Impact of Miller Cycle Strategies on Combustion Characteristics, Emissions and Efficiency in Heavy-Duty Diesel Engines 2020-01-1127
This study experimentally investigated the potential of Miller cycle strategies as a means to improve thermal efficiency and lower NOx emissions in heavy-duty diesel engines. The experiments were conducted at constant engine speed and load (1160 rev/min and 17.6 net IMEP) on a single cylinder research engine equipped with a fully-flexible hydraulic valvetrain system. Conventional valve profiles were compared to Miller cycle profiles, which included multiple Early Intake Valve Closing (EIVC) and Late Intake Valve Closing (LIVC) timing strategies. While the decrease in effective compression ratio associated with the use of Miller valve profiles was symmetric around bottom dead center, the decrease in volumetric efficiency (VE) was not. EIVC profiles were more effective at reducing VE than LIVC profiles. Despite this difference, EIVC and LIVC profiles with comparable VE decrease resulted in similar changes in combustion and emissions characteristics. Miller cycle strategies enabled lower NOx emissions and lower peak cylinder pressures at constant load, primarily due to lower end of compression temperatures and higher equivalence ratio mixtures, albeit with a thermal efficiency penalty. The use of increased intake pressure (boost) overcame the thermal efficiency penalty of Miller cycle applications, while maintaining a NOx emissions benefit. Higher intake boost also offered a considerable thermal efficiency gain when paired with conventional valve strategies, however, the efficiency gain came at the cost of higher peak cylinder pressures and lower exhaust temperatures.
Erick Garcia, Vassilis Triantopoulos, Andre Boehman, Maxwell Taylor, Jian Li
University of Michigan, Volvo Trucks North America