Ph.D Thesis


Ph.D StudentDruker Nir
SubjectTheoretical Study of Flame Initiation by Electrical
Discharge
DepartmentDepartment of Aerospace Engineering
Supervisors PROFESSOR EMERITUS Jerrold Greenberg
PROF. Eran Sher
Full Thesis textFull thesis text - English Version


Abstract

In Internal Combustion Engines, the combustible mixture is ignited at each cycle by a time-controlled spark discharge. The success of the flame initiation depends to a large extent on the operating conditions (engine speed, engine load, ambient conditions, and spark characteristics). It was found that with traditional ignition systems, the nature of the ignition process induces very significant cycle-to-cycle variations, and as compared to theoretical cycles, these are believed to be the main reason for lower engine torque, high fuel consumption and surplus engine emission. In order to improve the success level of the ignition process, quite a number of different ignition systems have been proposed. Higher discharge energies, longer discharge durations and multi-spark systems have been developed. During the last decade, cold-plasma multi-pulse assisted ignition systems have been proposed. In practice, these systems have shown much better results, though our understanding of how the energy evolves in this complex ignition process is still deficient. Such an understanding is valuable not only for widening and deepening our scientific knowledge, but also to optimize the systems in practical settings.

In this research, we conducted a preliminary, theoretical/numerical investigation into the possibility of use of on-board control of cold plasma processes, known to enhance ignition and combustion of reactive gas mixtures. The focus was on diagnostics and enhancement of energy deposition in specific modes, by application of bi-polar short duration voltage pulses in low-pressure air. Numerical evaluations using various pulse repetition frequencies and different pulse shapes were conducted. Special attention was given to the overall coupled energy, deposited during the discharge, and to energy channeled to known ignition supportive modes such as nitrogen electronic excitation and oxygen radicals generation.

The results of the analysis show that for the considered conditions, energy deposition can be divided into two main stages, characterized by high and low voltage magnitudes, respectively. It was found for the first time, that the (low voltage) second stage’s energy deposition can be higher than that of the first (high voltage) stage. At the second stage, the deposition of energy into specific modes can be tuned by setting appropriate voltage magnitudes. In addition, the energy deposited in modes important for ignition, exhibits a simple linear relation to the overall energy deposition. Furthermore, based on these findings, we demonstrate how a new sequence of voltage pulses can further increase enhancement of ignition and combustion supportive processes.

In addition, in this work, we present a preliminary development of a new, analytically-based finite difference numerical method. The motivation was to obtain high fidelity results and to tackle severe numerical restrictions. The latter usually arise during discharge evaluations at high pressure and voltage conditions. A thorough derivation of the new method is given. Comparisons with several known methods, including standard and non-standard finite difference methods and a commercial finite element code are shown. These suggest that the method can ultimately reduce computational costs and inaccuracies, which were previously reported and were also observed in some trials during the current numerical investigation.