Michael H. McMillian

Lean-Burn Stationary Natural Gas Reciprocating Engine Operation with a Prototype Miniature Diode Side Pumped Passively Q-switched Laser Spark Plug

To meet the ignition system needs of large bore lean burn stationary natural gas engines a laser diode side pumped passively Q-switched laser igniter was developed and used to ignite lean mixtures in a single cylinder research engine. The laser design was produced from previous work. The in-cylinder conditions and exhaust emissions produced by the miniaturized laser were compared to that produced by a laboratory scale commercial laser system used in prior engine testing. The miniaturized laser design as well as the combustion and emissions data for both laser systems was compared and discussed. It was determined that the two laser systems produced virtually identical combustion and emissions data.

Laser Spark Ignition: Laser Development and Engine Testing

Evermore demanding market and legislative pressures require stationary lean-burn natural gas engines to operate at higher efficiencies and reduced levels of emissions. Higher in-cylinder pressures and leaner air/fuel ratios are required in order to meet these demands. Contemporary ignition systems, more specifically spark plug performance and durability, suffer as a result of the increase in spark energy required to maintain suitable engine operation under these conditions. This paper presents a discussion of the need for an improved ignition source for advanced stationary natural gas engines and introduces laser spark ignition as a potential solution to that need. Recent laser spark ignition engine testing with natural gas fuel including NOx mapping is discussed. A prototype laser system in constructed and tested and the results are discussed and solutions provided for improving the laser system output pulse energy and pulse characteristics.Copyright

Laser Spark Ignition of a Blended Hydrogen-Natural Gas Fueled Single Cylinder Engine

Charge dilution, due to the reduced combustion temperatures that it brings about, has long been proven as effective means of reducing Nitrogen Oxides (NOx ) emissions in reciprocating engines. The extent of this dilution is practically bounded on the lean side of stoichiometric conditions by engine misfire or the point at which the combustion process is no longer sufficiently reliable to sustain engine operation within some specified limit. Extending this misfire limit of an engine becomes a worth while goal as it brings about further reductions in NOx emissions. Much work has been dedicated to reaching this end and several techniques have proven viable in natural gas fueled engines. This work explores potential synergies between two proven techniques for NOx reductions in lean-burn natural gas fueled engines, hydrogen enrichment of the natural gas fuel and application of laser spark ignition. Independently both techniques have been shown to provide significant NOx emissions reductions through lean limit extension in spark ignited gaseous fueled reciprocating engines [1–11, 13–15]. Here hydrogen is blended with natural gas at five different levels ranging from 0% to 40% by volume in a single cylinder engine. The mixtures are fired using a conventional spark plug based ignition system and then again with an open beam path laser induced breakdown spark ignition system. NOx emissions measurements were made at different levels including misfire conditions for each level of hydrogen enrichment with both ignition systems. Data are presented and the emissions and engine performance of two configurations are compared to determine realizable benefits that arise from combining the two techniques.Copyright

Misfire, Knock and NOx Mapping of a Laser Spark Ignited Single Cylinder Lean Burn Natural Gas Engine

Evermore demanding market and legislative pressures require stationary lean burn natural gas engines to operate at higher efficiencies and reduced levels of emissions. Higher in-cylinder pressures and leaner air/fuel ratios are required in order to meet these demands. The performance and durability of spark plug ignition systems suffer as a result of the increase in spark energy required to maintain suitable engine operation under these conditions. Advancing the state of the art of ignition systems for these engines is critical to meeting increased performance requirements. Laser-spark ignition has shown potential to improve engine performance and ignition system durability to levels required meet or exceed projected requirements. This paper discusses testing which extends previous efforts [1] to include constant fueling knock, misfire, thermal efficiency, and NO x emissions mapping of a single cylinder lean burn natural gas engine. Tests are conducted using an open beam path laser spark ignition system and a conventional spark plug based system for contrast. Under the conditions tested, the laser-spark ignition system increased the total operating envelope of the engine by 46% when compared to the conventional ignition system. Due to a wider misfire margin using the laser spark system, NO x emissions were half the minimum value of the spark plug ignition system with no appreciable degradation in thermal efficiency. Hydrocarbon emissions were comparable for both systems. The results of the testing are discussed in detail.

Theoretical Investigation of Autoignition of Combustible Gas Mixtures in Rapid Compression Machines

In any theoretical investigation of ignition processes in natural gas reciprocating engines, physical and chemical mechanisms must be adequately modeled and validated in an independent manner. The Rapid Compression Machine (RCM) has been used in the past as a tool to validate both autoignition models as well as turbulent mixing effects. In this study, two experimental cases were examined. In the first experimental case, the experimental measurements of Lee and Hochgreb (1998a) were chosen to validate the simulation results. In their experiments, hydrogen/oxygen/argon mixtures were used as reactants. In the simulations, a reduced chemical kinetic mechanism consisting of 10 species and 19 elementary reactions coupled to a CFD software, Fluent 6, was used to simulate the autoignition. The ignition delay from the simulation agreed very well with that from the experimental data of Lee and Hochgreb, (1998b). In the second case, experimental data derived from an RCM with two opposed, pneumatically driven pistons (Brett et al., 2001) were used to study the autoignition of methane/oxygen/argon mixtures. The reduced chemical kinetic mechanism DRM22, derived from the GRI-Mech reaction scheme coupled to Fluent 6, was applied in the simulations. The DRM22 scheme included 22 species and 104 reactions. When methane/oxygen/argon mixture were simulated for the RCM, the ignition delay deviated about 15% from the experimental results. The simulation approaches as well as the validation results are discussed in detail in this paper. The paper also discusses an evaluation of reduced reaction models available in the literature for subsequent Fluent modeling.© 2002 ASME

Development and Use of a Segregated-Solver for Detailed Modeling of End-Gas Detonation in a Lean-Burn Spark-Ignited Engine

End-gas detonation occurs in a spark-ignited engine when the advancing flame front compresses the end-gas mixture to its autoignition temperature. The rapid energy release results in shock waves which are undesirable due to resulting combustion noise and boundary layer breakdown leading to reduced engine performance and incipient engine damage. In a spark-ignited engine, end-gas knock can result from improper combinations of compression ratio, spark timing or inlet thermodynamic conditions (i.e. manifold temperature, pressure, and equivalence ratio). These variables exhibit very complex interactions, which require costly high dimensional experimental designs for proper evaluation. As a result, detailed modeling tools are needed to predict the onset of the end-gas detonation regime for engine design applications. Developing a solver to predict the end-gas detonation of gases ahead of the flame front in an operating engine is not trivial. In theory, the model would need to simultaneously resolve both the detailed fluid mechanics as well as describe the fuel decomposition using detailed chemistry. Calculations for this type can take weeks or months depending on the number of dimensions that are resolved. Since hundreds of computations may be necessary to optimize a given configuration, it is necessary to be able to not only compute the onset of auto-ignition and other parameters accurately, but efficiently. The objective of this work was to develop an efficient methodology that could be utilized to effectively predict detonation in an internal combustion spark-ignited engine. This paper presents the computational methodology, a review of the combustion tool capability, and a comparison to experiments. The work clearly demonstrates the existence of inhomogeneities in the temperature field and discusses their impact on the prediction of end-gas knock.Copyright