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Prospects of lean ignition with the quarter wave coaxial cavity igniter

Dissertation
Author: Franz Andreas Johannes Pertl
Abstract:
New ignition sources are needed to operate the next generation of lean high efficiency internal combustion engines. A significant environmental and economic benefit could be obtained from these lean engines. Toward this goal, the quarter wave coaxial cavity resonator, QWCCR, igniter was examined. A detailed theoretical analysis of the resonator was performed relating geometric and material parameters to performance characteristics, such as resonator quality factor and developed tip electric field. The analysis provided for the construction and evaluation of a resonator for ignition testing. The evaluation consisted of ignition tests with liquefied-petroleum-gas (LPG) air mixtures of varying composition. The combustion of these mixtures was contained in a closed steel vessel with a precombustion pressure near one atmosphere. The resonator igniter was fired in this vessel with a nominal 150 W microwave pulse of varying duration, to determine ignition energy limits for various mixtures. The mixture compositions were determined by partial pressure measurement and the ideal gas law. Successful ignition was determined through observation of the combustion through a view port. The pulse and reflected microwave power were captured in real time with a high-speed digital storage oscilloscope. Ignition energies and power levels were calculated from these measurements. As a comparison, these ignition experiments were also carried out with a standard non-resistive spark plug, where gap voltage and current were captured for energy calculations. The results show that easily ignitable mixtures around stoichiometric and slightly rich compositions are ignitable with the QWCCR using the similar kinds of energies as the conventional spark plug in the low milli-Joule range. Energies for very lean mixtures could not be determined reliably for the QWCCR for this prototype test, but could be lower than that for a conventional spark. Given the capability of high power, high energy delivery, and opportunity for optimization, the QWCCR has the potential to deliver more energy per unit time than a conventional spark plug and thus should be considered be as a lean ignition source.

iv TABLE OF CONTENTS

Abstract ...................................................................................................................... ........................................... ii

Acknowl edgement s .............................................................................................................. ................................ iii

Table of Contents ............................................................................................................. ................................... iv

List of Figures ............................................................................................................... ....................................... vi

List of Tables ................................................................................................................ ...................................... viii

Nomencla ture .................................................................................................................. .................................... ix

CHAPTER 1: Introduc tion ................................................................................................................. ................... 1

1.1 Motiva tion ................................................................................................................ ................................ 1

1.2 Obje ctive ................................................................................................................. ................................. 2

1.3 Appr oach .................................................................................................................. ............................... 2

CHAPTER 2: Literature Review ............................................................................................................ .............. 3

2.1 Sc ope ..................................................................................................................... .................................. 3

2.2 History of t he QWCCR I gniter .............................................................................................. ................... 4

2.3 Traditional DC Spark Ignition ............................................................................................. ..................... 6

2.3.1 Details of a DC Spark Di scharge ......................................................................................... ......... 9

2.3.2 The Brea kdown phase ..................................................................................................... ........... 10

2.3.3 The Ar c Phase ........................................................................................................... .................. 11

2.3.4 The Gl ow Phase .......................................................................................................... ................ 11

2.4 High Pressure Microwave Breakdown and Ignition ........................................................................... . 12

2.4.1 Characteristics of Microwave Plasmas .................................................................................... ... 13

2.4.2 Microwave Br eakdown in Gases ............................................................................................ .... 13

2.4.3 Linear Breakdow n Relati onship ........................................................................................... ....... 15

2.4.4 Relevance to Plasma I gnition ............................................................................................ ......... 18

2.4.5 Summary ................................................................................................................. .................... 19

CHAPTER 3: Design and Experiment al Appr oach ........................................................................................... 2 0

3.1 Design of a high performance re sonator .................................................................................... .......... 20

3.2 Electromagnetic Field in the QW CCR ........................................................................................ .......... 21

3.3 The Quality Factor, Relation to Tip Elec tric Field, Energy Storage and Lo sses. ................................ 21

3.3.1 Conductor and Di electric Losse s ......................................................................................... ....... 23

3.3.2 Radiatio n Losses ....................................................................................................... ................. 27

3.3.3 Cavity Ringup and Energy Storage ........................................................................................ .... 31

3.4 Analysis Results .......................................................................................................... .......................... 31

3.5 QWCCR Design and Implementation ................................................................................................... 32

v 3.6 Microwave Pulse Ignition and Measurement Electronics .................................................................... 34

3.7 Conventional Sparkplug Ignition Comparison System ........................................................................ 36

3.8 Combustion Vessel and Associated Instrumentation .......................................................................... 37

CHAPTER 4: Results and Conclusions ............................................................................................................ 40

4.1 Spark Ignition Results ........................................................................................................................... 40

4.2 Microwave Ignition Results ................................................................................................................... 42

4.3 Comparison of Results .......................................................................................................................... 45

4.4 Conclusion ............................................................................................................................................. 46

4.5 Recommendations ................................................................................................................................ 47

Bibliography ........................................................................................................................................................ 48

Appendix A ......................................................................................................................................................... 52

A.1 QWCCR Ignition Test Data .................................................................................................................. 52

A.2 Spark Plug Ignition Test Data ............................................................................................................... 80

A.3 Resonator Quality Factor Measurement .............................................................................................. 98

A.4 High bandwidth Microwave Detector ................................................................................................... 99

A.5 Pressure Sensors ............................................................................................................................... 102

A.6 Fuel Properties .................................................................................................................................... 103

A.7 Ignition Coil Efficiency Measurements ............................................................................................... 104

A.8 Combustion Vessel ............................................................................................................................. 105

vi LIST OF FIGURES

Fig. 1 Illustration of plas ma variety (L ieberman 2005) ....................................................................... ................ 3

Fig. 2 Sketch of spark plug anatomy (H eywood 1988) ........................................................................... ........... 6

Fig. 3 Minimum energy at 1/3 bar (Lewis and vo n Elbe 1948) ................................................................... ....... 8

Fig. 4 Minimum pressure with 8640 mJ of ener gy ............................................................................... .............. 8

Fig. 5 Minimum energy of propane at 0.17 bar ................................................................................ ................. 8

Fig. 6 Autoignition temperatures of hydrocarbons (Cowar d and Jones 1952) ................................................. 8

Fig. 7 Least igniting currents of paraffin hydrocarbon- air mixt ures .......................................................... ......... 9

Fig. 8 Voltage and current variations of c onventional coil spark i gnition (Maly 1976). ................................... 10

Fig. 9 Basic QWCCR coaxial st ructure ......................................................................................... ................... 20

Fig. 10 Contour plot of Q/2 as for Bass, at 2.45 GHz, as given by (49), neglecting radiation, and no dielectr ic .................................................................................................................... ............................ 26

Fig. 11 Contour plot of E a in (kV cm -1 W -1/2 ) for Bass at 2.45 GHz, as given by (30), .................................... 26

Fig. 12 Contour plot of Q/2 for Copper at 915 MHz as given by (49), neglecting radiation and no dielectric 26

Fig. 13 Contour plot of E a in (kV cm -1 W -1/2 ) for Copper at 915 MHz, as given by (30), ................................. 26

Fig. 14 Contour plot of the ratio of internal to exte rnal stored energy, U rad / U as given by (54) and (27). .... 28

Fig. 15 Contour plot of Q rad x 10 -5 as given by (56) .......................................................................................... 29

Fig. 16 Contour plot of the Q/2 for brass at 2.45 GHz, as gi ven by (57), air dielectric ................................... 30

Fig. 17 Contour plot of E a in (kV cm -1 W -1/2 ) for brass at 2.45 GHz, as gi ven by (30),air dielectric ............... 30

Fig. 18 Contour plot of Q/2 for copper at 915 MHz, as given by (57), air di electric ........................................ 30

Fig. 19 Contour plot of E a in (kV cm -1 W -1/2 ) for copper at 915 MHz, as give n by (30) air dielectric .............. 30

Fig. 20 QWCCR design and implem entation ...................................................................................... ............. 33

Fig. 21 Implemented rounded QWCCR center c onductor tip ....................................................................... ... 34

Fig. 22 Microwave pulse deliv ery and measurem ent system ...................................................................... .... 34

Fig. 23 CDI igni tion system .................................................................................................. ............................. 36

Fig. 24 Spark plug use in ignition experiments ............................................................................... ................. 37

Fig. 25 Combustion vesse l measuremen t system ................................................................................. .......... 38

Fig. 26 Successful spark igniti on of 3.9% LPG by volume ...................................................................... ........ 41

Fig. 27 Spark energy data (o ignited; + did not igni te) ...................................................................... ............... 42

Fig. 28 QWCCR successful igniti on of 3.8% LPG by volume ...................................................................... ... 43

Fig. 29 QWCCR successful igniti on of 3.5% LPG by volume ...................................................................... ... 44

Fig. 30 QWCCR energy data ( ignited; × did not ig nite) ................................................................................ 45

Fig. 31 Forward and reverse path loss test for 50 Ω load and open ............................................................... 52

Fig. 32 Spectrum analyzer measurement of si gnal .............................................................................. ........... 52

Fig. 33 QWCCR, successful igniti on of 5.9% LPG by volu me ..................................................................... ... 53

Fig. 34 QWCCR, successful igniti on of 4.9% LPG by volu me ..................................................................... ... 54

Fig. 35 QWCCR, successful igniti on of 4.1% LPG by volu me ..................................................................... ... 55

Fig. 36 QWCCR, successful igniti on of 3.5% LPG by volu me ..................................................................... ... 56

Fig. 37 QWCCR, successful igniti on of 3.7% LPG by volu me ..................................................................... ... 57

Fig. 38 QWCCR, successful igniti on of 3.4% LPG by volu me ..................................................................... ... 58

Fig. 39 QWCCR, successful igniti on of 3.8% LPG by volu me ..................................................................... ... 59

Fig. 40 QWCCR, successful igniti on of 5.4% LPG by volu me ..................................................................... ... 60

Fig. 41 QWCCR, successful igniti on of 5.6% LPG by volu me ..................................................................... ... 61

Fig. 42 QWCCR, successful igniti on of 4.4% LPG by volu me ..................................................................... ... 62

Fig. 43 QWCCR, successful igniti on of 4.0% LPG by volu me ..................................................................... ... 63

Fig. 44 QWCCR, successful igniti on of 7.5% LPG by volu me ..................................................................... ... 64

Fig. 45 QWCCR, unsuccessful atte mpt of 3.0% LP G by volume.................................................................... 65

Fig. 46 QWCCR, successful igniti on of 3.9% LPG by volu me ..................................................................... ... 66

Fig. 47 QWCCR, successful igniti on of 6.2% LPG by volu me ..................................................................... ... 67

Fig. 48 QWCCR, successful igniti on of 3.8% LPG by volu me ..................................................................... ... 68

Fig. 49 QWCCR, successful igniti on of 4.5% LPG by volu me ..................................................................... ... 69

vii Fig. 50 QWCCR, successful ignition of 3.5% LPG by volume ........................................................................ 70

Fig. 51 QWCCR, successful ignition of 6.9% LPG by volume ........................................................................ 71

Fig. 52 QWCCR, successful ignition of 4.2% LPG by volume ........................................................................ 72

Fig. 53 QWCCR, successful ignition of 5.0% LPG by volume ........................................................................ 73

Fig. 54 QWCCR, successful ignition of 3.3% LPG by volume ........................................................................ 74

Fig. 55 QWCCR, successful ignition of 7.4% LPG by volume ........................................................................ 75

Fig. 56 QWCCR, successful ignition of 5.3% LPG by volume ........................................................................ 76

Fig. 57 QWCCR, successful ignition of 3.4% LPG by volume ........................................................................ 77

Fig. 58 QWCCR, unsuccessful ignition attempt of 3.0% LPG by volume ....................................................... 78

Fig. 59 QWCCR, successful ignition of 3.7% LPG by volume ........................................................................ 79

Fig. 60 Spark, successful ignition of 3.9% LPG by volume ............................................................................. 80

Fig. 61 Spark, unsuccessful attempt of 3.4% LPG by volume ........................................................................ 81

Fig. 62 Spark, successful ignition of 3.9% LPG by volume ............................................................................. 82

Fig. 63 Spark, successful ignition of 3.8% LPG by volume ............................................................................. 83

Fig. 64 Spark, successful ignition of 6.0% LPG by volume ............................................................................. 84

Fig. 65 Spark, successful ignition of 7.3% LPG by volume ............................................................................. 85

Fig. 66 Spark, successful ignition of 7.9% LPG by volume ............................................................................. 86

Fig. 67 Spark, successful ignition of 9.0% LPG by volume ............................................................................. 87

Fig. 68 Spark, successful ignition of 8.3% LPG by volume ............................................................................. 88

Fig. 69 Spark, successful ignition of 4.7% LPG by volume ............................................................................. 89

Fig. 70 Spark, successful ignition of 4.4% LPG by volume ............................................................................. 90

Fig. 71 Spark, successful ignition of 3.9% LPG by volume ............................................................................. 91

Fig. 72 Spark, successful ignition of 3.6% LPG by volume ............................................................................. 92

Fig. 73 Spark, successful ignition of 3.8% LPG by volume ............................................................................. 93

Fig. 74 Spark, unsuccessful attempt of 3.4% LPG by volume ........................................................................ 94

Fig. 75 Spark, successful ignition of 4.2% LPG by volume ............................................................................. 95

Fig. 76 Spark, successful ignition of 5.4% LPG by volume ............................................................................. 96

Fig. 77 Spark, successful ignition of 3.7% LPG by volume ............................................................................. 97

Fig. 78 Impedance data for implemented QWCCR design prior to ignition testing; Q 0 .=515 ........................ 98

Fig. 79 Impedance data for implemented QWCCR design after ignition testing; Q 0 . = 513 .......................... 98

Fig. 80 Basic LTC5535 information .................................................................................................................. 99

Fig. 81 LTC5535 detector board schematic ................................................................................................... 100

Fig. 82 LTC5535 detector PCB board artwork ............................................................................................... 100

Fig. 83 LTC5535 detector response curve at 2430.73 MHz .......................................................................... 101

Fig. 84 LTC5535 response at 2.3 GHz to a 1 MHz pulse rate ...................................................................... 101

Fig. 85 Pressure sensors (left: MPX5010DP; right MPX4250AP)................................................................. 102

Fig. 86 Pressure sensors calibration curves (left: MPX5010DP; right MPX4250AP) ................................... 102

Fig. 87 Ignition coil primary to spark energy transfer efficiency ..................................................................... 104

Fig. 88 Ignition coil primary used in experiment ............................................................................................. 104

Fig. 89 Instrumented combustion vessel ........................................................................................................ 105

Fig. 90 Combustion vessel dimensions .......................................................................................................... 105

viii LIST OF TABLES

Table 1 Test-29 dat a for 5.9% LPG ............................................................................................ ...................... 53

Table 2 Test-30 dat a for 4.9% LPG ............................................................................................ ...................... 54

Table 3 Test-31 dat a for 4.1% LPG ............................................................................................ ...................... 55

Table 4 Test-32 dat a for 3.7% LPG ............................................................................................ ...................... 56

Table 5 Test-33 dat a for 3.7% LPG ............................................................................................ ...................... 57

Table 6 Test-34 dat a for 3.4% LPG ............................................................................................ ...................... 58

Table 7 Test-35 dat a for 3.8% LPG ............................................................................................ ...................... 59

Table 8 Test-36 dat a for 5.4% LPG ............................................................................................ ...................... 60

Table 9 Test-37 dat a for 5.6% LPG ............................................................................................ ...................... 61

Table 10 Test-38 da ta for 4.4% LPG ........................................................................................... ..................... 62

Table 11 Test-39 da ta for 4.0% LPG ........................................................................................... ..................... 63

Table 12 Test-40 da ta for 7.5% LPG ........................................................................................... ..................... 64

Table 13 Test-41 da ta for 3.0% LPG ........................................................................................... ..................... 65

Table 14 Test-42 da ta for 3.9% LPG ........................................................................................... ..................... 66

Table 15 Test-43 da ta for 6.2% LPG ........................................................................................... ..................... 67

Table 16 Test-44 da ta for 3.8% LPG ........................................................................................... ..................... 68

Table 17 Test-45 da ta for 4.5% LPG ........................................................................................... ..................... 69

Table 18 Test-46 da ta for 3.5% LPG ........................................................................................... ..................... 70

Table 19 Test-47 da ta for 6.9% LPG ........................................................................................... ..................... 71

Table 20 Test-48 da ta for 4.2% LPG ........................................................................................... ..................... 72

Table 21 Test-49 da ta for 5.0% LPG ........................................................................................... ..................... 73

Table 22 Test-50 da ta for 3.3% LPG ........................................................................................... ..................... 74

Table 23 Test-51 da ta for 7.4% LPG ........................................................................................... ..................... 75

Table 24 Test-52 da ta for 5.3% LPG ........................................................................................... ..................... 76

Table 25 Test-53 da ta for 3.4% LPG ........................................................................................... ..................... 77

Table 26 Test-54 da ta for 3.0% LPG ........................................................................................... ..................... 78

Table 27 Test-55 da ta for 3.7% LPG ........................................................................................... ..................... 79

Table 28 Test-10 da ta for 3.9% LPG ........................................................................................... ..................... 80

Table 29 Test-11 da ta for 3.4% LPG ........................................................................................... ..................... 81

Table 30 Test-12 da ta for 3.9% LPG ........................................................................................... ..................... 82

Table 31 Test-13 da ta for 3.8% LPG ........................................................................................... ..................... 83

Table 32 Test-14 da ta for 6.0% LPG ........................................................................................... ..................... 84

Table 33 Test-15 da ta for 7.3% LPG ........................................................................................... ..................... 85

Table 34 Test-16 da ta for 7.9% LPG ........................................................................................... ..................... 86

Table 35 Test-17 da ta for 9.0% LPG ........................................................................................... ..................... 87

Table 36 Test-18 da ta for 8.3% LPG ........................................................................................... ..................... 88

Table 37 Test-19 da ta for 4.7% LPG ........................................................................................... ..................... 89

Table 38 Test-20 da ta for 4.4% LPG ........................................................................................... ..................... 90

Table 39 Test-21 da ta for 3.9% LPG ........................................................................................... ..................... 91

Table 40 Test-22 da ta for 3.6% LPG ........................................................................................... ..................... 92

Table 41 Test-23 da ta for 3.8% LPG ........................................................................................... ..................... 93

Table 42 Test-24 da ta for 3.4% LPG ........................................................................................... ..................... 94

Table 43 Test-25 da ta for 4.2% LPG ........................................................................................... ..................... 95

Table 44 Test-26 da ta for 5.4% LPG ........................................................................................... ..................... 96

Table 45 Test-27 da ta for 3.7% LPG ........................................................................................... ..................... 97

Table 46 LTC5535 detec tor specific ations ..................................................................................... .................. 99

Table 47 Fuel com position and pr operties ..................................................................................... ................ 103

ix NOMENCLATURE

µ Magnetic permeability of medium a QWCCR center conductor radius AC Alternating current b QWCCR outer conductor radius B r

Shunt radiation susceptance of flanged coaxial cable aperture D Electron diffusion coefficient DC Direct current E Electric field intensity phasor E ( x )

Complete elliptical integral of the second kind E 0

Electric field strength E a

Electric field at radius a on surface of QWCCR tip E b

Root mean square electric field breakdown threshold ECR Electron cyclotron resonance E ef f

Frequency compensated effective electric field E rms

Root mean square electric field far

Molar fuel air ratio G r

Shunt radiation conductance of flanged coaxial cable aperture H Magnetic field intensity phasor H / /

Component of magnetic field intensity parallel to surface I 0

Assumed peak RF current IC Internal combustion I s

Sparkplug spark current ISM Industrial scientific medical J Surface current phasor l Electron mean free path m Electron mass M

Particle mass >> m

n e

Electron population n e 0

Initial electron population p Absolute pressure p *

Temperature adjusted equivalent pressure P ai r

Absolute pressure of air in combustion vessel prior to mixing with fuel

x P b

Power dissipated in QWCCR base conductor P ct r

Power dissipated by QWCCR center conductor P L

Dissipated power, load power P mix

Absolute pressure of air fuel mixture in combustion vessel prior to ignition P ou t

Power dissipated by QWCCR outer conductor P rad

Power dissipated as radiation p σe

Dielectric volume power density Q Quality factor Q b

Q of QWCCR base conductor Q ct r

Q of QWCCR center conductor Q in t

Q of QWCCR interior Q ou t

Q of QWCCR outer conductor QWCCR Quarter wave coaxial cavity resonator Q Γ=0 Loaded Q of QWCCR with 0 reflected power, perfect coupling, or 1:1 SWR Q σe Q of QWCCR dielectric r Radial distance from QWCCR center RF Radio frequency R s

Conductor surface resistance s Complex frequency-domain or s-plane variable SI Spark ignited T Absolute temperature T ai r

Absolute temperature of air in combustion vessel prior to mixing with fuel tan(δ e ) Dielectric medium loss tangent TEM Transverse electromagnetic T mix

Absolute temperature of air fuel mixture in combustion vessel prior to ignition U Stored energy U a

Energy absorbed by an electron between collisions U m

Time average stored magnetic energy U rad

Energy stored in radiative near field U s

Sparkplug spark energy V ab

Potential difference between QWCCR outer and inner conductor V i

Gas ionization potential V o

Assumed peak RF electric potential V s

Sparkplug spark potential

xi X s

Conductor surface reactance Y 0

Characteristic conductance of a coaxial cable z Distance along the axis of the QWCCR from the base Z s

Conductor surface impedance α Real part of propagation constant, attenuation constant β Imaginary part of electromagnetic propagation constant, phase constant Γ Voltage reflection coefficient ε Electric permittivity of medium ε’ Real part of dielectric complex permittivity ε” Imaginary part of dielectric complex permittivity ε c

Dielectric complex permittivity ζ Damping coefficient of a second order system η Intrinsic impedance of medium λ Electromagnetic wavelength Λ Characteristic diffusion length μ c

Conductor magnetic permeability ν a

Electron loss attachment frequency ν c

Effective momentum collision frequency of the electrons and neutral particles ν i

Electron production ionization frequency σ c

Conductor conductivity σ e

Dielectric medium effective conductivity ω Angular frequency ω n

Angular natural frequency of a second order system τ p

Ignition pulse duration U q w

Energy of microwave pulse accepted by QWCCR P p ls

Microwave pulse power as measured by spectrum analyzer P re f

Microwave pulse power reflected by QWCCR

1 C H A P T E R 1: I n t r o d u c t i o n

1.1 Motivation Since the invention of the modern internal combus tion (IC) engine over a hundred years ago, two basic methods have been used to ignite the combustion mixtures. Auto ignition of the air-fuel mixture through compression, as in the Diesel engine as invented by Rudolf Diesel in 1892, and spark ignition as is used in four stroke Otto-Cycle engines, as fi rst developed by Nikolaus Otto in 1876. Today a very large number of spark ignited (SI) engines are in use, consuming the planet ’s limited fossil fuel ener gy supply. A significant environmental and economic benefit could be obtained if these engines could be made more efficient. Higher thermal efficiencies for SI engines could be obta ined through operation with leaner fuel air mixtures and through operations at higher power densities and pr essures (Dale 1997). Unfortunately, experience has shown that as fuel-air mixtures are leaned or as cylinder pressure is increas ed, these mixtures become more difficult to ignite. More ener getic sparks can be used to ignite t hese mixtures, however, their overall ignition energy efficiency is reduced, and more energetic s parks with larger surfaces are required for reliable ignition (Maly et al. 1983). These higher energy levels are detrimental to the spar k plug lifetime, especially to the electrodes, and may also contribute to the form ation of undesirable pollutants. Alternatives to the traditional ignition spark could open the door to more efficient, leaner and cleaner combustion resulting in associated economic and environmental benefits. Such alternative ignition systems include unconventional electrical discharges as opposed to a direct current (DC) spark. These discharges form various plasmas. Research into plasma assisted ignition and combustion includes a frequency range from DC sparks to laser light. The various methods and devices each have their own peculiarities and complexities. For example, laser ignition requires optical transparency of the combustion chamber and compli cated laser equipment (Starikovskaia 2006). One particularly simple plasma device is the quarter wave coaxial cavity resonator (QWCCR). This device has been studied by researchers at the West Virginia University’s Mechanical and Aerospace Engineering Department for a number of years (Nas h 1988, Bonazza 1992, Stiles 1997, McIntyre 2000). The QWCCR consists of a quarter wavelength resonant c oaxial cavity into which electromagnetic energy is

2 coupled resulting in a standing electromagnetic field. Th is large field induces a break-down to occur in the gaseous medium surrounding the center electrode, creating a plasma discharge that has been demonstrated to have potential as an ignition source (McIntyre 2000). Theoret ical analysis of the QWCCR in the context of transmission line theory was perform ed by Nash, but experimental performance data has not been fully explained by this analysis. 1.2 Objective The objective of this work is to investigate the mi crowave QWCCR as an ignition source relative to a conventional DC spark plug and to determine if the quarte r wave coaxial cavity resonator can ignite a leaner fuel-air mixture than a conventional spark plug gi ven a similar amount of input energy. A secondary objective is to clarify the influences of geometry and materials on the design of a QWCCR. This will be accomplished through a theoretical analysis with emphas is on electrical efficiency, as represented by the quality factor of the QWCCR device, and the peak electric field generated. 1.3 Approach For experimental purposes, a coaxial cavity res onator, which can consistently produce microwave discharge, needed to be designed and constructed. The desi gn phase of this work examined, in detail, the quality factor, Q , of the coaxial cavity resonator through analysis using radio frequency (RF) cavity methods. The quality factor is a measure of a resonator’s electr ical efficiency and can also be related to the generated peak microwave electric field. A microwave coaxial cavity igniter was constructed out of brass; the quality factor was measured on a microwave network analyzer, and the igniter was coupled to a suitable microwave amplifier with associated power and control circuitry to generate a microwave pulse of the desired frequency and power. Following the design and construction, testing in a combustion bomb was performed, using a standard spark plug as a reference. The performance of the coaxial resonator for igniting lean charges will be evaluated by varying the fuel-air mi xture to determine the lean ignition th reshold for a given amount of input energy.

3 C H A P T E R 2: L i t e r a t u r e R e v i e w

2.1 Scope There are numerous alternative igni tion systems, including multiple s park plugs per cylinder systems, rail-plug igniters and corona spark plugs (Dale 1997). Mo st of these systems use plasma of one type or another. The conventional spark, as de scribed in detail by Maly (Maly 1984), is essentially a plasma. There are numerous ignition methods that can be classified as pl asma ignition systems. Some examples that have been investigated by various researchers include: laser plasma ignition (McMillian 2004), pulsed nanosecond discharges (Pancheshnyi 2005) , dielectric barrier discharges (Anikin 2003), radio frequency (RF) (Chintala 2006) and microwave discharges (B erezhetskaya 2005, Leonov 2006). There is a great variety of plasmas, often categorized by their tem perature and electron density as shown in Fig. 1.

Fig. 1 Illustration of plasma variety (Lieberman 2005)

4

Since operation of the QWCCR occurs at frequencies in the GHz (10 9 Hz) range, and at atmospheric or above pressures, the plasma generated by it is consi dered a high pressure microwave plasma similar to the plasmas of high pressure arcs, shock tubes and laser pl asmas. This literature review will therefore place emphasis specifically on high pressure microwave induced plasmas, the conventional DC spark and the prior work concerned specifically with the QWCCR. A broader scope of plasma assisted ignition which considers other types of plasma can be found in a recent review paper (Starikovskaia 2006). 2.2 History of the QWCCR Igniter The QWCCR was studied by Nash as an RF power processing element under the guidance of Dr. James Corum and Dr. James Smith at West Virginia University’s College of Engineering (Nash 1988). In his thesis, Nash analyzed the resonator using a tr ansmission line analogy and presented a lumped series resonant circuit model, the parameters of which were not worked out in detail, or in terms of the geometric variables. His work was focused on RF power proces sing by building up a high voltage, oscillating, charge reservoir and not on combustion ignition. Using 200 W of power, an RF plasma was produced by Nash with two 0.75m tall, 100 MHz cavities. His ex pression for the cavity quality factor, Q , given in (1), is the well known expression for the Q of a resonant quarter wave section of transmission line, where α + j β is the complex electromagnetic propagation constant.

α β ⋅ = 2 Q (1) In his conclusions Nash states that prediction of the performance measures based on his analysis are not entirely in agreement with experiment al results, even for such key val ues as the cavity quality factor, and the resonant frequency. In 1992, the QWCCR was proposed as an ignition source for internal combustion engines in two Society of Automotive Engineers (SAE) technica l publications (Bonazza 1992, VanVoorhies 1992). Bonazza proposes that an implement ation of the QWCCR replacing a spark plug would have to operate at about 2 GHz and be capable of ignition at pressures up to 10 bar. He speculated that an increase in ignition volume over the traditional DC spark would result in an advantage for igniting leaner fuel-air mixtures. VanVoorhies performed a theoretical rough-order-of- magnitude analysis of the QWCCR discharge at 2

5 GHz. His analysis indicated the formation of thermal plasma through electron impacts on a stationary gas ion background and the electrode, and that no plasma resonance effects would take place. His analysis was based on an assumed maximum electric field strength of 30 kV/cm attained by the resonator, which is sufficient for DC breakdown of air at 1 atm, and that an electron density of 10 23 m -3 (Baretto 1979) is sufficient to cause ignition. VanVoorhies’ analysis identifies a delay time on the order of 10 ns for imparting sufficient energy to the electrons undergoing multiple collisions to induce breakdown. He concluded that experimental verification of the ignition characteristics of the QWCCR was needed. By 1998, experimental devices had been constructed in the 440 and 900 MHz ranges (Stiles 1997, 1998). In this work, the operation of the cavities in a vessel pressurized up to nearly 7 atmospheres was confirmed at a power level near 150 W. Experimental cavities whose center electrode consisted of a thermocouple showed electrode tip temperatures in excess of 900°C capable of vaporizing Teflon TM for sustained discharges. Stiles’ experiments indicate a slight drop in electrode tip temperature with increasing pressure. Numerical modeling using the finite difference time domain method and experimental measurements confirmed a strong electromagnetic field concentration about the QWCCR’s center electrode tip. For input power levels of 1 mW, field strengths near 40 V/m were measured at approximately 3 mm from the center electrode tip of a prototype plasma igniter mounted in a Briggs and Stratton engine cylinder. Stiles also reported modulation of the RF signal at kHz frequencies (10 3 Hz) resulting in plasmas with modulation frequency in the audible range. Continuation of these experiments by McIntyre’s thesis work led to consecutive ignition events in a Briggs and Stratton engine cylinder by a Teflon TM filled coaxial cavity igniter, while the engine was motored by a dynamometer (McIntyre 2000). The microwave power levels used were approximately 100 W at frequencies in the 900 MHz range. Unfortunately, the Teflon TM filling used to keep the harsh combustion gases out of the resonator did not survive past a few ignitions. To address this shortcoming of the igniter, Lowery investigated several alternate dielectric filling materials and their effects on quality factor and efficiency through experimentation and numerical simulation. Lowery’s results show significant degradation of the quality factor by filling the cavity with a 98% pure alumina powder mix that could withstand the harsh in-cylinder conditions (Lowery 2006).

Full document contains 117 pages
Abstract: New ignition sources are needed to operate the next generation of lean high efficiency internal combustion engines. A significant environmental and economic benefit could be obtained from these lean engines. Toward this goal, the quarter wave coaxial cavity resonator, QWCCR, igniter was examined. A detailed theoretical analysis of the resonator was performed relating geometric and material parameters to performance characteristics, such as resonator quality factor and developed tip electric field. The analysis provided for the construction and evaluation of a resonator for ignition testing. The evaluation consisted of ignition tests with liquefied-petroleum-gas (LPG) air mixtures of varying composition. The combustion of these mixtures was contained in a closed steel vessel with a precombustion pressure near one atmosphere. The resonator igniter was fired in this vessel with a nominal 150 W microwave pulse of varying duration, to determine ignition energy limits for various mixtures. The mixture compositions were determined by partial pressure measurement and the ideal gas law. Successful ignition was determined through observation of the combustion through a view port. The pulse and reflected microwave power were captured in real time with a high-speed digital storage oscilloscope. Ignition energies and power levels were calculated from these measurements. As a comparison, these ignition experiments were also carried out with a standard non-resistive spark plug, where gap voltage and current were captured for energy calculations. The results show that easily ignitable mixtures around stoichiometric and slightly rich compositions are ignitable with the QWCCR using the similar kinds of energies as the conventional spark plug in the low milli-Joule range. Energies for very lean mixtures could not be determined reliably for the QWCCR for this prototype test, but could be lower than that for a conventional spark. Given the capability of high power, high energy delivery, and opportunity for optimization, the QWCCR has the potential to deliver more energy per unit time than a conventional spark plug and thus should be considered be as a lean ignition source.