Abstract
We report experimental observations of cyclic combus-tion variability during the transition between propagating flame combustion and homogeneous charge compres-sion ignition (HCCI) in a single-cylinder, stoichiometri-cally fueled, spark-assisted gasoline engine. The level of internal EGR was controlled with variable valve actua-tion (VVA), and HCCI combustion was achieved at high levels of internal EGR using the VVA system. Spark-ignition was used for conventional combustion and was optionally available during HCCI. The transition region between purely propagating combustion and HCCI was mapped at multiple engine speeds and loads by incre-mentally adjusting the internal EGR level and capturing data for 2800 sequential cycles. These measurements revealed a complex sequence of high COV, cyclic com-bustion variations when operating between the propagat-ing flame and HCCI limits. We were able to experimen-tally demonstrate an increase in the zone of acceptable HCCI-like combustion by using the spark assist. A de-tailed analysis of the cyclic variations in the intermediate zone indicates that they are dominated by nonlinear, nonrandom processes. Comparisons with previous studies of lean-limit cyclic variations suggest that nonlin-ear EGR feedback is probably the major source of the observed variations for this engine. The predictable na-ture of this feedback suggests the possibility of develop-ing on-line diagnostics and proactive control algorithms for expanding stable HCCI operation and improving transitions between conventional and HCCI modes.
