## 2 Methods  
The methodology followed to develop this review is a summarized version of the PRISMA Statement [13] in which some steps have been merged. In the previous section, the rationale for the review in the context of what is already known was described and an explicit statement of the questions being addressed with reference to participants, interventions, comparisons, outcomes and study design was provided. This method has been mainly used in medicine but, given its success in evidence-based approaches, its use has been extended to other fields such as economy, education and engineering. PRISMA adopted the definitions used by the Cochrane Collaboration [14]. The PRISMA Statement consists of a 27-item checklist and a four-phase flow diagram (see [13]). The aim of the PRISMA Statement is to help authors to improve the reporting of systematic reviews and meta-analyses. The general concepts and topics covered by PRISMA are all relevant to any systematic review. However, as PRISMA reports, some modifications of the checklist items or flow diagram will be necessary in particular circumstances. In Table 1, the five stages proposed in this work to follow the PRISMA Statement are shown. The first stage, the question formulation and objectives, is included at the end of the Introduction section. The two stages included in \"Methods\" are developed below. As in alternative methodologies, after presenting the justification and the objectives that motivate the research, the next stage is to locate the studies. The protocol followed to conduct the search is described as follows. The search began by querying citation databases using search strings and keywords. All the databases of the Web of Science and Scopus were considered. The keywords selected were those most often used in the literature to describe energy management systems (EMSs) that control the power flow of the components of hybrid electric vehicles (HEVs). Table 2 lists the 32 keywords identified in this stage. These keywords were combined to create the following string:  
* TI = (hybrid electric vehicle OR plug-in hybrid electric vehicle OR hybrid electric vehicle OR HEV OR PHEV) AND TI = (energy management OR control) NOT TI = (fuel cell OR FCV) \begin{table}
\begin{tabular}{c c} \hline \hline  & **INTRODUCTION** \\ \hline
1. & Question formulation and objectives \\ \hline  & **METHODS** \\ \hline
2. & Locating studies \\
3. & Study selection and evaluation \\ \hline  & **RESULTS** \\ \hline
4. & Analysis and synthesis \\ \hline  & **DISCUSION** \\ \hline
5. & Reporting and using the results \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Stages proposed in the systematic literature review following the PRISMA Statement. Specifically, following the PRISMA flow diagram, the use of this string resulted in 171 records in the identification phase. Then, a second phase of screening was carried out without excluding any record. Thirdly, the eligibility phase described below was conducted, reducing the records to 36. The criteria used for including or excluding studies were formulated to refine the search and to only retrieve the most relevant literature. The main reasons of exclusion were the following: (1) the vehicle considered for the study was a fuel cell vehicle or a pure electric vehicle; (2) the vehicle considered for the study had a topology that is not common in the HEV industry; (3) the vehicle considered for the study had components that are not commonly used in commercial HEVs; (4) the EMS proposed was tested using a driving cycle that was not standard and whose data were not available; (5) the topic of the paper did not correspond to the development of a new EMS for HEVs; (6) the study was focused on the charging infrastructure of EVs and/or HEVs and its EMSs; and (7) the study did not provide enough information to reproduce the test in the cases that it is necessary. Also excluded from our study were dissertations, textbooks, conference proceedings, working papers and other unpublished work.