Spark-induced breakdown spectroscopy for fuel-air equivalence ratio measurements at internal combustion engine-relevant conditions

Abstract

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.

Experimental Analysis of Breakdown With Nanosecond Pulses for Spark-Ignition Engines

“…, ln (2) If the formative time for a given condition shows minor scatter, an upper limit for the formative time is given by the minimum value for all observed delay times. The higher the number of observations, the closer one gets to the actual values.…”

“…In the present work, the constant volume cell introduced in [2] is used in conjunction with two commercial nanosecond pulse generators (FID 15-10NK, FID 30-100NM) coupled to an automotive sparkplug to characterize the single pulse discharge in air at different gas densities and sparkplug gap distances. Table 1 lists the main specifications for the pulse generator used.…”

“…300 ns. The streamer formative time is estimated using Equation (2) without accounting for electric field inhomogeneity with a reduced electric field equal to that at breakdown and neglecting attachment, the average formative time among all experiments is ca. 3 ns with a maximum value of 60 ns.…”

“…Subplot (a) outlines the minimal observed delay time, which is an upper bound for the formative time, and the yellow line represents the estimated formative time according to Equation (2). Subplot (b) in Figure 11 shows the mean statistical time for each maximal impulse factor, which is calculated as the mean of the difference between the measured delay and its minimum.…”

“…In this phase, the current rises to reach its maximal value, which is given by the ratio of the ignition voltage and near-gap impedance. At the same time, the gap voltage drops to low values [2]. The voltage drop rate depends solely on the inductive (ca.…”