Research on the Operating Characteristics of Hydraulic Free-Piston Engines: A Systematic Review and Meta-Analysis

Round 1

Reviewer 1 Report

Dear Authors,

This paper describes the known design principles and applications of the hydraulic free piston engine. The paper is written well regarding the contens and is properly structured. The advantages and disadvantages of particular solutions are described. In my opinion the overall efficiency could be compared for individual solution inclusive the the efficiency of the internal combustion engine, if such comparison is possible. Based on that the solution with the best design features could be defined.

Author Response

Thank you for your useful comments and suggestions on our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point.

Author Response File: Author Response.doc

Reviewer 2 Report

Dear Authors,

As the energies guideline says structured reviews and meta-analyses should use the same structure as research articles and ensure they conform to the PRISMA guidelines. The review was not done this way. It looks like the authors just telling the existing knowledge.

Almost every (38) point of the PRISMA guideline is missing.

Authors have an incredibly large bias to the mentioned universities and ignoring the rest of the world.

There are 32 figures in the paper without any citation, if figures were taken from publications, authors should present written permission from the authors.

From 2019 2pcs; 2018 1pcs; 2017 2pcs; 2016 4pcs (9/83 in the past 5 years) papers presented seems this technology is not relevant due to the lack of research papers. This lowers the scientific impact of the paper. Some reference can be considered to ancient (1941); readers would like to know from a review what is new, and where should they aim their research.

Referencing should be done according to the referencing guideline. Of the 83 publication, 25 is a conference paper, in reviews, these usually ignored and focusing on research papers.

The conclusion is subjective, it is more like a summary of the presented papers.

Yet I cannot elaborate the total work, since the literature research is not complete or readers do not know the limitations of the review.

What to do, how to improve the paper:

According to PRISMA guideline 2020, please make proper literature research focusing on the past 5 years research papers related to this topic.

Summarise the conclusion of previous review papers if it exists in this field (eg. ref 3 and 75).

Cite the figures and ask permission to use the figures.

Make an objective conclusion.

Avoid world etc and some and name or quantify everything in the review.

Author Response

Thank you for your useful comments and suggestions on our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point.

Author Response File: Author Response.doc

Reviewer 3 Report

There are few comments need to be addressed before considering publishing this manuscript: 

1. I wish I could see some theoretical parts in this manuscript.  There are already so many papers out there trying to analytically increase the efficiency of Piston Engine.  I would like to see more on what has been done and what has left as future works in terms of the analytical treatment of the problem under considetation.

2. Figures from different publications should be used once after getting the proper permission from the publisher.  Not sure if the authors have done this!

3.  English of the manuscript should be revised as some of the sentences are not really clear. 

4. Scale bar should be added to the figures to give the reader an idea about the length scale of the items in the figures. 

5. More information regarding the initial and boundary conditions should be given for the simulations. 

Author Response

Thank you for your useful comments and suggestions on our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point.

Author Response File: Author Response.doc

Reviewer 4 Report

The work deals with very important issues related to the development of HFPE engines.

However, this work is a review publication and does not contain any scientific research. Such works are also of great value. There is no significant mention in the introduction of the scientific value of the work. Please express it more. How important is such an analysis. In my opinion, there are also many publications on other issues missing.

It is worth mentioning the problem of the formation of an oil film and the lubrication of the piston rings and piston. It is one of the most important parts of the engine. The oil film distribution is strictly dependent on the reciprocating movement. In particular, the prevailing pressures in the combustion chamber and the displacement speeds. Additionally, it is influenced by the dynamic viscosity of the oil and the shape of the rings. This, in turn, affects the durability of HFPE engines and the amount of oil used. It is worth adding a few publications in this regard. I propose:

Wróblewski, P., Iskra, A., Problems with reduction of configuration friction losses piston-ring-cylinder in combustion piston engine with increased isochoric pressure increase, Event: SAE Powertrains, Fuels & Lubricants Meeting, SAE Technical Paper 2020- 01-2227, SAE International USA, 2020, https://doi.org/10.4271/ 2020-01-2227 + Add some from this range.

It is worth emphasizing and briefly describing it. The rest is analyzed in an interesting way. The drawings are correctly selected. Pay attention to the copyrights of the drawings.

If they are the authors of this publication, no problem. If not, permission from the publisher and author must be obtained. Sometimes a quote alone is not enough. The conclusions are correctly formulated. They should take into account the factors of engine durability and problems related to their operation from that point on.

Author Response

Thank you for your useful comments and suggestions on our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point.

Author Response File: Author Response.doc

Round 2

Reviewer 2 Report

The review still has a bias towards Chinese universities, it is not a problem since it is mentioned in the abstract. The literature searching method is still not mentioned (what was included and excluded). Please mention the limitations of your review also at the conclusions.

Author Response

Thank you for your useful comments and suggestions on our manuscript. We have modified the manuscript accordingly, and the detailed corrections are listed below point by point.

Author Response File: Author Response.doc