The role of non-thermal transient plasma for enhanced flame ignition in C₂H₄–air

Abstract

Transient plasma ignition, involving short ignition pulses (typically 10–50 ns), has been shown to effectively reduce ignition delays and improve engine performance for a wide range of combustion-driven engines relative to conventional spark ignition. This methodology is therefore potentially useful for many engine applications; however, at present there is limited understanding of the underlying physics. Evidence is presented here for two distinct phases of the plasma-ignition process: an initial non-equilibrium plasma phase, wherein energetic electrons transfer energy into electronically excited species that accelerate reaction rates, and a spatially distributed thermal phase, that produces exothermic fuel oxidation reactions that result in ignition. It is shown that ignition kernels are formed at the ends of the spatially separated streamer channels, at the cathode and/or anode depending on the local electric field strength, and that the temperature in the streamer channel is close to room temperature up to 100 ns after the discharge.

Transient Plasma Ignition for Small Internal Combustion Engines

“…Shown here are streamers generated by a 54 ns, 61 kV pulse across a 15 mm gap [4]. Production of active particles in streamer channels through electron impact dissociation, excitation, and ionization of atoms and molecules significantly impact chain branching reactions, reducing ignition delay times and allowing for more complete combustion.…”

Plasma-Assisted Ignition and Combustion

“…Left: streamers generated by a single 370 mJ, 56 kV, 54 ns pulse (maximum E/n ≈ 400 Td) in air (10 s gate time); Right: flame propagation from multiple ignition sites at the base of the streamers after a single pulse in φ = 1.1 C 2 H 4 -air mixture (1 ms gate time). Both images were captured from the same off-axis angle [Singleton et al, 2011].…”

“…Two distinct phases of the plasma-ignition process were demonstrated in [Singleton et al, 2011]: an initial non-equilibrium plasma phase, wherein energetic electrons transfer energy into electronically excited species that accelerate reaction rates, and a spatially distributed thermal phase, that produces exothermic fuel oxidation reactions that result in ignition. It is shown that ignition kernels are formed at the ends of the spatially separated streamer channels, at the cathode and/or anode depending on the local electric field strength, and that the temperature in the streamer channel is close to room temperature up to 100 ns after the discharge [Singleton et al, 2011]. The results presented in Figure 10 show that after a transient plasma discharge in a φ = 1.1 C 2 H 4 -air mixture at 1 atm, (i) ignition occurred within the streamer channel, (ii) flame initiation occurred within 1 ms of the discharge and (iii) flame propagation was faster than that initiated by conventional spark ignition.…”

“…In these studies, the electrical pulse delivered approximately 70 mJ to the mixture in 12 ns, distributed across many streamer channels (typically ≈ 50). Therefore, ~1.5 mJ was applied to the fuel-air mixture in each streamer, which was sufficient for ignition [Singleton et al, 2011]. Fig.…”

Volumetric Plasma Discharge in a Coaxial Electrode Configuration Using Repetitively Pulsed Nanosecond Discharges

“…A more recent approach to reducing ignition delay is ignition using transient, non-equilibrium plasmas. This method has proven to be more efficient compared to traditional spark ignition, [3][4][5] which uses a thermal form of plasma to create a pocket of hot gas to initiate combustion. In the case of non-equilibrium plasma ignition, a nanosecond-scale electrical discharge results in atoms and molecules that are excited to higher electronic, vibrational, and rotational states in which they remain for a few tens or hundreds of nanoseconds.…”