Combustion Sensing Methods - in Theory and Practice

Licentiate thesis, 2007

Accurate, reliable sensors are essential parts of any control system, and there are increasing needs for feedback systems to monitor and control key parameters of automotive internal combustion engines, operating in various combustion modes and configurations, to reduce fuel consumption and engine-out emissions. Thus, there are strong motives for studying combustion sensors. Combustion can be monitored by various types of sensors, all of which have the same ultimate purpose; to extract useful information about combustion processes in the engine. The work described in this thesis and the attached papers has included both a theoretical study of combustion sensors, and experimental studies using examples of the two most cost-effective and promising types of combustion sensors currently available; pressure sensors and ion current sensors. The first paper presents a study of inexpensive in-cylinder pressure sensors, in which experiments were performed in a single cylinder, direct injected spark ignition (SI) engine. The engine head was adapted to provide access for three different pressure sensors: a water-cooled, piezoelectric sensor (used as a reference), a piezoresistive sensor incorporated with a glow plug, and a fibre optic sensor, also incorporated with a glow plug. Furthermore, a piezoelectric force sensor of gasket type was used to measure the in-cylinder pressure. In addition to the comparative results for the sensors, some illustrative findings were obtained concerning the volume acoustic modes of the combustion chamber. The second paper included in the thesis describes an investigation of ion current sensing, in which experiments were performed in a single cylinder, optical, direct injected homogeneous charge compression ignition (HCCI) engine. This engine was equipped with a transparent, quartz glass piston, allowing the combustion progress to be monitored from below using a high speed camera. The images were correlated with the ion current traces. In addition, an ion current sensing electrode constructed in-house, in which the probe could be readily changed, was used to study the effects of varying the electrode’s surface area and shape. The results of the optical ion current investigation of HCCI combustion show (i) that there is a delay between the initiation of combustion and the occurrence of a corresponding ion current signal, and (ii) that this is partly because HCCI combustion seldom starts by the sensing electrode, but rather along the walls or by the hot exhaust valves, where the conditions are most favourable for the initiation of combustion. In addition, a study of the ion current supply voltage indicated that this variable strongly influences the ion current amplitude at lower voltages, implying that the supply voltage should be as high as possible.