Abstract
Keywords - Diesel, low pressure EGR, low temperature cooling, charge air cooler, condensation
Engine testing is performed on a 2.0 litre diesel engine comparing high and low pressure EGR architectures. With low pressure EGR, operating points chosen from the NEDC cycle achieve lower NOx emissions by 5 to 64% for similar fuel consumption and smoke emissions compared to high pressure EGR. The lower intake manifold gas temperature and the improved homogeneity of the mixture of EGR and fresh air are among the observed advantages of low pressure EGR. In order to achieve similar engine intake conditions with high pressure EGR, the EGR cooler would require a secondary low temperature coolant circuit and more than double the cooling power. An analysis of the cooling architecture for low pressure EGR demonstrates that a low temperature coolant circuit is not necessary for cooling the EGR gas, as the engine intake temperature is determined by the charge air cooler, and the compressor outlet temperature does not exceed material limitations when the EGR is cooled with engine coolant. Nevertheless, one of the challenges of low pressure EGR is the formation of condensates in the EGR cooler and charge air cooler. The EGR rate, relative fuel to air ratio, and ambient air humidity influence the temperature at which condensation occurs. For typical conditions, this condensation is observed at coolant temperatures below 35°C, in quantities that can reach 1.5 l/h at 20°C. The condensate pH is observed as low as 2.6 and is found to contain strong acids, sulphur, organic acids, aluminium and traces of other metals.