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 (∼ 77 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 possible 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 other equivalence ratios of a typical lean-burn engine.