| By tightly focusing the beam from a high-power pulse laser to form a combustion-initiating spark in flammable mixtures including those used in engines, we accomplish what is called laser ignition. Our research area is primarily in large (megawatt class) stationary gas engines that are typically used for power generation and natural gas compression. Technology drivers include the need for increased efficiency and reduced pollutant emissions, which are trending advanced engines toward lean (reduced emissions) and high- pressure (increased efficiency) operation. | |||
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| Laser ignition is viewed as an attractive candidate technology as studies have shown the potential for increased lean limits [1,2]. Further, in contrast to conventional spark ignition (where the voltage requirement, dielectric breakdown, and erosion increase with pressure), laser ignition becomes easier at elevated pressures (the breakdown threshold intensity decreases) [3]. Briefly, the differences (potential benefits) of laser spark ignition stem from two sets of effects: The first is associated with the ability to freely locate the spark within the combustion volume (by the selection of appropriate focusing optics) and the obviation of electrodes (which act as heat sinks and may provide catalytic chemistry), while the second is related to inherent physical differences between the two types of spark, for example, the higher pressure and temperature of laser sparks can lead to elevated (overdriven) early flame speed in laser ignition [4]. | |||
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| However, there is no practical industrial laser ignition system in use yet. Our research goal is to design and develop a practical laser ignition system using a single laser source multiplexed into multiple fiber optics (engine cylinders) by a multiplexer. | |||
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| We have successfully performed bench-top demonstration of atmospheric pressure sparking through a hollow core fiber and have also performed a single cylinder testing of a Waukesha Engine via a 1m long hollow core fiber. The chart above shows the combustion benefit (higher heat release rate) in the laser ignited cylinder. | |||
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| We have demonstrated bench-top atmospheric pressure sparking through two hollow core fibers using a multiplexer and are currently testing a system for multiplexed multiple cylinder engine testing of Caterpillar G3516 C engine (picture above). Below are solid model drawings for the multiplexer and the optical plug in use. | |||
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| We have also conducted extensive 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 (EPP), ii) non-fueled laser prechamber plug with laser spark in the middle of the prechamber (LLP), and iii) open chamber plug with laser spark in the main chamber (LOP). 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 Fraction Burn Duration (MFB) 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 as was expected because of incompatibility with the quiescent combustion chamber design of the engine. The figures below show the performance of the plugs for various loads. (EPP_50% denotes the engine performance with the non-fueled electric prechamber plug with electrodes at the base of the prechamber at 50% load). | |||
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It is hypothesized that the higher combustion variability observed with the laser prechamber plug is due to
high flow velocities and turbulence at the laser spark location (in the middle of the prechamber). Nonetheless,
test data for the laser prechamber plug showed the capability of the laser sparks (and associated early flame kernels)
to withstand non-ideal flow field conditions within the prechamber. In order to capitalize on the benefits of laser
sparks, and to achieve superior combustion, one should investigate a re-designed prechamber with optimized spark
location. We are currently working on investigating solid core step index silica fibers for high peak power delivery of laser beams and have shown the first demonstration of spark delivery at atmospheric pressure through a large core silica fiber. The figure below shows our accomplishments, all of which are first, till date. |
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