Application of Two-Dimensional Laser-Induced Fuel Tracer Fluorescence for Ion Current Evaluation

Abstract

An experiment is performed under well-controlled conditions to evaluate the prospects for fuel-to-air equivalence ratio determination by ion current measurements. To this end, two-dimensional laser-induced fluorescence (2D LIF) is utilized to acquire the true, cycle-resolved equivalence ratio conditions in the vicinity of the electrode gap. The experiment is conducted in an optically accessible combustion cell of constant volume, where methane doped with a fluorescent tracer, acetone, is injected into quiescent air and ignited by an electrical discharge. Different degrees of fuel homogeneity are achieved by varying the air/fuel mixing time prior to ignition. Apart from the local equivalence ratio, the following parameters are measured: the ion current, the combustion chamber pressure and the global equivalence ratio, which is derived from the oxygen content of the exhaust gases and is used for calibration of the 2D LIF data. After data extraction, the ion current and equivalence ratio parameters are examined for interrelations, both as 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 current peak. However, the ignition delay data alone can not be used for unambiguous determination of the local equivalence ratio, since the ignition delay exhibits a minimum for approximately stoichiometric conditions at the electrode gap. Further evaluation reveals that integrated portions of the ion current signal, i.e., the electrical charge, scale with the local equivalence ratio for lean/stoichiometric mixtures. At homogeneous and quiescent conditions, 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 field, however, generally reduces the correlation considerably. Nevertheless, applying the ion current diagnostics to a particular device, mapping the relevant operating conditions and acquiring a specific “fingerprint” behavior of the ion current parameters, can enable practical use of this technique.