In this PhD thesis different fundamental aspects and the practical usability of a laser ignition system as a new, innovative and alternative ignition approach for internal combustion engines were investigated in great detail mainly experimentally.
Ignition experiments in combustion chambers under high pressures and elevated temperatures have been conducted. Different fuels were investigated. Also the minimum breakdown energy in dependence of the initial temperature and pressure with the help of an aspheric lens with a high numerical aperture was studied. High-speed Schlieren diagnostics have been conducted in the combustion chamber. The different stages like the ignition plasma within the first nanoseconds via the shock wave generation to the expanding flame kernel were investigated. With the help of multi-point ignition the combustion duration could be reduced significantly. The controlled start of auto-ignition of n-heptane-air mixtures by resonant absorption of Er,Cr:YSGG laser radiation at 2.78 µm by additionally introduced water has been proven in combustion chamber experiments as a completely new idea.
Beside experiments in the combustion chambers and long term tests under atmospheric conditions, various tests in SI engines up to 200 h, have been made. Different sources of contamination of the window surface have been identified. First experiments with a longitudinally diode-pumped, fiber-coupled and passively Q-switched solid-state laser α-prototype system with maximum pulse energy of 1.5 mJ at about 1.5 ns pulse duration were performed which allowed to ignite the engine successfully over a test period of 100 h.
In cooperation with Lund University in Sweden, experiments have been performed on another engine test bed running in HCCI mode revealing the laser spark to be able to stimulate the auto-ignition process and to trigger the onset of combustion.
In another international cooperation conducted with the Southwest Research Institute in Texas, U.S.A., the potential of laser ignition in combination with the so called HEDGE concept was studied.
As a final direction of the work, first calculations and experiments of a β- prototype ignition laser of an own design have been conducted. The concept of a longitudinally diode-pumped, fiber-coupled and passively Q-switched solid-state laser was chosen as the most promising. Emitted pulse energy of 2 mJ within around 1 ns pulse duration was achieved easily allowing generating a laser-induced breakdown in air.
Contents
1 Introduction and goals of this work
2 Principles and advantages of laser ignition
2.1 Different types of laser ignition
2.2 Non-resonant laser-induced breakdown
2.2.1 Basic steps of a non-resonant breakdown
2.3 From the laser spark to combustion
2.4 Advantages of laser ignition
3 Overview on literature and patents dealing with laser ignition
3.1 Literature review
3.2 Patent review
4 Experimental investigation of laser ignition in a constant volume combustion chamber
4.1 Basic experimental setup
4.1.1 Employed combustion chambers
4.1.2 Mixture preparation, pressure measurement and experimental procedure
4.2 Extensive comparison of spark plug and laser ignition
4.2.1 Experimental setup
4.2.2 Results and discussion
4.3 Investigation of laser ignition at elevated temperatures
4.3.1 Investigation of the lean limit at elevated temperatures
4.3.2 Investigation of the minimum breakdown energy at elevated temperatures and high pressures
4.4 One-, two- and three-point ignition of hydrogen-air mixtures
4.4.1 Experimental setup
4.4.2 Results and discussion
4.5 Schlieren diagnostics of multi-point laser ignition and spark plug ignition
4.5.1 Experimental setup
4.5.2 Results and discussion
4.6 Resonant initiation of auto-ignition of n-heptane-air mixtures by an Er,Cr:YSGG laser
4.6.1 Experimental setup
4.6.2 Results and discussion
5 The optical focusing element
5.1 Relation of NA and Ethr for different focusing optics
5.2 The aspheric lens window (ALW)
5.3 Theoretical comparison of spheric and aspheric focusing lenses
5.4 Long term tests of different lens window materials at atmospheric conditions
5.4.1 Experimental setup
5.4.2 Results and discussion
6 Different experiments on the IC engine
6.1 Long term tests of different window materials and focusing systems
6.1.1 Experimental setup
6.1.2 Contamination of the combustion window
6.1.3 Comparison of separated and combined spheric focusing optics
6.2 Direct comparison of laser and spark plug ignition
6.2.1 Experimental setup
6.2.2 Results and discussion
6.3 Application of a α-prototype laser to an IC engine
6.3.1 Description and details of the α-prototype laser
6.3.2 First successful 100 h test with laser head from the engine decoupled
6.3.2 First successful test with laser head directly mounted on the cylinder head
7 Laser-triggered HCCI engine
7.1 Fuel: 80% isooctane & 20% n-heptane
7.1.1 Experimental setup
7.1.2 Results and discussion
7.2 Fuel: 100 % natural gas
7.2.1 Experimental setup
7.2.2 Results and discussion
8 Laser ignition of HEDGE engine operation
8.1 Constant volume combustion chamber experiments
8.1.1 Experimental setup
8.1.2 Result and discussion
8.2 Single-cylinder engine experiments
8.2.1 Experimental setup
8.2.2 Results and discussion
9 First design and realization of an own β-prototype laser ignition system
9.1 Basic description of a first β-prototype laser ignition system
9.2 Brief literature review on passively Q-switched, solid-state laser systems
9.3 First experimental setup and used components
9.4 Results and discussion
10 Summary, conclusions and outlook
Objectives & Key Themes
The primary objective of this thesis is to comprehensively investigate the potential of laser ignition as a reliable, high-performance alternative to conventional electrical spark plug ignition in internal combustion engines, particularly for demanding applications like stationary gas engines, direct injection engines, and HCCI combustion concepts.
- Theoretical and experimental analysis of non-resonant laser-induced breakdown mechanisms.
- Development and performance testing of specialized optical focusing elements and lens windows.
- Detailed comparative studies of laser ignition versus conventional spark ignition at varying pressures, temperatures, and fuel-air mixtures.
- Implementation of multi-point laser ignition systems to enhance combustion efficiency and reduce combustion duration.
- Design, assembly, and testing of prototype ignition laser systems (α- and β-types) for real-world engine applications.
Excerpt from the Book
2.4 Advantages of laser ignition
In this chapter the basic, fundamental advantages in comparison to conventional spark plug ignition should be presented and discussed. Especially for stationary, electricity producing gas engines like depicted in Fig. 10, with high demands on the ignition system, laser ignition can play out all of its main advantages. But also for triggering an HCCI engine or to ignite reliably a DI gasoline engine laser ignition is a promising alternative for the future like mentioned in the introduction chapter. This chapter is partly taken from the PhD thesis of Kopecek [20].
The following advantages of laser ignition in comparison to conventional spark plug ignition are mainly focused on gas engines: Ignition of leanest mixtures feasible => lower combustion temperatures => lower NOx emissions; No erosion effects occurring like in the case of spark plugs leading to significantly longer availability of laser ignition systems; Higher load/ignition pressures up to 35 bar applicable => increase in engine efficiency; Choice of arbitrary positioning of the ignition plasma in the cylinder available; advantageously in the center of the combustion chamber, to minimize the path length of the propagating flame front and to increase the engine efficiency especially in the case of very lean mixtures; Simplified possibility of multipoint ignition to speed up the combustion process for highest engine efficiencies especially for lean mixtures; Precise ignition timing possible for optimal engine performance and maximum efficiency; Shorter ignition delay time; Less space demand in the cylinder head because of the smaller components of a laser oscillator => larger inlet and outlet valve diameters => increase in engine efficiency. Some of these advantages are discussed in more detail just below.
Summary of Chapters
1 Introduction and goals of this work: This chapter introduces the challenges of conventional spark ignition in modern lean-burn and DI engines and outlines the potential of laser ignition as a superior alternative.
2 Principles and advantages of laser ignition: It details the physical principles of non-resonant laser-induced breakdown and compares laser ignition characteristics with conventional spark plugs.
3 Overview on literature and patents dealing with laser ignition: A comprehensive review of historical research and existing patent landscape regarding laser ignition technology is provided.
4 Experimental investigation of laser ignition in a constant volume combustion chamber: The author presents systematic experimental studies of laser ignition in controlled, non-engine conditions, focusing on lean limit and breakdown energy.
5 The optical focusing element: This chapter analyzes the optical requirements for effective focusing and introduces the aspheric lens window (ALW) design to maximize efficiency.
6 Different experiments on the IC engine: It documents the real-world performance of laser ignition systems in operational engines, including long-term reliability and component endurance tests.
7 Laser-triggered HCCI engine: This chapter investigates the influence of laser ignition on HCCI combustion modes using optical diagnostics to understand the transition from spark-triggered to auto-ignition.
8 Laser ignition of HEDGE engine operation: The focus is on the performance of laser ignition under HEDGE operational parameters, characterized by high exhaust gas recirculation rates.
9 First design and realization of an own β-prototype laser ignition system: The author outlines the development of a custom-designed, robust, and cost-optimized prototype laser system for industrial use.
10 Summary, conclusions and outlook: The work concludes by summarizing the primary findings and suggests future directions for the commercial development of laser-based ignition systems.
Keywords
Laser ignition, internal combustion engine, HCCI, HEDGE, optical diagnostics, non-resonant breakdown, combustion chamber, lens window, ignition delay, emission reduction, lean burn, numerical aperture, plasma formation, automotive technology, gas engine.
Frequently Asked Questions
What is the primary motivation for researching laser ignition systems?
Laser ignition is researched as a solution to overcome the physical limitations of conventional spark plugs, such as electrode erosion and the difficulty of igniting lean fuel-air mixtures effectively in modern, high-efficiency engines.
What are the central thematic areas covered in this work?
The work covers fundamental ignition physics, the design of optical focusing components, combustion chamber experiments, testing on real internal combustion engines, and the development of specialized ignition laser prototypes.
What is the core research question or objective of this thesis?
The core objective is to validate that laser-induced breakdown can provide a more reliable, efficient, and durable ignition source than conventional methods across various engine configurations, including HCCI and highly diluted gasoline engine operation.
Which scientific methods were employed to analyze the ignition process?
The research relies on experimental data from constant volume combustion chambers and real engine test beds, utilizing optical diagnostics such as high-speed Schlieren imaging and planar laser-induced fluorescence (PLIF) to visualize plasma and flame propagation.
What topics are specifically covered in the main experimental sections?
The main part details ignition experiments in static chambers at high pressures and temperatures, long-term endurance testing of various lens window materials, and advanced combustion analysis in HCCI and HEDGE-configured engines.
Which keywords best describe this research?
The research is best characterized by terms like laser ignition, internal combustion, lean burn optimization, HCCI combustion, emission control, plasma physics, and optical component design.
How does the author propose to solve the problem of window contamination?
The author suggests using specific materials with low thermal conductivity (like fused silica) and optimizing the adapter geometry to maximize window temperature, thereby preventing the adherence of combustion byproducts.
What is the significance of the "free burning" effect described in the engine tests?
"Free burning" refers to the deliberate use of high laser energy density to ablate and remove deposits from the combustion window surface during operation, thereby maintaining clear optical access for the laser beam.
- Quote paper
- Dr. techn. Dipl.-Ing. Martin Weinrotter (Author), 2006, Laser Ignition of Internal Combustion Engines, Munich, GRIN Verlag, https://www.hausarbeiten.de/document/169576
