BIBLIOMETRIC STUDY OF LIFE CYCLE ANALYSIS APPLIED TO MOTOR VEHICLES

Purpose: Conducting a bibliometric analysis of papers on the Life Cycle Assessment (LCA) of motor vehicles published over the last 20 years. Theoretical framework: Bibliometric analysis, a discipline within scientometrics, uses mathematical and statistical methods to synthesize and evaluate publications, identifying trends and structures in specific research fields. Method: The bibliometric analysis was conducted using data from Scopus and Web of Science, and it assessed the impact and trends of scientific production, correlating keywords to explore connections between themes. Articles from the last two decades were analyzed to quantify the relevance of the studied topic. Results and conclusion: Using the Bibliometrix algorithm in R, 512 published articles were analyzed. The results indicated a significant increase in research output in this field of study, as demonstrated by the substantial growth in the number of published articles and citations. Research implications: This bibliometric study reveals key trends and guides future innovations in the environmental and transportation sectors, highlighting the transition towards more sustainable options. Originality/value: This groundbreaking work provides valuable insights into the life cycle of vehicles with various propulsion technologies, emphasizing the evolution and environmental impacts of electric, hybrid, internal combustion, and fuel cell vehicles.


INTRODUCTION
The transport sector plays a vital role on the global stage and is one of the essential pillars for the economic, social development of countries.The sector accounts for 26% of final energy consumption in the world, with a strong dependence on petroleum products such as gasoline and diesel, corresponding to 91% of energy consumption (International Energy Agency, 2023).In relation to carbon dioxide emissions, the transport sector emitted about 8 gigatonnes of CO2 into the atmosphere in 2022.Of this total, around 5.9 gigatons are allocated specifically to road transport (International Energy Agency, 2023).
In Brazil, the share of the transport sector in energy consumption is 33%.This percentage, higher in comparison to the world is explained by the broad dominance of road transport in the country (Baran & Legey, 2013).In 2022, the total CO2 emissions equivalent to transportation in the country were 210.4 million tons, representing almost half of the anthropic emissions associated with the Brazilian energy matrix (Empresa de Pesquisa Energética, 2023).
The electrification of motor vehicles represents a significant transformation in the transport sector, with direct implications for greenhouse gas (GHG) emissions.Such change is crucial in the context of climate change and global efforts to reduce environmental impact.At this juncture, battery electric vehicles and fuel cell vehicles, as the most prominent electrification process technologies, will play a vital role in the energy transition to a lowcarbon economy in the transport sector (Bonsu, 2020).Although the electrification process has been slow in Brazil (Grangeia et al., 2023).However, some government efforts have been outlined.In 2022, the National Hydrogen Program (NHP2) was established with the aim of promoting the use of hydrogen, mainly produced from renewable energy sources, in the national energy matrix, including its application in fuel cell vehicles.
On the other hand, as regards biofuels, from the implementation of the National Alcohol Program -ProAlcohol (Gonçalves et al., 2021) in the 1970s until the establishment of the National Biofuel Policy -RenovaBio (Grangeia et al, 2022)  As noted, Life Cycle Analysis has been widely used in government programs as well as in more sustainable business management, assisting in more informed decision making (Guinee et al., 2011).In short, LCA is a method that estimates the energy consumption and greenhouse gas (GHG) emissions associated with a product throughout all stages of its life.Specifically, in the context of LCA for automotive fuels and other propulsion energy sources, the well-to-wheel analysis (WTW) has received considerable attention and can be divided into two sets of processes: well-to-tank (WTT) andtank-to-wheel (TTW).WTT encompasses processes such as raw material extraction, fuel production, storage, distribution to fueling stations, and refueling itself.TTW represents the operation of the vehicle, where the fuel is consumed to generate energy that drives the vehicle (Edwards et al., 2004).
Due to the growth of studies related to Life Cycle Analysis in recent years and the diversity of propulsion technologies of automotive vehicles, bibliometric mapping emerges as an important tool to present a quantitative and descriptive overview of this area of knowledge.In addition, bibliometric analysis methods minimize subjectivity and increase the reliability of discoveries (Bernatović et al., 2022).
The purpose of this study is to provide an in-depth assessment of the existing publications on Life Cycle Analysis (LCA) applied to internal combustion vehicles, hybrids, battery electric and fuel cell.The approach uses methods of bibliometric analysis, based on articles extracted from Scopus and Web of Science databases in the period 2003-2022 using the R programming language and VosViewer software.

THEORETICAL FRAME
Bibliometric analysis is a recent discipline within scientometry that uses mathematical and statistical methods to analyze and evaluate scientific publications.This allows researchers to explore the current status and emerging trends in academic literature of a specific field of research (Khanra et al., 2021).
As the number of studies in various fields grows, the main benefit of bibliometric research is its ability to summarize large amounts of scientific publications, thus presenting the intellectual structure and new research trends of the field examined (Donthu et al., 2021).Bibliometric analysis offers an advantage over alternative methods such as systematic literature reviews or meta-analyzes in terms of reducing subjective bias in literature reviews (Bernatović et al., 2021).
Bibliometric research combines performance analysis and scientific mapping analysis (Donthu et al., 2021).The performance analysis evaluates indicators of publication activity (Cobo et al., 2011), highlighting significant contributions to a field of research.Techniques include publication frequency, citation frequency, highly cited publications, and unit counts of analysis (authors, countries, institutions, sources, etc.).Scientific mapping analyzes relationships between research constituents, providing a spatial view of the connections between different scientific units (Caputo et al., 2021;Donthu et al., 2021).Techniques here include citation analysis, citation analysis, bibliographic coupling, keyword cooccurrence, coauthoring analysis, explosion detection analysis, and timeline analysis, all enriched by network metrics, clustering, and visualization, to portray the intellectual structure of the field (Donthu et al., 2021).
The analysis of citations evaluates the impact of a publication through citation counting (Donthu et al., 2021;Khanra et al., 2021).The cocitation analysis identifies publications linked by shared references in another publication bibliography, suggesting thematic similarity and providing a dynamic representation of a topic (Caputo et al., 2021;Donthu et al., 2021;Wanget al., 2021;Ferreira, 2018).
Another method, the bibliographic coupling analysis explores connections between articles that cite a third common article (Caviggioli & Ughetto, 2019), offering a static representation of the topic, because the references of an article do not change over time (Caputo et al., 2021;Ferreira, 2018).The cooccurrence of keywords, a form of content analysis, measures the frequency of the pairing of keywords in publications, illuminating the conceptual structure of a field of research (Khanra et al., 2021;Wang et al., 2021).
To identify emerging trends and challenges in a research field, you can apply explosion detection and timeline analysis.Explosion detection analysis highlights items that quickly gain attention, indicating new research trends (Li & Xu, 2022).Timeline analysis identifies new research trends over time, facilitated by clustering algorithms that reveal potential transformative clusters (Wang et al., 2021).
Recently, bibliometric analysis has gained popularity on issues related to the mitigation of greenhouse gas emissions (Zhang & Liang, 2020).Specifically in the area of Life Cycle Analysis (LCA), some researchers applied bibliometric methods and obtained remarkable results (de Souza & Barbastefano, 2011).
In one study, indexed LCA articles were investigated in the Web of Science database and the internal structure was analyzed in detail (Chen et al., 2014).In some papers, analyzes of articles related to Life Cycle Analysis were carried out in a general manner (Qian, 2014;Hou et al., 2015).While, other works focused on different areas of interest, such as bioenergy (Li et al., 2018), buildings (Nwodo & Anumba, 2019), treatment of waste water (Furness et al., 2021) and bioethanol production (Santoyo-Castelazo et al., 2023).The latter used the Web of Science and Scopus database for bibliometric analysis.

MATERIAL AND METHODS
Bibliometric analysis is an important tool for assessing the impact, relevance and trends of scientific production over the years.Furthermore, this analysis allows for the correlation of key words with the objetive of assessing the connections between various scientific themes.Accordingly, with the intention of quantifying the importance of the theme proposed in the scientific field, the bibliometric analysis was carried out on the basis of scientific articles from various magazines.
The information used in this analysis was collected from the Scopus databases, of the Elsevier company, and from the Web of Science (WoS), of the Clarivate Analytics company.They were selected for having two of the largest bibliographic materials in the databases (Pranckuté, 2021).In addition, they offer the ability to gather all the metadata needed for the assessment.
In relation to the research strategy, the identification and selection of the keywords associated with the object of the study were carried out through initial research on the topic, checking the main related articles.The research considered the articles published in the last two decades (2003 to 2022), being carried out in May 2023.Table 1 shows the search equations used for the two data sets.

RESULTS AND DISCUSSION
Through the filtering of the articles, 351 works were found at Scopus and 402 works on the Web of Science.From this, an algorithm developed in R was used for the processing of the collected data, called Bibliometrix.This tool transforms this data in order to obtain indicators related to bibliometric analysis (Aria & Cuccurullo, 2017).Besides transforming the data, the algorithm verified and removed the articles common to both databases, which added up to 241 articles.Thus, the compilation of the collected data resulted in 512 articles.Table 2 shows the number of articles (P), number of citations (TCS) and the average of citations per article (MCS), which are used to evaluate the scientific production of the articles collected in the last 10 years.When analyzing the number of articles, it can be observed that in 2013, 23 articles were published, while 93 papers were published in 2022.This means that, in the short period, there has been an increase of more than four times in the production of articles.
A second way of appreciating the preponderance of subjects is by examining the number of citations that have been made each year.It should be noted that 2015 was the year with the highest number of citations, with a total of 2,489.
Another way of gaging the relevance of a topic is to examine the number of authors per year, and in this case an exponential growth can be seen over the years, which confirms the relevance of the theme of life cycle analysis of motor vehicles in academia.In all, considering the last twenty years, 1,415 persons were indicated as authors in the scientific works presented.
A fourth indicator of the significance of the object of study is the average number of citations per article in each year.In the bibliometric analysis, it is important to note that in recent years it is common to present low average citations per document, because the reference time is shorter.In addition, the smaller the number of articles the larger the index.The year with the highest MCS index as shown in the table was 2013 with 85.87 and the lowest was 2022 with 6.30.The average number of citations per article of the papers collected in the survey was 41.53.
Finally, the number of authors does not vary significantly during the evaluation period, reflecting an absolute mean value of 4.11 and a growth trend, as can be seen in the last 5 years, in which they recorded average values higher than the absolute average.Figure 1 shows the exponential growth of the total number of published articles related to the study theme.In the first ten years, the increase is slower compared to subsequent years.As of 2019, curve growth increases and jumps each year are higher until they nearly double between 2020 and 2021, demonstrating a typical behavior of an exponential evolution.The annual growth rate was 26.94%.Figure 2 shows the number of citations made each year.From 2013 onwards, the number of citations increased considerably.The number of citations in that year more than doubled in relation to the previous year.Although there was a fluctuation in citations between 2013 and 2022, the average in the period remains above 1000 citations per year.The drop in the number of citations in 2022 is consistent, as it is the most recent year of published articles.
The keywords used to collect the articles in the research in the databases are crucial for the bibliometric study, since the frequency of these terms in the works expresses the pertinence of the subject in the academic environment.In this case, Table 3 establishes the twenty most frequent keywords over the last ten years of research.The key words "greenhouse gases", "life cycle" and "life cycle analysis"are predominant and show a trend of growth, while the others show an oscillation of their employment over the years.It is also worth highlighting the significant number of variations of the same term or subject.Figure 3 presents a graph with the 25 keywords most related to the proposed theme.Among the topics related to vehicle propulsion types, such as electric vehicle or gasoline, electric vehicles are among the most widely used subjects.It is also worth emphasizing that the keyword "fuel cells" has obtained a growth of occurrences in works over the last three years, which proves the recent attention given to this theme.In Figure 4, the most relevant keywords are shown again, however, this time in the form of a network diagram that allows the observation of the relationships between the keywords based on their proximity, quantity of connecting edges and the density measured by their respective occurrence.This view was generated through the VOSviewer software.
It can be seen from the diagram that the terms referring to electric vehicles are diametrically opposed to those related to hydrogen, which demonstrates the separation between the works on the so-called purely electric vehicles and the fuel cell electric vehicles.Another notable point is the low density of hydrogen keyword edges with the other keywords, attesting to the accuracy of performing work on this theme.Table 4 shows the twenty countries with the largest productions of articles related to the research theme.The first aspect to be highlighted is the presence in the top ten positions of countries located in the northern hemisphere (with the exception of Australia) and that have a competitive technological market.Another important point is that these countries are also the biggest emitters of greenhouse gases.The predominance of the United States and China in terms of both the number of articles and the number of citations can also be noted.Figure 6 shows the graph of the number of citations for each country.It should be noted that, in spite of the low number of articles, Brazil has an acceptable average of citations per article (35.7 citations/article) taking into consideration the other positions of the ranking.Even better is the situation in Norway, which, with only 10 articles, has 1589 citations, bringing an average of almost 159 citations per article.On the other hand, Germany presented an average of only 18 citations per article in the period.

CONCLUSIONS
In this study, a comprehensive bibliometric analysis was conducted to assess trends and development of academic research on the life cycle of motor vehicles of different propulsion technologies over the past decades.The results highlight a remarkable growth in scientific production in this area, as evidenced by the significant increase in the number of articles published.This growth reflects not only the growing interest, but also the growing importance of the topic in the global scientific community.
Furthermore, the study revealed a predominant focus on electric vehicles, an area that has received considerable academic attention.However, it is important to highlight the need to expand research to include life cycle analyzes related to fuel cell vehicles.This technology, a promising alternative in the electrification of the transport sector, has not yet been explored to the same extent as purely electric vehicles.
In short, this bibliometric analysis provides valuable insights into the development and trends in life cycle research of automotive vehicles, highlighting the area as a dynamic and expanding field in scientific research.This study not only reflects the growing interest and importance of the topic, but also serves as a guide for future investigations and collaborations in the field.
in 2017, these biofuels have acquired a central role in the country in order to achieve the National Commitments Determined by Brazil under the Paris Agreement (United Nations Framework Convention on Climate Change, 201015).RenovaBio includes Life Cycle Analysis (LCA) and financial instruments, supporting national energy security and reducing GHG emissions.It focuses on increasing the consumption of biofuels and expanding it in the Brazilian energy matrix, encouraging producers and creating an open market for carbon reduction credits called Decarbonization Credits (CBIO).In 2021, the Federal Government also launched the Fuel of the Future program with the aim of increasing the use of low-carbon fuels and decarbonizing the transport matrix.It proposes measures such as the full assessment of energy and environmental efficiency through LCA.In addition, this program seeks to integrate with other government programs, such as RenovaBio itself, the National Program for the Production and Use of Biodiesel (PNPB), the Air Pollution Control Program for Automotive Vehicles (Proconve), the Rota 2030 Program, the Brazilian Vehicle Labeling Program (PBE Veicular) and the National Program for the Rationalization of the Use of Petroleum and Natural Gas Derivatives (CONPET).

Figure 1 -
Figure 1-Evolution of the number of articles published per year.Source: Authors (2023).

Figure 2 -
Figure 2-Evolution of the number of citations per year.Source: Authors (2023).
Figure 5 graphically presents the production of academic works carried out in the last twenty years and related to the proposed research theme.This chart further highlights the hegemony of the United States and China over the topic.It is also worth pointing out the scarcity of work carried out by Brazil on the theme.

Table 1 -
Search Equations

Table 2 -
Scientific production in the last 10 years.

Table 3 -
Most relevant keywords in the last 10 years.

Table 4 -
Countries with the highest number of published articles.