Comparison of Alternative EGR Systems for a Medium Speed Diesel Engine

Meeting future regulations for diesel engine NO_x emissions with in-cylinder solutions will require a high rate of exhaust gas recirculation (EGR). For medium speed diesel engines, the exhaust manifold pressure is typically lower than that of the intake manifold, necessitating a rise in the exhaust gas pressure for exhaust flow to be introduced into the intake manifold. In this study, four high-pressure EGR engine concepts are investigated as a means to meet EPA Tier 4 NO_x emissions. These concepts include a system with an EGR pump, one with a power turbine downstream of the turbocharger (i.e., turbocompounding), one with dedicated donor EGR cylinders and the use of a backpressure valve. For each system, an optimum set of parameters that included intake valve timing, intake manifold pressure, and fuel injection timing were found that satisfy the emissions requirements while staying within the mechanical limits of the system. From an efficiency perspective, the turbocompound system is generally superior, followed by the donor cylinder concept. The EGR pumping system typically has lower overall efficiency due to the compressor power requirement and the use of a backpressure valve, representing the baseline for comparison, produced the lowest system efficiency.