Review Reports - Experimental Study of Air and EGR Dilution in a Pre-Chamber Spark-Ignited Engine Fueled by Methane

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study, through systematic experiments and simulations, deeply compared the application effects of air dilution and EGR dilution in methane PCSI engines. The research approach is clear, and the core conclusions have significant engineering significance. Although there are some deficiencies in certain aspects, it can be published after revision and improvement.

The details of the experimental setup are incomplete. The article only mentions that the engine is a modified Hatz 1D81Z single-cylinder air-cooled machine, and the compression ratio can be adjusted through the cylinder liner support ring and cylinder gasket. However, the specific adjustment accuracy of the compression ratio in the experiment is not specified. The compression ratio has a significant impact on the engine combustion process, and the lack of accuracy information may affect the repeatability of the experimental results.

The study did not mention the number of experimental repetitions for each working condition, nor did it provide the measurement error range of key parameters. Therefore, it is impossible to determine the stability and reliability of the experimental data.
The article points out that the 0D/1D numerical model has been widely verified based on previous research, but it does not clarify the verification method of the model in this experiment, such as which key operating points were selected for the comparison of experimental data and simulation results, nor does it provide the degree of consistency of the comparison parameters. Therefore, it is impossible to determine the applicability of the model under the conditions of this experiment.
The conclusion mentioned that "air dilution has the highest indicated efficiency under all operating conditions", but under the WOT operating condition, the highest indicated efficiency of air dilution (39.5%, λ=1.4) differs very little (only 0.1%) from that of EGR dilution (39.4%, EGR=19.9%). Without specifying whether the difference is within the measurement error range, directly determining that "air dilution is the highest" may not be rigorous enough. Supplementary error analysis is needed to verify the significance of the difference.
Please add the research background in the abstract.
What does the subscript of λ mean? It is not specified in the paper.

Comments on the Quality of English Language

Some figure captions need improvement.

Author Response

The details of the experimental setup are incomplete. The article only mentions that the engine is a modified Hatz 1D81Z single-cylinder air-cooled machine, and the compression ratio can be adjusted through the cylinder liner support ring and cylinder gasket. However, the specific adjustment accuracy of the compression ratio in the experiment is not specified. The compression ratio has a significant impact on the engine combustion process, and the lack of accuracy information may affect the repeatability of the experimental results.

Response: We are grateful to the reviewer for this comment. The compression ratio (CR) of the modified Hatz 1D81Z engine can be adjusted by using cylinder gaskets of different thicknesses. The volumes of specific parts of the engine were measured very precisely, piston bowl, prechamber volume, crevice Reagan. The distance between piston at TDC and cylinder head is the only variable part that is changed by cylinder gaskets thickness. Since the thickness of the gasket varies in mounted (compressed) state from unloaded conditions there is some uncertainty in obtained CR, which changes as the compression ratio changes. At CR016 the estimated uncertainty is 0.15. However, once the head was mounted it was not taken off during the entire design of experiment. Before each day of measurement motored pressure traces at predefined boundary conditions were taken to check the whole system. Therefore, the repeatability of conditions related to compression ratio are ensured. To clarify this in manuscript a more detailed explanation of the method for achieving the compression ratio has been included in the revised manuscript.: “The compression ratio of the engine can be varied by using cylinder support rings and head gaskets of different thickness which changes the distance between the piston top and cylinder head. Since the measured thickness in unloaded state varies from the thickness when the gasket is mounted the calculated uncertainty of compression ratio obtained in this way at compression ratio equal 16 is 0.15. During the entire experimental measurement, the engine was not dismantled, and the cylinder head gasket was kept inside the whole time. Also, before each day of measurement, motored pressure traces at predefined boundary conditions were taken to check the whole system including the compression ratio. Therefore, even though there is some uncertainty regarding the exact value of compression ratio it was kept constant during the entire experiment.”

The study did not mention the number of experimental repetitions for each working condition, nor did it provide the measurement error range of key parameters. Therefore, it is impossible to determine the stability and reliability of the experimental data.

Response: The authors appreciate the reviewer’s comment. Previous research* conducted using similar equipment has shown that the uncertainties of the derived values (IMEP, indicated efficiency, etc.) are very small (e.g. 0.48% points for indicated efficiency). Therefore, error bars are not shown in the figures presented in this work since they would not be noticeable. To further clarify how measurement uncertainty is minimized, the following sentence has been added to the manuscript: “The measurement is conducted by sampling low speed data of each operating point at a frequency of 1 Hz over a duration of 60 seconds, and by then averaging the results. For high-speed data (crank angle resolved data, e.g. cylinder pressure) the pressure profiles are sampled with frequency of 0.5°CA during the whole cycle and 0.1°CA dur-ing combustion over period of 300 cycles and then cycle averaged. The method was previously analyzed [27] and measurement uncertainty of 0.48 % points for in-dicated efficiency is calculated using standardized methods.”

*Kozarac, D., Taritas, I., Vuilleumier, D., Saxena, S., and Dibble, R.W., “Experimental and Numerical Analysis of the Performance and Exhaust Gas Emissions of a Biogas/n-Heptane Fueled HCCI Engine,” Energy 115:180- 193, 2016, doi:10.1016/j.energy.2016.08.055.

The article points out that the 0D/1D numerical model has been widely verified based on previous research, but it does not clarify the verification method of the model in this experiment, such as which key operating points were selected for the comparison of experimental data and simulation results, nor does it provide the degree of consistency of the comparison parameters. Therefore, it is impossible to determine the applicability of the model under the conditions of this experiment.

Response: The authors thank the reviewer for pointing out this unintentional oversight. To clarify, the revised manuscript now includes both a sentence and a figure that provide a more detailed explanation: “Each operating point was calibrated individually by adjusting the simulated main chamber and pre-chamber pressure profiles to match the corresponding experimental pressure profile and the model parameters (such as fuel flow, heat transfer, temperatures etc.) were tuned accordingly.”

The conclusion mentioned that "air dilution has the highest indicated efficiency under all operating conditions", but under the WOT operating condition, the highest indicated efficiency of air dilution (39.5%, λ=1.4) differs very little (only 0.1%) from that of EGR dilution (39.4%, EGR=19.9%). Without specifying whether the difference is within the measurement error range, directly determining that "air dilution is the highest" may not be rigorous enough. Supplementary error analysis is needed to verify the significance of the difference.

Response: The WOT operating condition does not provide a direct comparison under the same combustion characteristics, such as equal load. The first set of experiments served only as an initial study, aimed at providing insights into combustion parameters at different loads and mixture dilutions, both of which influence indicated efficiency and therefore cannot be directly compared. To eliminate the impact of varying mixture dilution and load, the subsequent two sets of experiments were carried out under the same load. The main conclusion was therefore drawn from these latter operating points. This has already been clarified in the manuscript, where it is stated: “It has to be noted that for IMEP =7.1 the excess air ratio in air dilution case is not optimal so the difference between air and EGR dilution might be caused by non-optimal operation. Therefore, the following two sets of experiments were performed. From the first set of experiments it could be concluded that the operation with EGR dilution while maintaining λglobal λ = 1.0, is possible and that in this setup the highest EGR level was around 20% when misfires start to occur.”

Please add the research background in the abstract.

Response: The research background was added to the abstract with the following sentence: “Improving the efficiency of spark-ignited (SI) engines while simultaneously reducing emissions remains a critical challenge in meeting global energy demands and increasingly stringent environmental regulations.”

What does the subscript of λ mean? It is not specified in the paper.

Response: λ_global was originally used to denote the average excess air ratio of the main chamber and the pre-chamber mixture measured at the exhaust pipe, while λ_PC refers to the excess air ratio in the pre-chamber at a specific spark timing (since this value depends on spark timing and PC DOI and is obtained from simulation). To avoid confusion, we have revised the text by consistently using only λ_global, as λ and λ_global describe the same quantity. The sentence with explanation is now added to the manuscript: “λ_global is used to denote the average excess air ratio of the main chamber and the pre-chamber mixture measured at the exhaust pipe.”

Comments on the Quality of English Language:

Some figure captions need improvement.

Thank You for the observation. The captions of the following figures have now been improved: Figures 2, 4, 5, 6, 8.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a valuable and well-structured experimental comparison of air and EGR dilution in a methane-fueled pre-chamber spark-ignited engine. The research is relevant to the field, addressing the important goal of improving engine efficiency while managing emissions. The study's strength lies in its carefully designed experiments across different engine loads, complemented by numerical simulations that provide deeper insights into pre-chamber phenomena. However, the paper requires significant revisions to enhance clarity, strengthen the analysis, and address several key omissions. The revision could focus on improving the methodological description, clarifying the data presentation, and expanding the discussion to more robustly support the conclusions drawn.

1. The asymmetry of the comparison framework and the unrealized potential of EGR dilution

The study's framework creates an unfair comparison because the air dilution method was tested to its peak efficiency, whereas the EGR method was stopped prematurely due to a misfire limit. While air dilution achieves a higher indicated efficiency in these tests, the EGR approach shows a consistent upward trend in efficiency right up to its operational limit, suggesting it has unrealized potential. The conclusion that air dilution is superior is therefore misleading, as it compares an optimized method against a constrained one; a more balanced discussion is needed to acknowledge that EGR could potentially be more efficient if the misfire issue, possibly through pre-chamber purging, were resolved.

2. Inconsistent experimental control and optimization of pre-chamber fueling

The experimental methodology is inconsistent regarding the optimization of pre-chamber (PC) fueling between the two dilution strategies. For the air dilution experiments, the pre-chamber duration of injection (PC DOI) was actively adjusted to maintain stable combustion as conditions changed. In contrast, for the EGR dilution experiments at WOT, the PC DOI was kept constant. This difference in control strategy introduces a significant variable, making a direct and fair comparison of the two methods' ultimate capabilities difficult.

3. Unaddressed contradiction with previous numerical work

This paper's experimental findings directly contradict the authors' own previously published numerical study, and this discrepancy is never addressed. The prior simulation work, cited as reference [20], concluded that an active pre-chamber engine with EGR dilution would achieve higher indicated efficiency compared to air dilution. However, the current experimental results consistently show air dilution to be the more efficient method. The manuscript must directly confront this contradiction, explain the potential reasons for the disparity between the simulation and real-world experiment, and discuss how these new findings revise the authors' previous conclusions.

4. Specific comments

The abstract contains too much specific numerical data, making it dense and hard to read. It should be revised to focus on the high-level findings and conclusions of the study rather than detailing the exact percentage increases in efficiency for each operating condition.
The introduction successfully establishes the context for the research but needs to better define the specific knowledge gap it aims to fill. While it mentions a lack of direct comparison between air and EGR dilution, it should more clearly articulate why this specific comparison is scientifically important and what unresolved questions it answers. Additionally, the reference to a previous numerical study by the authors ([20]) should be integrated more effectively to set up the contrast with the current experimental findings.
The methods section is missing several critical details that are necessary for reproducibility. Specifically, it needs to include the make and model of the gas analyzer, the precise formula used to calculate the EGR percentage, and a clearer explanation of the control strategy for adjusting pre-chamber injection timing and duration in different experimental sets. Clarification is also needed on how intake conditions were managed, particularly why boosting was used for air dilution at 7 bar while throttling was used for EGR.
The presentation of results is inconsistent, which confuses the reader. For instance, efficiency and emissions are sometimes shown in absolute values and other times as relative percentages, making direct comparisons difficult. The term "MBT" (Maximum Brake Torque) is used incorrectly, as the experiments were conducted at constant speed and IMEP, so it should be replaced with a more appropriate term like "Optimized Spark Timing". The analysis of the sudden misfire phenomenon is also too brief and lacks a deep physical explanation.
The main conclusion that air dilution is superior to EGR dilution is overstated and not fully supported by the presented data, as the EGR experiments were limited by misfire and not fully optimized. The conclusions should be rephrased to reflect this significant caveat and to more cautiously interpret the results. Furthermore, the suggestion to use pre-chamber air purging is a valuable idea but should be more explicitly linked back to the experimental findings as a direct solution to the observed EGR limitation.

5. Minor

The manuscript is generally well-written, but it contains several minor grammatical errors, typos, and awkward phrases that disrupt the flow. The quality of the figures is good, but some bar charts could be improved by adding data labels or error bars to enhance clarity. A thorough proofreading is recommended to correct these issues and improve overall readability.

line 98: citation [19] is unclear. It needs rephrasing
Typo: Line 257: "diluted furhet", Line 345: "EGRR dilution approach", Figure 10 caption: "Alndicated efficiency"
Ensure consistent use of terminology. For example, use either "excess air ratio" or "lambda" consistently throughout the text and figures.
The figure in table 3 is blurry, please improve it.

Author Response

We thank the reviewer for the positive and constructive comments. We have carefully addressed the points raised and hope that the revisions and our responses below have clarified all previously unfulfilled aspects of the manuscript.

The asymmetry of the comparison framework and the unrealized potential of EGR dilution

Response: Authors thank the reviewer for the comment. While we believed this point was already clearly explained, we have further clarified it in the Conclusion section with the following sentences: “However, EGR shows an increasing trend in indicated efficiency with higher EGR rates. Misfires currently limit the assessment of the full potential of the EGR dilution principle. EGR could potentially provide higher efficiency if the misfire issue were resolved, for example through pre-chamber purging.”

Inconsistent experimental control and optimization of pre-chamber fueling

Response: Thank you for pointing this out. To provide further clarification, the manuscript has been revised to include the following statement regarding the optimization of the pre-chamber fueling control strategy in the EGR dilution approach: “In the EGR dilution approach, the PC DOI was kept constant because the mixture in the main chamber was predominantly stoichiometric, which primarily influenced the occurrence of misfires, and modifying the PC DOI had little effect. To reduce misfires, pre-chamber purging should be applied.”

Unaddressed contradiction with previous numerical work

Response: The noted difference with the previous numerical study arises primarily from differences in the experimental and simulation conditions. In the prior study, the total fuel mass was kept almost constant (with variations of less than 4%) and the pre-chamber excess air ratio was fixed at λ_PC = 1.0. The excess air ratio in EGR cases was not set to stoichiometric conditions but changed at the EGR was increased. In contrast, in the experiments described in this study, λ_PC was adjusted for each operating point to ensure stable combustion. As a result, the pre-chamber excess air ratio in all experimental operating points was below 1.0, meaning more fuel was delivered to the pre-chamber, which directly affects indicated efficiency. Furthermore, in the EGR dilution approach, the excess air ratio was kept constant and stoichiometric to enable the use of the three-way catalyst.

Experimental results from the current study are comparable to the simulation results from previous study only for the specific case of CR = 16, λ_global = 1.2 at EGR = 0% and EGR = 20%, where λ_global = 1.0 was achieved. For this simulated operating point, with 20% EGR the indicated efficiency exceeded 38%, while for λ_global = 1.2 at EGR = 0% it was slightly above 36% (a 2 percentage points difference). In comparison, the experimentaly obtained indicated efficiency (this study) was 36.1% at EGR = 0% and 39.4% at EGR = 20% (a 3.3 percentage points difference) under WOT conditions and IMEP=7 bar, which is in reasonable agreement with the simulation results.

It should also be noted that the previous simulations were performed using a 0D/1D approach, which cannot predict misfires, allowing the model to explore higher EGR rates than are achievable experimentally. These factors collectively explain the observed differences between the numerical predictions and the experimental results.

To clarify this point in the manuscript, the following statement has been added: “The experimental engine's compression ratio is set at CR = 16.0, based on findings from the previous study [20], although the experimental procedure and conditions applied here differ substantially from those used in that work.”

Specific comments

The abstract contains too much specific numerical data, making it dense and hard to read. It should be revised to focus on the high-level findings and conclusions of the study rather than detailing the exact percentage increases in efficiency for each operating condition.

Response: The abstract has now been revised.

The introduction successfully establishes the context for the research but needs to better define the specific knowledge gap it aims to fill. While it mentions a lack of direct comparison between air and EGR dilution, it should more clearly articulate why this specific comparison is scientifically important and what unresolved questions it answers. Additionally, the reference to a previous numerical study by the authors ([20]) should be integrated more effectively to set up the contrast with the current experimental findings.

Response: To add more clarity why this specific comparison is scientifically important and what unresolved questions it answers, the following sentence is added to the introduction: “By directly comparing these two dilution approaches, it is possible to clarify the trade-offs between engine efficiency and emissions for each approach and suggest which dilution approach would be more favorable.”

The following sentence was added to link the previous numerical study to the current experimental: “The design of experiments and expected operating range of the engine is based on results from the previous numerical study done by the authors [20] and similar trends have now been confirmed experimentally.”

The methods section is missing several critical details that are necessary for reproducibility. Specifically, it needs to include the make and model of the gas analyzer, the precise formula used to calculate the EGR percentage, and a clearer explanation of the control strategy for adjusting pre-chamber injection timing and duration in different experimental sets. Clarification is also needed on how intake conditions were managed, particularly why boosting was used for air dilution at 7 bar while throttling was used for EGR.

Response: To give more information, next statements were added:

“THC emissions were measured with the Environnement Graphite 52M heated flame ionization detector (HFID) analyzer, CO and CO2 were measured with the Environnement MIR 2M non-dispersive infrared (NDIR) analyzer while NO_X emissions were measured by ECM NOx 5210t device.”

“The EGR percentage is determined by measuring the CO₂ concentration at the engine intake and exhaust. The ratio of the recirculated CO₂ volume fraction at the intake to the CO₂ volume fraction at the exhaust corresponds to the total fraction of recirculated exhaust gas. The formula for calculating the EGR fraction is given in the following expression:”

“The start of injection both into port and pre-chamber were set based on conclusions from the literature, as a compromise between providing sufficient time for mixture preparation in the pre-chamber and preventing injected fuel from escaping into the main chamber. PC DOI, on the other hand, is determined by the required fuel mass to be injected into the pre-chamber, since the injector is calibrated and its characteristic provides the relation between injection duration and injected fuel mass.”

“In the air dilution approach, boosting was applied in order to achieve very lean mixtures, whereas in the EGR dilution approach throttling was necessary to enable high EGR rates, i.e. a large amount of recirculated exhaust gas with a reduced intake air fraction.”

The presentation of results is inconsistent, which confuses the reader. For instance, efficiency and emissions are sometimes shown in absolute values and other times as relative percentages, making direct comparisons difficult. The term "MBT" (Maximum Brake Torque) is used incorrectly, as the experiments were conducted at constant speed and IMEP, so it should be replaced with a more appropriate term like "Optimized Spark Timing". The analysis of the sudden misfire phenomenon is also too brief and lacks a deep physical explanation.

Response: As noted in the manuscript, only the results of indicated efficiency of first set of experimental points were presented in absolute values, since it represented an initial experiment. For the other two experimental sets the results of indicated efficiency were shown in relative terms because the measurements were conducted on different days, which led to small differences in ambient conditions. To avoid confusing the reader with these variations, the results of the latter two sets were therefore presented in relative form.

Regarding the second comment, we have used the term MBT as defined in Heywood’s book Internal Combustion Engine Fundamentals (1988): “The optimum timing which gives the maximum brake torque – called maximum brake torque, or MBT, timing – occurs when the magnitudes of these two opposing trends (expansion stroke work transfer from the cylinfer gases to the piston) just offset each other. Timing which is advanced or retarded from this optimum gives lower torque.”

A detailed physical explanation of the misfire phenomenon is beyond the scope of the present study. Its investigation would require CFD analyses and optical engine experiments, which we could consider as directions for future work.

The main conclusion that air dilution is superior to EGR dilution is overstated and not fully supported by the presented data, as the EGR experiments were limited by misfire and not fully optimized. The conclusions should be rephrased to reflect this significant caveat and to more cautiously interpret the results. Furthermore, the suggestion to use pre-chamber air purging is a valuable idea but should be more explicitly linked back to the experimental findings as a direct solution to the observed EGR limitation.

Response: The authors are grateful for this valuable comment. As also noted in response to Comment 1, authors have revised the Conclusions section to more cautiously interpret the results and to acknowledge the unrealized potential of EGR dilution. Specifically, it is now stated: “However, EGR shows an increasing trend in indicated efficiency with higher EGR rates. Misfires currently limit the assessment of the full potential of the EGR dilution principle. EGR could potentially provide higher efficiency if the misfire issue were resolved, for example through pre-chamber purging.”

Minor

line 98: citation [19] is unclear. It needs rephrasing.

Response: The sentence around reference [19] is rephrased to be clearer. “In lean conditions NO_X can be lowered by lean NO_X traps or SCR catalytic converter. Both systems result in higher costs compared to a simple three-way catalyst, as the systems are more complex and they introduce additional operating cost, e.g. urea in SCR [19].”

Typo: Line 257: "diluted furhet", Line 345: "EGRR dilution approach", Figure 10 caption: "Alndicated efficiency"

Response: We thank the reviewer for pointing out typographical errors. They have now been corrected in the revised manuscript.

Ensure consistent use of terminology. For example, use either "excess air ratio" or "lambda" consistently throughout the text and figures.

Response: The term “lambda” was unintentionally used once in the expression “lambda sweep.” In the revised manuscript, this has been corrected to “excess air ratio sweep” to ensure consistency throughout the text and figures.

The figure in table 3 is blurry, please improve it.

Response: It has been improved in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors experimentally and numerically study the characteristics of methane combustion in an internal combustion engine, taking into account combustion product recirculation. They determine combustion efficiency parameters and the concentrations of individual gases.

This is a multiparameter study. The text contains a considerable number of results. The material is well-structured and relatively easy to read.

Overall, the material can be considered unique. The results may be of interest to readers studying the combustion characteristics of fuels, particularly methane, in combustion chambers. The manuscript may be accepted for publication. Some corrections are required.

Major.

All abbreviations should be defined at the first mention. This will facilitate reading. Of course, you can leave everything at the end of the article. However, this does not enhance reading comfort.

In figure and table captions, it is advisable to indicate how the data were obtained: numerically or experimentally.

Line 153. “A detailed description of the experimental equipment can be found in 153 [27].”

- Ok! Nevertheless, it's worth providing a brief description of the measurement methodology.

For example, the accuracy in determining the EGR parameter of 18.9% seems quite high (three decimal places). What instrument was used to determine the flow rate?

What are "combustion efficiency" and "highest indicated efficiency"? How were these parameters determined? What do they characterize?

Minor.

It's worth indicating in the abstract what the abbreviations EGR and IMEP define.
Line 29. "Load, for air dilution and" -> "Load for air dilution, and"
Line 65. What does the parameter "λglobal > 1.6" mean?
Line 74. "[14] [15]" -> "[14, 15]"
The figure in Table 3 should be shown in higher resolution. The legend in this figure is illegible.
There is a different font on page 6, lines 162-171.
It's also worth briefly describing the features of the numerical methodology in Section 2.2.
Line 173. What is the "CR"? I didn't find this designation in the Abbreviations section.
Line 182. "with a pressure of 50 bar" – Is this absolute pressure or overpressure?

Line 193. The same. "IMEP = 5 bar." – Is this absolute pressure or overpressure?

Line 185. "was performed from λ = 1.0 to λ = 1.8"

- This parameter should be defined in the text at the first mention.

Line 188. "EGR level was gradually increased from EGR =……%"

- by mass or by volume?

I didn't notice the information in the text. How many repetitions of the experiment with the same initial conditions were conducted?
Figure 6 shows the parameter "\Delta Indicated efficiency." What does the \Delta symbol mean?
Line 391. "Euro VI (2013) stage" – Is it possible to provide a permanent reference to the Euro VI standards?

Self-citation reaches 20%. I think that's quite a lot.

Author Response

This is a multiparameter study. The text contains a considerable number of results. The material is well-structured and relatively easy to read.

The authors sincerely thank the reviewer for the constructive and encouraging evaluation of our work. We have carefully considered all comments and implemented the suggested corrections to improve the clarity and quality of the manuscript. We hope that the revised version, together with our detailed responses, will now meet the reviewer’s expectations.

Major.

All abbreviations should be defined at the first mention. This will facilitate reading. Of course, you can leave everything at the end of the article. However, this does not enhance reading comfort.

Response: The authors thank the reviewer for careful reading and helpful suggestions. The manuscript has been revised, and all abbreviations are now defined at their first mention to improve readability.

In figure and table captions, it is advisable to indicate how the data were obtained: numerically or experimentally.

Response: The figure and table captions have now been revised.

Line 153. “A detailed description of the experimental equipment can be found in 153 [27].”

- Ok! Nevertheless, it's worth providing a brief description of the measurement methodology.

Response: More detailed description of measurement equipment is now added to the manuscript: “THC emissions were measured with the Environnement Graphite 52M heated flame ionization detector (HFID) analyzer, CO and CO2 were measured with the Environnement MIR 2M non-dispersive infrared (NDIR) analyzer while NOX emissions were measured by ECM NOx 5210t device. The measurement is conducted by sampling low speed data of each operating point at a frequency of 1 Hz over a duration of 60 seconds, and by then averaging the results. For high-speed data (crank angle resolved data, e.g. cylinder pressure) the pressure profiles is sampled with frequency of 0.5°CA during the whole cycle and 0.1°CA during combustion over period of 300 cycles and then cycle averaged. The method was previously analyzed [27] and measurement uncertainty of 0.48 % points for indicated efficiency is calculated using standardized methods.”

For example, the accuracy in determining the EGR parameter of 18.9% seems quite high (three decimal places). What instrument was used to determine the flow rate?

Response: It has now been added to the revised manuscript: “The EGR percentage is determined by measuring the CO₂ concentration at the engine intake and exhaust. The ratio of the recirculated CO₂ volume fraction at the intake to the CO₂ volume fraction at the exhaust corresponds to the total fraction of recirculated exhaust gas. The formula for calculating the EGR fraction is given in the following expression:”

What are "combustion efficiency" and "highest indicated efficiency"? How were these parameters determined? What do they characterize?

Response: According to Heywood (Internal Combustion Engine Fundamentals (1988)) combustion efficiency is defined as: “the fraction of the fuel energy supplied which is released in combustion process.”

We have revised and extended a sentence in the manuscript by adding an explanation to clarify the definition of indicated efficiency: “The operating points from the first set of experiments for the air dilution approach with the highest indicated efficiencies are given in Table 5 where the indicated efficiency is defined as the ratio of the indicated work to the fuel’s energy input.”

It's worth indicating in the abstract what the abbreviations EGR and IMEP define.

Response: The abbreviations are now defined.

Line 29. "Load, for air dilution and" -> "Load for air dilution, and"

Response: This has now been corrected.

Line 65. What does the parameter "λglobal > 1.6" mean?

Response: In the revised manuscript, the expression has been corrected and clarified as: “global excess air ratio higher than 1.6.”

Line 74. "[14] [15]" -> "[14, 15]"

Response: This has now been corrected.

The figure in Table 3 should be shown in higher resolution. The legend in this figure is illegible.

Response: This has been improved in the revised manuscript.

There is a different font on page 6, lines 162-171.

Response: The authors thank the reviewer for the careful reading and valuable suggestions. All noted issues have been corrected in the revised manuscript.

It's also worth briefly describing the features of the numerical methodology in Section 2.2.

Response: It has been added to the revised manuscript: “Each operating point was calibrated individually by adjusting model parameters (such as fuel flow, heat transfer, temperatures etc.) to match the experimental main chamber and pre-chamber pressure profiles. An example of the calibrated main- and pre-chamber pressure profiles for an experimental operating point is shown in Figure 3.”

Line 173. What is the "CR"? I didn't find this designation in the Abbreviations section.

Response: It is added to the Abbreviations section.

Line 182. "with a pressure of 50 bar" – Is this absolute pressure or overpressure?

Response: This has now been corrected.

Line 193. The same. "IMEP = 5 bar." – Is this absolute pressure or overpressure?

Response: IMEP is a standard engine performance parameter defined as the ratio of indicated work per cycle to displacement volume. It is a well-established quantity commonly used to express engine load and power, and therefore does not require specification in terms of absolute or overpressure.

Line 185. "was performed from λ = 1.0 to λ = 1.8"

- This parameter should be defined in the text at the first mention.

Response: The term λ here was referencing to global excess air ratio of the whole mixture which is measured in the exhaust pipe. In revised manuscript this is changed to λ_global and the term λ_globalis explained in introduction.

Line 188. "EGR level was gradually increased from EGR =……%"

- by mass or by volume?

Response: The EGR level is expressed by volume, and this has now been clarified in Subsection 2.1 of the revised manuscript.

I didn't notice the information in the text. How many repetitions of the experiment with the same initial conditions were conducted?

Figure 6 shows the parameter "\Delta Indicated efficiency." What does the \Delta symbol mean?

Response: The following sentence has been added to the revised manuscript: “∆ indicated efficiency stands for the difference between the reference operating point at λglobal = 1.0 and EGR = 0% and the other measured points.”

Line 391. "Euro VI (2013) stage" – Is it possible to provide a permanent reference to the Euro VI standards?

Response: A permanent reference to the Euro VI standards has now been added as reference [30]:

[30] M. Williams and R. Minjares, A technical summary of Euro 6/VI vehicle emission standards, 2016. [Online]. Available: www.theicct.org

Self-citation reaches 20%. I think that's quite a lot.

Response: The level of self-citation results from the fact that several of our previous works are directly connected to the present study and provide essential background as well as detailed explanations of both the experimental setup and the numerical approach.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made a commendable effort, and the manuscript is significantly improved. The work is valuable, and the dataset is of high interest to the community. However, the failure to substantiate the claim regarding PC fueling and, most importantly, the failure to incorporate the excellent discussion reconciling the experimental and numerical results into the manuscript itself are significant omissions.

The required revisions are as follows:

1. Integrate the discussion from the response letter regarding the previous numerical study into the main manuscript. This is the most critical revision required. The paper will be substantially stronger once this analysis is included for all readers, not just the reviewer.

2. Provide evidence or data to support the claim that varying PC DOI had "little effect" in the EGR case. If this data is not available, the authors must more explicitly acknowledge this as a limitation of the current study's comparative framework.

3. While the authors have defended their use of relative percentages for part-load data, the paper would be more transparent and rigorous if the data were presented in absolute terms, with any day-to-day variations noted in the text.

Upon the successful integration of these points, particularly the first two, I believe the manuscript will be ready for publication and will represent a solid contribution to the field.

Author Response

Integrate the discussion from the response letter regarding the previous numerical study into the main manuscript. This is the most critical revision required. The paper will be substantially stronger once this analysis is included for all readers, not just the reviewer.

Response: Authors thank the reviewer for this comment which has helped to further improve the manuscript. The discussion comparing the experimental and numerical results has now been incorporated into the 3. Results section (Section 3.1. First set of results - WOT conditions):

On first note the comparison of results presented here with the results in authors’ previous numerical study [20], might result in conclusion that there is a difference in results and conclusions. However, the difference to the results of the numerical study arises primarily from differences in the experimental and simulation conditions. In the prior study, the total fuel mass was kept constant (with variations of less than 4%) and the pre-chamber excess air ratio was fixed at λPC = 1.0. The excess air ratio in EGR cases was not set to stoichiometric conditions but changed as the EGR was increased. On the other hand, in the experiments described in this study for the EGR dilution approach, the excess air ratio was kept constant and stoichiometric to enable the use of the three-way catalyst. Also the λPC was adjusted for each operating point to ensure stable combustion. As a result, the pre-chamber excess air ratio in all experimental operating points was below 1.0, meaning more fuel was delivered to the pre-chamber, which directly affects indicated efficiency.

However, the experimental results from the experimental study are comparable to the simulation results from numerical study [20] for one specific case of CR = 16, λglobal = 1.2 at EGR = 0% and EGR = 20%, where λglobal = 1.0 was achieved in EGR dilution case. For this simulated operating point, with 20% EGR the indicated efficiency exceeded 38%, while for λglobal = 1.2 at EGR = 0% it was slightly above 36% (a 2-percentage points difference). In comparison, the experimentally obtained indicated efficiency was 36.1% at EGR = 0% and 39.4% at EGR = 20% (a 3.3 percentage points difference) under WOT conditions and IMEP = 7 bar, which is in reasonable agreement with the simulation results.

Provide evidence or data to support the claim that varying PC DOI had "little effect" in the EGR case. If this data is not available, the authors must more explicitly acknowledge this as a limitation of the current study's comparative framework.

Response: The following is now added to the manuscript:

The occurrence of misfire depends on high EGR ratios and cannot be corrected by changing the PC DOI. Figure 9 shows two cases with the same EGR percentage but different PC DOI values (0.8 and 0.4 ms). The variation in PC DOI did not lead to a stable operation without misfires.

Figure 9. The occurrence of misfires over engine 300 consecutive cycles for two operating points with the same EGR percentage and different PC DOI values.

While the authors have defended their use of relative percentages for part-load data, the paper would be more transparent and rigorous if the data were presented in absolute terms, with any day-to-day variations noted in the text.

Response: In the revised manuscript, all results previously presented in relative form have now been converted to absolute values in the figures.

Author Response File: Author Response.pdf