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Highlights • Analysis of Oxy-Hydrogen (HHO) use as fuel additive in internal combustion engines. • Energetic investigation of the interactions among engine and auxiliary components. • Innovative definition of HHO nature and of Unchanged Fuel Consumption Limit (UFCL). • Validation of energy needs for HHO production based on literature data. • UFCL evaluation for different engines and operations based on literature data. Abstract The present study aims at assessing, from a theoretical point of view, the feasibility of on-board production and use of Oxy-Hydrogen (HHO) as fuel additive in internal combustion engines. From the available literature, many works deal with the experimental investigation of the benefits achieved with HHO use in both compression and spark ignition engines in terms of brake thermal efficiency improvement and emissions reduction. Nevertheless, a thorough demonstration of the feasibility of HHO on-board production and use has not been performed yet, also due to the unclear nature of the generated HHO mixture. To bridge this gap, a rigorous analysis of the energetic needs related to HHO production through electrolysis, with the internal combustion engine as unique power supply, is here described in details. An innovative assumption is proposed to achieve a generalized representation of HHO mixture and univocally determine the energetic requirements related to its production. A conventional layout is considered to define realistic restrictions for HHO use and fuel savings achievement. Several case studies, based on works available in the literature, are investigated to prove the validity of the proposed analysis and assess the feasibility of the technological approach. [ABSTRACT FROM AUTHOR]