Laser ignition is a potential ignition technology to achieve reliable lean burn ignition in high brake mean effective pressure (BMEP) internal combustion engines. The technology has the potential to increase brake thermal efficiency and reduce exhaust emissions. This submission reports on engine testing of a Caterpillar G3516C stationary natural gas fueled engine with three types of ignition approaches: i) non-fueled electric prechamber plug with electrodes at the base of the prechamber (i.e., conventional ignition), ii) non-fueled laser prechamber plug with laser spark in the middle of the prechamber, and iii) open chamber plug with laser spark in the main chamber. In the second configuration, a stock non-fueled prechamber plug was modified to incorporate a sapphire window and a focusing lens to form a laser prechamber plug. A 1064 nm Q-switched Nd:YAG laser was used to create laser sparks. For these tests, a single cylinder of the engine was retrofitted with the laser plug while the remaining cylinders were run with conventional electric ignition system at baseline ignition timing of 24 degree before Top Dead Center (BTDC). The performances of the three plugs were compared in terms of Indicated Mean Effective Pressures (IMEP), Mass Burn Fraction Duration and Coefficient of Variation (COV) of IMEP, and COV of Peak Pressure Location. Test data show comparable performance between electric and laser prechamber plugs, albeit with a lower degree of variability in engine’s performance for electric prechamber plug compared to the laser prechamber plug. The open chamber plug exhibited poorer variability in engine performance. All results are discussed in the context of prechamber and engine fluid mechanics.

Past research has demonstrated the feasibility of using optical sparks for engine ignition, and has shown potential benefits associated with reduced cyclic variability and increased rate of cylinder pressure rise, thus extending the lean operating limit of natural gas engines. This contribution details the design and bench-top testing of a fiber-optic delivery system for ignition of natural gas engines. The system is designed for use on a Caterpillar G3516C engine and is comprised of a single Nd:YAG laser as the energy source, a multiplexer for switching the beam between cylinders, fiber optics to deliver the laser pulses to individual cylinders, and optical plugs to couple the beam into the cylinders. The optical fibers are a critical component of the system and discussion of use of both solid core silica fibers and cyclic olefin polymer-coated silver hollow fibers is included. The multiplexer design is presented and optical testing of the multiplexed fiber delivery on the bench-top is reported. Design considerations for engine integration are introduced.

A laser-based system should be advantageous to a spark-plug based ignition system. Free choice of the ignition spot and precise timing constitute two major advantages. Multi point laser ignition could lead to higher efficiencies, and laser ignition as such is capable of igniting leaner mixtures than a spark plug, thereby decreasing thermal NO x and soot emissions. This paper is devoted to advances in optical diagnostics of laser ignition for future internal combustion engines. The focus of this paper is on diagnostics at high pressures, that is engine-like conditions. Laser ignition tests were performed with the fuels methane, hydrogen and biogas in static combustion cells with dimensions comparable to stationary engines. A Nd:YAG laser (5 ns pulse duration, wavelength 1064 nm, 1–20 mJ pulse energy) was used to ignite gaseous fuel/air mixtures at initial pressures of 1–3 MPa. Schlieren photography and laser-induced fluorescence (LIF) were used for optical diagnostics (flame kernel development, shock wave propagation). The lean burn characteristics were investigated. Schlieren photography was used to determine the velocity of the shock wave and to study the influence of the shock wave on temperature rise and energy loss. Using planar laser-induced fluorescence (PLIF), the spatial distribution of the combustion intermediates OH and formaldehyde were recorded. The temporally resolved imaging shows that the initial stages of the flame front evolution closely follows the turbulence and density fluctuations caused by the shock and pressure wave induced by the laser spark. In this paper, results from LIF spectroscopy and Schlieren photography are compared. Depending on the laser pulse energy and focus size, at later stages after the ignition the flame front propagation approaches the laminar burning regime and flame front speed decrease. Flame front break up at lean conditions indicates the limit of the ignitable mixture fraction when the speed due to spark-induced convection exceeds the flame propagation rate.