Research on the Performance Parameters of a Compression-Ignition Engine Fueled by Blends of Diesel Fuel, Rapeseed Methyl Ester and Hydrotreated Vegetable Oil

Round 1

Reviewer 1 Report

The article entitled " Research on the Performance Parameters of a Compression-Ignition Engine Fueled by Blends of Diesel Fuel, Rapeseed Methyl Ester and Hydrotreated Vegetable Oil" is well written and arranged in a perfect way, but there are some issues need to be addressed as : 

1. Is HVO fuel road legal?

2. The authors should review the English grammar in the manuscript 

3. The references section should be updated as put various references in the manuscript as below:

1.https://doi.org/10.1016/j.jclepro.2023.135989

2.https://doi.org/10.1016/j.fuel.2020.120112

3.https://doi.org/10.1016/j.ijhydene.2023.02.020

4.https://doi.org/10.1007/s12649-022-01880-0

4. What additional infrastructure is needed for HVO fuel?

5. How can B100 Biodiesel RME be used effectively in the construction industry for the benefit of the environment and climate?

6. Can HVO fuel be used as an alternative to kerosene heating oil?

The authors should review the English language through the manuscript. 

Author Response

Comments and Suggestions for Authors

The article entitled "Research on the Performance Parameters of a Compression-Ignition Engine Fueled by Blends of Diesel Fuel, Rapeseed Methyl Ester and Hydrotreated Vegetable Oil" is well written and arranged in a perfect way, but there are some issues need to be addressed as: 

1. Is HVO fuel road legal?

Answer: Yes, this is legal fuel for the road transport. HVO meets the requirements of EN 15940 for paraffinic diesel fuel and allowed as a blending component in EN 590 B7 diesel fuel without any fixed maximum percentage. HVO is in EU’s fuel retail market but not attractive due to high price. More information can be found in manufacturer’s handbook https://www.neste.com/sites/default/files/attachments/neste_renewable_diesel_handbook.pdf

2. The authors should review the English grammar in the manuscript

Answer: English was edited by MDPI coordinated editors.

3. The references section should be updated as put various references in the manuscript as below:

1.https://doi.org/10.1016/j.jclepro.2023.135989

Answer: this reference is in the list.

2.https://doi.org/10.1016/j.fuel.2020.120112

Answer: added to the reference list.

3.https://doi.org/10.1016/j.ijhydene.2023.02.020

Answer: added to the reference list.

4.https://doi.org/10.1007/s12649-022-01880-0

Answer: added to the reference list.

4. What additional infrastructure is needed for HVO fuel?

Answer: HVO behaves just like conventional fossil diesel, whatever the blend, no additional investments are needed, either in new vehicles or fuel logistics. Fleets of diesel transport means can be switched to a cleaner fuel just right now – literally. Neste ProDiesel (later Neste MY) is well known in retail market of Northern Europe, which is blend of conventional diesel fuel and HVO.

5. How can B100 Biodiesel RME be used effectively in the construction industry for the benefit of the environment and climate?

Answer: Rapeseed methyl ester (fatty acid methyl ester) is known as 1^st gen. biodiesel that can reduce the emission of incomplete combustion compounds (generally: thermal hydrocarbons, carbon oxide and soot) as well as emission of particle matters and in the life cycle – carbon dioxide. This is very important in close construction sites where the change of fresh air is less intensive. However, the 100% RME will impact a significant increase of nitrogen oxides due to combustion specifics and weariness of engine rubber parts due to acidic nature. For this reason only the blends of conventional diesel fuel and fatty acid methyl ester are designed and allowed to use by engine manufacturers. Construction industry consists of number of diesel engine powered means as well as other land and water transport sectors, and the same technical issues exist. In order to increase the share of RME up to 100%, it is necessary to change rubber parts and ensure exhaust gas after-treatment facilities. Other issues can be met: impact of cold environment; filtration issue resulted by change of RME properties due to oxidation. Conclusion: blends with RME are more promising than pure RME as fuel for diesel engine powered construction means.

6. Can HVO fuel be used as an alternative to kerosene heating oil?

Answer: HVO has similar properties to conventional heating oil but it is renewable and 100% biodegradable. This makes it one of the best low-carbon alternative fuels for heating oil homes. No need a new heating system, only simple modifications made to your oil boiler by a qualified engineer. Boiler upgrades are much cheaper than installing a new heating system such as heat pumps. It burns much cleaner than heating oil, which improves air quality, reduces emissions and can improve boiler efficiency. HVO can be delivered and stored in the same way as traditional heating oil. No more intense heating oil smell like with heating oil.

Comments on the Quality of English Language

The authors should review the English language through the manuscript.

Answer: English was edited by MDPI coordinated editors (English-Editing-Certificate-71691). 

Author Response File: Author Response.pdf

Reviewer 2 Report

The research paper investigates the performance parameters of a compression-ignition engine fueled by blends of diesel fuel, rapeseed methyl ester (RME), and hydrotreated vegetable oil (HVO). The study covers various indicators such as air pollution (CO, CO2, HC, NOx, Smoke), energy (brake specific fuel consumption, thermal efficiency), and noise. Two-component fuel blends with HVO are found to be superior to conventional diesel fuel and two-component blends with RME in almost all cases. The research shows that HVO fuel, due to its chemical and physical properties, is relevant in controlling emissions from marine and coastal transport engines, helping to reduce fuel consumption, greenhouse gas emissions, and emissions of harmful compounds.

Hence, this study can be accepted with Minor Revision after incorporating the following comments.

1. There is no special section in the text for the economic analysis to which the last sentence of the introduction refers? (Gap analysis required)

2. It compares the performance of two-component blends with HVO and RME to conventional diesel fuel, highlighting the superiority of HVO in most cases. (What is the reason)

3. Author should elaborate on the reason for selecting Rapeseed Methyl Ester and Hydrotreated Vegetable Oil? What is the specific reason. What is the special property among others?

4. What is the novelty of your study?

5. Author should include error analysis for all instruments and other calculations.

6. Block diagram of the engine setup is given but no experimental evidence is added. An experimental setup should be included.

7. Justification for 15% and 30% Blend should be provided and also explain the method used for the preparation.

8. Further study about the emission reduction to justify the result should be included.

9. some of the figures (a) and (b) not properly cited, thoroughly checked and apply required corrections.

The text of the manuscript contains a few numbers of typographical & grammatical errors which should be thoroughly checked and apply required corrections.

Author Response

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2^nd Reviewer

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Comments and Suggestions for Authors

The research paper investigates the performance parameters of a compression-ignition engine fueled by blends of diesel fuel, rapeseed methyl ester (RME), and hydrotreated vegetable oil (HVO). The study covers various indicators such as air pollution (CO, CO2, HC, NOx, Smoke), energy (brake specific fuel consumption, thermal efficiency), and noise. Two-component fuel blends with HVO are found to be superior to conventional diesel fuel and two-component blends with RME in almost all cases. The research shows that HVO fuel, due to its chemical and physical properties, is relevant in controlling emissions from marine and coastal transport engines, helping to reduce fuel consumption, greenhouse gas emissions, and emissions of harmful compounds.

Hence, this study can be accepted with Minor Revision after incorporating the following comments.

1. There is no special section in the text for the economic analysis to which the last sentence of the introduction refers? (Gap analysis required)

Answer: This text is added to section “4. Conclusions”: One disadvantage of HVO must be noted. The price is non-competitive against conventional diesel fuel price in road transport fuel retail market as well as the RME. For an example, the price per liter (taxes included) of HVO in Europe gas stations varies ~2.5–2.9 Euro while conventional diesel fuel costs ~1.55–1.95 Euro per liter. This is one of the main problems with the spread of advanced fuels made from renewable energy sources.

Currently, due to economic considerations, the consumption of HVO advanced biodiesel is not high compared to the consumption of traditional 1st generation RME biodiesel. According to statistics, RME currently costs ~1210 USD/ton while HVO costs ~1500 USD/ton (taxes excluded) [...]. The price of HVO is about 24% higher than RME. For this reason, this fuel is not popular, but stricter environmental requirements and dependence on imported fuels can result to use of this fuel more widely. Features such as the low level of CO2 emissions when assessing the life–cycle of HVO and the absence of technical challenges in order to increase the use of HVO in vehicles and the availability of raw materials, increase HVO attractiveness and in the future prices may be-come close to petroleum-based diesel.

2. It compares the performance of two-component blends with HVO and RME to conventional diesel fuel, highlighting the superiority of HVO in most cases. (What is the reason)

Answer: RME is usual biodiesel as additive to conventional diesel fuel in the transport system of EU. In the same time, the retail HVO, known as Neste My, isn’t popular but promising renewable fuel due to better properties and performance features. Data of conventional diesel fuel is taken as the starting point to understand generally how the additives of 1^st biodiesel RME and advanced HVO change the performance of diesel engine. Generally, renewable fuels are compared with each other and in the same time with conventional diesel fuel.

3. Author should elaborate on the reason for selecting Rapeseed Methyl Ester and Hydrotreated Vegetable Oil? What is the specific reason. What is the special property among others?

Answer: Rapeseed Methyl Ester is the 1^st gen. biodiesel (fatty acid methyl ester) that is usually used in EU as additive to a conventional diesel fuel for transport means. Despite the environment-friendly features, fatty acid methyl ester has disadvantages, and a high concentration of RME in fuel blend or pure RME as fuel will lead environmental and technical issues. Main points of weakness are: poor low temperature properties (limited utilization in cold environment), acid environment (can damage rubber parts of engine), oxidation (change of properties due to bio-degradation), increased emission of nitrogen oxides (resulted by specifics of combustion in premixed combustion phase when delayed combustion generates higher peak of temperature). Advanced renewable fuel as HVO eliminates all of these disadvantages due HVO structure: can be utilized in low temperature due to different chemical composition and molecular relations, no acid environment and low oxidation (only pure HC in HVO structure), no delay in the combustion process due to high Cetane number which is determined by chemical composition of HVO.

4. What is the novelty of your study?

Answer: We carried out this study in order evaluate the impact of biodiesel on engine performance and emission parameters. Main target was to evaluate differences between 1^st gen. biodiesel, which is widely used in transport means as additive for the conventional diesel fuel, and advanced biodiesel, which is more expensive but has much better properties. As mentioned before, this is second part of continuing research of HVO impact on engine performance, first one was related to deep analysis of pure HVO, second one – represents the stage of analysis when less promising retail market biodiesel is changed by more advanced one.

5. Author should include error analysis for all instruments and other calculations.

Answer: Error data is represented in table 1 and in the paragraph between table 2 and figure 1.

6. Block diagram of the engine setup is given but no experimental evidence is added. An experimental setup should be included.

Answer: experimental setup is given in text right after table 3, experimental evidence is given in pic 1b.

7. Justification for 15% and 30% Blend should be provided and also explain the method used for the preparation.

Answer: Mixing ratios are justified by these statements: 1) usually retail market diesel fuel consists up to 7% of biodiesel, in this study authors were interested in data of impact of increased biodiesel share on engine’s performance, so 15% and 30% shares of biodiesel were taken in account; 2) according to the results of industrial and marine engine manufacturers data, the largest share of biodiesel, which is usually allowed in fuel, is up to 30%. It is well known that RME and HVO have no limitation when mixing with diesel fuel, so the blending process could be done mechanically.

8. Further study about the emission reduction to justify the result should be included.

Answer: This text is added to section “4. Conclusions”: This study is one of the constituent parts of the overall study of the use of HVO in diesel engines. In the first part [31], an in-depth analysis of the influence of HVO on engine parameters was carried out, in which conventional diesel fuel and HVO mixtures (0–100% HVO v/v) were studied. The second part (this study) covers the use of HVO as an advanced biodiesel in place of 1st generation RME blends with conventional diesel fuel. The next stage will be of a more practical nature and will be related to the adaptation of HVO to vehicle fleets and a deeper economic analysis.

9. some of the figures (a) and (b) not properly cited, thoroughly checked and apply required corrections.

Answer: Corrected.

Comments on the Quality of English Language

The text of the manuscript contains a few numbers of typographical & grammatical errors which should be thoroughly checked and apply required corrections.

Answer: English was edited by MDPI coordinated editors

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Author Response File: Author Response.pdf

Reviewer 3 Report

Dear author,

Very briefly, you can find below, some of my specific comments on your manuscript:

ü  The abstract must be revised in the sense that the context of the theme must be explained more clearly, what the manuscript contains, the results in brief.

ü  94-97 references must be specified.

ü  112-115 references must be specified.

ü  112-113 “HVO shows an advantage in field of compatibility,” to explain, in more detail, what compatibility means.

ü  Even if "conventional diesel fuel" is used quite frequently to name fuels obtained from petroleum, in my opinion, to avoid confusion, it should be replaced with "fossil fuel". This is a suggestion for the entire manuscript. Conventional fuel also represents a theoretical fuel with a calorific value of 7000 kcal/kg, used as a benchmark for comparing different real fuels.

ü  should be explained why RME was chosen for the first generation of biodiesel and not from another vegetable source (sunflower, palm, etc.)

ü  What is the raw material for HVO? It seems normal to me to use the same raw material processed by different methods. Please explain this aspect.

ü  127 Table 1- lacks the standards and equipment for determining Density, Lubricity, the standard for Kinematic viscosity.

ü  160 Table 3, I think that the limits stipulated in the norms specific to each fuel should also be added: EN590, EN14214; EN 15940

ü  Figures 2-8 (a) are not legible, should be modified.

ü  Figure 2. “Absolute characteristic (a) of mass break specific fuel consumption and relative deviation (b) from conventional diesel fuel” - the explanation is not clear.

ü  On figures 2-10, between the measured points, you have drawn, with a dashed line, the connecting curves. Please explain why and how you drew these curves. How did you determine the evolution trend of the parameters between the measured points?

ü  186-191 “For the analysis of BSFC parameter, as well as for others, two diagrams per parameter were done (Fig. 2). The first one marked with letter “a” is with absolute numbers and second one with letter “b” is with relative deviation or absolute deviation when percent is analyzed dimension. A “zero line” in diagrams marked with letter “b” represents conventional diesel fuel and positions of markers show the relative or absolute deviation in all tested load modes (BMEP = 0.2, 0.4, 0.6, 0.8 MPa).” It is not clear, and I feel that the explanation does not match the figure. I mean I can't find a "zero line", and there is only one curve for each type of fuel, even if you talk about relative deviation but also about absolute deviation. Please explain more clearly.

ü  You are talking about the fact that "only a part of HVO results is out of error level" ... which data are erroneous? Are they represented on the diagram? Etc... Can you draw a conclusion considering that some of the results are erroneous? Or are you talking about measurement error limits? What are the measurement error/accuracy limits? They should be specified. Can we do the analysis on such data?... It is not a clear expression, and you need to give additional explanations. This suggestion is valid for the entire manuscript (199, 202, 210, 212, 252, 323).

ü  215 Figure 3. "Absolute characteristic (a) of volumetric break specific fuel consumption and relative deviation (b) from conventional diesel fuel" I think you wanted to write "Figure 3. (a) Absolute characteristic of volumetric break specific fuel consumption; (b) relative deviation from conventional diesel fuel" or, I think, more correctly "Figure 3. Volumetric break specific fuel consumption: (a) absolute characteristic; (b) relative deviation from conventional diesel fuel". The same proposal for figures 4, 5, 6, 7, 8, 9, 10

ü  227 “is a is a colorless…” repetition of words

ü  247 “if the combustion conditions are equal” what do you mean by "equal", please, explain this

ü  Fig 10 missing diagram (b)  

ü  The conclusions talk about emissions, then noise, then emissions. I propose to respect the order in the manuscript.

ü  374-378 The conclusions chapter talks about the HVO price and does not talk about the RME price. On another note, mixtures of up to 30% are used, so what would be the economic impact in this case... perhaps an analysis in this sense should have been made in the manuscript, do not issue conclusions on a subject that has not previously treated.

Taking into consideration the above, I propose to accept the manuscript with major changes.

English very difficult to understand

Author Response

 Very briefly, you can find below, some of my specific comments on your manuscript:

ü  The abstract must be revised in the sense that the context of the theme must be explained more clearly, what the manuscript contains, the results in brief.

Answer: Corrected

ü  94-97 references must be specified.

Answer: References are provided.

ü  112-115 references must be specified.

Answer: References are provided.

ü  112-113 “HVO shows an advantage in field of compatibility,” to explain, in more detail, what compatibility means.

Answer: The sentence goes on to explain that " HVO shows an advantage in field of compatibility, while RME can be used as a blend with fossil diesel fuel, HVO is considered a drop-in fuel, meaning it can be used as a complete (100%) substitute for conventional diesel fuel without any engine modifications."

ü  Even if "conventional diesel fuel" is used quite frequently to name fuels obtained from petroleum, in my opinion, to avoid confusion, it should be replaced with "fossil fuel". This is a suggestion for the entire manuscript. Conventional fuel also represents a theoretical fuel with a calorific value of 7000 kcal/kg, used as a benchmark for comparing different real fuels.

Answer:  Throughout the article, the term “conventional diesel fuel” is replaced by “fossil diesel fuel”.

ü  should be explained why RME was chosen for the first generation of biodiesel and not from another vegetable source (sunflower, palm, etc.)

Answer: Text of the article has been expanded: “Diesel fuel sold in the Baltic States and many other countries is obtained by blending fossil diesel fuel with rapeseed methyl ester (RME), produced from locally sourced feedstock. The RME content of the blend is up to 7% by volume.  A blend of fossil diesel and hydrotreated vegetable oil (HVO) is also commercially available in various pro-portions. The aim is to use non-food feedstocks for the production of HVO and to in-crease the share of this biofuel in diesel fuel.”

ü  What is the raw material for HVO? It seems normal to me to use the same raw material processed by different methods. Please explain this aspect.

Answer: According to manufacturer’s statements, raw material for HVO processing can be fatty acids like animal fat from food industry waste, used cooking oil, vegetable oil processing waste and residues (e.g., palm fatty acid distillate, spent bleaching earth oil, palm effluent sludge), fish fat from fish processing waste, tall oil based raw materials, technical corn oil (a residue from ethanol production). A wide variety of waste oils and fats can be used as feedstock, and the resulting renewable diesel fuel meets global standards. During the NEXBTL production, hydrogen is used in the refining process to separate the feedstock molecules and remove oxygen from the fatty acids that make up the fuel’s completely renewable raw material base. The outcome is a renewable diesel with a similar chemical composition to fossil diesel. As a result, Neste MY Renewable Diesel (HVO) is a drop-in diesel that can easily be used in existing diesel-powered engines without modification.

ü  127 Table 1- lacks the standards and equipment for determining Density, Lubricity, the standard for Kinematic viscosity.

Answer: Text in table had to be center aligned, because both devices Anton Paar SVM 3000/G2 and Stabinger Viscometer measure density and lubricity according to ASTM D7042 standard. Manufacturers info: “Automatic kinematic viscometers determine kinematic and dynamic viscosity, density, viscosity index (VI), cloud point, freeze point, and many other parameters in one run. Exchange all your capillaries for a single unit. The SVM series covers the entire range with one cell for maximum flexibility. Enjoy all the benefits of ASTM D7042 and report in both D7042 as well as D445 with the integrated ASTM-defined bias corrections.”

ü  160 Table 3, I think that the limits stipulated in the norms specific to each fuel should also be added: EN590, EN14214; EN 15940

Answer: In suggested way the table would be too big, so we added the reference before table, where is possible to find the required limits of the fuel parameters.

ü  Figures 2-8 (a) are not legible, should be modified.

Answer: the main attention is focused on relative deviation figures with (b) which are clear, understandable and used for analysis. However, trying to represent more clear figures with (a), it is impossible to make adequate size figures for article due to wide data range and small difference, which increased size of figures.

ü  Figure 2. “Absolute characteristic (a) of mass break specific fuel consumption and relative deviation (b) from conventional diesel fuel” - the explanation is not clear.

Answer: Figures with (a) consist of absolute measured numbers and figures with (b) show the relative data deviation of fuel blends from fossil diesel fuel which is so called “zero line”.

ü  On figures 2-10, between the measured points, you have drawn, with a dashed line, the connecting curves. Please explain why and how you drew these curves. How did you determine the evolution trend of the parameters between the measured points?

Answer: Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10 are corrected.

ü  186-191 “For the analysis of BSFC parameter, as well as for others, two diagrams per parameter were done (Fig. 2). The first one marked with letter “a” is with absolute numbers and second one with letter “b” is with relative deviation or absolute deviation when percent is analyzed dimension. A “zero line” in diagrams marked with letter “b” represents conventional diesel fuel and positions of markers show the relative or absolute deviation in all tested load modes (BMEP = 0.2, 0.4, 0.6, 0.8 MPa).” It is not clear, and I feel that the explanation does not match the figure. I mean I can't find a "zero line", and there is only one curve for each type of fuel, even if you talk about relative deviation but also about absolute deviation. Please explain more clearly.

Answer: Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10 are corrected.

ü  You are talking about the fact that "only a part of HVO results is out of error level" ... which data are erroneous? Are they represented on the diagram? Etc... Can you draw a conclusion considering that some of the results are erroneous? Or are you talking about measurement error limits? What are the measurement error/accuracy limits? They should be specified. Can we do the analysis on such data?... It is not a clear expression, and you need to give additional explanations. This suggestion is valid for the entire manuscript (199, 202, 210, 212, 252, 323).

Answer: text and data is revised and corrected, the error limits are shown in BTE and BSFC figures, required accuracy of measurement devices are represented in methodology section.

ü  215 Figure 3. "Absolute characteristic (a) of volumetric break specific fuel consumption and relative deviation (b) from conventional diesel fuel" I think you wanted to write "Figure 3. (a) Absolute characteristic of volumetric break specific fuel consumption; (b) relative deviation from conventional diesel fuel" or, I think, more correctly "Figure 3. Volumetric break specific fuel consumption: (a) absolute characteristic; (b) relative deviation from conventional diesel fuel". The same proposal for figures 4, 5, 6, 7, 8, 9, 10

Answer: Figure titles have been adjusted to reflect the recommendation.

ü  227 “is a is a colorless…” repetition of words

Answer: corrected.

ü  247 “if the combustion conditions are equal” what do you mean by "equal", please, explain this

Answer: “combustion conditions are equal” text eliminated.

ü  Fig 10 missing diagram (b) 

Answer: added.

ü  The conclusions talk about emissions, then noise, then emissions. I propose to respect the order in the manuscript.

Answer: corrected.

ü  374-378 The conclusions chapter talks about the HVO price and does not talk about the RME price. On another note, mixtures of up to 30% are used, so what would be the economic impact in this case... perhaps an analysis in this sense should have been made in the manuscript, do not issue conclusions on a subject that has not previously treated.

Answer: This text is added to section “4. Conclusions”: One disadvantage of HVO must be noted. The price is non-competitive against conventional diesel fuel price in road transport fuel retail market as well as the RME. For an example, the price per liter (taxes included) of HVO in Europe gas stations varies ~2.5–2.9 Euro while conventional diesel fuel costs ~1.55–1.95 Euro per liter. This is one of the main problems with the spread of advanced fuels made from renewable energy sources.

Currently, due to economic considerations, the consumption of HVO advanced biodiesel is not high compared to the consumption of traditional 1st generation RME biodiesel. According to statistics, RME currently costs ~1210 USD/ton while HVO costs ~1500 USD/ton (taxes excluded) [...]. The price of HVO is about 24% higher than RME. For this reason, this fuel is not popular, but stricter environmental requirements and dependence on imported fuels can result to use of this fuel more widely. Features such as the low level of CO2 emissions when assessing the life–cycle of HVO and the absence of technical challenges in order to increase the use of HVO in vehicles and the availability of raw materials, increase HVO attractiveness and in the future prices may be-come close to petroleum-based diesel.

English corrected: English-Editing-Certificate-71691

Author Response File: Author Response.pdf

Reviewer 4 Report

1.    The article contains enough information and the considerations made for the analysis; however, it is recommended to indicate the design of experiments in a more explicit way. It is suggested to propose an experimental design that allows comparing the results between the gravimetric method and the solid particle number method, including a statistical analysis that allows statistically validate the information, and determine correlations of the measured data. Also, identify relevant variables in the production of smaller emissions particles size for the tested vehicles.

2.    The references 1-10, 12,13, 13-15 are publications prior 2017, most of the references used are not into the last 5 years, it is recommended to update the references mentioned above and extend search of researches related to the methods for emissions evaluation. It is recommended avoid self-citations if possible.

 Minor editing of English language required

Author Response

Comments and Suggestions for Authors

1. The article contains enough information and the considerations made for the analysis; however, it is recommended to indicate the design of experiments in a more explicit way. It is suggested to propose an experimental design that allows comparing the results between the gravimetric method and the solid particle number method, including a statistical analysis that allows statistically validate the information, and determine correlations of the measured data. Also, identify relevant variables in the production of smaller emissions particles size for the tested vehicles.

Answer: At the time of the research, there was no such equipment that would allow for the assessment of solid particle emissions using the gravimetric method and the solid particle number method, currently it is planned to update the equipment. For this reason, we did not have the opportunity to evaluate the solid particle gravimetric method and the solid particle number method.

2. The references 1-10, 12,13, 13-15 are publications prior 2017, most of the references used are not into the last 5 years, it is recommended to update the references mentioned above and extend search of researches related to the methods for emissions evaluation. It is recommended avoid self-citations if possible.

Answer: The reference list is updated, few new sources were added. Self-citations is necessary because this research is the second part of general research of HVO fuel. The first part consists of deep analysis only about HVO impact on engine performance, and this manuscript (2^nd part) consists of comparative analysis of RME and HVO to find out the impact of 1^st gen. biodiesel and advanced biodiesel on engine performance. For that reason self-citation is inseparable.

Comments on the Quality of English Language: Minor editing of English language required.

Answer: English was edited by MDPI coordinated editors.

English-Editing-Certificate-71691

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Author's Notes

 Very briefly, you can find below, some of my specific comments on your manuscript:

ü  The abstract must be revised in the sense that the context of the theme must be explained more clearly, what the manuscript contains, the results in brief.

Answer: Corrected

ACCEPTABLE ANSWER

ü  94-97 references must be specified.

Answer: References are provided.

ACCEPTABLE ANSWER

ü  112-115 references must be specified.

Answer: References are provided.

ACCEPTABLE ANSWER

ü  112-113 “HVO shows an advantage in field of compatibility,” to explain, in more detail, what compatibility means.

Answer: The sentence goes on to explain that " HVO shows an advantage in field of compatibility, while RME can be used as a blend with fossil diesel fuel, HVO is considered a drop-in fuel, meaning it can be used as a complete (100%) substitute for conventional diesel fuel without any engine modifications."

UNACCEPTABLE ANSWER. Do you mean it is compatible with the engine? Please, complete “engine compatibility” (119-v2) for easy understanding

ü  Even if "conventional diesel fuel" is used quite frequently to name fuels obtained from petroleum, in my opinion, to avoid confusion, it should be replaced with "fossil fuel". This is a suggestion for the entire manuscript. Conventional fuel also represents a theoretical fuel with a calorific value of 7000 kcal/kg, used as a benchmark for comparing different real fuels.

Answer:  Throughout the article, the term “conventional diesel fuel” is replaced by “fossil diesel fuel”.

ACCEPTABLE ANSWER

ü  should be explained why RME was chosen for the first generation of biodiesel and not from another vegetable source (sunflower, palm, etc.)

Answer: Text of the article has been expanded: “Diesel fuel sold in the Baltic States and many other countries is obtained by blending fossil diesel fuel with rapeseed methyl ester (RME), produced from locally sourced feedstock. The RME content of the blend is up to 7% by volume.  A blend of fossil diesel and hydrotreated vegetable oil (HVO) is also commercially available in various pro-portions. The aim is to use non-food feedstocks for the production of HVO and to in-crease the share of this biofuel in diesel fuel.”

ACCEPTABLE ANSWER

ü  What is the raw material for HVO? It seems normal to me to use the same raw material processed by different methods. Please explain this aspect.

Answer: According to manufacturer’s statements, raw material for HVO processing can be fatty acids like animal fat from food industry waste, used cooking oil, vegetable oil processing waste and residues (e.g., palm fatty acid distillate, spent bleaching earth oil, palm effluent sludge), fish fat from fish processing waste, tall oil based raw materials, technical corn oil (a residue from ethanol production). A wide variety of waste oils and fats can be used as feedstock, and the resulting renewable diesel fuel meets global standards. During the NEXBTL production, hydrogen is used in the refining process to separate the feedstock molecules and remove oxygen from the fatty acids that make up the fuel’s completely renewable raw material base. The outcome is a renewable diesel with a similar chemical composition to fossil diesel. As a result, Neste MY Renewable Diesel (HVO) is a drop-in diesel that can easily be used in existing diesel-powered engines without modification.

UNACCEPTABLE ANSWER. It is well known what you say, but I understand that you do not know the raw material from which the HVO used by you in the experiments is obtained. Just as the properties of FAME depend on the raw material from which it is obtained, I think that the properties of HVO also depend on the raw material. Obviously, these physico-chemical properties must fall within the limits provided by the current standards. Finally, I think you should write that you do not know the raw material and that you purchased HVO from the manufacturer……

ü  127 Table 1- lacks the standards and equipment for determining Density, Lubricity, the standard for Kinematic viscosity.

Answer: Text in table had to be center aligned, because both devices Anton Paar SVM 3000/G2 and Stabinger Viscometer measure density and lubricity according to ASTM D7042 standard. Manufacturers info: “Automatic kinematic viscometers determine kinematic and dynamic viscosity, density, viscosity index (VI), cloud point, freeze point, and many other parameters in one run. Exchange all your capillaries for a single unit. The SVM series covers the entire range with one cell for maximum flexibility. Enjoy all the benefits of ASTM D7042 and report in both D7042 as well as D445 with the integrated ASTM-defined bias corrections.”

UNACCEPTABLE ANSWER. So, I understand that you used both equipments to determine density, viscosity and lubricity? In other words, you made two determinations for each property. The lubricity is not specified in table 1, it would be good to add it. I don't understand why it was necessary to determine these properties on two devices. It would be good to explain. In table 3 you presented some values. You obviously did not get the same values on both devices. So, please tell me how you got that value. Is it the average of the values obtained on each equipment, or something else?

ü  160 Table 3, I think that the limits stipulated in the norms specific to each fuel should also be added: EN590, EN14214; EN 15940

Answer: In suggested way the table would be too big, so we added the reference before table, where is possible to find the required limits of the fuel parameters.

UNACCEPTABLE ANSWER. I really don't think that the space in the table is a problem, surely you will find a solution regarding the drafting. To make your research useful to the reader, you should add the limits according to the standards

ü  Figures 2-8 (a) are not legible, should be modified.

Answer: the main attention is focused on relative deviation figures with (b) which are clear, understandable and used for analysis. However, trying to represent more clear figures with (a), it is impossible to make adequate size figures for article due to wide data range and small difference, which increased size of figures.

ACCEPTABLE ANSWER. In a manuscript, if an information is not necessary, it would be better not to insert it... On the other hand, figure (a) in its present form is somewhat better than the previous version

ü  Figure 2. “Absolute characteristic (a) of mass break specific fuel consumption and relative deviation (b) from conventional diesel fuel” - the explanation is not clear.

Answer: Figures with (a) consist of absolute measured numbers and figures with (b) show the relative data deviation of fuel blends from fossil diesel fuel which is so called “zero line”.

ACCEPTABLE ANSWER

ü  On figures 2-10, between the measured points, you have drawn, with a dashed line, the connecting curves. Please explain why and how you drew these curves. How did you determine the evolution trend of the parameters between the measured points?

Answer: Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10 are corrected.

UNACCEPTABLE ANSWER. It's OK, you corrected the figures (b), but in (a) the same question remains

ü  186-191 “For the analysis of BSFC parameter, as well as for others, two diagrams per parameter were done (Fig. 2). The first one marked with letter “a” is with absolute numbers and second one with letter “b” is with relative deviation or absolute deviation when percent is analyzed dimension. A “zero line” in diagrams marked with letter “b” represents conventional diesel fuel and positions of markers show the relative or absolute deviation in all tested load modes (BMEP = 0.2, 0.4, 0.6, 0.8 MPa).” It is not clear, and I feel that the explanation does not match the figure. I mean I can't find a "zero line", and there is only one curve for each type of fuel, even if you talk about relative deviation but also about absolute deviation. Please explain more clearly.

Answer: Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10 are corrected.

ACCEPTABLE ANSWER

ü  You are talking about the fact that "only a part of HVO results is out of error level" ... which data are erroneous? Are they represented on the diagram? Etc... Can you draw a conclusion considering that some of the results are erroneous? Or are you talking about measurement error limits? What are the measurement error/accuracy limits? They should be specified. Can we do the analysis on such data?... It is not a clear expression, and you need to give additional explanations. This suggestion is valid for the entire manuscript (199, 202, 210, 212, 252, 323).

Answer: text and data is revised and corrected, the error limits are shown in BTE and BSFC figures, required accuracy of measurement devices are represented in methodology section.

ACCEPTABLE ANSWER

ü  215 Figure 3. "Absolute characteristic (a) of volumetric break specific fuel consumption and relative deviation (b) from conventional diesel fuel" I think you wanted to write "Figure 3. (a) Absolute characteristic of volumetric break specific fuel consumption; (b) relative deviation from conventional diesel fuel" or, I think, more correctly "Figure 3. Volumetric break specific fuel consumption: (a) absolute characteristic; (b) relative deviation from conventional diesel fuel". The same proposal for figures 4, 5, 6, 7, 8, 9, 10

Answer: Figure titles have been adjusted to reflect the recommendation.

ACCEPTABLE ANSWER

ü  227 “is a is a colorless…” repetition of words

Answer: corrected.

ACCEPTABLE ANSWER

ü  247 “if the combustion conditions are equal” what do you mean by "equal", please, explain this

Answer: “combustion conditions are equal” text eliminated.

ACCEPTABLE ANSWER

ü  Fig 10 missing diagram (b) 

Answer: added.

ACCEPTABLE ANSWER

ü  The conclusions talk about emissions, then noise, then emissions. I propose to respect the order in the manuscript.

Answer: corrected.

ACCEPTABLE ANSWER

ü  374-378 The conclusions chapter talks about the HVO price and does not talk about the RME price. On another note, mixtures of up to 30% are used, so what would be the economic impact in this case... perhaps an analysis in this sense should have been made in the manuscript, do not issue conclusions on a subject that has not previously treated.

Answer: This text is added to section “4. Conclusions”: One disadvantage of HVO must be noted. The price is non-competitive against conventional diesel fuel price in road transport fuel retail market as well as the RME. For an example, the price per liter (taxes included) of HVO in Europe gas stations varies ~2.5–2.9 Euro while conventional diesel fuel costs ~1.55–1.95 Euro per liter. This is one of the main problems with the spread of advanced fuels made from renewable energy sources.

Currently, due to economic considerations, the consumption of HVO advanced biodiesel is not high compared to the consumption of traditional 1st generation RME biodiesel. According to statistics, RME currently costs ~1210 USD/ton while HVO costs ~1500 USD/ton (taxes excluded) [...]. The price of HVO is about 24% higher than RME. For this reason, this fuel is not popular, but stricter environmental requirements and dependence on imported fuels can result to use of this fuel more widely. Features such as the low level of CO2 emissions when assessing the life–cycle of HVO and the absence of technical challenges in order to increase the use of HVO in vehicles and the availability of raw materials, increase HVO attractiveness and in the future prices may be-come close to petroleum-based diesel.

UNACCEPTABLE ANSWER. The addition you made is ok, but, as I recommended previously, I think a separate chapter should be added regarding the price of these fuels, that is, move this addition to a separate chapter. In the conclusions chapter, a short conclusion must obviously be presented.

English corrected: English-Editing-Certificate-71691

Author Response

ü  112-113 “HVO shows an advantage in field of compatibility,” to explain, in more detail, what compatibility means.

Answer: The sentence goes on to explain that " HVO shows an advantage in field of compatibility, while RME can be used as a blend with fossil diesel fuel, HVO is considered a drop-in fuel, meaning it can be used as a complete (100%) substitute for conventional diesel fuel without any engine modifications."

UNACCEPTABLE ANSWER. Do you mean it is compatible with the engine? Please, complete “engine compatibility” (119-v2) for easy understanding

Answer: Yes, it is compatible with the engine. Added (underline): "HVO shows an advantage in terms of compatibility (less aggressive effect on the surfaces of the parts, combustion process is similar to fossil diesel fuel, no operational challenges indicated) while RME can be used as a blend with fossil diesel fuel, HVO is considered a drop-in fuel, meaning it can be used as a complete (100%) substitute for conventional diesel fuel without any engine modifications [35,59]".

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ü  What is the raw material for HVO? It seems normal to me to use the same raw material processed by different methods. Please explain this aspect.

Answer: According to manufacturer’s statements, raw material for HVO processing can be fatty acids like animal fat from food industry waste, used cooking oil, vegetable oil processing waste and residues (e.g., palm fatty acid distillate, spent bleaching earth oil, palm effluent sludge), fish fat from fish processing waste, tall oil based raw materials, technical corn oil (a residue from ethanol production). A wide variety of waste oils and fats can be used as feedstock, and the resulting renewable diesel fuel meets global standards. During the NEXBTL production, hydrogen is used in the refining process to separate the feedstock molecules and remove oxygen from the fatty acids that make up the fuel’s completely renewable raw material base. The outcome is a renewable diesel with a similar chemical composition to fossil diesel. As a result, Neste MY Renewable Diesel (HVO) is a drop-in diesel that can easily be used in existing diesel-powered engines without modification.

UNACCEPTABLE ANSWER. It is well known what you say, but I understand that you do not know the raw material from which the HVO used by you in the experiments is obtained. Just as the properties of FAME depend on the raw material from which it is obtained, I think that the properties of HVO also depend on the raw material. Obviously, these physico-chemical properties must fall within the limits provided by the current standards. Finally, I think you should write that you do not know the raw material and that you purchased HVO from the manufacturer……

Answer: Authors thought that reviewer’s question “What is the raw material for HVO?” has general nature. Sentence, defining that we do not know the raw material, is added in text (2.3. 1^st paragraph “HVO was purchased from the manufacturer and the raw material is unknown.”).

Answer: Text in table had to be center aligned, because both devices Anton Paar SVM 3000/G2 and Stabinger Viscometer measure density and lubricity according to ASTM D7042 standard. Manufacturers info: “Automatic kinematic viscometers determine kinematic and dynamic viscosity, density, viscosity index (VI), cloud point, freeze point, and many other parameters in one run. Exchange all your capillaries for a single unit. The SVM series covers the entire range with one cell for maximum flexibility. Enjoy all the benefits of ASTM D7042 and report in both D7042 as well as D445 with the integrated ASTM-defined bias corrections.”

UNACCEPTABLE ANSWER. So, I understand that you used both equipments to determine density, viscosity and lubricity? In other words, you made two determinations for each property. The lubricity is not specified in table 1, it would be good to add it. I don't understand why it was necessary to determine these properties on two devices. It would be good to explain. In table 3 you presented some values. You obviously did not get the same values on both devices. So, please tell me how you got that value. Is it the average of the values obtained on each equipment, or something else?

Answer: Data of lubricity measurement is provided in table 1. Anton Paar SVM 3000/G2 device was primary tool to measure the density and viscosity, secondary one - Stabinger Viscometer for confirmation of results. If it is confusing – we can eliminate the secondary tool.

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ü  160 Table 3, I think that the limits stipulated in the norms specific to each fuel should also be added: EN590, EN14214; EN 15940

Answer: In suggested way the table would be too big, so we added the reference before table, where is possible to find the required limits of the fuel parameters.

UNACCEPTABLE ANSWER. I really don't think that the space in the table is a problem, surely you will find a solution regarding the drafting. To make your research useful to the reader, you should add the limits according to the standards

Answer: Corrected

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ü  On figures 2-10, between the measured points, you have drawn, with a dashed line, the connecting curves. Please explain why and how you drew these curves. How did you determine the evolution trend of the parameters between the measured points?

Answer: Figures 2, 3, 4, 5, 6, 7, 8, 9 and 10 are corrected.

 UNACCEPTABLE ANSWER. It's OK, you corrected the figures (b), but in (a) the same question remains

Answer: Dashed line is the polynomial curve of diesel fuel which gives a better view for visual evaluation and reflects the general trend of tested parameters. These polynomial curves reflect usual dependency of measured parameters on engine load. Trends have usual character for such engine testing results. There is no reason to believe that the trend must be skewed between the measured points.

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ü  374-378 The conclusions chapter talks about the HVO price and does not talk about the RME price. On another note, mixtures of up to 30% are used, so what would be the economic impact in this case... perhaps an analysis in this sense should have been made in the manuscript, do not issue conclusions on a subject that has not previously treated.

Answer: This text is added to section “4. Conclusions”: One disadvantage of HVO must be noted. The price is non-competitive against conventional diesel fuel price in road transport fuel retail market as well as the RME. For an example, the price per liter (taxes included) of HVO in Europe gas stations varies ~2.5–2.9 Euro while conventional diesel fuel costs ~1.55–1.95 Euro per liter. This is one of the main problems with the spread of advanced fuels made from renewable energy sources.

Currently, due to economic considerations, the consumption of HVO advanced biodiesel is not high compared to the consumption of traditional 1st generation RME biodiesel. According to statistics, RME currently costs ~1210 USD/ton while HVO costs ~1500 USD/ton (taxes excluded) [...]. The price of HVO is about 24% higher than RME. For this reason, this fuel is not popular, but stricter environmental requirements and dependence on imported fuels can result to use of this fuel more widely. Features such as the low level of CO2 emissions when assessing the life–cycle of HVO and the absence of technical challenges in order to increase the use of HVO in vehicles and the availability of raw materials, increase HVO attractiveness and in the future prices may be-come close to petroleum-based diesel.

 UNACCEPTABLE ANSWER. The addition you made is ok, but, as I recommended previously, I think a separate chapter should be added regarding the price of these fuels, that is, move this addition to a separate chapter. In the conclusions chapter, a short conclusion must obviously be presented.

Answer: This research mainly consists of engineering and technical data. The deep financial analysis was not in the scope during the research planning phase, for that reason the financial data was not collected and further steps are not possible. The prices of retailing market are given as well as comparative rates. We understand that remark about financial evaluation is important, however authors have no data for such evaluation.

Author Response File: Author Response.pdf