Spark-induced Breakdown Spectroscopy was used for local fuel-air equivalence ratio measurements of premix ed methane-air mix tures by a discharge from an inductive ignition system in a constant volume cell. Elemental emissions of hy drogen Balmer-alpha line (H α ), nitrogen (N7 46) and ox y gen (O777) were ex perimentally observed in the v icinity 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 max imum) ratios were analy zed and their relation with the local fuel-air equiv alence ratio at the spark plug was discussed, for operating pressures ranging from 1 0 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 v aried, 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 dev eloped technique is promising as a compact and v ersatile tool for applications inv olv ing local fuel-air equiv alence ratio measurements at different ambient conditions.
Optical diagnostics of in-cylinder processes in turbulent premixed combustion is a key element for improved understanding of governing physics occurring and indispensable for further development and validation of numerical models for computational fluid dynamic (CFD) simulations applied to engines. High-speed imaging combined with heat release analysis [1] has already been employed in the 1930s, demonstrating that engine combustion takes place in a premixed turbulent flame-front regime. Damköhler's pioneering work of turbulent combustion [2] indicated the scaling of the burn rate in proportion to the engine speed due to the increasing turbulent flame area by flame-front wrinkling. Application of planar laser diagnostics [3] confirmed the corrugated flamelet regime in an optical square
A diagnostic tool based on Spark Induced Breakdown Spectroscopy (INSIDE -INduced Spectroscopy for Ignition Diagnostics in Engines), designed and developed for providing information on the mixture composition at the spark plug location during spark timing, was successfully used for diagnostics in diluted methane/air mixtures at engine relevant conditions. The INSIDE technique allows deriving information on local fuel to air equivalence ratio in the vicinity of the spark plug and on operating pressure based on the features of the spectral emissions (peaks intensity ratios and peaks width). Aiming to future application of the INSIDE technique in running engines, the possibility to diagnose also the amount of exhaust gas recirculation (EGR) in the spark plug location is needed. In the present work, a capacitive discharge ignition (CDI) system is used in Spark Induced Breakdown Spectroscopy (SIBS) in a constant volume ignition cell with the aim to exploit the faster discharge at higher power when compared to an inductive discharge ignition system. To the authors' knowledge, this is the first time a CDI system is used for SIBS-based ignition diagnostics in spark-ignition engines' relevant conditions. The methane/air mixture dilution is performed following four different approaches: dilution with synthetic EGR mixture consisting of 20 vol.% CO2 and 80 vol.% nitrogen, dilution with pure nitrogen, dilution through fuel to air equivalence ratio variation without EGR addition, and dilution through equivalence ratio variation with constant EGR addition. Spectral signatures of the different mixtures are measured in the spark plug location, and the peaks intensities of four atomic lines (hydrogen Balmer-α at 656 nm, nitrogen at 744-746 nm, carbon at 765-769 nm and oxygen at 777 nm) are measured and compared to the baseline (stoichiometric methane/air mixture with no dilution). The atomic peak intensity ratios versus nitrogen percentage addition show distinct trends depending on the dilution strategy selected. Therefore, the atomic line ratios can be used to extract information on the local mixture composition also in the case of EGR dilution.
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