyond the recent advances made on the combined approaches? I would advise the authors to carefully carry out a close comparison as well in the discussion section to enumerate the advantages this study offers over other related works.
- The motivation of the paper is unclear, but it should be eye-catching in order to make more sense. In this regard, a separate section on motivation and contribution should be included.
- Introduction provides a great overview and introduces the topic. However, it misses the aim of the study or what is going to be done in this study. Please state clearly the aims and what this study does. However, highlight that innovative technology in the field of fuel injection systems, combustion and combined with alternative fuels are able to improve emissions and fuel economy in combination with advanced combustion concept and alternative fuels. Please take a look at the work performed by Di Blasio and J. Turner. These technologies/fuels could be used also to improve efficiency and performance. The authors could extend the introduction discussion reporting that innovative technologies could give a potential boost to the engine fuel economy and engine-out emissions reduction.
- More in-depth analysis of the author's contribution to this paper in the introduction section. I would like to see more discussion of the literature so that I can clearly identify the article relates to competing ideas.
- The language of the manuscript is fair; I would advise consulting a language editor to further polish the language of the manuscript. There are several grammatical mistakes. Please work closely with a native English speaker to refine the language of this paper.
- Challenges and future directions to improve and implement these technologies with big data analytics should be discussed.
- Further explanation of the advantages of the suggested approach should be added. What are the main positive and negative points of this approach?

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Accepts the article for publication in MDPI Energies