Theoretical and Experimental Studies of the Formation of Ionized Gases in Spark Ignition Engines

University dissertation from Lund Institute of Technology, Combustion Physics, P.O. Box 118 , 221 00 Lund , Sweden

Abstract: The processes that govern the formation, existence and consumption of ions in gases have been studied both experimentally and theoretically. The aim of the work is targeted at increasing the understanding of these phenomena in the working gases inside an engine. Both the cause and the application of ionized gases inside the cylinder of a spark ignited (SI) engine has been studied. The spark discharge process that eventually leads to a favorable combustion has been studied in order to determine the total amount of energy that enters the gas. A simplified theoretical model has been developed that includes the most dominant effects. The model includes molecular and atomic processes such as ionization, dissociation and thermal heating together with diffusion of cold gas molecules into the hot spark kernel. The energy input is then calculated from the electrical properties of the ignition system and the gas conditions between the electrodes. Electrode effects have not been included since the energy released close to the electrodes to a major extent is lost. In order to gain even more knowledge about the dynamic spark behavior, a hydrodynamic model for the description of the generated flow patterns emanating from a fast spark discharge was used. In addition, a detailed model for the generation of initial breakdown conditions was also used. The experimental techniques that have been used to support the model have been various, e.g. electrical power measurements, calorimetric measurements and interferometric measurements. The spark, that has been intensively studied, inflames a small region of gas in the vicinity of the electrode gap. This flame kernel will then develop into a propagating flame. While growing, the high gas temperature close to the spark gap reflects the progress of the combustion process. This fact has been used for engine diagnostics by measuring the current flowing through the spark plug gap. The phenomenon has been described in a model for flame ionization in order to deduce the cylinder pressure and the air/fuel ratio in vicinity of spark plug. There are mainly three contributing parameters, which are required for a correct description of the current namely; the ion/electron concentrations, the electric field distribution in space and the drift velocity of the electrons. It has been shown that the current is carried by the electrons to a higher extent than the ions although the electrons are easily attached to electronegative species, such as oxygen (O) and the hydroxyl radical (OH). A detailed chemical kinetic code has been used in order to calculate the concentrations of the most significant species in both the reaction zone and the postflame region. In addition to the theoretical work a large amount of experimental data has been collected for the purpose of model validation and obtaining a deeper understanding. Statistical treatment of the experimental data has also been performed. By this method the influence from some additional parameters such as the amount of EGR and variable fuels have been investigated.

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