An Analytical Study of the Elements of Airworthiness Certification Technology Based on the Development of the Conversion of Diesel Engines for Vehicles to Aviation

Round 1

Reviewer 1 Report

The authors perform a comparative study between two diesel engines. The OM640 designed for automotive applications and the AE300, designed for aerospace applications. The authors have disassembled both engines and analyzed the modifications made with the engine conversion. The authors have then analyzed the airworthiness of the aero engine and classified its components. The authors have also grouped the components according to CS-E items.

Overall, the manuscript is well structured with proper English. The reviewer acknowledges the remarkable work of engineering of disassembling two engines and figuring out the conversions done from one to another. The reviewer, however, fails to see novelty in the work presented, as well as the research gap(s) the manuscript attempts to fill. Both engines have been long available in the market and their documentation is ready available (as well as their certification specification).

The reviewer hopes that the authors can clarify this in the subsequent points in order to convince the reviewer:

- What is the intended novelty of the paper? What are the research gap/s intended to fill with this analysis? In other words, what does the manuscript bring to the state of the art?

- The AE300 engine is already 15 years old. How does its conversion technology still be the state of the art?

- Lines 50-54, 365-367: The study seems to be focused on Korea. However, the title is kept generic. 1. Do the authors plan to give broad recommendations that can be used worldwide? 2. Or are the recommendations more country-specific? On the first scenario this reviewer would ask to reconsider the aforementioned lines as they particularise on a specific country.  On the second scenario, this reviewer suggests a more specific title.

In any case, great care should be put when particularizing for a specific country. Does Korea's standards of airworthiness follow EASA's recommendations? If so and in the second scenario the authors should be more specific.

Minor points:

- Table 4, specify class A, B or C to be consistent with the discussion.

- Figure 5, if taken from the maintenance manual, should require a proper citation instead of a clarification between parenthesis. Especially since the citation exists (number 42 perhaps?).

- Citation 21, 28, 29, 32, 40, 42, 43, 44: who is the publisher? at which journal? Definitely more information is needed.

- Citation 38: please fix the names.

Author Response

<Response to Reviewer 1 Comments>

Point 1: Overall, the manuscript is well structured with proper English. The reviewer acknowledges the remarkable work of engineering of disassembling two engines and figuring out the conversions done from one to another. The reviewer, however, fails to see novelty in the work presented, as well as the research gap(s) the manuscript attempts to fill. Both engines have been long available in the market and their documentation is ready available (as well as their certification specification).

The reviewer hopes that the authors can clarify this in the subsequent points in order to convince the reviewer:

- What is the intended novelty of the paper? What are the research gap/s intended to fill with this analysis? In other words, what does the manuscript bring to the state of the art?

 Response 1: The main novelty of this paper is to provide a systematic understanding of the process of converting conventional vehicle diesel engines to aviation diesel engines. This makes it possible for us to analyze the basic structure for the transition from automotive to aviation engines and, through this, identify the necessary modifications to make the engines comply with airworthiness certification standards. Currently, there is little development of aviation diesel engines in each country, and most of them are dependent on imports. This study serves as an initial step towards addressing these research gaps. In addition, since it takes a lot of time and money to separately develop an engine for aviation, it is considered economical and efficient to convert and develop an engine for a vehicle that has already been developed for aviation. Nevertheless, because there is not much market demand for aviation reciprocating engines, there are not many known projects to convert vehicle engines to aviation. Considering that gas turbine engines have been developed for aviation from the early stage of development, the conversion and development of automotive engines to aeronautics is considered an area with sufficient research value in terms of economics and shortening the development schedule. This paper brings he state of the art to present a methodology for converting conventional vehicle diesel engines to aviation ones. Through this, we can expect substantial contributions to the growth of the UAM industry.

Point 2: The AE300 engine is already 15 years old. How does its conversion technology still be the state of the art?

Response 2: It was developed 15 years ago, continuously modified and supplemented, and is still being mass-produced while satisfying airworthiness certification and standards, so it was judged to be the latest technology. In other words, the AE300 engine is proof that it is continuously being upgraded, and it was selected as an engine that satisfies the minimum airworthiness standard. In terms of engine technology, direct fuel injection, which is currently applied to most vehicles, is applied to improve engine fuel efficiency, and FADEC (Full Authority Digital Engine Control) system is applied to automate engine control, thereby optimizing performance. In addition, a high-efficiency turbocharger is applied to minimize performance degradation as altitude increases. However, this is not different from the technology currently applied to diesel engines in general. Therefore, it was concluded that satisfying the airworthiness certification and standards is closer to the 'state of the art'. In aviation, even relatively cutting-edge technology cannot be developed unless it passes airworthiness certification and standards. In conclusion, even though mass production started 15 years ago, AD(Airworthiness Directive) and SB(Service Bulletin) continue to occur and are continuously improved, so it can be seen as the latest technology.

Point 3: Lines 50-54, 365-367: The study seems to be focused on Korea. However, the title is kept generic. 1. Do the authors plan to give broad recommendations that can be used worldwide? 2. Or are the recommendations more country-specific? On the first scenario this reviewer would ask to reconsider the aforementioned lines as they particularise on a specific country.  On the second scenario, this reviewer suggests a more specific title.

In any case, great care should be put when particularizing for a specific country. Does Korea's standards of airworthiness follow EASA's recommendations? If so and in the second scenario the authors should be more specific.

Response 3: As the title of the paper seems to cover a generally applicable topic, it was emphasized that this study was based on the most of country's UAM industry and diesel engine development within it. In other words, the research approach is applicable when many countries face similar problems. Therefore, the reason for specifying Korea is to emphasize the special context and relevance of this study, but does not imply that the findings cannot be applied in other countries. To clarify this point, the keyword Korea was deleted from the thesis.

Minor point 1: Table 4, specify class A, B or C to be consistent with the discussion.

Minor Response 1: Table 4 has been modified to Class A, B, and C to match the discussion.

Minor point 2: Figure 5, if taken from the maintenance manual, should require a proper citation instead of a clarification between parenthesis. Especially since the citation exists (number 42 perhaps?).

Minor Response 2: The caption of Figure 5 has been modified, and Reference No. 42, which is the maintenance manual for the AE300 engine, has been cited.

Minor point 3: Citation 21, 28, 29, 32, 40, 42, 43, 44: who is the publisher? at which journal? Definitely more information is needed.

Minor Response 3: Publisher and journal information of the references were entered.

Minor point 4: Citation 38: please fix the names.

Minor Response 4: The name of the reference has been corrected.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper “An analytical study of the elements of airworthiness certification technology based on the development of conversion of diesel engine for vehicles to aviation” has the following structure:

1.      Introduction

2.      Experimental devices and methods

2.1. Experimental devices

2.1.1. Engine selection

2.1.2. EASA airworthiness certification data

2.2. Research method

2.2.1 Classification of the engine components modified for aircraft use

2.2.2. Requirements for material selection and anti-loosening

2.2.3. Sensors subject to redundancy

3. Results

3.1. Classification of components modified for aircraft use

3.2. Requirements for material selection and anti-loosening

3.3. Sensors subject to redundancy

4. Conclusions

In the paper was analyzed a case where an automobile engine was applied as an aircraft engine. The engine components modified for the aircraft engine were classified. It was defined the major engine parts as fixed and alteration ones. The airworthiness-related alteration parts were identified. The conversion purposes into classes and was categorized. The criteria for selecting and applying sensors and separating sensors that must be made redundant from ones that are not subject to sensor redundancy was suggested. 

Remarks

1. There is no estimate of the cost of reengineering of the proposed technical solutions.

2. There is no description of the life test methodology.

3. It is not entirely clear why the engines in question were chosen as engine samples.

Minor editing of English language required

Author Response

<Response to Reviewer 2 Comments>

Point 1: There is no estimate of the cost of reengineering of the proposed technical solutions.

Response 1: In the case of automotive engines, it takes about 500 billion won in development costs to develop a new engine with the existing infrastructure in place. On the other hand, in the case of aviation engines, it is estimated that it will take less than 1/5 of the development cost when remodeling vehicle engines. (However, the operating cost is 10 to 50 times higher than that for vehicles, and this is because it is made to order on a small scale.) However, specific reengineering costs can vary depending on various factors. When taking reengineering costs for redundancy of sensors according to airworthiness requirements as an example, the type and complexity of sensors, the number of sensors, development costs (including software development, testing and verification, etc.), maintenance and operation costs, etc. must be considered. Therefore, in order to accurately estimate the re-engineering cost for sensor redundancy, specific information on the factors mentioned is required, but an accurate estimate was not known because the manufacturer did not disclose it. This means that the solution is still in its infancy, and because of the complexity of the technology, it can be difficult to accurately predict the time, resources, etc. required to redesign the entire system. Additionally, the market is uncertain, making it unclear to make cost estimates. Therefore, given these variables, it was judged that it is currently difficult to provide specific reengineering cost estimates.

Point 2: There is no description of the life test methodology.

Response 2: In practice, the maintenance cycle according to the aviation engine conversion is designed to be approximately 900 to 1800 hours. Since it has been verified through several years of testing and maintenance procedures, a replacement cycle of 1800 hours is generally applied to aviation engines that meet airworthiness certification without requiring a separate life test. TBO (Time Between Overhauls) of the AE300 engine specified in the maintenance manual is recommended to be 1800 hours, and for certain major parts, a maintenance cycle of 900 hours is recommended in consideration of safety. This TBO setting is a time calculated statistically and empirically through periodic inspection and maintenance in aircraft operation, and is widely recognized as a generally acceptable level considering the number and time of aircraft operation. For an injector, for example, 1000 hours → 1500 hours → 1800 hours were specified, but it was redefined as replacement after 900 hours of use due to various issues. The manufacturer did not disclose specific information about checking the wear and margin of the corresponding parts for each maintenance cycle. However, it is expected that the life test methodology for aviation engines will initially follow standard processes well known in the aero engine industry (Bench Tests, Endurance and Reliability Tests, Environmental Tests, Teardown Inspections, etc.)

Point 3: It is not entirely clear why the engines in question were chosen as engine samples.

Response 3: The target engine is an example of developing a vehicle engine (OM640) into an aircraft engine (AE300). In addition to the AE300 engine, there are engines such as the Centurion 2.0. The AE300 engine is still in constant revision and demand is occurring. Therefore, it was selected as a target engine for research because it is an engine that is currently in operation as well as a case of conversion development for vehicles. The AE300 engine has been applied to mass production since 2008 and about 8,000 units have been mass-produced by 2022, and is being applied to aircraft. And it has been used in various flight environments based on its stable efficiency and durability. In addition, after being developed in the past (15 years ago), it is still being mass-produced while satisfying airworthiness certification and standards. Therefore, it was determined that it is the latest technology applicable to the thesis. (This can be seen as the latest technology because AD(Airworthiness Directive) and SB(Service Bulletin) have been continuously issued and steadily improved since mass production began in the past.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear authors,

Thank you for your thorough review. In order to convince me for the points I raised during my previous review please provide justification (or example/s) for the following claims you make:

- "Currently, there is little development of aviation diesel engines in each country, and most of them are dependent on imports."

- "It was developed 15 years ago, continuously modified and supplemented, and is still being mass-produced while satisfying airworthiness certification and standards"

For your convenience, I am attaching some comments on your document. I have also highlighted these two claims on the document where I expect to see the justifications/examples requested.

Comments for author File: Comments.pdf

Author Response

<Response to Reviewer 1 Comments>

Point 1: In order to convince me for the points I raised during my previous review please provide justification (or example/s) for the following claims you make:

- "Currently, there is little development of aviation diesel engines in each country, and most of them are dependent on imports."

Response 1: Currently, airworthiness certification is being certified by FAA in the US and EASA in Europe. In order to obtain airworthiness certification for aviation diesel engines, it is necessary to understand airworthiness standards and develop related technologies, but it is very difficult to enter the aviation diesel engine market. The reason is that there are high technical barriers to self-development due to the difficulty of meeting airworthiness standards and implementing technology. As a result, countries around the world are dependent on imports from a small number of companies that develop and manufacture aviation diesel engines. Therefore, it was judged that information for implementation of airworthiness standards and related technologies of aviation diesel engines should be shared. In other words, the current aviation diesel engine market itself is not as large as that of vehicle engines, and since it is difficult to acquire related technologies such as airworthiness standards, the maturity of technology is not high, so most of them are developed and manufactured by specific companies. Therefore, through this paper, I would like to lay the groundwork for the development of aviation diesel engine technology. (Regarding this, we attach and share EASA certificates for each aviation diesel engine developer.)

Manufacturer	Model	Certification	Power
Technify Motors	TAE 125	2007-03-06	114 kW
DieselJet	TDA CR 1.9 8V	2010-06-11	118 kW
Austro Engine	E4P_AE300 Engine	2009-01-28	123.5 kW
SMA	SR305-230	2001-04-20	169 kW

 

Point 2: In order to convince me for the points I raised during my previous review please provide justification (or example/s) for the following claims you make:

- "It was developed 15 years ago, continuously modified and supplemented, and is still being mass-produced while satisfying airworthiness certification and standards"

Response 2: Even though mass production started 15 years ago, AD (Airworthiness Directive) and SB (Service Bulletin) are still generated and continuously improved, so it can be seen as the latest technology. In this regard, (E)AD and (O,R)SB issued from the 2009 to the 2023 are attached and shared. Additionally, from the engine technology perspective of the AE300, the fuel efficiency of the engine was improved by applying common rail direct fuel injection, and the engine control was automated by applying the FADEC (Full Authority Digital Engine Control) system. In addition, a high-efficiency turbocharger is applied to minimize performance degradation as altitude increases. This allows you to maintain optimal performance. In addition to these reasons, an additional reason for selecting the AE300 engine as a sample in this paper is that it is currently imported into Korea and can be used for analysis.

+ Response to Minor Points:

Regarding the quotation in the CTS part, it has been modified according to your feedback.

“Harness block diagram with CTS” →“Harness block diagram with CTS, extracted from the AE300 Maintenance Manual [45].”

Author Response File: Author Response.docx

Round 3

Reviewer 1 Report

Dear Authors,

Thank you for thoroughly replying to my questions.

Best,