Poznan University of Technology Faculty of Machines and Transport
MSc. Mech. Eng. Wojciech Bueschke
Experimental identification
of an engine lean burn gas-air combustion system with turbulent jet ignition
Dissertation
Promoter:
Prof. DSc. Eng. Krzysztof Wisłocki
Poznań, July 2017
2 Abstract
The study refers to lean gas combustion systems and is dedicated to IC engines. The main aim of conducted research was to investigate the possibility of increase in the ignitability of lean gas-air mixtures. Both the conventional ignition system, as well as the advanced concepts have been here described. Especially the application of spark-jet ignition into engine lean gas combustion system has been conducted. This system has been identified in the course of the investigations executed on the model test-benches using combined optical methods and indicating of fast changing operating data. In the final stage, a spark-jet ignition system has been implemented into the high-speed single-cylinder engine, which requires an auxiliary energy source to initiate the combustion process.
Research on spark ignition system has been carriedout in the rapid compression machine (RCM). The impact of variation in ignition timing on combustion course has been investigated.
The ignitability limit for various discharge energies has been examined and determined; an increase in air excess ratio has been obtained with a reduction of injected fuel quantity. The internal processes occurring in the ignition chamber were investigated in the test-stand with optically accessible, elongated model. The mixture formation process in the pre-chamber has been visualized and recorded using Schlieren imaging and high-speed CMOS camera. The influence of injection pressure and chamber back-pressure on the formation and development of gas jet have been parameterized. The positive impact of injection pressure on the course of mixture formation in the ignition chamber has been found as wellhigh braking effect of pre- chamber back-pressure.
The charge motion in dynamic conditions has been determined, with emphasis on the volume close to the spark-plug. The conditions occurring in the system, which havebeen indicated in the preliminary research campaign, were reintroduced in the research on the spark plug discharge initiating pre-combustion. The spark plug discharge was registered and analyzed regarding variable spark energy, the variable velocity of charge flowing through the spark gap and its direction. The research on spark-jet ignition implemented to lean gas combustion system has been carriedout on the RCM. The sensitivity to the changes in ignition angle has been tested, as well as various strategies of pre-chamber scavenging for determination of the ignitability limits of lean mixtures.
The spark-jet ignition system has been introduced into the single-cylinder research engine.
The investigations were conducted using lean mixtures, with λ=(1.20…1.38). The air excess
3 equivalence ratio has been changed by variation of throttle position resulting in a change of air mass supplied into the cylinder.
The charge motion in the pre-chamber is driven by the injected fuel and impacts sparking.
Bigger discharge energy provides its higher resistance to breaking effect of intensive flow through the spark gap. The privileged spark plug position has been determined. Application of spark-jet ignition resulted in a change of on-ignition mechanism. The occurrence of surface inflammation, mainly from reacting jet, has been found. This mechanism caused increased rate of heat release and extended lean ignitability limit, from λ=1.75 to λ=2.20 regarding defined conditions.
The operation of the system has been positively assessed regarding real engine combustion indicating the cyclic character of the process. The measurements conducted on real engine operating at constant speed and on different loads confirmed positive results reached from investigations on the Constant Volume Chamber and the Rapid Compression Machine. The operation of the combustion system with λ=1.38 comparing to the combustion of mixture with λ=1.20 resulted in 55% reduction of NOx concentration in the exhaust gasses occupied by 14%
rise of HC concentration.
ACKNOWLEDGMENTS
My special thanks I would like to express for Prof. DSc. DEng. Krzysztof Wisłocki for the leadership of my PhD course and the support in the research presented within this study.
I would like also to thank for the help in performing the research MSc. MEng. Maciej Skowron and the other members of Research Group of Engine Thermal Processes: Prof. DSc.
DEng. Ireneusz Pielecha, MSc. MEng. Wojciech Cieślik and MEng. Łukasz Fiedkiewicz.
Special thanks to EU for the financial support of some research in terms of the GasON project (GA No. 652816).
4 Tytuł: Eksperymentalna identyfikacja silnikowego systemu spalania ubogich mieszanek gazowych z zapłonem iskrowo-strumieniowym
Streszczenie
Niniejsza dysertacja traktuje o zagadnieniach spalania ubogich mieszanek gazowych. Celem przeprowadzonych badań było określenie możliwości zwiększenia zakresu zapalności ubogich mieszanek gazowo-powietrznych. Opisano zarówno konwencjonalny układ zapłonowy, jak i konstrukcje zaawansowane, szczególnie iskrowo-strumieniowy układ zapłonowy, który zastosowano w silnikowym systemie spalania ubogich mieszanek gazowych. System ten został zidentyfikowany w wyniku badań przeprowadzonych na modelowych stanowiskach badawczych z jednoczesnym użyciem optycznych metod badawczych oraz indykatorowych pomiarów wielkości szybkozmiennych. Został też zastosowany i przebadany w jednocylindrowym silniku badawczym.
Badania nad iskrowym układem zapłonowym zostały przeprowadzone na maszynie pojedynczego cyklu (RCM). Oceniono wrażliwość systemu spalania na zmiany wyprzedzenia zapłonu. Wyznaczono granicę zapalności ubogich mieszanek gazowych. Wartość współczynnika nadmiaru powietrza była regulowana poprzez zmianę dawki paliwa. Procesy zachodzące w komorze zapłonowej zostały zidentyfikowane na stanowisku z komorą wyposażoną w dostęp optyczny. Rozprzestrzenianie się strugi CNG zobrazowano przy użyciu techniki Schlieren i zarejestrowano kamerą CMOS o dużej częstotliwości filmowania. Wpływ ciśnienia wtrysku i przeciwciśnienia w komorze na rozwój strugi paliwa został oceniony na podstawie parametrów obliczonych dla wiodącego punktu czoła strugi. Wykazano pozytywny wpływ ciśnienia wtrysku na przebieg tworzenia mieszanki, oraz hamujący wpływ przeciwciśnienia w komorze. Wyznaczono warunki ruchu ładunku wewnątrz komory, z uwzględnieniem istotności analizy ruchu w okolicy świecy zapłonowej. Warunki te odwzorowano w badaniach nad wyładowaniem, które zostało zarejestrowane z uwzględnieniem zmiany jego energii, prędkości ruchu ładunku w szczelinie świecy, jak i jej pozycjonowania. Badanie iskrowo-strumieniowego układu zapłonowego zostało przeprowadzone na RCM. Przeanalizowano jego pracę przy różnych wartościach wyprzedzenia zapłonu, jak i strategiach wtrysku paliwa do komory zapłonowej. Wyznaczono granicę zapalności mieszanek ubogich. Opisywany system zastosowano w jednocylindrowym silniku badawczym. Przeprowadzono spalanie mieszanek ubogich, o współczynniku nadmiaru
5 powietrza λ=(1.20…1.38), którego wartość zmieniano poprzez zmianę położenia przepustnicy, a w konsekwencji, zmianę masy powietrza dostarczanego do cylindra.
Rozwój strugi gazu wpływa na ruch ładunku w komorze, a ten z kolei na parametry wyładowania świecy zapłonowej. Zwiększenie energii wyładowania wykazuje korzystny wpływ na stabilizację wyładowania w jego końcowej fazie. Zastosowanie iskrowo- strumieniowego układu zapłonowego skutkuje zmianą mechanizmu zapłonu mieszanki.
Zidentyfikowano występowanie zapłonu powierzchniowego od strugi zapłonowej zawierającej cząstki reaktywne. Zmiana mechanizmu zapłonu skutkowała zwiększeniem granicy zapalności ubogich mieszanek gazowych z λ=1.75 na λ=2.20. Wykazano stabilną pracę badawczego silnika spalinowego w przebadanym zakresie wartości współczynnika nadmiaru powietrza, która potwierdziła pozytywne wyniki badań przeprowadzonych z użyciem CVC i RCM.
Uzyskano mniejsze wartości ciśnienia w cylindrze oraz szybkości wywiązywania ciepła, jak również 55% spadek stężenia NOx w spalinach połączony 14% wzrostem stężenia węglowodorów.
PODZIĘKOWANIA
Szczególne podziękowania składam Panu prof. dr. hab. inż. Krzysztofowi Wisłockiemu za kierownictwo mojego przewodu doktorskiego i udział w kształtowaniu koncepcji badań przedstawionych w tej dysertacji.
Za nieocenioną pomoc w przeprowadzeniu badań dziękuję również mgr inż. Maciejowi Skowronowi oraz pozostałym członkom Zespołu Cieplnych Procesów Silnikowych: Panu prof.
nzw. dr hab. inż. Ireneuszowi Pielesze, mgr inż. Wojciechowi Cieślikowi i inż. Łukaszowi Fiedkiewiczowi.
Podziękowania pragnę również złożyć Komisji Unii Europejskiej za częściowe sfinansowanie badań w ramach projektu GasON (GA 652816).
6 Abbreviations and acronyms
Abbreviation Interpretation ... Unit
c concentration %
dQ/dt Heat release rate J/s
f Frequency Hz
I Intensity of luminescence Counts
Iα/β Intensity of scattered radiation for the scattering angle (α,β) W/cm2
IMEP Indicated Mean Effective Pressure bar
LHV Lower Heating Value MJ/kg
pch Pressure in the ignition chamber bar
pcyl Pressure in the cylinder bar
pinj Pressure in the fuel supply rail bar
Res Resolution px/px
RON Research Octane Number –
STD Standard Deviation
Sy Vertical component of fuel jet length mm
Q Heat released J
𝑊̇𝑖 Indicated work
ηt Thermal efficiency –
λ Air-fuel equivalence ratio –
7 Acronyms
AFR Air Fuel Ratio
ATDC after Top Dead Center BTDC before Top Dead Center
CR Compression Ratio
CV Caloric Value
CVC Constant Volume Chamber
CVCC Compound Vortex Controlled Combustion EGR Exhaust Gas Recirculation
EPA European Pollution Agency FAR Fuel Air Ratio
FJI Flame Jet Ignition
JCCI Jet Controlled Compression Ignition MBF Mass Fraction Burn
PDF Probability Density Function PJI Pulse Jet Ignition
RCM Rapid Compression Machine
SI Spark Ignition
SOI Start of Injection SOIgn Start of Ignition
YAG-laser Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12)
8 CONTENT
1. Introduction ... 10
1.1.Inspiration and motivation ... 10
1.2.Main definitions ... 15
2. Research topic, thesis, and scope of research ... 17
2.1.Justification of dissertation theme choice ... 17
2.2.Knowledge area and main research problem ... 17
2.3.Dissertation problematic ... 18
2.4.Thesis and dissertation scope ... 19
2.5.General concept of the study ... 19
3. Ignition systems for combustion engines ... 23
3.1.Spark ignition ... 23
3.2.Plasma ignition ... 26
3.3.Corona ignition ... 28
3.4.Laser-based ignition ... 29
3.5.Turbulent jet ignition ... 30
4. Lean burn engine combustion systems with pre-chamber ... 35
5. Optical research methods for the investigations on internal engine processes ... 40
5.1.Direct high-speed imaging ... 40
5.2.Laser Induced Fluorescence LIF ... 44
5.3.Shadowgraph and schlieren method ... 47
5.4.Chemiluminescence ... 51
6. Methodology of the experimental research ... 54
6.1.Research on air-fuel mixture ignitability using spark ignition system ... 54
6.2.Research on filling processes in the ignition chamber ... 56
6.3.Investigations on morphology of system-specific sparking ... 60
6.4.Research on ignitability using turbulent jet ignition ... 62
6.5.Turbulent jet ignition operation in real engine ... 63
6.6.Representativity criteria of research campaigns ... 67
7. Ignition capability of SI combustion system ... 69
7.1.Impact of ignition advance on combustion of lean mixtures ... 69
7.2.Ignitability limit of spark ignition system ... 73
8. Mixture formation in the pre-chamber ... 78
9
9. Discharge morphology in the spark-jet ignition system ... 86
10. Ignitability of turbulent jet ignition system ... 95
10.1. On-ignition mechanism ... 95
10.2. Impact of ignition timing on combustion ... 97
10.3. Scavenging of ignition chamber ... 100
10.4. Impact of pre-chamber fuel dose on combustion ... 101
10.5. Ignitability limit of spark-jet ignition ... 103
11. Comparison of spark ignition system and spark-jet ignition system ... 108
12. Assessment of turbulent jet ignition combustion system operation in real engine conditions ... 111
12.1. Analysis of combustion indicators... 111
12.2. Exhaust emissions indicators ... 115
13. Conclusions and directions of the further investigations ... 117
13.1. General feature of achieved results ... 117
13.2. Cognitive and practical conclusions ... 119
13.3. Directions of further investigations ... 122
14. Bibliography ... 124
15. List of figures ... 135
16. Appendix ... 138
16.1. Parameters of pressure sensors ... 138
16.2. Accuracy of measurement equipment ... 139
16.3. Optical data– rich-burn combustion in SI combustion system ... 140
16.4. Optical data– mixture formation in the pre-chamber ... 141
10
1. Introduction
1.1. Inspiration and motivation
The combustion engines are currently a significant source of the mechanical energy in the different keysectors of the human activity. The need of development of power train units is growing rapidly resulting from the means of transport expansion of the population. As the consequence of that, in recent years, the engine related combustion became a crucial research topic due to the number of units in production. As the history and forecasts[1]show, total automotive units sales are continually rising, a tendency that looks set to continue over the next few years.
Combustion engines are a flexible source of mechanical energy, serving high demand required in transport, power, and other industry sectors. Parallel with rising productionbetter fuel economy is observed[2], combined with the increased overall average efficiency of modern engines. Efficiencyis reflected in emission factors, which are limited by legislative regulations.
Different types of system-specific efficiencies can be distinguished as follows:
− total fuel efficiency (more often called as overall efficiency),
− thermal efficiency,
− mechanical efficiency,
− volumetric efficiency.
While mechanical efficiency is resulting from mechanical losses (mainly friction between mechanical parts and throttling loses in charge exchange processes) and actually is not prevalent in the engine energy balance, the thermal and volumetric efficiencies are dominating in the development of engine processes and its design. For the assessment of combustion system, the most important factor is thermal efficiency, defined as:
𝜂𝑡 = 𝑊̇𝑖
𝑄̇𝑖𝑛 (1)
where 𝑊̇𝑖– indicated work, 𝑄̇𝑖𝑛– heat supplied to the system.
Limits of emissions are systematically lowered to reduce the harmful impact of the combustion engines, but the potential in the development of combustion systems is still indicated. Because the rise in emissions of some toxic compounds is combined with energy losses, the thermal efficiency of combustion systems needs to be further increased.
An important element for combustion systems to functioning effectively is the choice of fuels used. As fuels for combustion engines mostly the fuels in the liquid state of matter are currently used. Conventional liquid fuels for combustion engines are produced from crude oil.
11 Crude oil resources are limited, and forecasts predict the end of proven reserves in approximately 30 years, based on current exploitation levels [3].This is driving intensive research into alternative fuels. Following gas fuels look like a very promising alternative because of their properties:
− natural gas in the gaseous state of matter (CNG),
− natural gas in the liquid state of matter (LNG),
− liquefied petroleum gas (LPG),
− hydrogen.
The main properties of these fuels are outlined in Table 1-1.
Table 1-1. Properties of selected gas fuels
Parameter Unit LPG CNG LNG H2
Main components
Propane,
Butane Methane Methane Hydrogen
LHV (typically) MJ/kg 44−46 50.5 48.6 120.2
RON 94−112 Over 120 Over 120 Over 130
Flammability limits
%vol in the
mixture with air 2−9 5−15 5−15 4−75
Stoichiometric
ratio AFRstoich kg air/kg fuel 15.4 17.2 17.2 34.3
Caloric value of stoich. gaseous mixture CV
MJ/Nm3 3.43 3.39 3.66 3.03
State of matter - liquid gaseous liquid gaseous
Storage
temperature deg C Norm. Cond. Norm. Cond. -163 Norm.
Cond.
Storage pressure
(typically) bar 6 200 9 700
Spec. CO2
emission kg CO2/kg fuel 3.27 2.67 2.67 0
From this table one can notice, that CNG, LNG, H2 are having highest LHV, however among them CNG and LNG are getting the highest calorific value CV of stoichiometric gaseous mixture. Only CNG and H2 are being stored in the gaseous state of matter which makes the mixture formation much faster and better when compared to liquid fuels. But storage of H2 under much higher pressure (700 bar) to preserve better energy density of the fuel kept in the
12 tank makes the fueling system in the vehicles much more complicated, than for CNG stored with only 200 bar overpressure. The fuels mentioned aboveare classified in the group of fuels with a high octane number compared to conventional liquid fuels [4, 5]. The importance of natural gas in fuelling of current engines has been highlighted with its advantages by Heywood in 80s [6]. A higher octane number represents a lower ability to knocking. Knocking is an abnormal combustion, which indicates the multisource occurrence of combustion in the volume of combustion chamber resulting in oscillations of cylinder pressure.
Knockingleads to reduced process thermal efficiency, combined with higher thermal stress in the combustion chamber components[7]. It occurs when the values of pressure and the temperature in the unburned area are sufficient to ignite the mixture spontaneously [8]. During the knocking, high-frequency pressure variations occur in the combustion chamber [9].The capability to knocking depends on the system’s construction and the values of regulatory parameters. The constructional factor influencing engine efficiency, limited by knocking is the engine effective compression ratio [10], which results from geometrical compression ratio and charging pressure ration. The higher the engine compression ratio, the higher efficiency[11].
In the previously mentioned study[11], a research engine with variable compression ratio was used to analyze the connection between the operational parameters’ values and the value of the geometric compression ratio. Its increase from 5 to 9 resulted in 6% higher thermal efficiency and significantly reduced specific fuel consumption. Authors of another study [12]
indicated an approximately linear increase of NOx emission with an increase in compression ratio, which caused in parallel higher cylinder pressure.
An important control parameter also affecting cylinder pressure is ignition timing, which is being intensively investigated [13, 14]. In the study [15] analyzed operational parameters and emissions for the gasoline-fueled engine were consideredin relation tovarying ignition timings.
For the ignition advanced from 10 deg ATDC to 30 degBTDC, IMEP value increased by 34%was indicated at constant fuel dose. In parallel, a 105% larger value of cylinder peakpressure and a 650% higher NOx concentration in exhaust gasses were noted. A strong correlation between engine efficiency and ignition timing has been confirmed. Middleton A. et al. [14]conducted the research on the spark-ignition gas-fueled engine regarding a variable ignition advance angle. Under the significantly earliersparking (from 7 to 20 deg BTDC), can be concluded, that the 15% lower specific fuel consumption and reduced NMHC-emissions were noted.
13 The fuels, data of which were collected in Table 1-1, are characterized bydifferent stoichiometric ratio, which depends on fuel composition. The stoichiometric combustion equation for hydrocarbon fuels can be represented as [16]:
𝐶𝑛𝐻𝑚+ (𝑛 +𝑚
4) (𝑂2+ 3.78𝑁2) → 𝑛𝐶𝑂2+𝑚
2 𝐻2𝑂 + 3.78 (𝑛 +𝑚
4) 𝑁2+ 𝑄𝑐𝑜𝑚𝑏+ 𝑄𝑟𝑎𝑑𝑖𝑡𝑖𝑜𝑛 so every mole of fuel requires 4.78 (𝑛 +𝑚
4)mol of air to complete oxidation. The relationship between actual air-fuel ratio and the stoichiometric factor is represented with factor– air-fuel equivalence ratio, described in the chapter1.1. The significant impact of air excess ratio on engine performance and emissions is indicated (Figure 1.1):
Figure 1.1. Impact of lambda on some operational parameters and emissions [17]
The rapid drop of NOx emission is observed in the lean burn conditions because of the drop in intensity of combustion process and reduced flame front temperature. However, this is occupied with an increase in THC-emission. The increase of the air excess ratio value results in higher BMEP value, which is caused by increased cycle efficiency, what will be than explained in Figure 1.2.
%, bar, g/hph
14 Figure 1.2. The impact of air equivalence ratio variation on engine thermal efficiency[18]
From the curves presented in Figure 1.2,it can be found, that the increase in thermal efficiency of the cycle takes place when air equivalence ratio is increased. Thisis caused by reduced amount of heat delivered to the charge in the analyzed ideal process, while the drop in the specific value of thermal efficiency ηc/λ can be observed.
The fuels mentioned in Table 1-1, compared to the conventional liquid fuels, are characterized with smaller mass-specific CO2-emission. As the research results deliver[19], from the combustion of 1kg pure methane 25% less CO2 is emitted, than from octane combustion, which makes methane especially important in the group of alternative fuel.
As it was mentioned, the lean burn combustion results in higher indicators (which ones) values and also in the NOx-emission benefits. However, more energy is required to initiate the combustion process[20].The minimum ignition (activation) energy (MIE) depends on the type of fuel and the air-fuel equivalence ratio (Figure 1.3):
Figure 1.3. The impact of fuel/air equivalence ratio on minimum ignition energy for the chosen alkanes [21]
c
15 In the case of an air mixture with methane or propane or butane, the lowest ignition energyis ratedat the level of 0.25mJ. For hydrogen, the lowest MIE-value is approx. 10 times lower than for methane [22].When the leaner mixture is being combusted, the higher initiating energy is required to avoid misfires.
The limitation of engine lean-burn combustion related to the misfiring can be described with IMEP cyclic variability: CoV(IMEP). In the study [23]rise of cyclic variations, over the acceptable value, was noticed for the values over λ~1,7. For the further increase of airexcess ratio value, advanced ignition systems are necessary. The comparison of conventional ignition system with turbulent jet ignition system was undertaken by Aleiferis P. G. et al. [24]. In the first case, the rapid increase of CoV(IMEP) value over 2%, was observed for λ~1,3. For the combustion system with the turbulent jet ignition, the rapid drop in combustion stability was noticed barely over λ~1,85.Other advanced concepts, including turbulent jet ignition, also compared to the basic system, will be further described in chapter 3.
From the above presented short overview of some recent investigations and research one can concluded, that engine related combustion systems of lean air/gas mixtures should be further investigated while searching for systems with relatively high thermal efficiencies and strongly limited NOx and CO2 emissions. This statement was the basis for the research undertaken by the author of this thesis, which is described in the following chapters.
1.2. Main definitions
There are some terms used in the title of this study, which should be defined more accurately.
Experimental identification has been here usedin a sense for experimental based determination of the most important parameters affecting system functioning. These parameters will be described in the following chapters.
The investigative analysis was conducted based on experimental investigations, which led to the explanation of the main research questions. The investigations have been focused on an identification of combustion system for are ciprocating engine, classified as high-speed-type, with combustion initiated from the auxiliary energy source, named spark ignition engine. The experiments have been conducted on test-beds described in chapter 2.Combustion systems enable mixture formation through proper air-fuel proportion; it’s ignition and control of the combustionin every moment. Combustion systems consist of following main elements:
- a combustion chamber,
- channels delivering fresh charge to the chamber,
16 - elements managing charge movement,
- a fuel supply system,
- an activation (ignition) energy source (internal or auxiliary),
- channels to pipe the combustion products and other remainders from the combustion chamber.
The correct operation of such a system is evaluated using several parameters where following magnitudes are considered in the analysis, such as:
− cylinder pressure,
− increase in cylinder pressure (cylinder pressure rate),
− the amount of heat released during combustion and heat release rate,
− charge temperature,
− indicated mean effective pressure,
− thermal efficiency,
− main emission factors.
The term gas-air mixture, in the scope of this work, has been used to describe the mixture of air and fuel in a gaseous state of matter consisting of a very high percentage of methane. The magnitude representing the proportion of the fuel mass mfuel and air mass mair is a fuel-air ratio:
𝐹𝐴𝑅 =𝑚𝑓𝑢𝑒𝑙
𝑚𝑎𝑖𝑟 (2)
which is commonly converted into air-fuel ratio:
𝐴𝐹𝑅 = 1
𝐹𝐴𝑅= 𝑚𝑎𝑖𝑟
𝑚𝑓𝑢𝑒𝑙 (3)
In relation to the combustion stoichiometry, the parameter air-fuel equivalence ratio is introduced, sometimes called as air excess ratio (in this work as well):
𝜆 = 𝐴𝐹𝑅𝑎𝑐𝑡𝑢𝑎𝑙
𝐴𝐹𝑅𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐 = 𝑚𝑎𝑖𝑟
𝑚𝑓𝑢𝑒𝑙∙𝐴𝐹𝑅𝑠𝑡𝑜𝑖𝑐ℎ𝑖𝑜𝑚𝑒𝑡𝑟𝑖𝑐 (4)
Lean burn combustion concerns a mixtures oxidation process with air excess coefficient value over λ>1.2 and specified ultra-lean variant: λ=(1.5÷2). The lean-burn combustion is limitedregarding inflammation capability, which is different in the current concepts of ignition systems. In this study, ignitability limit will be assessed for the conventional ignition system and the turbulent jet ignition, which has been chosen from the proposed advanced ignition systems. The ultra-lean range of combustion is especially important in this study, due to the reasons described in the following chapter.
17
2. Research topic, thesis, and scope of research
2.1. Justification of dissertation theme choice
As already mentioned, the combustion engines are currently a very important source of the mechanical energy in the different keysectors of the human activity. Because combustion engines are widely used, improvements should be prioritized to minimize their impact on the environment. Further development of the reciprocating engines is combined with the rise of their thermal efficiency– the improvement potential is still indicated. Higher thermal efficiency can be achieved due to the combustion of the mixtures with higher air equivalence ratio, what has been described in the chapter 1.1., especially when local air deficit will not take place and better burning-out of C and H will be achieved. This tendency can also be seen in very lean mixtures. In the mentioned conditions, misfiring was noticed during the research presented in the cited studies [22, 23]. The misfiring causes significant reduction of the process efficiency and the undesirable emission of the unburned fuel components.
According to chapter 1, the crude oil resources limitation is a significant incentive for seeking alternative energy carriers. In the case of the fuel supply for the combustion engines, the gaseous fuels have important advantages, as a high RON, relative high CV, and high energy density. The use of gaseous fuel also indicates some drawback– during their expansion, volumetric efficiency is significantly reduced. However, mentioned advantages constitute big potential of these fuels and are important for their further implementation to the supplying combustion engines and have been considered when choosing fuel to this research.
In the scope of this study, a novel advanced ignition system concept has been implemented to expand the lean combustion limit, avoid misfiring and therefore increase thermal efficiency when keeping emissions on an acceptable level. The system needs to be better identified. The author conducted analyses of the internal system phenomena and its impact on the combustion system functioning. The scope of investigations will be described in detail in chapter2.4.
2.2. Knowledge area and main research problem
The challenge of the dissertation is rooted in the technological sciences. These sciences study manmade objects defined in functional terms, in contrast to those focused on natural objects[25]. Another important feature of this science area is the implementation of engineering design, which will necessitate the creation of research objects. As research objects, elements of the specific combustion system were chosen within this study. The main research problem is combined with the functionality of this combustion system and is defined as:
18 How should the combustion system be configured to provide enhanced ignitability of lean mixtures of air with gas fuel?
The configuration of the combustion system, in the meaning of this study, consists of the following specific issues:
– determining the proper chamber filling-it strategy with gaseous fuel and with an oxidizer,
– determining injection parameters, – adjustingthe ignition control parameters.
Taking into account these elements, the following aim of this study is defined as the:
application of small-scale turbulent jet ignition to a high-speed engine lean gas combustion system and its experimental identification of its physical and technical properties.
2.3. Dissertation problematic
The features, detailed in the research results that are mentioned in the first chapter, impact combustion system operation. Between these features, the important role of the control parameters has been indicated and proved in the cited research results.
The dissertations’ author focused on the lean-burn combustion systems, which show in general higher thermal efficiency than of rich-burn systems. Lean-burn combustion systems with an auxiliary source of combustion initiating energy will be considered in this study. In this case, for the combustion of the specified mixture, a defined minimum energy portion to initiate combustion (activation energy) is required. The conventional ignition systems deliver a limited portion of this energy. Because the leaner mixtures require a higher initiatingenergy, the possibility of very lean combustion is adequately reduced. An advanced system has been implemented to extend this limit. The operation of this system will be analyzedregarding engine combustion (see chapter 12).
The small-scale jet ignition system replaced conventional spark ignition in the gas combustion system. Its identification was carried out using mostly experimental methods. The optical measurements were applied to characterize the movement of the energy carrier in the ignition chamber and the mixture formation. The system-specific movement was determined with numerical methods to compliment the definition of the mixture formation in the chosen operational conditions. After the definition of the charge movement, the impact of system- specific conditions on spark plug discharge was analyzed. The analysis was conducted in model conditions for different spark energies, also needed to initiate the combustion of different
19 mixture compositions. The proposed system causes spark plug discharge distortions. Their occurrence was registered on the model test bench in the adequate charge movement conditions (see chapter 9).
2.4. Thesis and dissertation scope
Based on the assumptions and the features of the combustion systems approach, the following thesisis formulated:
it is possible to extend the ignitability limit of lean gas-air mixtures using spark-jet ignition system.
Using the method described in chapter 5, this thesis will be confirmed, if the ignitability limit determined for jet ignition system will be higher than the limit for conventional spark ignition.
This will be assessedin the way of comparison of misfiring limits determined in the similar measurement conditions and values of control parameters, on the same test bench using the same combustion system with both ignition systems.
The research presented in this study has been conducted to identify the combustion system in a chosen, unique configuration. The presented system is equipped with a spark-jet ignition to help investigate the ignitability of the lean gas-air mixtures in the extended range.
The study consists of an explanation of the used terms and features. The literature review is focused on state of the art in the direction of lean burn combustion systems (see chapter 3 and 4). Chosen examples are described, like the optical methods for the research on the internal combustion engines (chapter 5). The basic ignition system is described as well as alternative concepts with jet ignition implemented in this research (chapter 6). The ignition system has been identified in the following way: the injection to the ignition chamber was visualized to detect the impact of injection parameters and chamber pressure on the gas spray development in the chamber (chapter 7). The impact of the chamber fulfillment strategy was identified in quasi-static conditions within the chamber as well as under defined pressure rise (Chapter 8).
These charge movement conditions were simulated on the ignition test-bed to investigate deformations of spark plug discharge in the system-specific conditions (chapter 9).
2.5. General concept of the study
The research problem requires preliminary investigations to create a necessary background to implement a comparative method to solve the research problem. The experimental investigationsare divided into four main steps (Figure 2.1).
20 Figure 2.1. General course of experimental investigations
According to Figure 2.1,the first experimental research campaign led to the estimation of lean-burn ignitability limits. It was conducted to determine the properties of the conventional system. These investigations have beenfocusedon obtaining the answer to the following question:
What is the ignitability limit on the model engine using a conventional ignition system in the chosen configuration?
According to the chapter 1.1, combustion of lean mixtures shows a bigger demand forignition energy, which can be covered using advanced ignition systems. The proposed advanced concept– turbulent jet ignition–has been implemented and identified in the second
Identification of phenomena occurring inside the ignition chamber
21 step of research. This identification was performedin two parts and led to the definition of the internal processes occurring in the chamber.
The firstpart of the second research campaign has been conducted on the test-bench with CVC equipped with an optically accessible ignition chamber. According to these investigations, the following questions are posed:
− How do the injection parameters influence the mixture formation process?
− How does the pressure in the chamber impact the mixture formation process?
− How does the strategy of mass transfer from the main chamber influence the mixture formation process?
The gas jet has been registered and evaluated, in both the quasi-static and in dynamic ambient conditions to answer these questions. The movement of charge in the ignition chamber has been identified.
The previously defined charge movement was simulated in the second part of the second research campaign, combined with spark plug discharge. The results from this research allow to answer the following questions:
− How does the charge movement impact the early stages of sparking?
− Will the specific charge movement brake the discharge process?
− How big should the ignition energy be to avoid significant discharge imperfections (which can result in misfiring)?
The implementation of a turbulent jet ignition to the RCM has been executed to prepare the third research campaign. The combustion process using this type of ignition system has been compared with combustion initiated with basic spark ignition based on the optical and indicating data. Based on the results of this comparison, the evaluation of the thesis formulated in chapter 2.4 has been conducted.
In the above-mentioned measurement campaigns, which have been conducted on the model test stands, the in-cylinder phenomena were investigated using indicating research coupled with optical analyses. Such a combination of techniques allows analyzing the distribution of the complex processes occurring in the volume of the combustion chamber. The identification and assessment of gas distribution in the prechamber during mixture formation has been performed using Schlieren technique. The direct registration of luminous processes allowed to detect the ignition spots. The flame propagation, by means of its velocity and development direction, has been parameterized due to the generation of luminescence, which was captured and analyzed
22 in the further steps. The systematics of the chosen optical research techniques has been presented in chapter 5.
The verification of the turbulent jet ignition in the engine conditions consists of the last part of the experimental investigations. Within this campaign, the turbulent jet ignition has been introduced into the research engine with the necessary equipment. Thiswas done to evaluate its applicability regarding cyclic lean combustion of an engine supplied with gaseous fuel.
In these investigations some basic measurements of the emission of toxic compounds were conducted and comparatively presented in the chapter 12.2, just for confirmation of the expected trends in the engine behavior concerning its operational indexes.
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3. Ignition systems for combustion engines
3.1. Spark ignition
The spark ignition is currently the conventional energy source in combustion systems with an auxiliary energy source to initiate the combustion. The main element generating active igniting medium is a spark plug, invented in 1839 by E. Berger [26]. The active medium is being produced mostly from the capacitance sparks and takes place in the ionized volume. The degree of the ionization is described with Eggert-Saha equation [5]:
𝑛𝑖2
𝑛𝑎 = 2√(2∙𝜋∙𝑚𝑒∙𝑘∙𝑇)3
ℎ3 𝑒𝑥𝑝 (− 𝐸𝑖
𝑘∙𝑇), ( 5)
where:
ni– number of ionized molecules, na– total number of moles,
me– electron mass (9.109∙10-31 kg),
k– Boltzmann’s constant (1.38∙10-23 [J/K]), T– temperature [K],
h– Planck’s constant (6.6256∙10-34 [J·s]), Ei– substance’s ionization energy [eV].
The ionization energy is highly dependent on the temperature and the composition of the medium. The influence of the spark ambient conditions on the discharge parameters is also indicated. The impact of the system-specific ambient conditions on the discharge has been investigated within this study and will be presented in the next chapters.
The spark plug discharge time is rated at the level from 0.01μs to 10 ms [27]. The course of the discharge can be defined by the electrical values in the spark plug gap (Figure 3.1).
24 Figure 3.1. Voltage and current course history during the spark plug discharge [28]
After the cut of the primary current, the high voltage value in the spark plug gap has been identified.From this moment, the pre-breakdown process appears. The discharge start (breakdown phase) can be observed when the current rise occurs [29]. After the breakdown process, the rapid voltage drop takes place, which results from the current outflow from the central electrode (anode) and beginning of the discharge of electrical capacity. The secondphase is the arc discharge, due to the capacitive character of the high-voltage leads and coil. In the third phase, the stored energy is dumped into the discharge circuit. The time period of this stage is rated as the longest part of the discharge process and has an inductance-based source. As the studies deliver [30], the total discharge time depends on the spark gap resistance.
On the way of processes mentioned above, in the spark gap area, active plasma is being created. The discharge length can be over 20-times bigger than the spark gap length. The biggest energy concentration is placed in the middle volume, far from the electrodes[31]. The mixture inflammation starts due to the energy transferred from discharge. However, the energy transferred to the mixture is significantly smaller, than the energy supplied to the spark gap area. Big energy amount is being dissipatedin the way of thermal energy flow. The heat dissipation from the spark plug discharge through the spark plug was defined as demonstrated in Figure 3.2:
25 Figure 3.2. Dissipated heat flow through the spark plug [32]
With respect to the numbers in Figure 3.2, 20% of the energy delivered to the spark gap is being transferred to the cylinder charge. Significant heat amount increases the spark plug element’s temperature and is dissipated on the cylinder head and to the spark plug ambient.
Taking into account the impact of various ambient and constructional factors, the research task to reduce a mixture inflammation lack, became very important and was a source of spark ignition system development and new concepts, described in the following chapters.
There are many different methods applied to provide more reliable mixture ignition using spark ignition system. One of the methods is to multiply the number of discharges during the engine cycle. The double spark plug is invented in the scope of the bigger discharge volume from one igniting device and is presented in the study[33]. Bigger discharge volume has been obtained as a result of the larger sparks length. The successful discharge creation has been confirmed with their very long lifetime–150-250 ms–during the research in the pressure chamber. This solution was also tested in the engine conditions and resulted in proper engine operation.
Another, already patented method was the double sliding spark plug [34]. The spark plug generates sparks which execute the movement on the conical way between the electrodes. The invention was created to achieve a higher level of heat and electrical energy transfer to the
26 mixture. As a consequence, the increased ignition speed and a shortened combustion duration were claimed.
The influence of the spark plug ambient and the mixture properties were investigated within the scope of the study [35]. The strong magnetic field and additives in the mixture were applied.
The distortions in spark plug functioning and its degradation were indicated. The impact of the ambient pressure on the breakdown voltage was also a subject of the research [36]. The discharge parameters were analyzed in the pressure vessel, where up to 4 bars of the vessel overpressure was applied. In this case the electrodes were coated with carbon nanotubes layers.
Linear dependency of the breakdown voltage with pressure rise was found. The three-fold bigger breakdown voltage was registered for the overpressure of 4 bars in comparison to the discharge in the atmospheric ambient pressure. Additionally, the introduction of the nanotubes layers caused reduced initial voltage by 30-50%. By these means thespark duration was increased.
The impact of the spark plug electrodes number was analyzed and presented in the study [37]. The research was conducted in the engine conditions. The spark plugs with four, two, one and without ground electrode were compared. As the comparative values, the Pmax, CoV(Pmax), CoV(n) and time to 10% mass fraction burned, were used. The spark plug without a ground electrode, in comparison to the spark plug with four electrodes, provided in the analyzed operating point, 7.3% shorter time to burn 10% of mixture mass and the 4.4% higher IMEP value. The positive effect of the ground electrodes number reduction was noted. The sources of this phenomena are the electric fields caused by small diameter nanotubes.
It should be mentioned that the discharge processes were mainly investigated for engines fueled with liquid fuels and not so intensively for gaseous ones.
Bigger energy demand for the combustion initiation of the lean fuel-air mixtures (compare with Figure 1.3) results in the higher requirements for the ignition systems. There are several methods to supply bigger energy to ignite the mixture, which will we further described.
3.2. Plasma ignition
To create a significantly bigger volume of the plasma in the combustion chamber [38] the achievement of the highly reliable mixture ignition under the consequently increasing back- pressure was the aim. In the plasma ignition system, the discharge covers the large region and has got streamer form. The streamers propagate from the central electrode on multiple paths.
The result of increased plasma volume is also thevolumetric type of ignition. The high-speed plasma creation requests high-voltage short-pulse generation, so the energy consumption is
27 relatively low. The exemplary plasma igniter patent was requested in 2009 and published in [39]. The construction of the igniter tip and exemplary flame propagation shows Figure 3.3.
Figure 3.3. The view of the plasma igniter tip with the plasma streamer (a) and exemplary flame propagation view (b); 1– cathode, 2– anode, 3– plasma streamers [40]
The mixture accumulates in the volume between the centrally mounted anode (2, in Figure 3.3) and the cathode (1). After the pulse generation, plasma (3) is generated in this volume. In contrast to the high-temperature plasma generated in the conventional spark plug system, the low-temperature plasma is related only to the higher energy level of the electrons [41]. These electrons meet mixture molecules and generate reactive radicals. The active radicals start the chain oxidation reaction.
The plasma ignition system has been applied to the small-scale one-cylinder engine within the scope of the study mentioned above. It was indicated that the rise of the system voltage led to the transition of the streamer discharge to the arc. The level of this voltage depends on the pressure. The higher the ambient pressure, the bigger the transition voltage. The dependency doesn’t have linear character (compare with Fig. 8,[41]).
The ignition delay using plasma-assisted ignition was a subject of the analysis [42]. The mixture of the air with propane has been investigated. The ignition delay was measured while the air flow in the combustor was increased. The elongated delay was noticed. The impact of the plasma energy on the ignitability was researched. Increased arc current resulted in an easier mixture inflammation. Shorter ignition delay, in comparison to the spark ignition, was also confirmed.
1 2 3
28
3.3. Corona ignition
Another ignition system concept is designed to provide spatial discharge kernels. The corona ignition system is based on the high-frequency electrical field generated by the igniter tip. Such an igniter provides controlled ionization in the multiple ion streams developing in the combustion chamber [43]. These streams are located in the area of the electrical field where the ionization strength isexceeded. The fields strength decreases in the forward direction and, after the achievement of the corona length, is separated from the electrical ground (wall of the chamber) with the charge isolating layer [44]. The comparison of the discharge produced in the conventional system and the corona ignition is presentedinFigure 3.4.
Figure 3.4. Corona discharge (a) and the spark plug discharge (b)[45]
The ion streams are generated due to the resonance transformation. The corona discharge duration is rated on the 100–300 s. This time is approx. 10-fold shorter in comparison to the spark plug discharge, what provides a more precise estimation of the ignition start point. With respect to the research results [45], higher inflammability of the lean mixture has been observed.
For the comparable demand of energy for one discharge, the ignitability limit for spark ignition was estimated on the level of ~1.5 and for the corona ignition~2.
Based on these results it should be stated that corona-type ignition with its higher discharge energy is offering better ignitability of lean and very lean mixtures.
Idicheria C. A. and Najt P. M.[46] conducted research on the corona ignition system using the optically accessible single-cylinder engine and application aspects of in the multi-cylinder engine operation conditions. In the course of the optical investigations significantly advanced
a) b)
29 combustion phasing compared to the spark ignition was observed. Several ignition points were registeredas well as the shorter ignition delay value. From the analysis results of measurements conducted on multi-cylinder engine, it can be concluded that in the range of 0 to 10 MBF combustion duration was improved. However, the transition events from the corona mode to the arcing mode were noticed. In this mode, significantly smaller benefits from the corona ignition application were indicated due to the reduced number of ignition sources. The necessity of the bigger chamber depth is confirmed to create a thicker separation layer and avoid the aforementionedarcing.
3.4. Laser-based ignition
Another art of the ignition system is based on the ignition energy transferred from the laser light beam. Acronym LASER means Light Amplification by Stimulated Emission of Radiation.
The amplification in such a device is obtained when the excited-state atom meets the electric field of the photon with the defined frequency ω. Then the excitationlevel drops to ground level and the second photon is released with the same frequency as the photon-stimulator. The laser- generated light beam is highly coherent. Its application is considered mainly due to the relative big possible energy density of such a beam.
Griffiths J. et al. [47] conducted research on the application laser light to ignite the mixture.
The investigations were focused on the assessment of the impact of ambient conditions on the ignition characteristics. The investigations were done in the atmospheric combustion facility with ND:YAG laser-based ignition system operated at 532 nm wavelength. A mixture of air with natural gas was used. The minimum ignition energy for different air excess ratio values was determined, at the variable flow velocity and temperature. The aforementioned parameterswere identified as keyfactors for the combustion initiation. It was indicated, that higher flow velocity results in higher pulse energy demand. Exemplary, for some investigated air excess ratio value, by the 13% increased flow velocity, 30% higher pulse energy was necessary to initiate the combustion. The positive temperature impact on the mixture ignitability was noticed at the leaner mixture operation.
Prasad R. K. et al. [48]undertook the work on the flame kernel characterization of the laser- ignited fuel-air mixture. As a fuel, the CNG was used. The fuel was enriched with hydrogen to increase flame front velocity. The research was conducted in the constant volume chamber, and the flame was visualized using shadowgraph. The trend of shorter combustion duration was noticed in the case of lower air excess ratio, bigger hydrogen additive as well as for lower initial
