Applied Energy, Vol.249, 377-391, 2019
Experimental and numerical study of pre-injection effects on diesel-n-butanol blends combustion
A trade-off inevitably exists between soot and NOx emissions in a traditional engine. The addition of an oxygen containing biofuel can decrease the soot emission but increase the NOx emission. Multi-injection strategy can be used to decrease the NOx emission. Therefore, both simulations and experimental tests were conducted to evaluate the influences of pre-injection on the combustion and emission characteristics of pure diesel (D100) and diesel/n-butanol (30% n-butanol and 70% diesel by volume, B30). The results showed that as the pre-injection interval decreased, the in-cylinder pressure peak increased, the pre-injection heat release ratio increased, and the brake-specific fuel consumption (BSFC) decreased. Only at a small pre-injection interval, the BSFC under pre injection condition was lower than that obtained using the single injection strategy. When the same pre-injection strategy was used, B30 had a higher BSFC and lower brake thermal efficiency than D100. The soot emission decreased with the increase in pre-injection interval but increased with the increase in pre-injection ratio. More soot was emitted using the pre-injection strategy than that using single injection strategy. B30 had a lower soot emission than D100 due to a longer ignition delay. As the pre-injection interval increased, the decreasing amplitude of NO production became smaller than the increasing amplitude of NO2 production, thus increasing the net NOx emission. As the pre-injection ratio increased, more CO was produced through the reaction paths CH2O -> HCO -> CO, whereas the transformation of CO to CO2 almost remained unchanged, thus increasing the net CO emission. The increase in pre-injection ratio inhibited the oxidation of CO to CO2 and thus increased the CO emission. The amount of unburned hydrocarbons increased with the increase in both pre-injection ratio and interval.