A Parametric Model for Ionization Current in a Four Stroke SI Engine

Journal article, 2009

A model for the thermal part of an ionization signal is presented that connects the ionization current to cylinder pressure and temperature in a spark ignited internal combustion engine. One strength of the model is that it after calibration has only two free parameters, burn angle and initial kernel temperature. By fitting the model to a measured ionization signal it is possible to estimate both cylinder pressure and temperature, where the pressure is estimated with good accuracy. The model approach is validated on engine data. Cylinder pressure and ionization current data was collected on a Saab 4-cylinder SI engine for a variation in ignition timing and air-fuel ratio. The main result is that the parameterized ionization current model can be used to estimating combustion properties as pressure, temperature and content of nitric oxides based on measured ionization currents. The current status of the model is suitable for off-line analysis of ionization currents and cylinder pressure. This ionization current model not only describes the connection between the ionization current and the combustion process, it also offers new possibilities for EMS to control the internal combustion engine.