Cooled exhaust gas recirculation (EGR) is a common way to control in-cylinder NOx production and is used on most modern HSDI Diesel engines. However, EGR has different effects on combustion and emissions production that are difficult to distinguish (increase of intake temperature, delay of rate of heat release (ROHR), decrease in O2 concentration and flame temperature, increase of fuel-air ratio at lift-off length,…), and thus the influence of EGR on NOx and PM emissions is not perfectly understood, especially under high EGR rates.
An experimental and numerical study has been conducted on a 2.0 litters HSDI automotive Diesel engine under low load and part load conditions in order to distinguish and quantify some effects of EGR on combustion and NOx/PM emissions, as the increase of inlet temperature, the decrease in AFR, and the delay of combustion process. A 6-zones phenomenological combustion model, developed at the Ecole Centrale de Nantes, based on Dec and co-workers' “conceptual model” and Siebers and co-workers’ spray model, has been used to analyse experimental data. Calculated ROHR were compared to experimental ones and gave good results, except at low load conditions at high EGR. This model provided “local” informations in the cylinder: the penetration length, the spread angle, the liquid length, the fuel-air equivalence ratio in the different zones, and the lift-off length. It gave some new explanations on the influence of EGR on spray development and combustion, and NOx/PM emissions. Finally, some new trends were observed for specific operating conditions, particularly when holding a constant AFR to try to limit soot and BSFC penalty with increased EGR rate.