LASER SPARK-IGNITION AND COMBUSTION CHARACTERISTICS OF METHANE-AIR MIXTURES

Ignition breakdown kernels of methane-air mixtures initiated by laser- induced sparks and by conventionaI electric sparks are compared during initial stages. Experiments were conducted using a four-stroke (Otto- cycle) single-cylinder typical high-pressure combustion chamber. The p iston is cycled in the cylinder by using an electric motor driven hydr aulic ram. An excimer laser beam, either produced from krypton fluorid e gas (lambda = 238 nm) or argon fluoride gas (lambda = 193 nm), or a Nd:YAG laser beam (lambda = 1064 nm) is focused into a combustion cham ber tu initiate ignition. Conventional electric spark ignition is used as a basis for comparison between the two different ignition methods and the resultant early breakdown kernel characteristics. A streak cam era is used tu investigate and record the initial stages of Kernel for mation. Both a breakdown and a radial expansion wave of the ignition p lasma are observed for certain Laser ignition conditions of methane-ai r mixtures under typical internal combustion (IC) engine conditions. R esults indicate that only certain wavelengths used for producing laser ignition produce a radial expansion wave. Laser ignition kernel size is calculated and laser-supported breakdown velocity is calculated by using Raizer's theory and is compared with measured results. Laser ign ition results in a 4-6 ms decrease in the time for combustion to reach peak pressure than is obtained when using electric spark ignition in the same combustion chamber and under the same ignition conditions. (C ) 1998 by The Combustion Institute.