Review Reports

Round 1

Reviewer 1 Report

I do have one issue to address: the CO, HC, NO models are based on equations specifically designed for the tested engine (based on the experimental results); could these models be generalized for any other type of engine?

Author Response

In the introduction of the paper, we say :

The main disadvantage of empirical hypothesis is that the models obtained are valid for a specific engine. Indeed, they are difficult to adapt to other engines because they do not take into account specificity such as combustion chamber parameters (geometry, surface/volume ratio, turbulence intensity). It is therefore necessary to calibrate the models for the engine under study.

 

 

Nevertheless, for similar engine architectures, one can expect trends to be the same, although quantitatively there may be variations. The general optimization methodology and results exposed in our work should therfore apply and keep its validity for SI engines with equivalent features. The units considered for pollutant models are concentrations, so an increase in the mass flow will naturaly increase the amount of pollutants. Engine models introduce a displacement volume parameter, wich can be used for scaling purposes.

 

We also mention int the discussion Section :

Next project will include the latest technologies for direct injection spark ignition engines (DISI) and corresponding control parameters in the optimization process (turbo charging, variable valve lifting, ...).

 

To conclude with this question, Shayler *et al* made the same kind of work. I notice from their article the following sentence that can be applied to our work: "*The other parameters reflect influences of operating conditions and calibration settings. It is a more difficult task to define the influence of engine design details. The generic functions developed in the following are mainly relevant to variations with operating conditions

and calibration. The functions have been derived from data sets from four-valve cylinder DOHC engines with typical pentroof, open-chamber features. It would be unreasonably optimistic to expect these functions to apply to all varieties of chamber design with similar

accuracy. Nevertheless, the trends characterized are universal and adjustments are simple to make. The application of the results to predict brake specific emissions values is relatively straightforward.*"

 

Shayler PJ, Chick J, Darnton NJ, Eade D. Generic functions for fuel consumption and engine-out emissions of HC, CO and NOx of spark-ignition engines. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering. 1999;213(4):365-378. doi:10.1243/0954407991526937

 

Reviewer 2 Report

The manuscript deals with an optimisation study on a particular SI engine using a few engine operating parameters. The engine out emissions models are based on the experimental data. The results are interesting, and the manuscript can be published, however, minor corrections are required in the manuscript to improve the readability.

1. The second paragraph of the Introduction section talks about the research gap and everything that follows from there is the modelling methodology. The third and the fourth paragraph of the Introduction does include some literature review of the modelling approaches / models, etc. Section 4.1 again talks about the novelty of the work. Although the Introduction was not hard for me to follow, the readability of the article can be improved if the discussion is arranged systematically. Usually, the approach taken by researchers is to start the Literature review from a broad perspective and narrow it down to end at the research gap, which is then followed by the research methodology, results and discussion. This provides a fairly good readability. I suggest the readability of the article can be improved if the sections are rearranged properly.
2. The term ‘Spark Advance’ or ‘Spark Retard’ in an SI engine can be easily understood by the readers. Using the terms such as ‘Negative Advance’ or ‘Ignition Delay’, may cause confusion and makes it hard to follow.
3. Ignition Delay is a term usually used in CI engines and not in SI engines. I suggest, it is defined at the beginning of the discussion.
4. Spelling of ‘fresh’ in line 98.
5. The phenomenon of knocking in SI engine is different to how it has been stated in the manuscript in line 98. This sentence seems to be defining knock in a CI engine. It should be made clear.
6. Section 2.2, Figure 2 is just bolted in without any explanation of what it is. There should be some text before this Figure is introduced to the reader.
7. Line 105, check the sentence for the missing word and grammar.
8. Line 106, use CO, as it is already defined the first time in line 87.
9. Line 113,’figure 3’ should be ‘Figure 3’. Check at other places too.
10. Line 117, it should be Equation 1. Check at other places too.
11. Line 119, Table 1 instead of table 1.
12. Equation 1 - The equation symbols should be defined with the equation. Simply stating that they are defined in the nomenclature is not enough.
13. You have to be consistent how you use fuel-air (fuel-air or fuel/air). You have both at the moment and at some places you are writing Fuel/Air, while at some you write fuel/air.
14. Again, the symbols used in Figure 3 are nowhere defined before.
15. There should be more explanation of Figure 3 than what exist at the moment.
16. Caption of Figure 4 - ‘measure’ should be clarified if it means experimental measurement. Same to be done for other figures too.
17. Reference is needed in the discussion from line 131-135.
18. Reference is needed in the discussion from line 148-152.
19. Line 157 - Equation 2 and not equation 2. Check at other places too.
20. Symbols of Equation 2.
21. Caption of Figure 7 (‘measure’).
22. The bulleted point NOx formation mechanisms need reference, only fuel NOx is referenced at the moment.
23. Line 178-183, reference is required.
24. Line 165 – NO model – I think it would be quite reasonable to explain the readers at the beginning of this section why NO is modelled and not both NO and NO2 or NOx is modelled. Explain about the proportions of NO and NO2 in NOx and why NO is more important.
25. Again, the discussion about NO formation agrees with the fundamentals of NOx formation, however, no reference is found in this discussion. There can be numerous papers that agree with the authors’ results but none of them are referenced here.
26. Line 279 - probably a typo in the bracket ‘et’.
27. Line 307-309 – when the combustion happens late, the piston is already coming down in the expansion stroke, with the increase in volume taking place. This will result in lower combustion temperature, which is what usually happens in late combustion. How do you have more temperature increase then?
28. Line 318 – should it be ‘coupled’? Please make it clear.
29. Line 333 – Spell check – Stoichiometric. Same at other places too (Line 410). Check them all.
30. All Table captions are below the table, usually, they should be above the table. Check with the publishers guidelines.
31. Figure 30 caption: ’emissions’ spelling.
32. Line 430 – ‘Sense’ spelling
33. Section 4.1 – this is the research gap the authors’ talk about here and should be towards the end of the Introduction section where research gap is discussed and why this work is taken up is discussed.
34. Line 440 – remove air to fuel, only say mixture.
35. There are a number of issues with the language and grammar noticed in the article. I suggest that the article be proof-read to improve / correct the language / grammar (e.g. use of articles: a, an, the are seen to be incorrect at a number of places, several typos, spelling mistakes)

Author Response

The manuscript deals with an optimisation study on a particular SI engine using a few engine operating parameters. The engine out emissions models are based on the experimental data. The results are interesting, and the manuscript can be published, however, minor corrections are required in the manuscript to improve the readability.

 

1 The second paragraph of the Introduction section talks about the research gap and everything that follows from there is the modelling methodology. The third and the fourth paragraph of the Introduction does include some literature review of the modelling approaches / models, etc. Section 4.1 again talks about the novelty of the work. Although the Introduction was not hard for me to follow, the readability of the article can be improved if the discussion is arranged systematically. Usually, the approach taken by researchers is to start the Literature review from a broad perspective and narrow it down to end at the research gap, which is then followed by the research methodology, results and discussion. This provides a fairly good readability. I suggest the readability of the article can be improved if the sections are rearranged properly.

 

In order to comply with a better structuring of the article, we add a sentence at the end of the introduction, specifying that:

 

The main objective of this work is to explore trade-offs between fuel consumption and pollutant emissions while adopting a systemic approach. With this expression, we mean considering the entire vehicle as a system, starting from a drive cycle up to modeling the after treatment system, and considering drive train, engine and so on. The models are described in Section 2, followed by the results in section 3. The originality of this approach will be further discussed in section 4.

 

2 The term ‘Spark Advance’ or ‘Spark Retard’ in an SI engine can be easily understood by the readers. Using the terms such as ‘Negative Advance’ or ‘Ignition Delay’, may cause confusion and makes it hard to follow.

 

We clarified the text

 

3 Ignition Delay is a term usually used in CI engines and not in SI engines. I suggest, it is defined at the beginning of the discussion.

 

spark advance is used in the discussion

 

 

4 Spelling of ‘fresh’ in line 98.

 

Done

 

5 The phenomenon of knocking in SI engine is different to how it has been stated in the manuscript in line 98. This sentence seems to be defining knock in a CI engine. It should be made clear.

 

The sentence : “This phenomenon corresponds to the auto-ignition of part of the fresch air/fuel mixture and gives rise to pressure peaks that damage the engine.” has been replaced by :

This phenomenon corresponds to a spontaneous-ignition of a part of the end-gas ahead of the flame. This ignition rapidly releases a large energy and causes high-frequency pressure oscillations which may damage the engine. (ref Heywood).

 

 

6 Section 2.2, Figure 2 is just bolted in without any explanation of what it is. There should be some text before this Figure is introduced to the reader.

 

We propose to add the following presentation of the Figure 2:

 

The Figure 2 is a graphical illustration of the measurements for CO concentration. Each graph represents the CO concentration in function of one control parameter, i.e. (a) engine speed, (b) fuel/air equivalence ratio, (c) spark advance and (d) intake pressure. Each curve represents a series of measurements where only the parameter in abscissa changes.

 

7 Line 105, check the sentence for the missing word and grammar.

 

The sentence has been modified

 

8 Line 106, use CO, as it is already defined the first time in line 87.

 

Done

 

9 Line 113,’figure 3’ should be ‘Figure 3’. Check at other places too.

 

Done

 

10 Line 117, it should be Equation 1. Check at other places too.

 

Done

 

 

11 Line 119, Table 1 instead of table 1.

 

Done

 

12 Equation 1 - The equation symbols should be defined with the equation. Simply stating that they are defined in the nomenclature is not enough.

 

 

We propose to add the following presentation of the Equation 1:

 

In Equation 1, phi represents the fuel/air equivalence ratio, co_1 and co2 are the adjusted coefficients.

 

13 You have to be consistent how you use fuel-air (fuel-air or fuel/air). You have both at the moment and at some places you are writing Fuel/Air, while at some you write fuel/air.

 

We harmonize the text and choose fuel/air ratio.

 

14 Again, the symbols used in Figure 3 are nowhere defined before.

 

See question 15

 

15 There should be more explanation of Figure 3 than what exist at the moment.

 

This answer to the questions 14 and 15. We propose the following modification :

 

 

The figure 3 is a correlogram between intake pressure, [CO] concentration, fuel/air ratio phi, engine speed N and Spark Advance SA. It allows to analyse the relationship between each pair of numeric variables in a dataset. We notice that neither spark advance, engine speed nor intake pressure have high correlation factor with CO concentration.

 

16 Caption of Figure 4 - ‘measure’ should be clarified if it means experimental measurement. Same to be done for other figures too.

 

Figures 4, 7, 10, 13, 15 and 18 have been modified. Labels have been clarified, and the units have been added. Captions are also clearer.

 

17 Reference is needed in the discussion from line 131-135.

 

We added the following reference :"Hydrocarbon Emissions from Spark Ignition Engines",Springer Netherlands,2010,by Lakshminarayanan

 

18 Reference is needed in the discussion from line 148-152.

 

The same reference describes the effect of IMEP at fixed engine speed, equivalence ratio and MBT timing on HC emissions.

 

19 Line 157 - Equation 2 and not equation 2. Check at other places too.

 

Done

 

20 Symbols of Equation 2.

 

We propose to add the following presentation of the Figure 6 and Equation 2:

 

As shown in Figure 6, HC emissions are positively correlated with Spark Advance SA and the augmented variable (phi - 0,9)^2, and negatively correlated with intake pressure Pintake and engine rotation speed N. The complete HC model expresses the causalities with the variables where hc1 to hc4 are the regression parametrers.

 

21 Caption of Figure 7 (‘measure’).

 

See question 16

 

22 The bulleted point NOx formation mechanisms need reference, only fuel NOx is referenced at the moment.

 

References have been added : Heywood, Lavoie, Bowman, Zeldovich, Raine, Fenimore, Yang

 

 

23 Line 178-183, reference is required.

 

Reference added :

Yang et al. "Variation of Exhaust Gas Temperature with the Change of Spark Timing and Exhaust Valve Timing During Cold Start Operation of an SI Engine". Transactions of The Korean Society of Mechanical Engineers B, 2005

 

 

24 Line 165 – NO model – I think it would be quite reasonable to explain the readers at the beginning of this section why NO is modelled and not both NO and NO2 or NOx is modelled. Explain about the proportions of NO and NO2 in NOx and why NO is more important.

 

The oxides of nitrogen NO, N2O and NO_2 are all present in the exhaust of a spark ignition engine. However, nitrous oxide is not significant as it represents less than 1% of NO emitted. Nitrogen dioxide can be more present with the maximum occuring at wide open throttle and a rich mixture. Nevertheless, its concentration is less than few pourcent of NO \cite{Raine1995}. This is confirmed by Bowman et al \cite{Bowman1975}, which states that : "NO is the predominant nitrogen oxide emitted by combustion devices". In this context, the present work will focus on NO species, which can be formed by three distinct mechanisms:

 

 

25 Again, the discussion about NO formation agrees with the fundamentals of NOx formation, however, no reference is found in this discussion. There can be numerous papers that agree with the authors’ results but none of them are referenced here.

 

References have been added : Heywood, Lavoie, Bowman, Zeldovich, Raine, Fenimore, Yang

 

26 Line 279 - probably a typo in the bracket ‘et’.

 

Done

 

27 Line 307-309 – when the combustion happens late, the piston is already coming down in the expansion stroke, with the increase in volume taking place. This will result in lower combustion temperature, which is what usually happens in late combustion. How do you have more temperature increase then?

 

I think there is a misunderstanding about this particular point. As stated in several articles, a late combustion in the chamber gives less time for the mixture to burn, less time for the fluid to work, and also less time to exchange energy with the cylinder wall. As a consequence, a degradation of the efficiency is usually observed, accompagned with a higher exhaust temperature. See for example :

Zareei, J., Kakaee, A.H. Study and the effects of ignition timing on gasoline engine performance and emissions. Eur. Transp. Res. Rev. 5, 109–116 (2013). https://doi.org/10.1007/s12544-013-0099-8

 

28 Line 318 – should it be ‘coupled’? Please make it clear.

 

29 Line 333 – Spell check – Stoichiometric. Same at other places too (Line 410). Check them all.

 

Done

 

30 All Table captions are below the table, usually, they should be above the table. Check with the publishers guidelines.

 

31 Figure 30 caption: ’emissions’ spelling.

 

Done

 

32 Line 430 – ‘Sense’ spelling

 

Done

 

33 Section 4.1 – this is the research gap the authors’ talk about here and should be towards the end of the Introduction section where research gap is discussed and why this work is taken up is discussed.

 

The introduction has been modified.

 

34 Line 440 – remove air to fuel, only say mixture.

 

Done

 

35 There are a number of issues with the language and grammar noticed in the article. I suggest that the article be proof-read to improve / correct the language / grammar (e.g. use of articles: a, an, the are seen to be incorrect at a number of places, several typos, spelling mistakes)

 

The paper has been extensively corrected

 

 

Author Response File: Author Response.pdf