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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 use of a spark plug as an ionization sensor in an engine, and its physical and chemical explanation has been investigated. By applying a small constant DC voltage across the electrodes of the spark plug and measuring the current through the electrode gap, the state of the gas can be probed. An analytical expression for the current as a function of temperature is derived, and an inverse relation, where the pressure is a function of the current, is also presented. It is also found that a relatively minor species, NO, seems to be the major agent responsible for the conductivity of the hot gas in the spark gap.

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.

A model based on an ionization equilibrium analysis, that can relate the ion current to the state of the gas inside the combustion volume, has been presented earlier. This paper introduces several additional models, that together with the previous model have the purpose of improving the pressure predictions. One of the models is a chemistry model that enables us to realistically consider the current contribution from the most relevant species. A second model can predict the crank angle of the peak pressure and thereby substantially increase the accuracy of the pressure predictions. Several other additions and improvements have been introduced, including support for part load engine conditions.