Numerical study on the nitrogen oxide emissions of biodiesel-diesel blends in a light-duty diesel engine

摘要

The nitrogen oxide emissions characteristics of biodiesel-diesel blends in a light-duty diesel engine operating at a moderate load are investigated using KIVA coupled with chemical kinetics. Pure diesel, 20 vol % biodiesel-80 vol % diesel and 50 vol % biodiesel-50 vol % diesel are investigated. A reduced mechanism concerning methyl butanoate and n-heptane is applied in the combustion model. The characteristics of the combustion and nitrogen oxide emissions for different fuels are compared. The effects of the injection timing and the exhaust gas recirculation rate on the nitrogen oxide emissions are of particular interest. The results show that 50 vol % biodiesel-50 vol % diesel has the shortest ignition delay, the longest spray penetration and the lowest fuel-to-oxygen equivalence ratio at the spray tip under the same initial conditions, which results in the highest nitrogen oxide emissions. As the injection timing is retarded, the nitrogen oxide emissions of blended fuels gradually decrease. The nitrogen oxide emissions of 50 vol % biodiesel-50 vol % diesel are close to those of the diesel case with increasing exhaust gas recirculation rate from 19.6% to 25%. By further increasing the exhaust gas recirculation rate to 28%, the nitrogen oxide emissions can be reduced further. It is thus suggested that increasing the exhaust gas recirculation rate by a small amount is more effective in controlling the formation of nitrogen oxides for blends with a high biodiesel content than injection timing retardation is at a moderate engine load.