Numerical and thermodynamic study on effects of high and low pressure exhaust gas recirculation on turbocharged marine low-speed engine

Marine low-speed engines are one of the most favorable power devices for oceangoing vessels. However, their energy consumption reduction is currently limited because of applying the exhaust gas recirculation (EGR) to reduce the NOx emission. Consequently, high pressure (HP) and low pressure (LP) EGR are studied in this study to explore the interactive relationship among the EGR, the turbocharger and the engine. The HP EGR reduces the turbine power, causing the deterioration of scavenge air pressure and compressor surge margin. As a solution, the cylinder bypass rate should be kept approximately equivalent to the EGR rate. Moreover, to avoid unfavorable impacts on the NOx emission, it should be decoupled with the exhaust gas receiver. While, the LP EGR has opposite effects on the turbocharger: the turbine power is raised and compressor operating points move to the opposite direction. It could be preliminarily attributed to the better utilization of the increased exhaust gas temperature. From a thermodynamic analysis, the fundamental reason is the difference of compressor inlet composition. Demands of them for the compressor are concluded subsequently: the HP EGR requires a wide high efficiency area at a constant pressure ratio, while the LP EGR needs the high efficiency area as long as possible in its demand direction. Finally, detailed comparisons between them are conducted based on the re-matched turbochargers and optimized engine parameters: the control and the structure of LP EGR are simpler and more effective, but its cost and compactness are inferior to the HP EGR.