Assessment of Carbon Nanotubes as Ignition Boosters Under Dual-Fuel Combustion with Hydrogen-Derived Fuels

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by Gallego et al. contains the results of a study that is apparently part of an ongoing program designed to examine and evaluate the effects of carbon nanotubes (CNTs) as an additive to the base fuel on the performance and emissions of diesel engines, including dual-fuel combustion systems. The review of the relevant literature in Introduction indicates that addition of various nanomaterials has a positive but somewhat mixed effect on combustion of diesel fuels, as summarized in Table 1. It is also pointed out therein that partial replacement of a diesel fuel with hydrogen, methane, or ammonia can reduce pollutant emissions but tends to increase ignition delay (ID) and lead to combustion instability. The investigation of the effects of CNTs on combustion of a diesel fuel mixed with each of these gases, which have never been addressed previously, is therefore a new important contribution to the field of engine-related combustion.

            The equipment employed and measurement procedures are appropriate. However, it is advisable to explain why the intersection of zero with the secant line connecting one-half and one-quarter of the maximum pressure derivative (dp/dt) was chosen to determine ID (Fig. 1), because it is a rather uncommon definition (e.g., see [41]).

            The paper can be accepted without major revisions, but the following remarks and questions should be paid attention to before publication:

Line 108: The cited publication by Mei et al. [16] is not found in References.

Line 224: The missing reference source must be inserted.

Line 225: The authors need to be more careful with abbreviations: some are not explained, while others are expanded several times. For example, ID first appears on line 25 and is spelled out on line 225.

Line 236: Table 3 does not reflect all the experiments described below.

Lines 334 and 341: Should the phrases peak pressure and pressure curves be interpreted as peak pressure gradient and pressure gradient curves, respectively?

Lines 359 and 364: Is Fig. 5 the reference source omitted in both cases?

Lines 369 and 384: The missing reference sources must be inserted.

Section 3. Results and discussion is followed directly by 5. Conclusions instead of 4.

The figures should be moved closer to their descriptions; their current positions make the text difficult to read.

It would be useful to supplement the study with an analysis of exhaust products for the presence of nanomaterial residues.

It is better to avoid the word "fundamental" giving appreciation to own work.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

• The authors claim that the effect of CNTs diminishes at high temperatures because accelerated chemical kinetics counteracts the influence of nanomaterials; that CNTs act as a heat sink to reduce local temperature and inhibit ignition; and that CNTs trap fuel droplets and impede evaporation. Can these conclusions be further supported by experimental mechanism validation or theoretical/modeling analysis?  
• Is there any potential randomness or contingency in using only SDBS as the sole surfactant without comparison with other dispersants?  
• The authors state that “CNTs offer no benefits at elevated temperatures”. Could the authors provide a threshold temperature or critical temperature for this effect, to better support engineering applications?

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript reports a carefully designed constant-volume combustion chamber (CVCC) study that examines whether pristine multi-walled carbon nanotubes can act as an ignition enhancer when diesel is used as the pilot fuel in dual-fuel operation with methane, hydrogen, or ammonia. The work fills an important methodological gap by moving from engine-level tests to a controlled phenomenological platform. Overall, the study is technically sound, timely, and provides useful negative results that temper enthusiasm for nanomaterial additives in dual-fuel systems. With corrections to several editorial and formatting issues, the manuscript meets the standards of a reputable journal such as Processes.

1. The work is incremental relative to the authors’ prior publications (refs. 29 and 39). A clearer one- or two-sentence statement in the Introduction that distinguishes the present CVCC dual-fuel study from those engine and single-droplet investigations would help readers appreciate the specific advance.
2. Abstract: The conclusion sentence is strong; consider moving the phrase “practical benefits … remain limited” to the first half so readers immediately grasp the main outcome.
3. Figure 3 caption, Figure 5 caption, and mismatched table titles (Table 2 repeats the title of Table 1).
4. The authors report 15 replicates and 95% confidence intervals, but many differences in ID fall within overlapping intervals. Using pairwise t-tests, ANOVA, or effect-size reporting would let readers judge significance more clearly, rather than just visually inspecting error bars.
5. The droplet-trapping hypothesis is plausible and well-linked to the literature, yet it remains largely inferential. Because the CVCC lacks optical access, there is no direct evidence (e.g., high-speed imaging, schlieren, or droplet-size measurements) that CNTs physically entrap diesel droplets. The authors should either (a) acknowledge this limitation more explicitly or (b) propose targeted follow-on experiments to test the hypothesis.
6. The temperatures bracket the standard DCN test point (590 °C), but modern CI engines often reach 800–900 K. The authors acknowledge this discrepancy, but including a quantitative comparison (e.g., in-cylinder temperature at SOC versus CVCC wall temperature) and a brief discussion of scaling would better connect with practical engine operation.
7. Language and style: The English is generally clear but contains occasional awkward phrasing (e.g., “the impact of CNTs on ID is negligible and highly temperature dependent”; “vanished at higher temperatures”). A native-speaker proofread is recommended.
8. Table 2 carries an erroneous title and should be relabelled “Properties of the test fuels”. The WSD increase (187.5 → 296.4 μm) warrants a brief comment on potential implications for injector wear.
9. Conclusions are labelled “5.” immediately after “3. Results and discussion”; section 4 appears to be missing.
10. Beyond long-term dispersion stability, what additional criteria (cost, catalytic activity, commercial availability) guided the selection of exactly 100 mg/L CNTs and 200 mg/L SDBS?
11. How was the mass of each gaseous fuel (CH₄, H₂, NH₃) precisely quantified and verified to achieve the target energy-substitution ratios?
12. How do the authors expect the observed ID and pressure trends to translate to a real engine environment that includes turbulence, wall heat transfer, and multiple injections?
13. The wear-scar diameter (WSD) increased markedly with CNTs; do the authors anticipate long-term injector or fuel-filter fouling issues in engine operation?
14. Have preliminary tests been conducted with single-walled CNTs, functionalized CNTs, or alternative surfactants that might alter the droplet-trapping behavior?

 

Comments on the Quality of English Language

Profesional english editing is required

Author Response

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Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised version of the manuscript could be published in Processes journal.

Author Response

Thank you

Reviewer 2 Report

Comments and Suggestions for Authors

Why are the accuracies of physical property data different? What are the respective uncertainties?

 

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript carefully, and it can be accepted after minor English language revision. Some of the points given below:

1. Section 3.1 (p. 8): “CNTs does not enhance ignition delay process” should be “CNTs do not enhance the ignition-delay process."
2. Section 3.2 (p. 9): “This delay prolongs the evaporation process and thus extends the evaporation process.” Here,  delete the redundant second phrase (“This delay prolongs the evaporation process …”).
3. Abstract (final sentence):
• “in diesel engine operating” → “in diesel engines operating” (or “in a diesel engine operating”).
• Rephrase for clarity and formality: “the main engine-related phenomena which could be affected, which is autoignition, is not really enhanced.” e.g.: “…the main engine-related phenomenon that could be affected by autoignition is not enhanced.”
4. Minor style notes (optional but recommended for polish):
• Standardize “dual-mode” / “dual fuel” / “dual-fuel operation” throughout.
• One awkward sentence in results (“a reduction of approximately 3% of the pressure peak”) could be written as “a reduction in the pressure-peak value of approximately 3 %”.
• Cover letter itself: “has been proofreading and editing” could be written as “has been proofread and edited." 

Author Response

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