In-Cylinder Soot Reduction Using Microwave Generated Plasma in an Optically Accessible Small-Bore Diesel Engine

The present study explores the effect of in-cylinder generated non-thermal plasma on hydroxyl and soot development. Plasma was generated using a newly developed Microwave Discharge Igniter (MDI), a device which operates based on the principle of microwave resonation and has the potential to accentuate the formation of active radical pools as well as suppress soot formation while stimulating soot oxidation. Three diagnostic techniques were employed in a single-cylinder small-bore optical diesel engine, including chemiluminescence imaging of electronically excited hydroxyl (OH*), planar laser induced fluorescence imaging of OH (OH-PLIF) and planar laser induced incandescence (PLII) imaging of soot. While investigating the behaviour of MDI discharge under engine motoring conditions, it was found that plasma-induced OH* signal size and intensity increased with higher in-cylinder pressures albeit with shorter lifetime and lower breakdown consistency. Results also indicated that a decreasing pressure gradient extends the lifetime of plasma-induced OH* signals while an increasing pressure gradient suppresses plasma-induced OH* formation and increases the rate of signal decay. Studies on the effect of MDI at the start of high temperature reaction when plasma is discharged during the ignition delay phase were also carried out. Despite plasma-induced OH* signals being detected from the motoring experiments, no significant difference in OH* or OH-PLIF signals were observed under fuel injected conditions in comparison to the baseline case. However, PLII imaging revealed that discharging the MDI during the sooting period of combustion resulted in both suppressed formation and enhanced oxidation of soot. This effect became more prominent with the implementation of multiple pulse MDI discharge strategies, leading to significantly reduced soot over all crank angles.