Effects of exhaust gas recirculation temperature on diesel engine combustion and emissions

When hot exhaust gas is mixed with inlet air, the charge to a diesel engine is modified in three ways: the charge temperature increases, the total cha rge mass is reduced and the charge composition changes. This paper is conce rned with the effects on combustion and pollutant emissions of only the fir st two items. The last item has been investigated by the authors previously and reported in references [1-4]. The investigation was conducted on a hig h-speed direct-injection diesel engine at about 40 per cent of full load 20 00 r/min and constant fuelling rate. The investigation included in-cylinder heat release and exhaust gas analysis. It was found that increasing inlet charge temperature, at constant inlet charge mass and composition, increase d oxides of nitrogen (NOx) and particulate exhaust emissions but reduced un burnt hydrocarbon (UHC) emissions. When the inlet charge mass was reduced a t constant inlet temperature, the exhaust particulates and UHC emissions in creased substantially, principally because the oxygen available for combust ion was reduced. In contrast, NOx emissions increased only slightly, probab ly because the effects on NOx of a higher combustion peak temperature tende d to be offset by substantially lower oxygen availability.