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The effect of the electrodes on the early signal of the ionization sensor has been studied experimentally and with a model for the sensor. Experiments in a constant-volume combustion chamber with a generic electrode configuration allowed to investigate the role of electrode contact and the main path of the current. A framework for a model allowing the inclusion of electrode processes is introduced, and a first implementation of this model is presented. The results from the simulations are in good qualitative agreement with the experimental observations. Our conclusion is that electrode processes can limit the current during early combustion, and that the geometry of the electrodes, and especially the cathode, govern the characteristic shape of the first current peak.

A combined experimental and theoretical effort has been made to identify the most important contributors to equilibrium ionization in the post-flame gas. In the past, nitric oxide (NO) has always been assumed to be the main electron donor in the compressed hot post-flame gases. However, correlations observed between the amount of NO in the exhaust gases and the current amplitude may be deceiving due to the fact that both the formation of NO and the ionization process are strongly temperature dependend. The temperature-current relationship in data from various experiments in constant volume combustion chambers and engines was utilized to check the hypothesis that NO acts as the major contributor to ionization. Based on a well-motivated model for the current, the effect of temperature and electron donor concentration has been separated.

The location of the peak pressure can serve as a control parameter to adjust ignition timing and optimize engine performance. The ionization sensor, an electrical probe for combustion diagnostics, can provide information about the peak pressure location. However, the reliability of such information is rather poor. In-cylinder gas flow at the electrodes may be one reason for this. We present results from an investigation of the relationship between ionization sensor current and pressure under various gas flow conditions. The gas flow velocity in the vicinity of the electrode gap was measured by LDA. From the results one may infer how the in-cylinder gas flow affects the reliability of the prediction of pressure peak location from the ionization sensor signal. One finding is that high bulk gas flow impairs the precision of the prediction in certain configurations.

The spark energy delivered to the gas has been measured calorimetrically for five different ignition systems at six pressure points. Both inductive and capacitive systems have been investigated. Simultaneous electrical measurements were used to estimate the fraction of electrically stored energy actually deposited in the gas time-resolved for two systems. A separation of breakdown and glow energy was attempted using a simple model for the heat distributed to the gas and the electrodes by the spark. The results are considered valuable for the understanding of spark energy deposition as well as the design of adaptable ignition systems.