As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7-1.0), EGR rates (10-25%), and cross flow speeds (0-30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7-1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

This study reports enhancement in the electrostatic precipitation (ESP) of diesel engine exhaust particulates using high voltage nanosecond pulse discharge in conjunction with a negative direct current (DC) bias voltage. The high voltage (20 kV) nanosecond pulses produce ion densities that are several orders of magnitude higher than those in the corona produced by a standard DC-only ESP. This plasma-enhanced electrostatic precipitator (PE-ESP) demonstrated 95 % remediation of PM and consumes less than 1 % of the engine power (i.e., 37 kW diesel engine at 75 % load). While the DC-only ESP remediation increases linearly with applied voltage, the plasma-enhanced ESP remains approximately constant over the applied range of negative DC biases. Numerical simulations of the PE-ESP process agree with the DC-only experimental results and enable us to verify the charge-based mechanism of enhancement provided by the nanosecond high voltage pulse plasma. Two different reactor configurations with different flow rates yielded the same remediation values despite one having half the flow rate of the other. This indicates that the reactor can be made even smaller without sacrificing performance. Here, this study finds that the plasma enhancement enables high remediation values at low DC voltages and smaller ESP reactors to be made with high remediation.

It is now recognized that nanoscale particulate matter (PM) represents a substantial health hazard for our society, including PM from restaurant smoke. In this study, we explored the use of a transient pulsed plasma in conjunction with an applied DC bias to treat oil aerosols that closely resemble restaurant (i.e., charbroiler) smoke emissions. For polyaromatic olefin PAO-4 and soybean oil, we found that a threeorder-of-magnitude reduction in particulates (i.e., 99.9% remediation) could be achieved with this system. Here, the plasma discharge was produced in a 4-in.-diameter cylindrical reactor with a 5-10 ns high voltage (30 kV) pulse generator together with applied DC bias voltages up to 10 kV. The distribution of nanoparticle sizes was measured using a scanning mobility particle sizer (SMPS) with diameter centered around 225 nm. Here, the main mechanism of remediation occurs in a two-step process in which the oil nanoparticles are first ionized by the free electrons and free radicals in the plasma and then the charged particles are swept out to the sidewalls of the reactor by the applied DC potential. We believe this general approach opens up new degrees of freedom in the design of electrostatic oil aerosol pollution control devices. (C) 2020 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Abstract Transient plasma ignition (TPI) employs highly non-equilibrium plasmas driven by nanosecond high-voltage pulses for combustion ignition. The effects of rise time and pulse repetition frequency (PRF) on combustion ignition are evaluated when transient plasmas, powered by 10-ns, kilovolt pulses, are employed to initiate combustion of lean, stoichiometric, or rich methane-dry air mixtures at atmospheric pressure. It was found that the plasmas driven by high voltage pulses with a faster rise time and PRF between 6–10 kHz enhanced combustion with higher peak pressure and/or shorter ignition delay. A mode transition in the plasma, corresponding to the combustion initiation, was observed based on voltage and current waveforms of four consecutive pulsed plasmas driven by a pulse burst at kilohertz PRFs. Although benefits in combustion ignition from a shorter rise time and optimal PRF were observed for all three equivalence mixture ratios, the highest peak pressure and shortest ignition delay were associated with the stoichiometric mixtures. In addition, the gas temperature of the transient plasma was measured by determining the rotational temperature of the 2nd positive system of nitrogen to be ∼1000 K for the first pulse and increased up to 2000 K after the application of the 4th pulse in a four-pulse burst TPI.

We have demonstrated nearly 80% remediation of diesel particulates from an 18 kW (i.e., 24.4 HP) diesel engine with just 1.6% of the engine power using a transient pulsed plasma in conjunction with an applied DC bias voltage. This result was achieved using a high voltage (~10 kV) DC bias together with nanosecond high voltage pulses (~20 kV) in what we refer to as a plasma-enhanced electrostatic precipitator (PE-ESP). Here, the remediation takes place in a two-step process in which the particles are first charged by the ions in the plasma and then they are swept out to the collecting electrode by the applied DC bias. The fast rise times (i.e., dV/dt ≈ 1012V/sec) associated with these nanosecond pulses produce a streamer discharge with ion densities that are more than one order of magnitude higher than conventional DC coronas. These extremely high ion densities produce the enhanced electrostatic precipitation observed here. In fact, little or no remediation was observed with the DC bias only or with the nanosecond pulses only, and it is the combination of these two applied fields (pulsed and DC) that produces the robust enhancement observed here. This general approach of using nanosecond pulse discharge to produce plasma-enhanced electrostatic precipitation opens up new degrees of freedom in the design of new and more compact ESPs.

In the present study, the performance and emissions characteristics of three low-temperature plasma (LTP) ignition systems were compared to a more conventional strategy that utilized a high-energy coil (93 mJ) inductive spark igniter. All experiments were performed in a single-cylinder, optically accessible, research engine. In total, three different ignition systems were evaluated: (1) an Advanced Corona Ignition System (ACIS) that used radiofrequency (RF) discharges (0.5–2.0 ms) to create corona streamer emission into the bulk gas via four-prong electrodes, (2) a Barrier Discharge Igniter (BDI) that used the same RF discharge waveform to produce surface LTP along an electrode encapsulated completely by the insulator, and (3) a Nanosecond Repetitive Pulse Discharge (NRPD) ignition system that used a non-resistor spark plug and positive DC pulses (∼10 nanoseconds width) for a fixed frequency of 100 kHz, with the operating voltage-controlled to avoid LTP transition to breakdown. For the LTP ignition systems, pulse energy and duration (or number) were varied to optimize efficiency. A single 1300 revolutions per minute (rpm), 3.5 bar indicated mean effective pressure (IMEP) homogeneous operating point was evaluated. Equivalence ratio (ϕ) sweeps were performed that started at stoichiometric conditions and progressed toward the lean limit. Both the ACIS and NRPD ignition systems extended the lean limit (where the variation of IMEP < 3%) limit (ϕ = 0.65) compared to the inductive spark (ϕ = 0.73). The improvement was attributed to two related factors. For the ACIS, less spark retard was required as compared to spark ignition due to larger initial kernel volumes produced by four distinct plasma streamers that emanate into the bulk gas. For the NRPD ignition system, additional pulses were thought to add expansion energy to the initial kernel. As a result, initial flame propagation was accelerated, which accordingly shortens early burn rates.

Recent studies have shown that nanoscale particulate matter produced in commercial charbroiling processes represents a serious health hazard and has been linked to various forms of cancer and cardiopulmonary disease. In this study, we propose a highly effective method for treating restaurant smoke emissions using a transient pulsed plasma reactor produced by nanosecond high voltage pulses. We measure the size and relative mass distributions of particulate matter (PM) produced in commercial charbroiling processes (e.g., cooking of hamburger meat) both with and without the plasma treatment. Here, the plasma discharge is produced in a 3" diameter cylindrical reactor with a 5-10 ns high voltage (17 kV) pulse generator. The distribution of untreated nanoparticle sizes is peaked around 125-150 nm in diameter, as measured using a scanning mobility particle sizer (SMPS) spectrometer. With plasma treatment, we observe up to a 55-fold reduction in relative particle mass and a significant reduction in the nanoparticle size distribution using this method. The effectiveness of the nanoscale PM remediation increases with both the pulse repetition rate and pulse voltage, demonstrating the scalability of this approach for treating particulate matter at higher flow rates and larger diameter reactors.

Combustion efficiency and rate of ignition were shown to be improved when fuel-air ignition was initiated with highly non-equilibrium plasmas generated by highvoltage, nanosecond pulses, also known as transient plasma ignition (TPI). In order to optimize the pulse power parameters for plasma ignition for combustion, detailed experimental investigations of the effect of risetime and pulse repetition frequency (PRF) were conducted for atmospheric pressure static methane/air ignitions. Plasmas driven by 10 ns, 12 kV pulses at a range of PRF from 1 kHz to 10kHz were generated for combustion ignition with a conventional spark plug electrode configuration. Experiments revealed that a different mode in the plasma was initiated when the fuel/air mixture was ignited. At constant pulse duration and PRF, this plasma occurred earlier for the faster rise time (e.g. 4 ns) compared to the longer one (e.g. 8 ns) [1]. In addition, faster PRF favored the earlier plasma mode change or earlier ignition. Importantly, the kinetics of reactive plasma species that were generated during the TPI and combustion were investigated using optical emission spectroscopy (OES). Filtered high speed imaging in combination with electrical measurements are to help understand the plasma rotational temperature related to combustion that is initiated with different pulse rise times and PRFs at a constant pulse width of 10 ns. Gas temperature of the repetitively pulsed plasma ignition for combustion is discussed by measuring the rotational temperature of the second positive systems of nitrogen N2 (C-B).

Transient plasma ignition (TPI) uses short (typ. nanosecond durations) high voltage pulses to generate highly non-equilibrium plasmas for combustion ignition. TPI allows lean-fuel combustion, improves ignition or combustion efficiency with potentially reduced emission [1], [2]. This study evaluates the effect of pulse risetime of >10 kV, 6 ns pulsed plasmas on the ignition plasma formation and combustion chemistry in a static chamber containing methane and dry air at atmospheric pressure. Repetitive (up to 10 kHz), 6 ns, up to 20 kV pulses with different pulse risetimes (i.e. 4 ns and 8 ns) were used to generate transient plasmas between a tip-to-pin electrode configuration, which was the same as a typical spark-plug igniter. Ignition delay, peak pressure and pressure risetime were compared among the transient plasmas driven by different nanosecond pulses. The presence of hydroxyl radicals during the ignition process in forms of OH(A) emission was recorded and the OH role contributing to the subsequent combustion chemistry was discussed.

Lean-burn combustion dominates the current reciprocating engine R&D efforts due to its inherent benefits of high BTE and low emissions. The ever-increasing push for high power densities necessitates high boost pressures. Therefore, the reliability and durability of ignition systems face greater challenges. In this study, four ignition systems, namely, stock Capacitive discharge ignition (CDI), Laser ignition, Flame jet ignition (FJI), and Nano-pulse delivery (NPD) ignition were tested using a single cylinder natural gas engine. Engine performance and emissions characteristics are presented highlighting the benefits and limitations of respective ignition systems. Optical tools enabled delving into the ignition delay period and assisted with some characterization of the spark and its impact on subsequent processes. It is evident that advanced ignition systems such as Lasers, Flame-jets and Nano-pulse delivery enable extension of the lean ignition limits of fuel/air mixtures compared to base CDI system.

The efficiency improvement and emissions reduction potential of lean and EGR dilute operation of spark-ignition gasoline engines is well understood and documented. However, dilute operation is generally limited by deteriorating combustion stability with increasing inert gas levels. The combustion stability decreases due to reduced mixture flame speeds resulting in significantly increased combustion initiation periods and burn durations. A study was designed and executed to evaluate the potential to extend lean and EGR-dilute limits using a low-energy transient plasma ignition system. The low-energy transient plasma was generated by nano-second pulses and its performance compared to a conventional transistorized coil ignition system operated on an automotive, gasoline direct injection (GDI) single-cylinder research engine. The experimental assessment was focused on steady-state experiments at the part load condition of 1500 rpm 5.6 bar IMEP, where dilution tolerance is particularly critical to improving efficiency and emissions performance. Experimental results suggest that the energy delivery process of the low-energy transient plasma ignition system significantly improves part load dilution tolerance by reducing the early flame development period. Statistical analysis of relevant combustion metrics was performed in order to further investigate the effects of the advanced ignition system on combustion stability. Results confirm that at select operating conditions EGR tolerance and lean limit could be improved by as much as 20% (from 22.7 to 27.1% EGR) and nearly 10% (from λ=1.55 to 1.7) with the low-energy transient plasma ignition system.

The efficiency improvement and emissions reduction potential of lean and exhaust gas recirculation (EGR)-dilute operation of spark-ignition gasoline engines is well understood and documented. However, dilute operation is generally limited by deteriorating combustion stability with increasing inert gas levels. The combustion stability decreases due to reduced mixture flame speeds resulting in significantly increased combustion initiation periods and burn durations. A study was designed and executed to evaluate the potential to extend lean and EGR-dilute limits using a low-energy transient plasma ignition system. The low-energy transient plasma was generated by nanosecond pulses and its performance compared to a conventional transistorized coil ignition (TCI) system operated on an automotive, gasoline direct-injection (GDI) single-cylinder research engine. The experimental assessment was focused on steady-state experiments at the part load condition of 1500 rpm 5.6 bar indicated mean effective pressure (IMEP), where dilution tolerance is particularly critical to improving efficiency and emission performance. Experimental results suggest that the energy delivery process of the low-energy transient plasma ignition system significantly improves part load dilution tolerance by reducing the early flame development period. Statistical analysis of relevant combustion metrics was performed in order to further investigate the effects of the advanced ignition system on combustion stability. Results confirm that at select operating conditions EGR tolerance and lean limit could be improved by as much as 20% (from 22.7 to 27.1% EGR) and nearly 10% (from λ = 1.55 to 1.7) with the low-energy transient plasma ignition system.