Numerical Investigation on the Combustion and Emission Characteristics of Diesel Engine with Flexible Fuel Injection

Round 1

Reviewer 1 Report

1.      The analysis in the abstract is mostly qualitative analysis, and the quantitative analysis results are lacking.

2.      What do the authors mean by "ultrahigh injection pressure"? 150 MPa injection pressure is a fuel line pressure currently used in today's common rail diesel engines. This makes the word ultrahigh questionable!

3.      More recent studies should be included in the introduction to reduce NOx and particulate emissions, which are the most important disadvantages of diesel engines. For instance; https://doi.org/10.1177/0958305X20960701

4.      The numerical model is not adequately explained. The use of the popular Lagrangian-parcel Eulerian-fluid method is not a sufficient justification for the readers to perceive the numerical model. Government equations, boundary conditions, and assumptions related to the numeric model should be added to the paper.

5.      The paper presents the experimentally obtained visual and numerical results for validation of the simulation results. However, no information and figures about the experimental setup and equipment from which these experimental results were obtained are included. Question: Were these experimental studies carried out within the scope of the paper? If it's the results of another paper, why weren't they cited? It creates serious confusion.

6.      In addition, to prove the validity of the numerical model, the relationship between the experimental results and the experimental results should be expressed numerically.

7.      Was it not possible to verify the simulation results with experimental results for both cases? Please indicate your limitations for this topic.

8.      Increase the visibility and legibility of numerical expressions in figures. In many Figures.

9.      The authors used many mysterious abbreviations in the paper. Mysterious because many of them are not given full names. For instance; H2RECT, L2RECT, L2RECTLt.

10.   Use the degree expression instead of the symbol "°" for spray angel.

11.   After giving the mass flow rate values comparatively in Figure 4, why did you feel the need to give them separately again? Here, it is sufficient to give a more comprehensive comparison of the mass flow rate values. Also, in practice, it is not possible to create such a sharp profile structure (rectangular or square) for the injected fuel mass. This makes the simulation results far from usable in practice. The authors should give a satisfactory explanation of this topic.

12.   Why is it necessary to give mass flow rate curves in each figure? Then, the same curves are repeated many times. This unnecessarily longs the paper.

13.   Deep and mechanistic discussions are required to explain the results obtained. In the section "Results and Discussion", the study's comparative analysis with others must be thoroughly discussed. The findings of the paper should be described with quantitative analysis results, such as the percent or numerical results.

14.   The conclusion is insufficient to summarize the findings of the article. Numerical results should be added in the conclusion section. In addition, the conclusion section is missing some perspectives related to future research work.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors made an interesting discussion about the injection strategy, investigated the combustion and emission characteristics under the flexible fuel injection and explored an optimal injection strategy for high-efficiency combustion. The simulation results show that the combustion efficiency has been improved whilst the NOx formations have been reduced. The paper is well-written and organized; however, some issues need to be addressed.

1. the authors conducted several simulations, the results seem close to the experimental data; however, the simulation has not been verified by grid independence, so the authenticity and effectiveness are worthy of consideration.

2. The description of spray behavior is not clear enough, what is the meaning of the blue line in Fig.2(a)?

3. In section 3.2, the effects of injection rate and interval were discussed, Why does Fig 5 introduce the line chart of injection Angle? Is it a bit illogical?

4.    Why are the (a) and (b) pictures in FIG. 6 different in horizontal and vertical? For the sake of neat layout, it is suggested to unify horizontal or vertical.

 

5.    FIG. 7 does not indicate a and b, but it says FIG. 7 (b) in line 294. Is it inconsistent?

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors,

the article presents an interesting study of fuel injection strategy and its effect on engine performance and toxic emissions. The paper is written in a good style and includes adequate sections. The analysis of the results could have been a bit more elaborate especially toxic emissions, since it appears in the title of the article. I recommend the article for publication, but I suggest that some information need to be supplemented (listed below).

Numerical setup:

1)      Line 192: Please add more information about SAGE software (0 or 1 D, chemical mechanism, etc.) Which kinetic mechanism was used for the SAGE combustion modelling?

2)      Line 200: Please, specify criteria for the grid adaptation method and range of the grid elements after the adaptation.

3)      Please, specify how high is a percentage share of solved TKE for this LES simulation? Based on the contours and the validation chart there are some places where discrepancy is present.

 Results and discussion

1)      Line 216: Based on the data shown in Figure 2a, the authors indicate good agreement between simulation and experimental results. Please add numerical values showing differences in relative or absolute values.

2)      Figure 3: What phenomenon can be explained by the different nature of the HRR pattern for the simulation and experimental results, especially for Case 1 in the initial injection area around the 0.5-1 ms.

3)      Figure 9a (NOx emission) presents interesting results. Did the authors analyze what phenomena occur in fuel oxidation and nitrogen oxide formation/reduction especially in the second stage of combustion. Is the lack of increase in NO emissions for H2RECT due to temperature reduction or perhaps, for example, the reduction process as reburning.

 

4)      Figure 9a: Please add unit for NOx emission (%, ppm Vol or mass?)

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Accept as is