Influence of Hydrogen on the Performance and Emissions Characteristics of a Spark Ignition Ammonia Direct Injection Engine
Round 1
Reviewer 1 Report
Using direct injection of ammonia is a potentially key measure to secure engine power density, however, this kind of ammonia injection is rarely reported. This paper presents engine performance using ammonia direct injection and deserves publication. However, a revision is still needed. My comments are as follows.
1. I suggest that you convert Nm to IMEP in both figures and main texts.
2. What kind of ammonia injector and pump (both low- and high-pressure ones) were used? Please provide detail information about their model numbers and manufactures.
3. How did you prevent vapor lock in the fuel supply system, since ammonia has a pretty high vapor pressure?
4. Figure 2, what is the intake pressure?
5. Figure 8, why NH3 emission keeps increasing as the torque increase from 160 Nm to 280 Nm.
I think some sentences read strange and are difficult for me to understand, maybe the authors need to proofread it again carefully.
Author Response
The response to the reviewer's comments is enclosed. Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Title: Influence of Hydrogen on the Performance and Emissions Characteristics of a Spark Ignition Ammonia Direct Injection Engine
Document formats: with the draft as it is now, is very hard to do punctual notations.
For an easier and more precise review, the authors should insert the number of rows.
Presentation
In this study, authors identify the area where stable combustion of the engine is possible by direct injection of ammonia fuel at high pressure in a 2.5 L engine and evaluate how much it is possible to extend its operating area by adding minimal quantity of hydrogen.
The topic of investigation is not new but still to be analyzed in all its aspects.
Comments
1. A general review of English is recommended;
2. Nowhere it is specified whether the percentages are volumetric or based on mass.
3. It’s very strange that the authors, in all the work, use torque as an independent variable. Normally torque is a derived quantity.
4. In the introduction it is needed to insert other bibliography entries. We note that the bibliography references only one paper from SAE! In the last years, SAE published a lot of papers concerning decarbonization using hydrogen.
Furthermore, authors are invited to insert and comment the following paper which presents an extremely similar and very recent investigation:
‘Effect of ammonia/hydrogen mixture ratio on engine combustion and emission performance at different inlet temperatures’, Binbin Wang, Hechun Wang *, Baoyin Duan, Chuanlei Yang, Deng Hu, Yinyan Wang; Elsevier, Energy 272 (2023) 127110;.https://doi.org/10.1016/j.energy.2023.127110;
5. for a better reading quality, insert the formal definition of ‘thermal efficiency’, COVimep, and ‘energy fraction of H2’;
Punctual notations
1. All figures are very clear but perhaps too large;
2. Verify the latent heat of vaporization of liquid Ammonia (327 kcal/kg).
3. As per the consolidated literature, the stability of the combustion is evaluated by observing the trend of the IMEP and COVIMEP as a function of the excess air ratio. Owning the declared measures of air-fuel ratio, and then excess air ratio, the authors should include this type of information.
4. The throttle conditions are specified only in Figures 9, 10, and 11; why? This information is needed in all other operation conditions. As an example, at the beginning of paragraph 3.1, ‘…the maximum torque increased by 20.26% to 263.5 Nm compared to the case of using only ammonia fuel-’.
5. kg/h instead of kg/hr.
6. Page 7: ‘Combustion speed increased and combustion stability improved, resulting in an increase in thermal efficiency by about 2% points under similar load conditions.’ This sentence should be justified later using the concerning graphs and not inserted here as a simple statement and also not referenced. Maybe, the authors can just mention this result in this position adding that it will be explained in detail afterwards; in fact, a few rows after, there is the explanation: ‘…turbulent flame propagation speed according to the increase in the combustion rate and turbulence intensity due to thermal diffusion instability’
7. Page 7: ‘As discussed above, because the combustion speed of the ammonia and air mixture is relatively very low compared to other hydrocarbon-based fuels, with a maximum of 7 cm/s, from a practical point of view in the engine, the ignition timing must be advanced in order to overcome the low combustion speed’. To give the reader indications of the operating conditions, this notation should be moved to the beginning of the discussion showing the optimization of the SA.
Page 8: ‘…. the required flow rate of intake air decreased because the decrease in the supply flow rate of ammonia was large and the increase in the supply flow rate of hydrogen, which had high energy per mass, was relatively small in the condition where hydrogen was added’. This sentence needs further clarification.
Minor editing of English language required.
Slight typing errors and, sometimes, inappropriate use of "the".
Author Response
The response to the reviewer's comments is enclosed. Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
It would be good for the authors to add, at the end of the manuscript, a brief description of future works that will continue this line of research.
