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Appl. Sci.2026, 16(1), 77;https://doi.org/10.3390/app16010077 
(registering DOI)
by
  • Diego Contreras1,
  • Luis Miguel García-Cuevas1 and
  • Francisco José Arnau1,*
  • et al.

Reviewer 1: Anna Zylka Reviewer 2: Hao Guo

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the performance of a spark-ignition oxy-fuel combustion engine equipped with an in-situ oxygen production system using a MIEC membrane. The study analyses part-load behaviour and altitude effects for two compression ratios. The topic is relevant, the paper is clearly structured, and the results are coherent with the modelling approach. Overall, the manuscript is technically sound and fits well within the scope of Applied Sciences.

Before publication, several minor issues should be addressed to improve clarity and presentation. These include small editorial corrections, clarification of selected descriptions, improved consistency in notation, and enhanced readability of some figures. Detailed minor comments are listed below:

  1. There is some inconsistency in the naming and numbering of system components between Figure 1 and Figure 2. The same elements are occasionally referred to using different labels or notation styles. For example, the component marked as a compressor in Figure 1 is described using a different naming convention in Figure 2. Unifying the terminology across figures and text would improve clarity.
  2. The formatting of Equations (1)–(6) is not fully consistent. I recommend unifying the style of the equation descriptions (capitalization, spacing between symbols) and ensuring uniform line spacing around the equations to improve readability.
  3. Starting from Figure 3 onward, several figures contain contour labels or numerical values with relatively low contrast against the background colour gradients, which makes them difficult to read. Increasing the contrast or slightly adjusting label placement would improve the overall clarity of the graphical results.
  4. The Conclusions section is overly long and contains many descriptive details that repeat material from the Results. It lacks a concise summary of the main findings and does not clearly separate conclusions from discussion or operational observations. As a result, the section loses focus and does not fulfil the typical role of a brief, synthesizing conclusion.
  5. The Conclusions section does not explicitly summarise the main limitations of the study. Although several operational constraints are discussed throughout the manuscript, these limitations are not clearly synthesised in the final section. The authors are encouraged to add 1–2 sentences explicitly outlining the key limitations of the study.

Author Response

See the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

  1. Figure 1 and Figure 2 Integration: Figure 1 provides a good system overview, and Figure 2 details the heat exchanger network (HEN). To improve readability, consider labeling the heat exchangers in Figure 1 (e.g., HE6, HE7, etc.) to match their labels in Figure 2, creating a clearer link between the two figures.

  2. Definition of I*: The definition of I* in Equation 4 is mathematically correct but complex. Consider adding a brief sentence immediately after Equation 4 to explain its physical significance in plain language. For example: "Therefore, a higher I* value indicates a more favorable balance between the engine's brake power and the energy required per unit of oxygen produced, signifying a more self-sustaining system."

  3. Reference Consistency: Reference [27?] on line 165 contains a question mark, which appears to be a typo. Please verify and correct the citation.

  4. Clarity on the Primary Limiting Factor at Part-Load: The conclusion identifies two distinct limits for part-load operation: an energy availability limit (I* < 0.3) and a physical backflow risk (intake pressure below atmospheric). However, the discussion would benefit from a clearer hierarchy or interaction between these two limits. Is the energy limit (I* < 0.3) the first constraint encountered as load is reduced, or does the backflow risk become critical earlier? The text states that for CR9.6, the minimum load is 40%, and for CR20, it is 50%, but it's not explicitly stated which of the two identified mechanisms (energy or backflow) is the dominant factor for each case. A brief analysis or a summary table indicating the limiting mechanism for each CR would strengthen the conclusion.

Author Response

See the attached file

Author Response File: Author Response.pdf