Impact on Fouling of Different Exhaust Gas Conditions with Low Coolant Temperature for a Range of EGR Cooler Technologies

摘要

Degradation of anti-pollutant devices must be taken into account in design so durability of the function is guaranteed over the vehicle lifetime. As for NO_x reduction of diesel engines, Exhaust Gas Recirculation (EGR) systems are a well-known and robust solution. However, exposure of the EGR cooler inside this system to the exhaust gas conditions leads to a degradation of its function, affecting to the EGR rate and therefore to potential for NO_x reduction. So, sizing and technology of these heat exchangers must be selected to avoid malfunction during vehicle operation. Current scenario in Europe with new homologation cycles and focus on NO_x emissions of diesel vehicles under real driving conditions is challenging for the design of EGR systems. This leads to the increase of the areas of the engine map using EGR. Moreover new homologation includes also the use of low ambient temperature. In this context, it is important to understand the impact of critical exhaust gas conditions coupled with low EGR coolant temperature. That is to say, the use of low coolant temperature adds condensation to the typical phenomenon of soot deposition, thus making it even more critical since condensation puts in to the thermophoresis a significative impact in the formation of the fouling layer. As the impact varies according to the heat exchanger technology, a set of tests has been performed in order to analyze the effect in the two main functions: thermal efficiency and pressure drop. Six different technologies have been tested: from round to rectangular corrugated tubes, and tubes with fins with different design parameters. Most critical conditions of exhaust gas have been taken in terms of opacity and HC content, as well as the effect of using nominal coolant temperature versus low coolant temperature have also been analyzed. This study enables to discriminate most suitable technologies for different engine conditions. Further work is planned in order to study soot characterization and representativeness in lifetime of the vehicle.