Load-Dependent Efficiency and Emission Trade-Offs of n-Butanol–Diesel Blends in a Naturally Aspirated Diesel Engine

This work systematically evaluates the combustion and emission characteristics of n-butanol–diesel blends to clarify load-dependent trade-offs. A single-cylinder diesel engine was operated under low (25%)- and high (75%)-load conditions using commercial diesel and n-butanol blends (5–15 vol%). The results indicate that n-butanol addition tends to improve brake thermal efficiency (BTE) and reduce brake-specific energy consumption (BSEC), particularly at high loads, likely due to enhanced premixed combustion and fuel oxygenation. Emission trends exhibited distinct load-dependent behaviors: nitrogen oxides (NOx) emissions decreased at low loads, ostensibly because the charge-cooling effect of n-butanol’s high latent heat dominated, whereas they increased at high loads driven by elevated temperatures and oxygen availability. Smoke opacity, carbon monoxide (CO), and carbon dioxide (CO2) emissions were consistently reduced across all operating conditions, suggesting benefits from improved oxidation and the lower carbon content. In contrast, unburned hydrocarbon (HC) emissions increased significantly, which is primarily attributed to prolonged ignition delay and local quenching arising from the fuel’s low cetane number and high latent heat. These findings demonstrate n-butanol’s potential to enhance efficiency and mitigate smoke, CO, and CO2 emissions, though the trade-offs with HC and high-load NOx necessitate optimized control strategies.