Spectroscopy Based Tool for Temperature Evaluation during the Spark Discharge 2019-32-0502

In this work, a new tool is proposed and tested to investigate the early phase of spark ignition (SI) processes. The diagnostic tool is based on Spark-Induced Breakdown Spectroscopy (SIBS), a consolidated technique in which the plasma formed by spark generation between two electrodes is used as the excitation source for optical emission spectroscopy (OES).

The spark discharge of a commercial ignition system was analyzed through OES to correlate the characteristic evolution of the discharge with the formation of reactive species inside the activated volume. Specifically, an open-source spectrum simulation program (Lifbase) together with the NIST database was used for defining relations between the ultraviolet emission bands of nitrogen first negative system (FNS_N₂) in the glow phase for different plasma temperature and pressure values.

Besides plasma density and ion energy, electron and gas temperatures are important parameters that govern the reaction rate of active species generation through dissociation, excitation, and ionization processes and thus influence the chemistry of the spark discharge. It is well known that the electrical discharge occurring between the spark plug electrodes can be divided into three phases (breakdown, arc and glow discharge), characterized by different time scales. The breakdown occurrence causes the gas molecules in the ignition area to break into atoms and ions. Molecular recombination starts after some hundreds nanoseconds from breakdown, thus leading to significantly different spectral emissions. Consequently, if measurements are triggered after the time at which breakdown occurs, molecule and molecular radical bands will be dominating in the spectral emission instead of the atomic lines.

The proposed methodology takes advantage of the peculiarity of N₂ molecules to exchange rotational and translational energy with heavy particles faster than with electrons. For this reason it is possible that rotational distributions quickly achieve thermodynamic equilibrium with the bulk gas. Therefore, a convenient way to determine the gas temperature is through the measurement of the roto-vibrational band spectrum of nitrogen.

The validation of the developed tool was performed by considering the emission of excited species detected in ambient conditions. Successively, the methodology was applied in an optically accessible combustion chamber of a spark ignition research engine under motored and fired conditions, and further validated by temperature evaluations based on CN and OH emission bands ratio. The proposed tool allowed obtaining deeper insight into the complex physical and chemical phenomena underlying the ignition event.