Ionisation and ionisation rate of a two-stroke HCCI engine fuelled with E85 for control feedback

Homogenous Charge Compression Ignition (HCCI) combustion phasing and stability provides a challenging control problem over conventional combustion technologies of Spark Ignition (SI) and Compression Ignition (CI). Due to the auto ignition nature of the HCCI combustion there are no direct methods for actuation, the combustion and the phasing relies on indirect methods. This in itself creates a nonlinear dynamic problem between the relationships of control actuators and the combustion behavior. In order to control the process, an accurate feedback signal is necessary to determine the state of the actual combustion process. Ideally to ensure that combustion remains stable and phased correctly an in-cylinder feedback of each cylinder for multi cylinder engines would be preferable. Feedback has been seen in studies using piezoelectric pressure sensors for visually monitoring the pressure in the combustion chamber. This is expensive and requires redesign of the combustion chamber. A potential alternative feedback is to use the conventional spark plug as a sensor. This is achieved by applying a voltage across the spark plug to provide a sensor for ion current. The ions are created through the combustion event, and the current is created by the flow of the ions between the spark plug gap. The work presented in this paper provides a comparison between ion current feedback and pressure trace for a two stroke HCCI combustion from a control perspective. The emphasis of the work is to show the capability of using the ion current system as feedback for information for the start of combustion, combustion duration, and rate of combustion and estimations on peak pressure magnitude. These key parameters from the signal could be useful when applied as feedback for closed loop control.