An Experimental Investigation of the Burn Rates of Naturally Aspirated Spark Assisted Compression Ignition Combustion in a Single Cylinder Engine with Negative Valve Overlap.

The implementation of homogeneous charge compression ignition (HCCI) in an engine remains a challenge due to the limited operating range and lack of a direct ignition timing control mechanism. Spark assisted compression ignition (SACI) has been shown by several research groups, including the work presented here, to provide such a mechanism, helping to control the phasing and stability of a primarily auto-igniting charge, as well as provide a means of extending the high load limit of HCCI while maintaining high thermal efficiency. The approach used in this study is unique in that flexible engine valve timing allowed for independent control of the thermal/compositional stratification associated with a large internal residual fraction, allowing its effect to be isolated from other thermophysical parameters. In these experiments, a single-cylinder engine equipped with fully-flexible valve actuation was used to explore the effects of spark assist in controlling peak heat release rates. With spark assist, a small portion of the heat release occurred via flame propagation, increasing the overall duration of the combustion event and dramatically reducing peak rates of heat release. At constant engine load and combustion phasing, peak heat release rates were reduced by 40% by controlling spark timing and unburned gas temperature via changes in internal and external EGR rates. Internal EGR was adjusted by varying the duration of negative valve overlap (NVO); for the range of NVO investigated, potential variations in in-cylinder mixing and thermal/compositional stratification were found to have a weak effect on burn characteristics, confirming the notion that temperature and spark timing are the primary variables affecting SACI burn rates for a fixed mixture composition. In the experiments, heat release analysis showed that the behavior of SACI was consistent with the theoretical kinetics associated with turbulent flame propagation and auto-ignition, supporting the hypothesis that SACI is essentially two distinct energy release events coupled by compression heating from an expanding flame front. The results of this work provide new insights into the physical and chemical mechanisms important during low temperature combustion. The results confirm proposed representations of SACI, and thereby provide direction for developing new advanced low temperature engine strategies.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99979/1/manofsky_1.pd