Computational investigation of the effect of biodiesel fuel properties on diesel engine NOx emissions

A detailed numerical spray atomization, ignition, combustion and nitrogen oxides (NOx) formation model was developed for direct injection diesel engines by using KIVA3V code. Several modified or recalibrated sub-models including a Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) spray breakup model, a Shell ignition model, a single-step kinetic combustion model and a Zel'dovich NOx formation model were incorporated into KIVA3V. This modified model was validated by experimental data obtained from a John Deere 4045T direct injection diesel engine that was fueled with a natural soybean methyl ester, a yellow grease methyl ester, a genetically modified soybean methyl ester and No.2 diesel fuel. Errors between predictions of the brake-specific NOx and measured values were less than 1% at full load. For biodiesel fuels, either the Zel'dovich mechanism overpredicted NOx emissions, the ratio of NO to NOx should be less than diesel fuel, or both. As observed from the modeling results, the higher latent heat of vaporization and higher surface tension of biodiesel relative to diesel fuel did not result in increased NOx emissions. The higher viscosity of biodiesel could be one of the reasons for increased NOx, but its effect was relatively small compared with the effect of decreased spray cone angle and advanced start of injection timing on NOx. Decreased spray cone angle and advanced start of injection were the main reasons for increased NOx emissions of biodiesel.