Abstract
Low-temperature combustion concepts like homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) offer the benefit of higher engine efficiency while keeping lower exhaust emissions. In this study, a full-transparent optical engine is employed to investigate the effect of fuel spray/wall interactions on combustion characteristics for transition in combustion mode from HCCI, via PPC to CI. Planar laser-induced fluorescence (PLIF) imaging of acetone and high-speed natural flame luminosity imaging techniques are applied to visualize fuel spray/wall interactions, fuel distribution, and combustion development processes. The combustion phasing, IMEP as well as engine-out emissions are analyzed to study the relationships between fuel spray, combustion, and emissions. The experimental results show three distinct zones with different fuel injection timings: the transitional zone between HCCI and PPC, the PPC zone, and the transitional zone between PPC and CI. The fuel/wall impingement position, fuel penetration and distribution, as well as combustion stratification, are investigated in these three zones. Both CA10 (the crank angle when 10% of the overall heat is released) and CA50 followed similar trends at different injection timings and present obvious "bowl" shapes in the PPC zone. Moreover, more fuel distribution near the cylinder wall and crevice is related to the increased UHC emission. The trade-off trend between UHC and NOx emissions, as well as the CO and CO2 emissions during the combustion modes transition from HCCI to CI via PPC, are investigated and analyzed based on the fuel/wall interaction and combustion processes.