Sachin Joshi Frank Loccisano Azer P. Yalin Department of Mechanical Engineering, Colorado State University. Fort Collins, CO 80521 Dave T. Montgomery Technical Center-F-515. Caterpiliar Inc. P.O. Box 1875, Mossviiie.iL 61552 On Comparative Performance Testing of Prechamber and Open Chamber Laser Ignition Laser ignition is a potential ignition tectmotogy to achieve reliable lean burn ignition in high brake mean effective pressure (BMEP) internal combustion engities. The technology has the potential to inctease btake thertnat ejficiency and t educe exhaust emissions. This submission reports on engine testing of a Catetpittar G35I6C stationary natural gas fueled engine with thtee types of ignition apptoaches; (i) nonfueted etecttic ptechamber ptug with etecttodes at the base of the prechamber, (ii) nonftteled laser ptechatnber plug with laser spark in the middle of the prechamber, atid (Hi) open ctiatnber plug with laser spark in the main chamber. In the second configuration, a stock twtifueted prechamber plug was modified to incorporate a sapphire window and a focusing lens to fot m a laser ptechamber plug. A 1064 nm Q-switched Nd;YAG laser was used to cteate la.ser .ipatks. For these tests, a single cylinder of the engine was tetrofitted with the taser plug while the remaining cylinders were tun with cottventional electric ignition system at basetine ignition titning of 24 deg befóte top dead center (BTDC). The petformances of the thtee plugs were compated in terms of itidicated mean effective pre.ssures (IMFP), mass burn fraction duration attd coefficient of variation (COV) oflMEP, and COV of peak pres,sure location. Test data show compatable petformance between electric and la.ser ptechatnber plugs, albeit with a lower degree of variability in engine s petformance for electric pre- ctiamber ptug compated to the laser prechamber plug. The open chamber plug exhibited poorer variability in engine perfotmanee. All results ate di.sctissed in the cotite.xt of pre- chctmber atid engitte fluid mechatiics. [DOI: 10.1115/1.4003972] 1 Introduction Laser sparks are a potential ignition source for lean fuel air mixtures. When a sufficiently high powered Q-switched la.ser is focused in gas, the gas breaks down into its constituent atoms, electrons, and ions to form plasma (spark). Laser sparks with suf- ficient energy to overcome any quenching can be u,sed to ignite fuel-air mixtures. In recent years there have been studies carried out using laser ignition in automotive ga.soline engines [1], aircraft engine turbines |2|, and reciprocating natural gas engines [3-5]. Laser ignition offers many potential advantages over electric spark ignition, primarily the ability to locate the spark at an opti- mum position within the combu.stion volume (by selection of appropriate focusing optics), obviation of electrodes (which other- wise acts as heat sinks), and ability to ignite lean air-fuel mixtures [4,6,7]. Another difference (and potential advantage) between electric sparks and laser .sparks stems from the different initial plasma environment (i.e., higher pressure and temperature of the la.ser plasma) that lead to elevated ("overdriven") early fiame speed [8]. Since laser ignition inherently provides an optical win- dow to the combustion chamber, it also provides the opportunity to perform in-cylinder la.ser induced breakdown spectroscopy for monitoring ignition, and air-fuel ratio measurements [9]. The primary interest of this research is in fiber delivered laser ignition of large reciprocating stationary natural gas engines which are generally used for gas compression and power genera- tion. Technology drivers for these engines are increased efficiency and reduced emissions both of which are trending these gas engines towards high pressure and lean operation. At these condi- tions, electric spark ignition suffers from a perpetual cycle of increasing breakdown voltage requirement and electrode erosion (which increases the electrode gap) ultimately leading to dielectric breakdown and reduced spark plug life [4]. In contrast, laser igni- Comributed by the IC Engine Division of ASME for publication in the JOURNAL OF ENGINEKRINCI FOR GA.S TuRBtNcs AND POWKR. Manuscript received October 26. 2OtO: final manuscript received October 29. 2010; published online August 26, 2011. Editor: Dilip R. Ballal. tion becomes attractive at elevated pressure since the breakdown threshold of the gas decreases with increasing pressure [10]. Dale et al. conducted the first experimental investigation into the benefits of using laser ignition in a gasolitie engitie [6]. The CO2 laser operating at 10.6 //m permitted extension of the lean limit, and rapid in-cylinder pressure rise (i.e., faster heat release) and improved fuel econotny. They also showed the use of exhaust gas recirculation (EGR) in the laser ignited engine to reduce nitro- gen oxides (NOx) emissions significantly. Similarly, Kopecek et al. also demonstrated laser ignition on a 1 MW gas engine running at 1500 rpm [7]. A single cylinder was ignited with a 5 ns 1064 nm Nd:YAG la.ser. The authors reported low levels of NO, output compared to direct electric spark ignition and a reliable operation of the laser ignited engine for a te.st period of 100 hrs. Similarly, previous work by the current authors on fiber delivered laser igni- tion of a VGF-18 Waukesha engine showed shot ter ignition delay and low coefficient of variation (COV) of indicated mean effec- tive pre.ssure (IMEP) [11], All published results on laser ignition of automotive (gasoline) and stationary natural gas engines have been with open chamber plugs, i,e., the laser beams were focused inside the main combus- tion chamber to form a single combustion initiating spark. On the other hand, experiments have also shown increase in fuel effi- ciency and reductions in emissions through the use of indirect (prechamber) electric spark ignition 112,13]. By generating turbu- lent gas jets in the main combustion chatnber, a prechamber inten- sifies and accelerates the combustion process in the main chamber. However, the choice of the associated ignition approach; for example, fueled prechamber plug (in which a prechamber is equipped with a separate fuel supply system), nonfueled precham- ber plug (where the fuel and air mixture is led into the prechamber through the jet holes during the compression stroke), or conven- tional open chamber ignition is engine specific. For example, the choice of the ignition approach depends on various factors such as in-cylinder fluid mechanics, engine heat transfer, in-cylinder mix- ing, etc. which are optimized to increase engine efficiency and lower emissions. Journal of Engineering for Gas Turbines and Power Copyright © 2011 by ASiVIE DECEMBER 2011, Vol. 133 / 122801-1