## 4 Conclusions  
This investigation reports the potential application of dual-fuel operation in SI engines using acetone-butanol-ethanol (ABE) and hydroxy gas (HHO) enrichment. We characterized the main operation and design constraints of an experimental setup that combines a 3.5 kW YAMAHA engine and HHO generation. The evaluated fuel blends were ABE 5, ABE 5 + HHO, ABE 10, and ABE 10 + HHO, and gasoline was the baseline fuel. The core findings of the investigation can be summarized as follows:  
* ABE standalone blends reduced both in-cylinder pressure and heat release rate compared to pure gasoline. Contrarily, hydroxy enrichment intensified the former and the latter while promoting a homogeneous fuel mixture. * Engine load directly affected the combustion phasing leading to advanced or retarded combustion in the range of 0.2\({}^{\circ}\)-1.2\({}^{\circ}\). * ABE-based blends increase BSFC between 10-25 g \(\bullet\) kWh\({}^{-1}\) compared to pure gasoline due to lower calorific value and lower energy density. The partial fuel substitution with hydroxy gas counterbalanced this rise while obtaining a net BSFC reduction compared to the baseline fuel. * The implementation of dual-fuel operation promoted a significant minimization of CO, HC, and smoke levels. However, CO\({}_{2}\) and NOx emissions escalated due to enhanced combustion oxidation and higher combustion temperatures, which opens a new path for incorporating advanced fuel injection systems and after-exhaust treatment technologies. * Energy losses represented a predominant share (37-52%) from the chemical energy input depending on the load. Increasing ABE and HHO content in the dual-fuel operation maximizes the power output by up to 2.2%. In contrast, high-load conditions promoted the minimization of energy losses, which implies higher combustion efficiency. * ABE 10 + HHO featured the highest thermal efficiency (28-33%) from the fuel blends. Moreover, hydroxy doping increased efficiency up to 1.8%. * Energy destruction represents up to half of the exergy distribution, demonstrating the predominant share of internal irreversibilities in the combustion phenomena. Dual-fuel mode and higher engine loads result in enhanced useful exergy and power output. The implementation of dual-fuel operation in SI engines demonstrated promising results towards emissions reduction and enhanced combustion performance. Further studies should assess the effect of engine speed on engine performance and control of the spark timing. Emissions maps are a robust tool to predict emissions levels and thermal performance. The incorporation of advanced technologies such as control strategies, waste heat recovery, and exhaust gas recirculation stand as promising avenues in the long-term perspectives of ICEs to further enhance the fuel utilization ratio. Conceptualization, W.G.-E., D.M.-C. and J.D.-F.; Methodology, W.G.-E., D.M.-C., A.B.-S. and J.D.-F.; Software, W.G.-E., D.M.-C., A.G.-Q. and J.D.-F.; Validation, A.B.-S., A.G.-Q. and J.D.-F.; Formal Analysis, W.G.-E. and D.M.-C.; Investigation, W.G.-E. and D.M.-C.; Resources, A.B.-S., A.G.-Q. and J.D.-F.; Writing--Original Draft Preparation, W.G.-E., D.M.-C. and A.G.-Q. ; Writing--Review and Editing, A.B.-S. and J.D.-F.; Funding Acquisition, W.G.-E. and A.B.-S. All authors have read and agreed to the published version of the manuscript. This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable.