Laser Ignition of Internal Combustion Engines

Table of Contents

• Introduction and goals of this work
• Principles and advantages of laser ignition
  • Different types of laser ignition
  • Non-resonant laser-induced breakdown
    • Basic steps of a non-resonant breakdown
  • From the laser spark to combustion
  • Advantages of laser ignition
• Overview on literature and patents dealing with laser ignition
  • Literature review
  • Patent review
• Experimental investigation of laser ignition in a constant volume combustion chamber
  • Basic experimental setup
    • Employed combustion chambers
    • Mixture preparation, pressure measurement and experimental procedure
  • Extensive comparison of spark plug and laser ignition
    • Experimental setup
    • Results and discussion
  • Investigation of laser ignition at elevated temperatures
    • Investigation of the lean limit at elevated temperatures
    • Investigation of the minimum breakdown energy at elevated temperatures and high pressures
      • Spherically corrected meniscus lens system
      • Aspheric lens with high NA
  • One-, two- and three-point ignition of hydrogen-air mixtures
    • Experimental setup
    • Results and discussion
  • Schlieren diagnostics of multi-point laser ignition and spark plug ignition
    • Experimental setup
    • Results and discussion
  • Resonant initiation of auto-ignition of n-heptane-air mixtures by an Er,Cr:YSGG laser
    • Experimental setup
    • Results and discussion
• The optical focusing element
  • Relation of NA and Ethr for different focusing optics
  • The aspheric lens window (ALW)
  • Theoretical comparison of spheric and aspheric focusing lenses
  • Long term tests of different lens window materials at atmospheric conditions
    • Experimental setup
    • Results and discussion
• Different experiments on the IC engine
  • Long term tests of different window materials and focusing systems
    • Experimental setup
    • Contamination of the combustion window
    • Comparison of separated and combined spheric focusing optics
  • Direct comparison of laser and spark plug ignition
    • Experimental setup
    • Results and discussion
  • Application of a -prototype laser to an IC engine
    • Description and details of the -prototype laser
    • First successful 100 h test with laser head from the engine decoupled
    • First successful test with laser head directly mounted on the cylinder head
• Laser-triggered HCCI engine
  • Fuel: 80% isooctane & 20% n-heptane
    • Experimental setup
    • Results and discussion
  • Fuel: 100 % natural gas
    • Experimental setup
    • Results and discussion
• Laser ignition of HEDGE engine operation
  • Constant volume combustion chamber experiments
    • Experimental setup
    • Result and discussion
  • Single-cylinder engine experiments
    • Experimental setup
    • Results and discussion
• First design and realization of an own -prototype laser ignition system
  • Basic description of a first -prototype laser ignition system
  • Brief literature review on passively Q-switched, solid-state laser systems
  • First experimental setup and used components
  • Results and discussion
• Summary, conclusions and outlook
• Nomenclature
• References
• Appendix A
• Appendix B
• Collaborations
• Scientific achievements
  • Journal Publications
  • Conference presentations
  • Patents
  • Co-supervision of diploma and baccalaureate theses
• Curriculum vitae

Objectives and Key Themes

This PhD thesis examines the various fundamental aspects and practical usability of a laser ignition system as a new, innovative, and alternative ignition approach for internal combustion (IC) engines. The main focus is on stationary, electricity-producing gas engines, direct injection (DI) gasoline engines, laser-triggered homogeneous charge compression ignition (HCCI) engines, and laser-ignited high efficiency dilute gasoline engines (HEDGE). The dissertation investigates the challenges associated with conventional spark plug ignition, particularly in the context of these engine types, and presents laser ignition as a potential solution. Key themes explored in the thesis include: * **Expanding the lean limit of combustion:** Demonstrating laser ignition's capability to ignite leaner fuel-air mixtures compared to spark plugs, leading to lower combustion temperatures and reduced NOx emissions. * **Improving ignition performance:** Exploring the benefits of laser ignition regarding shorter ignition delays, enhanced ignition reliability, and reduced cycle-to-cycle variations in combustion. * **Optimizing the optical focusing element:** Investigating the influence of the numerical aperture (NA) and lens design on the breakdown threshold energy (Ethr) required for plasma formation. * **Understanding long-term effects:** Analyzing the impact of laser-induced plasma emissions and energy density on different window materials and developing strategies to mitigate contamination and ensure window longevity. * **Exploring the potential of laser ignition in specific engine types:** Presenting experimental findings on the application of laser ignition in HCCI engines, HEDGE engines, and direct injection gasoline engines, demonstrating its potential to enhance efficiency and reduce emissions.

Chapter Summaries

The dissertation presents a comprehensive exploration of laser ignition, covering fundamental principles, experimental investigations, and real-world engine applications. Here is a concise overview of the main themes and findings of the first eight chapters, excluding the conclusion and final chapter to avoid spoilers: * **Chapter 1:** Introduces the concept of laser ignition and its potential applications for various types of IC engines, emphasizing the advantages of laser ignition over conventional spark plug ignition, particularly for stationary gas engines and advanced combustion concepts. * **Chapter 2:** Discusses the different types of laser ignition, focusing on non-resonant laser-induced breakdown, the process of plasma formation through multiphoton ionization and electron cascade processes, and the dynamics of laser spark propagation and combustion. * **Chapter 3:** Provides an extensive review of the current literature and patent landscape related to laser ignition, highlighting key research findings and technological advancements in the field. * **Chapter 4:** Presents a detailed analysis of laser ignition experiments conducted in constant-volume combustion chambers, showcasing the ability of laser ignition to ignite leaner mixtures than spark plugs, reduce ignition delays, and achieve shorter combustion durations. The chapter also explores the impact of elevated initial temperatures on ignition limits and breakdown energy, investigates multi-point ignition strategies, and examines the feasibility of resonant absorption for triggering auto-ignition. * **Chapter 5:** Focuses on the crucial role of the optical focusing element in laser ignition systems. The chapter investigates the relationship between the numerical aperture (NA) and the minimum breakdown energy (Ethr), highlighting the advantages of aspheric lens windows (ALWs) for achieving lower Ethr values. The chapter also details the design and construction of ALWs for use in engine applications and discusses the long-term performance of various lens materials under atmospheric conditions. * **Chapter 6:** Delves into laser ignition experiments conducted on real IC engines, investigating the performance of different window materials and focusing systems in long-term tests. The chapter explores the impact of window temperature, laser energy density, and contamination on engine operation. Additionally, it provides a direct comparison between laser ignition and spark plug ignition, demonstrating the advantages of laser ignition in terms of reduced emissions, improved combustion stability, and higher efficiency. Finally, the chapter presents the successful implementation of a -prototype laser ignition system, both in a decoupled configuration and directly mounted on the cylinder head. * **Chapter 7:** Examines the potential of laser ignition for triggering homogeneous charge compression ignition (HCCI) in IC engines. Experiments conducted on a modified Scania D12 engine using a mixture of 80% isooctane and 20% n-heptane and later with 100% natural gas as fuel, demonstrate the ability of the laser plasma to advance the onset of HCCI combustion. The chapter analyzes the influence of ignition timing, laser energy, EGR rate, and valve timing on HCCI combustion and explores the use of optical diagnostics like PLIF, Schlieren photography, and chemiluminescence imaging to study the combustion process. * **Chapter 8:** Investigates the potential of laser ignition for igniting highly diluted fuel-air mixtures in the HEDGE (High-Efficiency Dilute Gasoline Engine) concept. Experiments conducted in a constant-volume combustion chamber and on a single-cylinder Labeco CLR engine with varying EGR rates, compression ratios, and loads show that laser ignition offers significant advantages over spark plugs in terms of reduced cycle-to-cycle variations, lower emissions, and improved combustion stability. The chapter also explores the effects of multi-point ignition strategies on combustion duration.

Keywords

This dissertation investigates the various aspects of laser ignition, a promising alternative to conventional spark plug ignition for internal combustion engines, with a focus on improving engine efficiency and reducing emissions. The work covers topics like: laser-induced breakdown, lean combustion, minimum breakdown energy, numerical aperture, aspheric lens windows, HCCI engine, HEDGE engine, and multi-point ignition. The results demonstrate the significant potential of laser ignition to address the future challenges of internal combustion engine technology.