Sreenath B. Gupta

Development of Advanced Laser Ignition System for Stationary Natural Gas Reciprocating Engines

Laser ignition is considered the prime alternative to conventional coil based ignition for improving efficiency and simultaneously reducing NOx emissions in lean-burn natural gas fired stationary reciprocating engines. In this paper, Argonne’s efforts towards the development of a viable laser ignition system are presented. The relative merits of various implementation strategies for laser based ignition are discussed. Finally, the performance improvements required for some of the components for successful field implementation are listed. Also reported are efforts to determine the relative merit of laser ignition over conventional Capacitance Discharge Ignition (CDI) ignition. Emissions and performance data of a large-bore single cylinder research engine are compared while running with laser ignition and the industry standard CDI system. It was primarily noticed that NOx emissions reduce by 50% under full load conditions with up to 65% reductions noticed under part load conditions. Also, the lean ignition limit was significantly extended and laser ignition improved combustion stability under all operating conditions. Other noticeable differences in combustion characteristics are also presented. Efforts wherein ignition was achieved while transmitting the high-power laser pulses through optical fibers showed performance improvements similar those achieved by using free-space laser ignition.© 2005 ASME

Ignition Characteristics of Methane-Air Mixtures at Elevated Temperatures and Pressures

Lean operation of natural gas fired reciprocating engines has been the preferred mode of operation as it allows low NO x emissions and simultaneous high overall efficiencies. In such engines, the operation point is often close to where the ignition boundary and the knock limiting boundary cross-over. While knocking is, to a large extent, limited by engine design, ignition of lean-mixtures is limited by the mode of ignition. Since significant benefits can be achieved by extending the lean-ignition limits, many groups have been researching alternate ways to achieve ignition reliably. One of the methods, laser ignition, appears promising as it achieves ignition at high pressures and under lean conditions relatively easily. However, most of the current knowledge about laser ignition is based on measurements performed at room temperature. In this paper, ignition studies on methane-air mixtures under in-cylinder conditions are presented. A Rapid Compression Machine (RCM) was designed to reproduce typical in-cylinder conditions of high temperature (∼ 490°C) and pressure (∼ 80 Bar) at the time of ignition. Experiments were performed comparing conventional coil based ignition (CDI) and laser ignition on methane-air mixtures while varying pressure and equivalence ratio systematically. It was observed that substantial gains are likely with the use of laser ignition as it extends the lean-ignition limit to the flammability limit, I.e.,. Φ=0.5. On the other hand, conventional CDI ignition could not ignite mixtures leaner than Φ = 0.6. Also, faster combustion times and shorter ignition delays were observed in the case of laser ignition. Through scans performed for minimum required laser energies (MRE), it was noted that the measured values were substantially higher than those reported elsewhere. However, the trends of these values indicate that a laser ignition system designed for Φ= 0.65 will successfully operate under all equivalence ratios of a typical lean-burn engine.

Low Temperature Combustion Using Nitrogen Enrichment to Mitigate NOx From Large Bore Natural Gas Fueled Engines

Low Temperature Combustion (LTC) is identified as one of the pathways to meet the mandatory ultra low NOx emissions levels set by regulatory agencies. This phenomenon can be realized by utilizing various advanced combustion control strategies. The present work discusses nitrogen enrichment using an Air Separation Membrane (ASM) as a better alternative to the mature Exhaust Gas Re-circulation (EGR) technique currently in use. A 70% NOx reduction was realized with a moderate 2% nitrogen enrichment while maintaining power density and simultaneously improving Fuel Conversion Efficiency (FCE). The maximum acceptable Nitrogen Enriched Air (NEA) in a single cylinder spark ignited natural gas engine was investigated in this paper. Any enrichment beyond this level degraded engine performance both in terms of power density and FCE, and unburned hydrocarbon (UHC) emissions. The effect of ignition timing was also studied with and without N2 enrichment. Finally, lean burn versus stoichiometric operation utilizing NEA was compared. Analysis showed that lean burn operation along with NEA is one of the effective pathways for realizing better FCE and lower NOx emissions.

Performance Analysis of a Natural Gas Generator Using Laser Ignition

A single cylinder spark ignited gasoline engine was modified to operate with natural gas. In such an engine, laser ignition was successfully demonstrated while transmitting the high-power laser pulses via solid core optical fibers. Subsequently, ignition studies were performed while using laser ignition (LI) and conventional spark ignition (SI). However, due to limitations imposed by the engine hardware the adverse conditions for ignition could not be simulated, i.e., of lean operation and high-pressures. As a result, the scope of the study was limited to comparing LI and SI ignition characteristics at various ignition timings. It was observed that both LI and SI resulted in reliable combustion over all ignition timings. Furthermore, LI resulted in higher rates of pressure rise and higher peak cylinder pressures. However, the higher NOx emissions resulting from such conditions might not be representative as the final performance of an engine as it is determined by optimizing ignition timing and other operating parameters.Copyright

Laser Based Ignition of Natural Gas-Air Mixtures

In current natural gas engines, lean operation to reduce NOx emissions along with the requirement to maintain high specific power results in in-cylinder conditions that demand spark voltages beyond the capabilities of present ignition systems. Unable to overcome such limitations, presently these engines are operated well below their full potential (about 15% less). Additionally, undue maintenance demands are placed for the upkeep of ignition systems. Laser based ignition (LBI) on the other hand, overcomes the above limitations and potentially reduces emissions and increases efficiency. Experimental studies were performed to identify such potential benefits while using lasers to ignite quiescent methane-air mixtures. Quiescent methane-air mixtures at various conditions (φ = 0.6–1.0, fill pressure = 2–20 Bar) were established in a pressure vessel and were ignited using lasers and by conventional ignition systems. Such tests showed lasers to ignite mixtures with initial pressures 30% higher than those limiting ignition by conventional ignition systems. However, extension of the lean ignition limit appeared to be marginal and was defined by φ = 0.675. Also, for single point ignition followed here, the rates of pressure rise and ignition delays were identical and did not depend upon the method of ignition. Other characteristics in terms of (a) effect of focal length, (b) effect of fuel composition, and (c) effect of laser beam polarization are presented. In practice, in-cylinder conditions such as turbulence, velocity and temperature are likely to have an additional bearing on the ignition characteristics. Such effects will be determined through future investigations.© 2003 ASME

Natural Gas Fired Reciprocating Engines for Power Generation: Concerns and Recent Advances

© 2012 Gupta et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Natural Gas Fired Reciprocating Engines for Power Generation: Concerns and Recent Advances

Nitrogen Enriched Combustion of a Natural Gas Engine to Reduce NOx Emissions

Nitrogen enrichment of intake air is proposed as an alternative to Exhaust Gas Recirculation (EGR). Experimental results of nitrogen enriched combustion of a Kohler M12 engine (converted to operate with natural gas) are presented in this paper. A 70% reduction in NOx emissions was observed at full load (4 kW) and ignition timing (IT) equal to −20 ATDC with 2.1% NO2 enrichment (40 slpm). However, NOx reduction was minimal at lower loads. The effect of spark or ignition timing along with nitrogen enrichment is also reported for full load. It is recognized that advancing the ignition timing from conventional values has more advantages than retarding the same. A 68% reduction in NOx and a 0.8% drop in Fuel Conversion Efficiency (FCE) were observed at −30 ATDC ignition timing. However, the maximum ignition timing advance with stable engine operation was limited to −40 ATDC. Some of the drawbacks encountered were engine misfire at higher concentrations of nitrogen-rich air and retarded spark timing resulting in poor FCE.Copyright

Natural Gas Engine Performance Ignited by a Passively Q-Switched Microlaser

A robust end pumped, passively Q-switched, air-cooled, microlaser was designed and prototyped, and tested in a natural gas fueled single-cylinder engine, and in one cylinder of a turbocharged 6-cylinder engine.

Performance Evaluation of a DENSO developed Micro-Laser Ignition System on a Natural Gas Research Engine

The performance of a prototype micro-laser ignition system developed by DENSO was evaluated on a single cylinder natural gas research engine. Several geometric combinations of pre-chamber spacing and hole sizes were tested at two engine load conditions. Significant improvements in brake thermal efficiency (BTE) were realized due to the extension of the lean ignition limit with a slight penalty in NOx.

In-Cylinder Equivalence Ratio Measurements in an EGR Equipped Engine

A single-cylinder natural gas fueled engine equipped with a Exhaust Gas Recirculation (EGR) system was ignited using a laser. The broadband emission from the spark kernel was spectrally resolved and the peaks corresponding to Hα , N and O atoms were measured for a range of conditions with global equivalence ratios ranging between 0.6 and 1.0, and for Exhaust Gas Recirculation fractions up to 29%. The (Hα /O) and (Hα /N) peak intensity ratios from the spectral scans correlated extremely well (R2 > 0.97) with local oxygen based equivalence ratios. Appropriate relations were developed to relate such values to global equivalence ratios and the EGR rate. For homogenous intake charge, the present LIBS diagnostic enables estimation of one of the two values, global equivalence ratio or EGR rate, with the knowledge of the other.Copyright