An experimental investigation by optical methods of the physics and chemistry of transient plasma ignition

The use of nonequilibrium plasma generated by nanosecond discharges to ignite fuel/air mixtures, known as transient plasma ignition (TPI), has been shown to effectively reduce ignition delay and improve engine performance relative to spark ignition for combustion engines. While this method is potentially useful for many engine applications, at present the underlying physics are poorly understood. This work provides a review of previous engine implementation work as well as previous experimental work seeking to provide an understanding of the physical and chemical mechanisms of TPI. Work on producing the pulses needed for TPI, both engine testing and optical diagnostic is presented. The emission of TPI is analyzed in order to determine the spectral, spatial, and temporal behavior of the discharge. Temperature mesurements of TPI using optical emission spectroscopy (OES) show that the temperature in streamer discharge and afterglow increases, though it is difficult to quantify the increase with this method. The results of coherent anti-Stokes Raman spectroscopy temperature measurements are reported and discussed, with temperature increases up to 1500 K above ambient observed in the discharge afterglow in fuel/air mixtures. The impact of this temperature increase on TPI and the possibility of thermal ignition is considered. In addition, CARS measurements show that generation of vibrationally excited states of nitrogen is inefficient during the discharge in air but that generation occurs at a high rate roughly 5 µs following the discharge; with the addition of fuels vibrationally excited states are observed during the discharge but an increase in population is still seen at 5 µs. Possible mechanisms for this behavior are discussed. Additionally, this work uses two-photon absorption laser induced fluorescence to measure oxygen atom concentrations in streamer discharge afterglow in a variety of fuel/air mixtures in order to account for the oxygen pathways in transient plasma ignition. It is demonstrated that oxygen atoms are generated in high quantities with lifetimes on the order of hundreds of microseconds, but that fuel chemistry alters the oxygen pathways even without the presence of sustained combustion. Finally, future experiments are proposed to continue developing an understanding of the combustion improvements of TPI.