A Numerical Investigation of Potential Ion Current Sensor Applications in Premixed Charge Compression Ignition Engine

Simultaneous reduction of engine pollutants (e.g., CO, THC, NOx, and soot) is one of the main challenges in the development of new combustion systems. Low-temperature combustion (LTC) concepts in compression ignition (CI) engines like premixed charged compression ignition (PCCI) make use of pre-injections to create a partly homogenous mixture. In the PCCI combustion regime, a direct correlation between injection and pollutant formation is no longer present because of long ignition delay times. In LTC combustion systems, the in-cylinder pressure sensor is normally used to help the combustion control. However, to allow the control of PCCI engines, new sensor concepts are investigated to obtain additional information about the PCCI combustion for advanced controller structures. In LTC combustion systems like gasoline-controlled autoignition (GCAI) concepts, the application of ion current sensors enables additional monitoring of the combustion process with real-time capability. In analogy to GCAI, the use of an ion current sensor for the control of PCCI combustion in diesel engines could allow effective pollutant and combustion control. To investigate the potential of the application of an ion current sensor for controlling a PCCI engine, numerical engine investigations have been performed and are presented in this work. Experimental data of a single cylinder engine (SCE) are used to validate a RANS 3D-CFD simulation framework focusing on the prediction of engine-out emissions. The assembled chemical kinetic model accounts for ion and NOx formation inside the combustion chamber. After model validation, operating conditions with varying pre-injection patterns were analyzed to find correlations between pollutant and ion formation. The simulation results show a correlation between NOx and ion formation, suggesting that engine controls relying on ion current measurements potentially allow for a reduction of NOx emissions. Applying ion current sensors to control PCCI combustion seems promising to reduce pollutant emissions and improve the engine's overall performance through real-time in-cycle control strategies.