Laser-induced spark for simultaneous ignition and fuel-to-air ratio measurements

This work investigates the use of laser-induced gas breakdown for simultaneously igniting and measuring fuel-to-air ratio of CH₄–air and H₂–air combustible mixtures. The fuel-to-air ratio is determined using the measured spectral peak ratio I_o,Hα/I_o,OI. Sparks are produced using a single-mode, Q-switched Nd–YAG laser. The laser produces a beam of 6 mm in diameter at the wavelength of 1064 nm and pulse duration of 5.5 ns. The beam optics is designed to have mainly a beam splitter and a focusing lens. The beam splitter is coated to reflect the laser beam and transmit emission lines with wavelengths from 600 to 900 nm which are then collected by a fiberoptic cable and detected by an imaging spectrometer–detector assembly. The results showed a linear dependence of the spectral peak ratio on the equivalence ratio that can be generally expressed by φ=a(I_o,Hα/I_o,OI)+b, where a and b are the parameters that depend on the gas pressure. Using the least-square curve fitting technique to fit the experimental data, a calibration curve for calculating the equivalence ratio as a function of the ratio of (I_o,Hα/I_o,OI) was generated.