Investigation of the Impact of Engine Injection Strategy on the Biodiesel NOx Effect with a Common-Rail Turbocharged Direct Injection Diesel Engine

Abstract

An investigation of the impact of engine injection strategy on the biodiesel NOx effect was conducted with a common-rail turbocharged direct injection diesel engine at moderate speed and load. The fuels included a baseline ultralow sulfur diesel fuel (ULSD) and a B40 (v/v) blend of a soybean methyl ester (SME)-based biodiesel and ULSD. When an engine is held at fixed speed and load, the increase of fuel consumption when biodiesel is used leads to two possible changes in injection strategy: (1) increase of injection pressure and (2) extension of injection duration. Either because of these differences in injection parameters or because of the inherent physics and chemistry of the diesel combustion process, combustion of the B40 blend leads to higher NOx emissions than for ULSD. Experiments seeking to investigate the impact of each of these two fuel injection parameters showed that neither was the dominating factor that determined the NOx increase. The observations in this work confirmed that either an increase of injection pressure or an advance of the start of injection can significantly increase NOx emissions. Meanwhile, no significant difference in brake fuel conversion efficiency was observed with changes to the injection strategies including the start of injection, injection pressure, and duration for both fuels. Heat release analysis showed a faster and premixed combustion at higher injection pressure, which resulted in increased NOx emissions. A numerical model was employed to characterize the fuel spray and lift-off length, and a good correlation between the oxygen equivalence ratio at the autoignition zone near the lift-off length, and NOx emissions were observed for each start of injection timing, regardless of fuel type. These results confirmed that the dominant mechanism leading to the NOx increase is higher local temperatures and earlier maximum cylinder temperatures due to leaner combustion in the premixed and mixing controlled combustion phases.