Ultra-High Speed Fuel Tracer PLIF Imaging in a Heavy-Duty Optical PPC Engine 2018-01-0904
In order to meet the requirements in the stringent emission regulations, more and more research work has been focused on homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) or partially premixed compression ignition (PCCI) as they have the potential to produce low NOx and soot emissions without adverse effects on engine efficiency. The mixture formation and charge stratification influence the combustion behavior and emissions for PPC/PCCI, significantly. An ultra-high speed burst-mode laser is used to capture the mixture formation process from the start of injection until several CADs after the start of combustion in a single cycle. To the authors’ best knowledge, this is the first time that such a high temporal resolution, i.e. 0.2 CAD, PLIF could be accomplished for imaging of the in-cylinder mixing process. The capability of resolving single cycles allows for the influence of cycle-to-cycle variations to be eliminated. This ability to study individual cycles aids the understanding of the mixture formation process as well as the cycle-to-cycle variations. Strong air entrainment at the boundary layer can be clearly observed and followed as the mixing process progresses. The formation of eddies created by the shear force and their rotational motion can be continuously observed during the mixing process. The interaction between two adjacent spray plumes in the recirculation zone is well captured and studied. In addition, the mixing process resulting in the stratified fuel charge being located in the recirculation zone before the SOC while the areas along the original spray axis are leaned out after the end of injection, can be followed in one time sequence. Moreover, the auto-ignition position and early flame development can be studied, from the high-speed chemiluminescence imaging, together with the fuel distribution in the combustion chamber.