title: Laser Ignition of an Internal Combustion Engine
Creators: name: David Dodd, Robert
Publisher: name: University of Liverpool Repository
Year: 2023

description: The continued development of increasingly compact and efficient laser sources, advanced laser optics and optical techniques has provided the basis for significant steps forward in research towards practical proof-of-concept demonstration of LI in IC engines. Before this goal could be realised, however, research was needed to characterise the laser beam parameters required for LI under true engine conditions and to. demonstrate the feasibility of an optical system capable of delivering such laser beam parameters from the most' appropriate high power pulsed laser source available. This dissertation discusses research undertaken in the Department of Engineering at the University of Liverpool, in which a prototype laser ignition system has been developed and used to successfully ignite, and run for extended periods, one cylinder of a 4-cylinder petrol-fuelled IC test engine. The laser ignition system was based on a Q-switched lamp-pumped Nd: YAG laser operating at 1064 nm wavelength and with pulse duration configurable between 6ns and 15ns. The research was supported by the Department of Trade and Industry (DTI), through a 3Y2 year project 'Laser Ignition in IC Engines (LASIIC)' awarded under the Foresight Vehicle LINK initiative to partners Ford Motor Company, GSI Group and the University of LiverpooI. The experimental work has largely addressed the development and implementation of free-space (or direct) laser beam delivery in order to practically realise the prototype LI system, although optical fibre delivery was also addressed in part. The variation of several laser parameters and their effect on the control of engine performance are discussed, but primarily centring on the influence of pulse energy and focused beam waistÃ‚Â· diameters at the combustion point. The engine performance was measured in terms of changes in Coefficient of Variation (COV) of mean and peak engine cylinder pressure values with laser input parameters. Laser pulse energies of up to 30 mJ at the cylinder were used to obtain power densities at the focusedÃ‚Â· beam waist of up to 4470 GWcmÃ‚Â·2 Ã‚Â• The minimum ignition energy (MIE) required for successful ignition (no misfires) was measured as 4 mJ for a pulse length of 6 ns, which compares to 30 mJ in I ms for a typical spark plug and suggests the likely Ã‚Â·laser and power supply requirements of a future laser ignition system. The power density required to be delivered into the combustion volume has a minimum value of 545 GW/cm2 , which gave comparable or better COVs, than conventional spark ignition. These power densities are significantly less than that required to produce dielectric breakdown in air, due to the high values ofpressure in the cylinder up to 20 bars, but are high enough to conclude that dielectric breakdown is the dominant mechanism ofLl in this case. One potential benefit of LI realised during the work has been the ability to vary the focal position within the combustion chamber, which is not available with spark ignition due to the fixed spark plug. Using a 'design, build and test' approach, the work therefore investigated the effect of varying the focal point position of the beam waist within the combustion volume through a number of configurations of the laser ignition system optics. Once stable combustion had been achieved, further investigations were carried out into optimising laser parameters for ignition control. Earlier work had suggested that laser-induced damage in optical media could affect the achievement of a robust LI system for continuous delivery of such high pulse energies and short pulse lengths, or at least detennine the most appropriate fonn of beam delivery for such high power pulsed laser sources. Hence, a study was also undertaken on laser parameters and beam conditions that could give rise to laser-induced optical damage in system components and the damage mechanisms involved. The work also considers a self-cleaning effect of the pulsed laser radiation and its role in preventing 'sooting' of optical window materials placed at the boundary of the combustion chamber, which has been one of the key problems in earlier LI studies. To complete the experimental work, the prototype laser ignition system was further developed for implementation on a spray guided direct injection (SGDI) test engine at Ford Motor Company's European test facility at Merkenich in Germany. The results obtained were both unique and publishable.