Numerical and Experimental Study on the Impact of Mild Cold Exhaust Gas Recirculation on Exhaust Emissions in a Biodiesel-Fueled Diesel Engine

This work evaluated the effect of cooled exhaust gas recirculation (EGR) on fuel consumption and pollutant emissions from a diesel engine fueled with B8 (a blend of biodiesel and no. diesel 8:92% by volume), experimentally and numerically. Experiments were carried out on a diesel power generator with varying loads from 5 kW to 35 kW and 10% of cold EGR ratio. Exhaust emissions (e.g., total hydrocarbons (THCs), nitrogen oxides (NO_x), carbon monoxide (CO), etc.) were measured and evaluated. The results showed mild EGR and low biodiesel content have minor impact of engine-specific fuel consumption, fuel conversion efficiency, and in-cylinder pressure. Meanwhile, the combination of EGR and biodiesel reduced THC and NO_x up to 52% and 59%, which shows promising effect on overcoming the particulate matter–NO_x tradeoff from diesel engine. A three-dimensional computational fluid dynamics engine model incorporated with detailed biodiesel combustion kinetics and NO_x formation kinetics was validated against measured in-cylinder pressure, temperature, and engine-out nitric oxide emission from diesel engine. This valid model was then employed to investigate the in-cylinder temperature and equivalence ratio distribution that predominate NO_x formation. The critical results showed that the reduction of NO_x emission by EGR and biodiesel is obtained by a little reduction of the local in-cylinder temperature and, mainly, by creating comparatively rich combusting mixture, which makes the combustion path pass through lower NO_x zone in the ϕ–T diagram.