(A) Study on fouling characteristics of EGR coolers in diesel engine

Abstract

In recent years, HSDI (high speed direct injection) diesel engines have been largely applied to RV, SUV and passenger cars due to good fuel economy, high thermal efficiency and very low carbon dioxide emissions. However, worldwide environmental issues such as global warming have led to ever tightening exhaust emission legislations for passenger diesel vehicles, especially on NOx and PM emissions. For example, Euro-6 emission legislation enforced since 2014 mandates that NOx emissions should be reduced by 56% more than the current Euro-5. An exhaust gas recirculation (EGR) system has been widely applied to diesel engines to reduce NOx emissions without a large modification on the engine body. The principle of EGR is to reduce the temperatures of flame and the oxygen concentration of working fluid in a combustion chamber

thereby, reducing NOx emissions. The existing EGR system uses only EGR valve to reduce NOx emissions, resulting in increase in PM emissions. Therefore, modification on the system is required to avoid the PM penalty. Recently, numerous researches have been actively conducted to reduce NOx emissions as well as to overcome the disadvantages of deteriorating PM and fuel consumption through a new EGR cooler system. In order to satisfy the strict Euro-6 NOx emission standard, the use of a high EGR rate becomes more crucial. If a high EGR rate is applied, PM emissions will be inevitably increased. Practically, high concentration of PM in the exhaust gas is relatively easy to be deposited on the surface of an EGR cooler. This is so-called fouling which deteriorates the thermal effectiveness of an EGR cooler and increases the pressure drop through it. As a result, the NOx reduction rate and the performance of an engine will be decreased. On the other hand, recent combustion technologies of HSDI diesel engine such as LTC (low temperature combustion) diesel engine and ultra-high pressure fuel injection system lead to severe fouling phenomena because smaller sized particles deposit on the inner surface of an EGR cooler. Accordingly, many studies have been conducted in the field of EGR cooler fouling. However, there are few researches concerning the cross correlation on the fouling phenomena between a test rig and a real engine. In this study, the design features of existing and modified EGR coolers were reviewed to maintain desired performance against fouling resistance. The behavior of fluid such as velocity vector, temperature and pressure contour of exhaust gases in EGR cooler was analyzed through 3-D CFD (Computational Fluid Dynamics). The results which consist of outlet temperatures, heat transfer effectiveness and pressure drops and so on were obtained by using 2 kinds of EGR coolers in production and another 2 kinds of modified EGR coolers. Then, the characteristics of the above EGR coolers on a test rig were investigated at clean and fouled conditions. During this step, fouling resistances which are directly related to the heat transfer effectiveness were verified with gas velocities. In addition, the characteristics of EGR coolers on engine dynamometers were evaluated at clean and fouled conditions. The fouling test mode was made on the basis of ESC 13 mode and proposal from an EGR cooler manufacturer. The characteristics of heat transfer effectiveness and pressure drop were examined with EGR mass flow rates. For the further analyses on the emission level of NOx and PM were performed according to EGR ratio. This work has the advantages to identify the change rate of NOx and PM for the purpose of evaluation of EGR coolers. The fouling characteristics of each step, such as design review, rig test and engine test were evaluated through the aforementioned studies. On the basis of these results, the improved design specification was proposed. The effectiveness of improved EGR cooler was confirmed through CFD analysis. From these, it is possible to suggest proper methodology to obtain good characteristics against fouling of EGR cooler.