Benjamin Wolk

The role of hydrodynamic enhancement on ignition of lean methane-air mixtures by pulsed nanosecond discharges for automotive engine applications

ABSTRACT The downsizing and boosting of automotive engines for increased fuel economy poses challenges in both obtaining stable ignition at boosted intake pressures and high dilution conditions. Pulsed nanosecond discharge ignition technologies have shown promise in more reliably igniting dilute charge mixtures in internal combustion engine experiments. However, reasons for this combustion enhancement remain unclear. In this study, we ignited lean methane-air mixtures in a constant volume chamber at 2 bar absolute pressure to evaluate pulsed discharge ignition using a novel electrode geometry. The in-chamber pressure history indicates faster flame development times than those produced by traditional inductive spark. High-speed schlieren imaging reveals a significant hydrodynamic component to the observed enhancement: a more wrinkled flame kernel structure and increased burning rates from increased flame surface area. Increasing the number of pulses increased expulsion of the flame kernel. Our results clarify the enhancement observed by other researchers in internal combustion engine experiments.

Calorimetry and Atomic Oxygen Laser-Induced Fluorescence of Pulsed Nanosecond Discharges at Above-Atmospheric Pressures

The conversion efficiency of secondary electrical energy into thermal energy was measured in air using an optically accessible spark calorimeter for high-voltage (28 kV peak) pulsed nanosecond discharges with secondary streamer breakdown (SSB) and similar low-temperature plasmas (LTP) without. Initial pressures were varied between 1 and 5 bar absolute, with the anode/cathode gap distances likewise varied between 1 and 5 mm. Secondary electrical energy was measured using an in-line attenuator, with the thermal energy determined from pressure-rise calorimetry measurements. The SSB probability at each initial pressure and gap distance was also recorded. The calorimetry measurements confirm that, similar to inductive spark discharges, SSB discharges promote ignition by increasing the local gas temperature. LTP discharges, on the other hand, had very little local gas heating, with electrical-to-thermal conversion efficiencies of ~1 %. Instead, the LTP was found to generate substantial O-atom populations — measured using two-photon laser-induced fluorescence near the anode where electric field strengths were strongest — that persisted for 100’s of microseconds after the discharge. The influence of 10 repetitive pulses spaced 100 µs apart was also evaluated for a fixed 5 mm electrode gap distance, with the conditional SSB probability for each pulse evaluated using an available photodiode, with the SSB probability found to have increased for each successive pulse. The influence of chemical and thermal preconditioning by the preceding LTP pulse was evaluated, with the increase in SSB occurrence attributed predominantly to mild gas heating that decreased number densities between the electrodes and hence the gas resistance for the subsequent pulse.