Laser Based Ignition of Natural Gas-Air Mixtures

In current natural gas engines, lean operation to reduce NO_x 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.