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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.

The influence on the ion current from a variety of fuels and additives has been investigated. The ion current has been measured in an engine as well as in a continuous atmospheric burner. The acquired ion current data has been statistically treated in order to find general trends and behaviors. The measurements were found to be in agreement with established theories for the ion current formation. This work indicates that the use of alternative fuels will not endanger the large engine control potential supplied by the ion current technique, since the changes induced by the fuel variety can be accounted for, when using the different fuel types of today.

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.