Research on influence and demand of Miller cycle based on the coupling of marine low-speed engine and turbocharger

Abstract

• Predictive model is built to study Miller cycle on the whole low-speed engine. • Impacts of Miller cycle is studied based on coupling of engine and turbocharger. • Uniflow scavenging is the fundamental reason for Miller cycle’s unique effect. • Turbine re-match and demand of Miller cycle is summarized by thermodynamics. • Above turbine re-match and demand are general in engine of uniflow scavenging. Miller cycle has been gradually applied on marine low-speed engines to reduce NOx emission, due to its convenient implement with lower cost. However, current studies about relevant effects, influence mechanism and turbocharger re-match all have neglected the coupling relationship between engine and turbocharger, which will have non-negligible impact on final results. Therefore, based on this relationship, the influence of Miller cycle is studied through a numerical simulation. Then, a thermodynamic analysis is conducted to summarize the relevant impact mechanism and provide guidance for turbocharger re-match. Numerical results show that the in-cylinder charger temperature is actually reduced, but the combustion duration is prolonged. Hence, the exhaust gas temperature is increased, causing compressor operating points moving to the direction with higher pressure ratio and mass flow rate. Furthermore, from the thermodynamic analysis, the fundamental reason of this phenomenon is the uniflow scavenging adopted by marine low-speed engine: for this scheme, the more the Miller cycle is applied, the more the mass will flow across the cylinder. For compensating the losses caused by Miller cycle, achieving the original cylinder compression pressure is an effective method, hence a higher boost pressure is needed and the turbocharger should be re-matched. Finally, based on the thermodynamic analysis, the demand of Miller cycle for turbocharger is summarized and the relevant guidance for turbocharger re-match is proposed: the relationship between boost pressure and Miller cycle should follow the original poly-tropic process in cylinder; the high efficiency area of compressor should include the operating point with maximum Miller cycle, where the pressure ratio and the mass flow rate are decided by the maximum Miller cycle and the index of original poly-tropic process.