Application of two-dimensional laser-induced fuel tracer fluorescence for ion current evaluation

An experiment is performed under well-controlled conditions to evaluate the prospects for fuel-to-air equivalence ratio determination by ion current m easurements. To this end, two-dimensional laser-induced fluorescence (2D LI F) is utilized to acquire the true, cycle-resolved equivalence ratio condit ions in the vicinity of the electrode gap. The experiment is conducted in a n optically accessible combustion cell of constant volume, where methane do ped with a fluorescent tracer, acetone, is injected into quiescent air and ignited by an electrical discharge. Different degrees of fuel homogeneity a re achieved by varying the air/fuel mixing lime prior to ignition. Apart fr om the local equivalence ratio, the following parameters are measured: the ion current, the combustion chamber pressure and the global equivalence rat io, which is derived from the oxygen content of the exhaust gases and is us ed for calibration of the 2D LIF data. After data extraction, the ion curre nt and equivalence ratio parameters are examined for interrelations, both a s ensemble averages and as individuals. A significant correlation is identified between the local equivalence ratio at the electrode gap and the ignition delay, which is defined as the time delay from spark initiation to the occurrence of a detectable second ion cu rrent peak. However, the ignition delay data alone can not be used for unam biguous determination of the local equivalence ratio, since the ignition de lay exhibits a minimum for approximately stoichiometric conditions at the e lectrode gap. Further evaluation reveals that integrated portions of the io n current signal, i.e., the electrical charge, scale with the local equival ence ratio for lean/stoichiometric mixtures. At homogeneous and quiescent c onditions, high levels of correlation are found for both ensemble averaged and cycle-resolved data. The presence of strong fuel inhomogeneities, and a simultaneously distinct flow held, however, generally reduces the correlat ion considerably. Nevertheless, applying the ion current diagnostics to a p articular device, mapping the relevant operating conditions and acquiring a specific "fingerprint" behavior of the ion current parameters, can enable practical use of this technique.