By discharging nanosecond high-voltage (5 kV) pulses across an insulating substrate containing Au, Pt, or Cu nanoparticles, a 3 order of magnitude (1000x) enhancement in the generation of plasma can be achieved through local field enhancement on the surface of the nanoparticles. The low-temperature nature of this transient plasma is crucial to maintaining the structural integrity of these delicate nanoparticles. These nanoparticles provide up to a 1000-fold enhancement in the generation of the plasma, which is localized to the surface of the nanoparticles where it is potentially useful (e.g., for catalysis). We performed both time-domain and frequency-domain calculations of the electromagnetic response of the nanoparticles based on high-resolution transmission electron microscope (HRTEM) images, which show local field enhancement of the nanosecond high-voltage pulse on the order of 3x. Since the plasma initiation depends exponentially on the peak electric field strength, this 3-fold increase in the local electric field can result in a several orders of magnitude increases in the generation of plasma at a given applied external field strength. In order to rule out plasmon-resonance enhancement, which is often associated with small metal nanoparticles, we performed finite difference time domain (FDTD) simulations in the optical frequency domain, which show that the effect of plasmon resonance is negligible for Pt nanoparticles. We therefore attribute the nanoparticle-based enhancement to the generation of plasma (an electrostatic effect) rather than enhanced coupling of light from the near field to the far field via the plasmon resonance phenomenon (an optical effect).

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.

Transient plasma ignition using nanosecond pulses has demonstrated the potential to enable improved fuel economy and reduced emissions by enabling lean and EGR limit extension in dilute burn engines. Existing spark ignition technology is not adequate because the energy transfer mechanisms between the spark and the fuel-air mixture are not efficient enough to guarantee stable ignition for dilute mixtures at high-load conditions. Additionally, long duration sparks and other advanced ignition solutions that require increased energy delivered accelerate spark plug electrode wear. To date, non-thermal plasma ignition with nanosecond pulses have demonstrated a lean ignition limit beyond an air/fuel ratio of 24 [1], demonstrated high-pressure ignition at densities equivalent to over 100 bar at the time of ignition [2], and demonstrated stable (COV <3 %) ignition at EGR dilution levels >20 % [3]. While low-energy nanosecond pulses have demonstrated strong performance compared to existing solutions, they currently only exist on the market in laboratory systems, rather than a production ready system in a single rugged, weather-proof, under-the-hood enclosure. Transient Plasma Systems (TPS) has recently demonstrated the potential for a retroffitable solution similar to coil-on-plug architecture that allows a direct replacement of existing ignition technology without any engine modification. The system was run on a gasoline direct injection engine at Argonne National Laboratory and demonstrated the same trends as previously observed with research grade systems, including lean and EGR limit extension and more stable ignition across a range of loads. The system was capable of delivering 30 kV pulses in bursts of up to 20 pulses at 30 kHz, and demonstrated stable combustion at an air/fuel ratio of 23.5, exhaust gas recirculation of 23 %, and ignition at 19.2 bar with COV <3 % using only 20 kV pulses.

The interplay of humidity and non-equilibrium, transient plasma was studied via ignition experiments in a C2H4–air mixture, concentration measurements in humid air, and detailed simulations. Hydroxyl (OH) and ozone (O3) produced via non-equilibrium plasma were characterized in a flowing H2O–air mixture at atmospheric pressure with varying the levels of humidity using planar laser-induced fluorescence (PLIF) and UV absorption, respectively. The OH, which was created in the discharge streamers, peaked at a concentration of ∼5×1014/cm3 and then decayed below 1×1014/cm3 after ∼100µs. O3, which is long lived, peaked at a concentration of 1.4×1015/cm3. An increase in humidity from XH2O≈0.2% to 1% resulted in a monotonic increase in the concentration of OH and a 67% decrease in that of O3. Zero-dimensional Boltzmann modeling of non-equilibrium plasma discharges in humid air showed qualitative agreement with these results and points to the decrease in O concentration (with increasing humidity) as the reason for the decreased O3 concentration. In spite the dramatic decline in XO3 with increased humidity, there was no strong commensurate effect on ignition and flame propagation in C2H4–air mixtures: Peak pressure rise rate was at its maximum value at XH2O=1% but was only 25% less at XH2O=5%.

High-speed schlieren imaging of nanosecond pulsed discharges in a near-atmospheric lean methane/air mixture reveals enhanced ignition over traditional inductive spark. The chamber pressure history also indicates faster flame development time.

Summary form only given. The application of a 12ns unipolar pulse-or `half of a microwave'-for plasma production is compared in applications to more traditional microwave generated plasma, and evidence of improved pressures and efficiencies will be presented. These include initiation of combustion in fuel-air mixtures wherein the electron energy distribution is fundamentally different, sometimes appearing as streamers with space-charge limited streamer heads. Understanding the dynamics of nanosecond streamer discharges in air and in fuel-air mixtures at multi-atmospheric pressures is needed for applications of the non-equilibrium plasma assisted combustion processes in a variety of engines and with various fuels[1]. Pressure inside internal combustion engines, where transient plasma can be applied to improve combustion efficiency and peak pressure, can be very high. Cathode-directed streamer discharges and streamer propagation characteristics in synthetic air at pressures ranging from 1 to 22 bar are reported, and extension of these methods to fuel-air mixtures, including methane and diesel fuel, will be discussed. The discharges are investigated by optical, electrical and theoretical (phenomenological) methods. Streamer velocity scaling for higher pressures as a function of applied voltage, pressure and reduced electric field, E/P, is measured, and the scaling compared with the result of dimensional analysis. Transient plasma is shown to be useful for improving ignition and combustion in a range of fuels.

Nanosecond scale pulsed high voltage discharges in air/fuel mixtures can generate radicals which in turn have been shown to improve combustion efficiency in gasoline fueled internal combustion engines. We are exploring the possibility to extend such transient plasma generation and expected radical species generation to the range of pressures encountered in compression-ignition (diesel) engines having compression ratios of ~20:1, thereby improving lean burning efficiency and extending the range of lean combustion. Our preliminary experiments on streamer propagation velocities have shown deviations from simple (pd = const.) based similarity law in the pressure range of 1-8 bar in synthetic air [1]. Here we report the results of streamer propagation experiments in the extended range of air pressures, 6-18 bar. In order to extend our work to 18 bar, the gap size is reduced to 1.75 mm. Optical data obtained from PI-MAX 3 ICCD camera is complemented with electrical measurements to deduce average streamer velocities.

Achieving effective ignition of reacting mixtures using nanosecond pulsed discharge non-equilibrium transient plasma (TP), requires that the effects of several experimental parameters be quantified and understood. Among them are the electrode geometry, the discharge location especially in non-premixed systems, and the relative ignition performance by spark and TP under the same experimental conditions. In the present investigation, such issues were addressed experimentally using a cylindrical constant volume combustion chamber and a counterflow flame configuration coupled with optical shadowgraph that enables observation of how and where the ignition process starts. Results were obtained under atmospheric pressure and showed that the electrode geometry has a notable influence on ignition, with the needle-to-semicircle exhibiting the best ignition performance. Furthermore, it was determined that under non-premixed conditions discharging TP in the reactants mixing layer was most effective in achieving ignition. It was also determined that in the cases considered, the TP induced ignition initiates from the needle head where the electric field and electron densities are the highest. In the case of a spark, however, ignition was found to initiate always from the hot region between the two electrodes. Comparison of spark and TP discharges in only air (i.e. without fuel) and ignition phenomena induced by them also suggest that in the case of TP ignition is at least partly non-thermal and instead driven by the production of active species. Finally, it was determined that single pulsed TP discharges are sufficient to ignite both premixed and non-premixed flames of a variety of fuels ranging from hydrogen to heavy fuels including F-76 diesel and IFO380 bunker fuel even at room temperature.

Transient plasma ignition, a method of fuel-air ignition involving streamer discharges produced by nanosecond high-voltage pulses, is attractive, and that it reduces ignition delay, increases the burning rate, and can ignite leaner mixtures compared with the traditional spark ignition. In this paper, images of the transient plasma discharges and subsequent flame development in a C2H4-air mixture were captured using a single-lens reflex camera and a high-speed camera, respectively.

In this paper recent studies of compact power modulators, used to produce nonequilibrium plasma in the transient, formative phase of an arc, and applied to ignition of a quiescent fuel-air mixture in a constant-volume reactor, are reported. In this work, ignition delays produced by transient plasma were measured and compared in pre-mixed C2H4-air at atmospheric pressure. Two compact power modulators studied included; 1) a 54 ns pseudospark switched line-type power modulator that delivered 365 mJ per pulse, and 2) a 12 ns SCR-switched magnetic compression based power modulator that delivered 75 mJ per pulse. Despite the difference in energy delivered, both systems achieved similar ignition delays across a broad range of fuel-air equivalence ratios, and produced ignition delays up to two times shorter than those produced using traditional spark ignition. The results indicate that lower energy and therefore more compact power modulators may be used for this application.

Non-equilibrated (non-thermal) plasma generated by short (ns), high-voltage (kV) pulses has attractive electron characteristics that fundamentally and favorably alter pre combustion chemistry and physics. This technology has been demonstrated in applications for airborne engines, including pulse detonation engines, in collaborative studies with Nissan in applications for internal combustion engines, as well as for more fundamental studies. We report recent studies where two compact power modulator systems are used to produce non equilibrium plasma in the transient, formative phase of an arc, and are applied to ignition and combustion (transient plasma ignition or TPI) in a constant volume reactor. In this work, ignition delays, a key parameter in the application to pulse detonation engines, were measured in transient plasma ignited C 2 H 4 -air in a constant volume reactor at atmospheric pressure. Two compact power modulators were used; a 12 ns SCR switched magnetic compression based pulse generator and an 85 ns pseudospark switched line-type pulse generator. The results show that despite the difference in energy delivered (70 mJ vs. 400 mJ), both systems achieve similar ignition delays across a broad range of equivalence ratios, and produce ignition delays up to two times shorter than with traditional spark ignition, confirming previous results obtained in an flowing system (pulse detonation engine). The results indicate that lower energy, more compact power modulators may be used for this application.

The use of a compact solid-state pulse generator and compact igniters for transient plasma ignition in a pulse detonation engine (PDE) is reported and compared with previous results using a pseudospark pulse generator and threaded rod electrode. Transient plasma is attractive as a technology for the ignition of PDEs and other engine applications because it results in reductions in ignition delay and has been shown to ignite leaner mixtures which allows for lower specific fuel consumption, high-repetition rates, high-altitude operation, and reduced NOx emissions. It has been applied effectively to the ignition of PDEs as well as internal combustion engines. Nonequilibrium transient plasma discharges are produced by applying high-voltage nanosecond pulses that generate streamers, which generate radicals and other electronically excited species over a volume. The pulse generator used is in this experiment is capable of delivering 180 mJ into a 200-¿ load, in the form of a 60-kV 12-ns pulse. Combined with transient plasma igniters comparable with traditional spark plugs, the system was successfully tested in a PDE, resulting in similar ignition delays to those previously reported while using a smaller electrode geometry and delivering an order of magnitude less energy.