Cylinder Pressure and Ionization Current Modeling for Spark Ignited Engines

Engine management systems (EMS) need feedback on combustion performance to optimally control internal combustion engines. Ion sensing is one of the cheapest and most simple methods for monitoring the combustion event in a spark ignited engine, but still the physical processes behind the formation of the ionization current are not fully understood. The goal here is to investigate models for ionization currents and make a connection to combustion pressure and temperature. A model for the thermal part of an ionization signal is presented that connects the ionization current to cylinder pressure and temperature. 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 parameterized ionization current model is composed by four parts; a thermal ionization model, a model for formation of nitric oxide, a combustion temperature model and a cylinder pressure function. The pressure function is an empirical function design where the parameters have physical meaning and the function has the main properties of a solution to the cylinder pressure differential equations. The sensitivity of the ionization current model to combustion temperature and content of nitric oxide is investigated to understand the need of sub-model complexity. Two main results are that the pressure model itself well captures the behavior of the cylinder pressure, and that the parameterized ionization current model can be used with an ionization current as input and work as a virtual cylinder pressure sensor and a combustion analysis tool. 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.