Transient Plasma Ignition (TPI) for Automotive Applications

Transient plasma ignition (TPI) – where high-energy, non-equilibrium plasma ignites flammable mixtures – has been found to promote faster flame propagation rates through a combination of larger volume ignition kernels and the generation of active radicals that enhance flame speeds. For the present study, ignition and early flame propagation characteristics of TPI were investigated within a custom-built optically accessible spark calorimeter for near-atmospheric stoichiometric mixtures of propane and air. Transient plasma was generated using an available high-voltage (~ 25 kV peak), short duration (~12 nanosecond) pulse generator. Two electrode configurations were investigated: (1) a wide-gap pin-to-pin and (2) a groundless partial dielectric barrier discharge (DBD) igniter with a flush mounted and exposed anode tip. Each electrode was expected to promote faster initial burn rates through some combination of reduced heat transfer losses, formation of radical species favorable for ignition, and distributed ignition sites within the combustion chamber. Important post-discharge products were bulk-sampled from non-flammable fuel/air mixtures and speciated via gas chromatography. The impact of the post-discharge products on the flame speeds and auto-ignition delay times were evaluated using the 0-D combustion commercial solver CHEMKIN PRO. High-speed schlieren imaging was used to characterize discharge streamer phenomena and flame propagation rates. Flame propagation measurements were benchmarked against a similar operating point that used a highenergy inductor coil spark plug (93 mJ). For transient plasma discharges in air, the pin-to-pin electrodes generated strong twin streamers that bridged the electrode gap. Complementary discharge modeling indicates elevated electron densities and atomic oxygen concentrations, especially around the anode tip. Conversely, air discharge imaging of excited state atomic oxygen for the groundless partial DBD igniter indicates strong negative corona streamers that propagate up along the insulator surface toward the expose anode. Corresponding post-discharge sampling and speciation via gas chromatography reveals substantial dissociation of parent fuel molecules into smaller hydrocarbon constituents. Complementary CHEMKIN PRO modeling indicates these species will increase laminar flame by up to 20%. Finally, it was found that both the pin-to-pin and groundless partial DBD igniters increased flame propagation rates by a factor of 2 relative to the inductor coil spark igniter due to a combination of reduced electrode heat losses, larger ignition volumes, and the formation of radicals that increase initial flames speeds