Understanding the transition between conventional spark-ignited combustion and HCCI in a gasoline engine

We describe experimental observations of the gradual transition between conventional spark-ignited (SI) propagating flame combustion and homogeneous charge compression ignition (HCCI) in a single-cylinder, stoichiometrically fueled gasoline engine. Our objective is to better understand the transition process in terms of characteristic changes to the combustion stability as indicated by patterns of cyclic variations. The transition was experimentally achieved by incrementally adjusting the level of internal exhaust gas recirculation (EGR) using variable exhaust valve actuation. Throttle adjustments were also made to maintain a constant fueling rate. For low levels of EGR, conventional spark ignition was stable, while at the highest EGR levels, HCCI was stable. The spark was used to ignite conventional combustion and was optionally available during HCCI. The character of the cyclic combustion oscillations that occurred between the conventional and HCCI limits suggests that it can be described as a sequence of bifurcations in a low-dimensional dynamic map. Comparisons with previous studies of lean-limit cyclic variations suggest that nonlinear EGR feedback is probably a major factor in these dynamics.