Experimental investigation of flame propagation and dilution effects in ethanol and ethanol–fuel blends within an optical SI engine

Abstract

© 2026 Elsevier Ltd.The development of cleaner and more efficient combustion systems is essential to reduce the environmental impact of transportation and to integrate renewable fuels into future energy strategies. Understanding how combustion dynamics change under diluted conditions is crucial for improving efficiency and reducing emissions of carbon monoxide, unburned hydrocarbons, and nitrogen oxides. In this context, the present research investigates the influence of flame propagation speed on the combustion of ethanol, isooctane, and gasoline mixed with ethanol in a spark ignition optical research engine. Two dilution strategies were examined: combustion with excess air and with a synthetic mixture of carbon dioxide and nitrogen representing exhaust gas recirculation. High-speed optical diagnostics and thermodynamic analysis were combined to correlate flame development with engine performance, combustion stability, and gaseous emissions. The results show that moderate dilution enhances efficiency until the flame speed falls below twenty meters/second and combustion ends forty-five degrees after the upper dead center. Exhaust gas dilution strongly decreases nitrogen oxide formation, while air dilution promotes faster combustion and lower carbon monoxide emissions. The study demonstrates the central role of flame propagation speed in linking combustion efficiency, stability, and emissions, providing guidance for the design of engines operating with renewable and blended fuels.

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