Determination of Optimal Piston Trajectories for High Efficiency 4-Stroke Cycles by Using Predictive Combustion Modeling

Round 1

Reviewer 1 Report

The present manuscript presents determination of optimal piston trajectory for high efficiency

4-stroke cycles by using predictive combustion modelling. This work is interesting, and the manuscript is well presented however the topic is not novel and it has been described many times.

 

The work is interesting. It describe the optimization of the volume change as a function of pressure (heat cycle). The developed solution does not have to concern the piston trajectory. This pressure change as a function of volume don’t need to be effected by a piston in the conventional sense of a crank-piston arrangement. The required volume change can be partially achieved by, for example, an additional piston in the engine head. (see publication)

 

A. DABROWSKI1), M. GLOGOWSKI2) and P. KUBIAK3)* IMPROVING THE EFFICIENCY OF FOUR-STROKE ENGINE WITH USE OF THE PNEUMATIC ENERGY ACCUMULATOR-SIMULATIONS AND EXAMINATION International Journal of Automotive Technology, Vol. 17, No. 4, pp. 581−590 (2016)

 

In my opinion the title should be change and the article should be rewritten. It is better to write “Determination of optimal engine cycle for high efficiency 4-stroke cycles by using predictive combustion modelling”

 

Then the last paragraph is not necessary since there will be no discussion about piston or crank mechanism.

I would suggest the authors to proofread and do some changes based on the following points:

It will be interesting to add more references in the introduction. The references should focus on engine cycles and its efficiency more than on crankshaft mechanisms.

What is the novelty in this paper? Please compare with the other publication relative to this topic and underline your contribution to this research area.

What is the limit for the piston acceleration? Base on what the value 100 000m/s^2 is assumed.

 

Figure 4 shows the results for infinite acceleration – I understand the simulation were done without acceleration limitation but what was the value of acceleration calculated by software?

 

Figure 4 shows the results for infinite acceleration – Table 1 -  520 000 m/s^2 instead

 

Figure 6 – there are 4 curves on the graph (2 black, red and blue), the legend presents only 3.

 

Figure 5 and 14 also shows infinite value but there is a finite point on the graph.

 

Summarizing the paper novelty is low but it can be published after major revision

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors present a very interesting piece of work, very relevant for the engine community, taking into account the new technologies available nowadays. Somehow, they are using critical thinking to reconsider some well-established facts in standard engines, as is the case of the piston movement imposed by the crank-rod-piston mechanism. Even if the scope of the paper is very interesting and relevant, there are some issues that could be improved. My comments try to help in this sense.

Here are some major comments:

·         Page 3, line 127. What is “pinion shaft”?? Shouldn’t it be “transmission shaft” or similar??

·         Page 4. Based on equation (2), I deduce that the authors are neglecting the blow-by losses (i.e. in-cylinder gas leakages through the piston-liner small passage). I’m convinced that this is probably the most reasonable way to proceed (since nothing will change in relative terms, nor in conclusions of the study, if the blow-by is considered), but probably it is important that the authors indicate this assumption. Like this, you show that you are conscious of your choice, and it is not an oversight.

·          Page 5, equation (12). I cannot see this equation in the reference provided by the authors. Where exactly they have taken it?? In my opinion, the equation provided by the authors is better that the one provided by Heywood, but I wonder where the equation comes from.

·         Next page, equation (13). Same comment: this equation is not from Heywood. I’ve represented its evolution against y_res, and it seems totally correct. But where have you taken the equation??

·         Same page, section 2.1.5. Model verification. It is not enough to simply say “in this case, the simulation shows good agreement with the measurement”. The reader will be more convinced of the accuracy of the model if you show him this validation. Probably, you should provide a plot with the corresponding comparison, with some details on the engine you have tested, as well as the operating conditions.

·         Page 7, equation (19). Something is wrong with this equation: as it is written, the piston position is the same for 0 and 180 CAD, which makes no sense… L

·         In this equation, what r is?? I assume that is the stroke / 2, but you should clearly indicate this.

·         Equation (20). It is also incorrect, because the product of a volume and a distance cannot be “mixed” with a volume. The second term should be Ap (piston area) * (xp-r) or similar.

·         Equation (21). This equation is hard to be understood. This is indicated as being P(t), but nowhere on the right of the equation the independent variable, t, appears. How can I use the equation??

·         End of page 8, line 287. The authors indicate that the piston velocity is not a problematic factor (compared to acceleration), and they introduce a limit of 100 m/s. I disagree with the authors, since the average piston speed is limited due to lubricity and friction losses constraints. Consequently, you should reconsider this excessively high level for the piston speed. If a limit in average piston speed is ~20 m/s (in production engines), probably the maximum piston speed could be 50 m/s, as a limit (or probably still lower). How can this impact the results??

·         Page 9, figure 4. It is unclear which plot corresponds to full load, and which one to part load (besides, it is more appropriate to write “part load” rather than “partload”). Just indicate: “…full (top) and part (bottom) load operation”.

·         Page 14, figure 13. Same problem than before. Indicate if full load is the right or the left plot, and accordingly with part load.

·         Page 16. Besides the section “abbreviations”, you should probably include a “nomenclature” section, to indicate the meaning of the different symbols used in the paper. This is always helpful for the reader.

And now some minor comments:

·         Page 2, line 73. “Both approaches aim to reduced the air…” should be “reduce”.

·         Page 3, line 116. “…considered as a appropriate solution” should be “an”.

·         Page 4, line 163 (at the end): “…during motoring operation and…” A colon should be added before “and”: “…operation, and…”

·         Page 8, line 255. “…design includs a gradual…” should be “includes”.

·         Caption of figure 3. End of first line: length is misspelled (lenght).

·         Line 276. “naturally aspired engine” should be “aspirated”.

·         Page 12, line 357: “…also due the simulation…” should be “due to”.

·         Page 13. Caption of figure 12. At the end you put “..”, and it should be “.”.

·         Page 14, line 386. “…which highl temperatures…” should be “high”.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

thank you very much for your answer. I read the paper in the new form. I still think that you focus too much on the piston trajectory instead of volume change, however the theoretical equation that you have provided in your paper are very valuable for the readers. Also, based on your work, the engine designers could set the goal for their work. Good job!