Experimental Investigation of Ion Formation for Auto-Ignition Combustion in a High-Temperature and High-Pressure Combustion Vessel

One of the main challenges in internal combustion engine design is the simultaneous reduction of all engine pollutants like carbon monoxide (CO), total unburned hydrocarbons (THC), nitrogen oxides (NOx), and soot. Low-temperature combustion (LTC) concepts for compression ignition (CI) engines, e.g., premixed charged compression ignition (PCCI), make use of pre-injections to create a partially homogenous mixture and achieve an emission reduction. However, they present challenges in the combustion control, with the usage of in-cylinder pressure sensors as feedback signal is insufficient to control heat release and pollutant emissions simultaneously. Thus, an additional sensor, such as an ion-current sensor, could provide further information on the combustion process and effectively enable clean and efficient PCCI operation. This study performed experiments in a high-temperature, high-pressure, constant-flow combustion vessel to verify the ion-current application for premixed charge compression ignition (PCCI) engine control approaches. In this vessel, a metallic plate has been installed with a 40° orientation in front of the injector. A positively charged ion-current probe has been positioned close to the plate in the region where the fuel is injected. The electrons formed in the combustion process are drained to the probe because of the generated electrical field between the probe and the plate. The number of electrons is quantified as an ion-current signal. N-dodecane, representing a single-component surrogate fuel, has been used in the measurements to facilitate model validation. Additionally, diesel and a corresponding surrogate fuel formulation for diesel fuel have been investigated to validate the concept for a more complex fuel. The ion-current signal is measured at various conditions. These ion-current measurements will then serve as validation targets to correlate the combustion process with pollutant formation. Additionally, the local inhomogeneity of the mixture around the ion-current sensor head is analyzed regarding its impact on the measured ion-current signal. The results show promising evidence that ion-current sensors can control PCCI.