**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:


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.

**Author Contributions:** 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.

**Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Acknowledgments to Universidad del Atlántico through the project ING81- CII2019 "Estudio experimental de la sustitución parcial de combustible con HIDROXY (HHO) en motores térmicos de encendido por compresión y la influencia sobre sus prestaciones", Universidad del Norte, Sphere Energy Company for the support provided for the investigative internship of Daniel Maestre, and Colombian Institute for Scientific and Technological Development (COLCIENCIAS) through the "Convocatoria 809 Minciencias, Formación de capital humano de alto nivel para las regiones- Atlántico" for the support provided.

**Conflicts of Interest:** The authors declare no conflict of interest.
