Experimental and Numerical Study on the Effect of NO₂ on n-Butanol/Biodiesel Dual-Fuel Combustion in a Compression...

Nitrogen dioxide (NO₂) is an active species of exhaust gas recirculation gas, and it has a significant impact on the autoignition and combustion processes of fuels. This study presented a comprehensive investigation of the effect of NO₂ on the combustion characteristics of the n-butanol/biodiesel dual fuel. Experiments were conducted on a single-cylinder engine with 0, 100, 200, and 400 v/v ppm NO₂ addition at two fuel injection ratios. The findings of the experiments indicated that adding NO₂ resulted in an earlier start of heat release and an increase in peak in-cylinder pressure as compared to experiments where no NO₂ was added. The evolutions of n-butanol, biodiesel, and OH radicals were evaluated using the computational fluid dynamics software coupled with the n-butanol-biodiesel-NO₂ mechanism. The results revealed that when 400 v/v ppm NO₂ was added, the consumption of n-butanol and biodiesel occurred earlier, and the formation of OH radicals was approximately an order of magnitude higher before the biodiesel was injected. Furthermore, reaction rate and flux analyses were performed to understand the effect of NO₂ addition on the reaction process. When NO₂ was added, 35% of the HO₂ radicals reacted with NO which converted from NO₂ via the reaction NO + HO₂ ⇌ NO₂ + OH, promoting the formation of OH radicals in the reaction system. The addition of NO₂ can also enhance the consumption of CH₃ radicals via the reaction CH₃ + HO₂ ⇌ CH₃O + OH.