Investigations into the Effects of Thermal and Compositional Stratification on HCCI Combustion - Part II: Optical Engine Results

The effect that thermally and compositionally stratified flowfields have on the spatial progression of iso-octanefueled homogeneous charge compression ignition (HCCI) combustion were directly observed using highspeed chemiluminescence imaging. The stratified incylinder conditions were produced by independently feeding the intake valves of a four-valve engine with thermally and compositionally different mixtures of air, vaporized fuel, and argon. Results obtained under homogeneous conditions, acquired for comparison to stratified operation, showed a small natural progression of the combustion from the intake side to the exhaust side of the engine, a presumed result of natural thermal stratification created from heat transfer between the in-cylinder gases and the cylinder walls. Large differences in the spatial progression of the HCCI combustion were observed under stratified operating conditions. Qualitative observations of the manner in which the combustion proceeded indicated that +/- 20 degrees C temperature stratification, +/- 15% fuel concentration stratification, and +/- 5 air-fuel ratio stratification all similarly affected the combustion progression, with the combustion proceeding, in general, from high to low temperature, high to low fuel concentration, or high to low air-fuel ratio. Concurrent and opposite stratification of these parameters were often present and mitigated the effects that the stratified flowfields had on the manner in which the combustion progressed spatially. The possibility of using the thermal and compositional stratification studied herein for HCCI combustion control was evaluated in a hypothetical recompression-type engine. The magnitude of stratification that could be maintained until combustion commenced was estimated to be small, and competing effects were often present, likely limiting the effects the stratification could have on the combustion.