## 6 Conclusions  
The engineering process (EP) of a digital System of Systems (SoS) requires a change in perspective and approach to address the evolving requirements of the Industry 4.0 domain: flexibility and evolvability, an increased level of automation, multistakeholder management, full life cycle support, cost reduction, and environmental sustainability. Architectural models addressing the life cycle and the value chain, such as RAMI 4.0, are available, but are affected by scalability limitations and are difficult to adopt in real industrial use cases.  
Therefore, in this paper, we present an enriched set of models, maps, and tools to simplify and make the modelling and execution of the engineering process of the Industry 4.0 use case more efficient and cost-effective. The proposed solution models and highlights a large amount of complementary information associated with the EP, including the phases composing the EP, the relations and the connections between them, the tool chains, the tools, the structure of the associated value chain, the stakeholders involved in system life cycle management, and their relationships.  
Finally, we discuss how the Arrowhead-EP model can be used to manage the entire life cycle of the smart boiler use case, and we discuss the advantages of adopting this new EP model in a real vertical application. Thus, we address the main objectives identified in the literature gap analysis.  
At the moment, we define the Arrowhead-EP and VCEP-map ontology for describing the EP of relevant industrial use cases, while future works will be aimed at integrating the proposed technologies in automatic working pipelines designed to manage the life cycle of products and services. A future solution will be based on the Eclipse Arrowhead framework, which provides the services and functionalities required to simply automate EP tools and tool-chains.  
Conceptualisation, G.U. and P.A.; methodology, G.U., P.A., J.v.D. and J.D.; software, J.v.D.; formal analysis, G.U. and A.M.; investigation, G.U. and P.A.; resources, A.M., E.M. and J.D.; writing--original draft preparation, G.U., P.A., J.v.D. and J.D.; writing--review and editing, G.U., P.A. amd J.v.D.; supervision, G.U. and P.A.; project administration, J.D. and E.M.; funding acquisition, J.D., P.A. and E.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the European Commission through the EU H2020 research and innovation programme, ECSEL Joint Undertaking, and National Funding Authorities from 18 countries under the Arrowhead Tools research project with Grant Agreement no. 826452.  
Not applicable.  
The authors declare no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} Arrowhead-EP & Arrowhead Engineering Process \\ c & Connection \\ EAf & Eclipse Arrowhead framework \\ EP & Engineering Process \\ EPC & Engineering Process Connection \\ EPI & Engineering Process Interface \\ EPM & Engineering Process Mapping \\ EPMS & Engineering Process Management System \\ EPP & Engineering Process Phase \\ EPU & Engineering Process Unit \\ EP-I & Engineering Process Phase Input \\ EP-O & Engineering Process Phase Input \\ HW & Hardware \\ IIoT & Industrial Internet of Things \\ IoT & Internet of Things \\ RAMI 4.0 & Reference Architectural Model Industrie 4.0 \\ SC & System Component \\ SBS & Smart Boiler System \\ SOA & Service Oriented Architecture \\ SoS & System of Systems \\ StkH & Stakeholder \\ SW & Software \\ T & Tool \\ TC & Tool-Chain \\ UC & Use Case \\ VCEP-map & Value Chain Engineering Process Map \\ \end{tabular}