Comprehensive Experimental and Thermodynamic Sustainability Assessment of a Spark-Ignition Engine Fueled with n-Butanol/Gasoline Blends

This study experimentally investigates the effects of n-butanol/gasoline blends on combustion, exhaust emissions, and sustainability indicators in a single-cylinder, air-cooled spark-ignition engine. Two fuel mixtures containing 5% and 15% n-butanol by volume (NB5 and NB15) were tested under full load at engine speeds of 1500, 2000, 2500, and 3000 rpm. Results show that the addition of n-butanol improves combustion quality and significantly reduces CO emissions by up to 50% at high speeds, while CO₂ emissions increase slightly at low-to-medium speeds due to more complete combustion. HC emissions decreased with the NB5 blend but increased with NB15, particularly at lower speeds, due to reduced volatility and mixture inhomogeneity. An integrated energy–exergy–exergoenvironmental–exergoenviroeconomic analysis was performed to evaluate thermodynamic efficiency, environmental cost, and sustainability performance. Exergy loss exhibited a logarithmic increase with engine speed, while the exergoenvironmental impact showed a logarithmic decrease. The exergoenviroeconomic index (Ψ) demonstrated a logarithmic growth pattern, indicating improved sustainability with n-butanol addition. Among the tested fuels, NB15 achieved the lowest exergy destruction (≈9.6 MJ/s at 3000 rpm) and the highest sustainability index (Ψ ≈ 3019), demonstrating a 10–15% improvement in thermodynamic and environmental performance compared with gasoline. These findings confirm that n-butanol, due to its oxygenated nature and high-octane rating, enhances combustion efficiency and offers a viable pathway toward cleaner and more sustainable spark-ignition engine operation.