Study on Diesel Engine Characteristics under Large Transient EGR

Abstract

Modern diesel engines tend to employ up to 50-70% exhaust gas recirculation (EGR) together with high intake pressure and injection strategies to enable low temperature combustion (LTC) cycles for reducing NOx and soot emissions simultaneously. Obviously, the combustion conditions and exhaust emissions are sensitive at such high EGR rate. And any slight fluctuation in the EGR quantity will bring unintended deviations from the desired engine performance, thus LTC mode is only limited at partial engine operation points. So the engine has to switch combustion mode frequently between compression ignition (CI) and LTC region within a few engine cycles in real application, which may result in combustion cyclic variations and even misfire, especially during transient operation. In order to investigate effect of heavy EGR transient process on engine combustion cycles, the experimental work was carried out on a four-cylinder VM common-rail turbocharged diesel engine. The results show that the oxygen concentration in the intake charge almost maintains at steady level at EGR steady conditions, while the exhaust oxygen concentration is affected by exhaust values opening/exhaust values closing (EVO/EVC), and result in intra-cycle fluctuation, which will approximately bring 2% calculation error bandwidth for EGR ratio. From 37% to 55% EGR ratio, the EGR gas is mainly driven by the pressure ratio of intake and exhaust duct, and it will experience a long accumulating process to reach a new equilibrium. And the inordinate delayed injection timing will promote in-cylinder cycle-to-cycle variation and even misfire, especially during transition from CI to LTC region.