OEM & Lieferant Ausgabe 2/2019 - OEM & Supplier 2/2019 by VEK Publishing

156 Exhaust gas recirculation Well-tempered: the next generation of CFC EGR coolers By José A. Grande (Technology Transfer Group Manager), Manuel J. Dieguez (Senior Product Engineer), Julio A. Carrera (Senior Product Engineer), Felix Lopez (Senior Product Engineer), BorgWarner Emissions Systems in Vigo (Spain) Thanks to the flexible water-cooled thermal damper directly integrated into their inner core, BorgWarner’s new compact floating core (CFC) exhaust gas recirculation (EGR) coolers are a convincing proposition due to their high robustness against thermal stresses and their ability to reduce the impact of transients. EGR systems for commercial vehicles Generally speaking, EGR is an in-cylinder emission control technology designed to re- duce NO X formation. The working principle is based on recirculating part of the exhaust gas back to the cylinders and reintroducing it into the combustion process after mixing it with in- take air. Due to the rough working conditions and the long lifetime of commercial vehicles, particularly high demands are made on the durability of EGR systems. During the design process, therefore, engi- neers focus on the lifetime requirements of EGR coolers. To withstand everyday stresses, the parts have to be able to operate for up to 1.5 * 10 6 kmor 20,000 h in an environment with the maximum temperatures reached on die- sel engines, or even 800 ºC on Compressed Natural Gas (CNG) systems. To meet these high demands, BorgWarner developed a CFC solution which covers numerous specifica- tions andworks under the roughest conditions imaginable. As the commercial vehicle market has low sales volumes, another design chal- lenge was to make the systems adaptable for use in different applications to reduce costs. Besides common diesel applications, however, alternative fuels such as CNG were taken into account too. The EGR cooler family includes a minimum of four standard designs, Figure 1, that can op- erate at a gas flow of between 50 and 100 %. High thermal resistance When the coolant boils, there can be several possible reasons. In situations of this kind, the high thermal stress within the cooler can lead to malfunctions at an early stage. Nev- ertheless, the generation of small bubbles on the hot skin of the cooler core components, also known as nucleate boiling, is generally uncritical. On the basis of long-term experi- ence in this field, it can be said that the ar- eas with a higher risk of boiling are mostly located very close to the hot side header and the inner tubes. To avoid film boiling, it is im- portant to take care that the coolant flows round each section of the cooler core to an adequate extent. In a first step during the de- velopment process, a scenario of the worst possible operating conditions was created for each of the four standard types. The coolant side was then designed on the basis of this data. The final result is for example a combi- nation of maximum gas flow and maximum gas temperatures with a low coolant flow and Images: © BorgWarner Figure 1: Four standard CFC EGR cooler designs Figure 2: Optimization of the coolant system