Fiber optic sensing technology for measuring in-cylinder pressure in automotive engines

Abstract

A new fiber optic sensing technology for measuring in-cylinder pressure in automotive engines was investigated. The optic sensing element consists of two mirrors in an in-line single mode fiber that are separated by some distance. To withstand the harsh conditions inside an engine, the Fiber Fabry-Perot Interferometer (FFPI) element was coated with gold and copper. The metal-protected fiber sensor was embedded into a small cut in the metal casing of the spark plug. At first, the sensing element was dipped in liquid gold and cured. Then the gold-coated fiber sensor was electroplated with copper. Finally, the metal-coated fiber sensor was embedded in the spark plug. The spark-plug-embedded FFPI sensor was monitored using a signal conditioning unit. Field tests were carried out in a 3-cylinder automotive engine with a piezoelectric pressure sensor as a reference transducer up to about 3500 rpm. The fiber optic sensor data generally matched those measured by the piezoelectric reference sensor. The use of a Vertical Cavity Surface Emitting Laser (VCSEL) diode as a light source in an FFPI optic sensor system was investigated. Reflected light from the FFPI sensing element was used to measure the optical path difference. With a 1550nm VCSEL as the light source in a 12mm cavity length Fiber Fabry-Perot Interferometer, spectral characteristics were examined to determine the proper combination of dc bias current, modulation current amplitude and modulation frequency. Single VCSEL operation and regular fringe patterns were achieved. The laser tuning was -41.2 GHz/mA and was determined from measurements of the shift in the spectral peak of the VCSEL diode output as a function of dc bias current. By testing the fringe movement as the FFPI sensor was heated, the temperature tuning coefficient for the optical length was determined to be 11 x 10-6 ºC. The results of these experiments indicate that the use of VCSEL diode as a light source for the FFPI sensor offers a viable alternative to the use of Distributed Feedback (DFB) laser diodes for monitoring at a lower bias current and modulating current amplitude.