Experimental Study on Combustion and Emission of a Dual-Fuel Engine Fueled with Diesel and N-Butyraldehyde

N-butyraldehyde is a highly oxygenated and promising biofuel to alleviate the energy crisis and environmental pollution problems. To the effects of n-butyraldehyde on diesel engine, this paper investigated the combustion characteristics and emission characteristics of a diesel/n-butyraldehyde dual-fuel engine at different loads and n-butyraldehyde substitution ratio (BSR). The results indicate that the addition of n-butyraldehyde exhibits significant load dependence in terms of combustion characteristics. The ignition delay (ID) decreases as the load increases and increases as the BSR rises; however, this prolonging effect weakens at high loads. The combustion duration (CD), on the other hand, increases with both load and BSR; particularly under high-temperature, high-load conditions, the effect of BSR on prolonging CD is more pronounced. Furthermore, due to the lower calorific value of n-butyraldehyde, cylinder pressure and heat release rates in dual-fuel mode were essentially on par with or slightly lower than those of pure diesel at low loads. Under medium-to-high load conditions, however, n-butyraldehyde significantly enhanced premixed combustion intensity and optimized the combustion phase by extending the ID and increasing the oxygen content of the mixture. In terms of emission characteristics, the addition of n-butyraldehyde synergistically reduces NOx and particulate matter (PM) emissions, but causes a slight increase in HC and CO emissions. Its mechanism for suppressing NOx varies with load. At low loads, it primarily lowers combustion temperature through a dilution effect, whereas at medium to high loads, it effectively reduces local high-temperature zones by utilizing low-temperature combustion of the dual-fuel mixture, resulting in a total contribution to NOx emission suppression of up to 28.4%. At the same time, the extended ignition delay caused by n-butyraldehyde, combined with the highly reactive oxidation of fuel-bound oxygen, significantly promotes the oxidation of soot and PM, reducing their emissions by up to 47.4% and 69.9%, respectively. However, because n-butyraldehyde reduces the overall reactivity of the fuel, HC and CO emissions increase slightly. In all, n-butyraldehyde can be considered as a promising alternative fuel for diesel engine in dual-fuel mode, which has proved its worth on combustion improving and emissions reduction.