Combustion Engine Spectroscopy
Spark-induced breakdown spectroscopy for fuel-air equivalence ratio measurements at internal combustion engine-relevant conditions
Spectrochimica Acta Part B: Atomic Spectroscopy, Volume 155, 2019, Pages 79-89, https://doi.org/10.1016/j.sab.2019.03.006.
Spark-induced Breakdown Spectroscopy was used for local fuel-air equivalence ratio measurements of premixed methane-air mixtures by a discharge from an inductive ignition system in a constant volume cell. Elemental emissions of hydrogen Balmer-alpha line (Hα), nitrogen (N746) and oxygen (O777) were experimentally observed in the vicinity of the electrode gap, using a lens-coupled spectrometer and an intensified camera. After optimization of the gating strategy, the spectral emission features, i.e. line intensity ratios and peaks width (full width at half maximum) ratios were analyzed and their relation with the local fuel-air equivalence ratio at the spark plug was discussed, for operating pressures ranging from 10 to 20 bar.
Results showed that a change in pressure and temperature did not affect the spectral emissions' atomic peaks ratios, as long as the density was kept constant. For tests at constant temperature and at constant pressure, the peaks intensity ratios showed a linear correlation with local fuel-air equivalence ratio, while peaks width did not show noticeable changes. When pressure was varied, while keeping temperature and fuel-air equivalence ratio constant, a different behavior was observed: peaks intensity ratio were only slightly affected by pressure change, more prominently at fuel-rich conditions, while a peak broadening was clearly recognizable, as the peak width linearly increased with pressure.
Moreover, an additional optical setup involving a fiber-optic spark plug was implemented with the aim to adapt the proposed diagnostic tool to engines in operando. Comparisons with the lens setup showed that the developed technique is promising as a compact and versatile tool for applications involving local fuel-air equivalence ratio measurements at different ambient conditions.
Spark-induced breakdown spectroscopy of methane/air and hydrogen-enriched methane/air mixtures at engine relevant conditions
Spectrochimica Acta Part B: Atomic Spectroscopy, Volume 148, 2018, Pages 152-164, https://doi.org/10.1016/j.sab.2018.06.013.
An experimental study of temporally and spatially resolved spark-induced breakdown spectroscopy (SIBS) of methane and hydrogen-enriched methane mixtures is reported for premixed combustion in internal combustion engines. Experiments were conducted at quiescent conditions in a small constant volume chamber, varying pressure, stoichiometry and hydrogen admixture rates. Spectral emissions of hydroxyl (OH) at 306 nm, NH at 336 nm, the cyanogen (CN) band at 388 nm and the nitrogen second positive system (N2) were spatially resolved on a time-integrated setup. OH emissions were found to be strongest between the electrodes, whereas CN emissions were more pronounced at the center and ground electrode. Metal emission lines were observed, partially interfering with the radicals of interest. Energy dispersive X-ray spectroscopy showed nickel, copper and iron shares in the noble metal electrodes, confirming the origin of these metal emissions. Simultaneously to time-integrated spectra, temporally-resolved spectra at a frame rate of 100 kHz were recorded during the glow phase of the electrical discharge. Comparison of both spectra revealed a good agreement in terms of spectral characteristics and resolved species. Temporally-resolved spectra highlighted the strong dependence on the electric discharge characteristics of the inductive coil ignition system in the early phase of ignition. Ratios of CN/OH and CN/NH were found to correlate with the fuel-air equivalence ratio, but shot to shot repeatability was up to a factor 3 larger than the dependence on the mixture composition. Moreover, these ratios changed with increasing pressure at ignition timing, and with the higher pressure metal emission lines became more pronounced. Additionally, at pressures higher than 2 bar, the equilibrium position shifted to the disadvantage of the second positive system of nitrogen, suppressing this emission. Hydrogen admixture to methane of up to 50 vol% were found to not significantly alter the spectral signature between 300 and 400 nm, only marginally affecting the signal ratios of CN/OH and CN/NH.