OPTICAL INVESTIGATIONS OF THE EFFECTS OF STRATIFICATION ON HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION
Herold, Randy E.
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The effects that thermal and compositional stratification have on homogeneous charge compression ignition (HCCI) combustion were studied using an optically accessible internal combustion engine. A stratified flowfield was developed in the engine by feeding each intake valve of the four-valve engine with independent intake systems. Planar laser-induced fluorescence (PLIF) of 3-pentanone that was introduced through only one intake valve indicated significant mixing between the two intake streams. A number of different intake-flow modifying devices were used in an attempt to maximize the amount of bulk stratification maintained throughout compression, but only when using top- and inside-directing intake baffles were significant improvements over a simple, straight-runner system observed. The bulk stratification maintained throughout compression, measured as the average deviation of the mean fluorescence profile from the mean homogeneous fluorescence profile, increased by 36% when using the topdirecting baffles and by 30% when using the inside-directing baffles compared to when using the same runner with no baffles. The combination of cylinder pressure, engine-out emissions, and high-speed chemiluminescence measurements were used to evaluate the effects that stratification had on HCCI combustion. The cylinder pressure and emissions data showed little-to-no difference when comparing the combustion under homogeneous operation to combustion under stratified operation at a constant location of peak pressure. Large differences, however, could be observed in the spatial progression of the HCCI combustion. Qualitative observations of the manner in which the combustion proceeded indicated that �60 K temperature stratification, �25% fuel concentration stratification, and �7 air-fuel ratio stratification all similarly affected the combustion progression. A dual-tracer PLIF temperature imaging technique was calibrated in situ and applied under motored and fired engine operation. Initial experiments under motored engine operation showed that the dual-tracer PLIF technique, with 3-pentanone and triethylamine as the tracers, achieved sufficient temperature precision to measure singleshot temperature variations of �2.3 K (�1?) with an intensified camera or �1.4 K (�1?) with an unintensified camera. Mean temperature profiles acquired with thermally stratified intake conditions showed a 5 K gradient across the combustion chamber. When applied under fired operation, the high in-cylinder temperatures resulted in low fluorescence signals and limited the ability to precisely measure temperature variations resulting from introduced thermal stratifications.