Spark-plug-mounted fiber optic sensor for measuring in-cylinder pressure in engines

Abstract

A new design for an in-cylinder fiber Fabry-Perot interferometer (FFPI) pressure sensor suitable for automotive engines has been investigated experimentally. The FFPI sensor consists of a single mode fiber containing two internal mirrors which form the cavity for the interferometer. To protect the FFPI sensor from the harsh environment of an internal combustion engine, the FFPI element is coated with metal. The sensing element is first dipped in liquid gold and cured in an oven. Then this gold-coated fiber sensor is electroplated with copper. Finally, the metal-protected fiber sensor is embedded in a groove cut in the spark plug casing. Spark-plug-embedded FFPI sensors were used to monitor pressure in internal combustion engines, with an optoelectronic signal conditioning unit containing a chirped distributed feedback laser used to interrogate the sensor. The response of the fiber sensor was compared with that of a piezoelectric pressure sensor used as a reference transducer. Tests were carried out in two engines, a single cylinder Ajax engine and a four-cylinder Onan engine. In each case, the fiber optic sensor data closely matched those measured with the reference sensor. In the case of the Onan engine, the spark plug containing the embedded fiber optic sensor was used to ignite combustion in one of the cylinders.