On the Nature of Cyclic Dispersion in Spark Assisted HCCI Combustion

We report experimental observations of cyclic combustion variability during the transition between propagating flame combustion and homogeneous charge compression ignition (HCCI) in a single-cylinder, stoichiometrically fueled, spark-assisted gasoline engine. The level of internal EGR was controlled with variable valve actuation (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 incrementally adjusting the internal EGR level and capturing data for 2800 sequential cycles. These measurements revealed a complex sequence of high COV, cyclic combustion variations when operating between the propagating flame and HCCI limits. We were able to experimentally demonstrate an increase in the zone of acceptable HCCI-like combustion by using the spark assist. A detailed 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 nonlinear EGR feedback is probably the major source of the observed variations for this engine. The predictable nature of this feedback suggests the possibility of developing on-line diagnostics and proactive control algorithms for expanding stable HCCI operation and improving transitions between conventional and HCCI modes.