This photograph, exposed over several milliseconds, shows both the laser-induced spark and the subsequent ignition of the flame. (U.S. Air Force photo) Laser-Induced Ignition Eliminates Need for Spark Plug (Diversity) by Thomas Brown Propulsion 4/19/2011 - WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL researchers successfully demonstrated resonant laser-induced ignition of a propane-air flow mixture, a concept in an aircraft combustor that would eliminate the protruding spark plug. In place of the spark plug would be an insulated wall electrode where the fiber-optic-coupled laser enters and induces the spark, using the combustor wall as the ground electrode. The innovative spark initiation scheme uses a relatively low-energy, single-pulsed ultraviolet laser to create a pre-ionized channel, allowing a relatively low applied voltage to create an electrical spark that directly follows the channel and ignites the fuel. Because the spark follows the channel, this may provide a more reliable ignition by delivering the spark to an optimal location inside the ignition volume with a rich fuel-air mixture. The ultraviolet laser pulse is tuned to the precise wavelength to generate a pre-ionized path through the air via resonant enhanced multiphoton ionization (REMPI) affecting the molecular oxygen in the flow. In these initial laser-ignition experiments, the 10 nanosecond laser pulse created the pre-ionized path that generated breakdown and a spark within a microsecond. The eventual propane flame formed within milliseconds.