Modeling and Control of Nanosecond Repetitively Pulsed Discharge for Plasma-Assisted Ignition

Abstract

Robust ignition of hard-to-ignite fuels is essential for future spark ignited internal combustion engines, particularly for introducing efficiency-enhancing diesel-like process parameters like air excess or high amounts of exhaust gas recirculation (EGR). Adopting novel plasmabased ignition systems like Nanaosecond Repetitively Pulsed Discharge (NRPD) is promising in extending the ignition limits and the early flame development speed. The aim of this thesis is to propose a characterization and modeling of the relevant aspects necessary for the application of NRPD ignition in engines and to investigate the effect of NRPD on ignition and combustion. This thesis is divided into two parts: the first where the fundamental aspects of single nanosecond Pulse Discharge (NPD) are discussed and analyzed, and the second where the effects of multiple NPD at high repetition frequencies (NRPD) on ignition and combustion are investigated on various setups. Three main research topics are discussed in the fundamental part. First, the electrical characterization of NPD, which includes the modeling of the electrical circuit active during the discharge and the development of a measurement technique for voltage and current measurements. Second, the investigation of the mechanism that leads to an electrical breakdown in sub-mm gas gaps under nanosecond rising voltages, and third, the development of a 0D multiphysics discharge model. The second part incorporates four investigations dealing with plasma-assisted ignition and combustion. The first investigation is performed in a constant volume setup where the plasma to early flame transition under NRPD is analyzed. Two investigations are performed on a Rapid Compression Expansion Machine, one dealing with an ignition detection technique and another investigating hydrogen jet-guided combustion. The fourth investigation is on a 4-cylinder full metal engine setup, where NRPD ignition is investigated both in terms of efficiency and emission. Nanosecond pulses similar to high-frequency signals are transmitted from the pulse generator at high repetition frequencies (≥ 10 kHz) to the load (a spark plug) through a high-frequency transmission line. The electrical characterization is crucial for the characterization of NRPD ignition. The voltage and current during the nanosecond discharges are measured using a shunt measurement in the middle of the coaxial cable that connects the pulse generator to the spark plug. The used pulse generators deliver between 8 and 80 mJ in dependence on the pulse generator’s specific pulse duration and on the variable amplitude. Ideally, the pulse energy could be completely absorbed by the load in the case of ideal impedance matching between the coaxial cable and the load. In reality, the load impedance varies from high to low values during the discharge, resulting in partial reflection of the pulse energy. When energies in the order of some mJ are delivered to the load, a breakdown followed by the establishment of a well-developed spark channel is always formed, which is recognizable from the high currents at low voltages measured. The ratio of pulse energy to delivered energy is an important optimization factor that is affected by the electrode geometry, the stray capacitance and inductance of the load, the pulse shape, and the initial conditions of the gas. The breakdown of the gas gaps is of great importance for NRPD ignition; in fact, the breakdown voltage and time affect the energy deposited during the discharge. Under pulsed conditions, the breakdown voltage is always higher than the static breakdown voltage (DC). The breakdown voltage depends on the static breakdown voltage value and the voltage rise rate. The delay between the static breakdown voltage and the breakdown voltage decreases with increasing overvoltage. This delay is composed of a formative and a statistical time lag; interestingly, both the formative and statistical times decrease with overvoltage, contrary to the established expectation that the statistical time would remain somehow constant. The breakdown voltage is therefore investigated in a well-defined geometry offering a quasi-uniform electric field where evidence of a field-assisted emission of seed electrons with a pressure dependent offset is detected, explaining the statistical time-lag reduction with overvoltages. To understand the electrical measurements, a 0D multiphysics model that incorporates the high-frequency transmission line, a lumped circuit of the load, a two-temperature model for the electrons and the heavy particles, and an ionization scheme is introduced. The comparison of the experimental results with the simulations shows that the plasma electrical resistance rapidly drops after the breakdown. The resistance decrease is present due to the formation (within a few ns) of a micrometric-sized fully ionized spark in thermal equilibrium with temperatures above ∼30’000 K. Optical emission spectroscopy measurements support this conclusion. Plasma to early flame kernel transition under NRPD is investigated and compared against a state-of-the-art inductive ignition system using Schlieren imaging in the pre-chamber of an optical constant volume setup. The evolution of the flame position shows two different phases. The first one is where the expansion is not affected by the initial conditions (Air to fuel ratio (AFR), turbulence level), which lasts approximately 1,5 ms. In this first phase, the expansion rate with NRPD is much higher. The second phase is the region where the AFR and turbulence impact the propagation speed. In this region, the influence of NRPD ignition and increased number of pulses is only apparent when the flame speed is low (low AFR and low turbulence) and has the positive effect of increasing the flame velocity. Today’s high efficiency internal combustion engines are developed in an increasingly narrower range between stable operation, misfire, and knocking. A method for detecting successful ignition under NRPD discharges is therefore investigated. After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established or not. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse depends on the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. The results show that lower breakdown voltages of the control pulse are indeed present if a robust early flame kernel has developed The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to react, if necessary. Combustion researchers, facing increasingly stringent regulations on efficiency and emissions along with the introduction of novel renewable fuels, are investigating a multitude of innovative combustion concepts. Two of these concepts, where NRPD ignition could be beneficial, are analyzed and summarized in the following paragraphs. Hydrogen combustion in engines usually relies on the Otto cycle. This usually results in poor power density and non-ideal efficiencies due to the lean premixed operation and low compression ratio required to avoid preignition and knocking. Alternatively, hydrogen can be directly injected at high pressures into the combustion chamber, and spark ignited at the periphery of the jet, with fuel conversion taking place in jet-guided mode while injection remains active. The high-pressure jet carries significant momentum and exhibits high velocities. Considerable turbulence is produced in the shear layer. Using NRPD ignition, the delay between the spark and ignition is shorter, and the completeness of combustion is higher, highlighting the positive impact of a fast ignition when high turbulence is present. NRPD ignition also results in a lower scatter of the initial combustion phase (premixed phase) that is expected to be the main driver of cycle-to-cycle variations for this combustion concept. Lastly, the combination of NRPD ignition and Turbulent Jet Ignition (TJI) is investigated using a full engine setup. The aim is to use a technology for a robust early flame phase (NRPD) in combination with a technology for a fast combustion of the bulk charge (TJI). The engine can be fitted with a classical open chamber spark plug or with pre-chambers. NRPD ignition is compared against an inductive discharge ignition system. Despite the faster inflammation present with NRPD ignition, similar peak efficiencies, and emissions are reached in open chamber configuration using the inductive discharge and NRPD ignition systems, which are achieved by varying AFR and EGR rates. Above dilution levels for peak efficiency, the efficiency using NRPD ignition decreases at a slower pace and tolerates higher AFR and EGR rates, thanks to a more complete and shorter combustion. For the pre-chamber experiments using NRPD ignition, an efficiency increase and a reduction of emissions compared to inductive discharge ignition are present. The efficiency increases are present due to a stronger prechamber discharge and thanks to a faster end phase of combustion. Overall, this thesis has outlined that NRPD generates a series of powerful sparks where the plasma is in thermal equilibrium, rapidly expanding to establish a fast and reproducible early flame kernel. This capability allows engines to run at much higher dilutions without misfires. NRPD is particularly interesting for new combustion concepts where a powerful ignition source is beneficial, such as in pre-chambers or for jet-guided combustion. NRPD ignition is still in the early phase of development, and many optimizations remain possible; two of those are developing a tailored igniter for NRPD ignition and a detailed investigation into the effect of pulse duration on ignition and combustion.