Experimental study of the combustion and emission characteristics of oxygenated fuels on a heavy-duty diesel engine

Abstract

Bio-derived alcohol fuels and oxygenated aromatic hydrocarbons are interesting alternative fuels applicable in low temperature combustion concepts due to their low reactivity. In this paper, three oxygenated fuels are mixed with n-heptane respectively as binary fuel blends to investigate combustion and emission characteristics of the oxygenated fuels with different molecular structures. The n-butanol/n-heptane blend has an identical stoichiometric air/fuel ratio as benzaldehyde/n-heptane blend while anisole is used to assess the importance of the aromatic structure. Standard diesel is also used as a baseline fuel. Experiments are performed on a single-cylinder heavy-duty diesel engine at medium and high loads. It is found that the n-butanol/n-heptane blend presents a higher premixed burn fraction and a higher pressure rise rate at both loads. Increasing injection pressure leads to lower soot emissions and higher NOx emissions are found regardless of fuel type. The n-butanol/n-heptane blend achieves 0.027 g/kWh and 0.2 g/kWh for engine-out soot and NOx emissions with high exhaust gas recirculation rate at high load. For the benzaldehyde/n-heptane blend and diesel, the classical NOx/soot tradeoff is observed under the same operating conditions. The particle size distribution mainly shows accumulation mode particles increase at a high exhaust gas recirculation rate, irrespective of the fuel structure. Only the n-butanol/n-heptane blend emits more nucleation mode particles and much less accumulation mode particles compared to the benzaldehyde/n-heptane and anisole/n-heptane blends. Clearly, the fuel structure influences the soot formation and is therefore not fully determined by the stoichiometric air/fuel ratio of the base fuel.