The Development and Evaluation of Robust Combustion Systems for Miller Cycle Engines 2018-01-1416
Miller Cycle engines employ a high expansion ratio to achieve high part-load efficiency, while minimizing knock sensitivity by using valve events that limit the effective compression ratio. The Miller effect may be achieved using either early or late intake valve closure. Combustion systems for these engines must be carefully designed to obtain adequate trapped charge to achieve full-load objectives as well as charge motion characteristics supporting good mixture preparation and flame propagation. This paper summarizes the results of a holistic project tasked with developing robust combustion systems for both early and late intake valve closure strategies. Based on best practices from conventional engines and preliminary Miller cycle requirements, a series of combustion systems was designed. These were analyzed using 3-dimensional computational fluid dynamics and those showing favorable combustion characteristics were experimentally evaluated using a modular single cylinder engine. Through a rapid assessment process, evaluations of part-load, full-load, light-load, and emissions performance were completed. The results of these experiments supported refined designs. These refined combustion systems were, again, evaluated analytically before being assessed experimentally. It was found that both early and late intake valve timing could be used to achieve high efficiency at part load while achieving full-load performance objectives. At peak power, the early intake closure systems better limited the effective compression ratio and were, consequently, more robust against knock. Both systems demonstrated similar part-load efficiency potential. Both strategies were sensitive to boost system capability. Based on the experimental results, with context added by further analytical studies, important design considerations and other implications of early and late intake valve closure Miller Cycle combustion systems are outlined.