Non-Intrusive Engine Piston Temperature Measurement Using a Fiber Bragg Grating

Abstract

An embedded fiber Bragg grating was used to examine the operating temperature of a small gasoline engine piston. Optical system analyses were conducted to enhance the accuracy and consistency of the measured piston temperature. The goal of the work was to experimentally characterize light transmission through embedded fibers, with the aim of improving the entire measurement system to acquire data as the engine was running. An aspheric collimating lens and a polyimide-buffered fiber Bragg grating were found to create a transmission profile with a much more noticeable Bragg dip, and the optical system was more tolerant to misalignment while maintaining measurement accuracy. A wavelength-agile VCSEL diode laser provided a low-cost means to interrogate the embedded fiber Bragg grating at speeds up to 100 kHz, albeit for a small wavelength domain. Stationary engine measurements successfully determined the piston temperature to be 31.5 �C. However, irreparable damage was caused to the engine when a small piece of electroplated nickel became loose and wedged between the piston and cylinder wall shortly after initiating motored tests. After verifying the integrity of the embedded FBG, the cylinder was rebored and the piston reinstalled. A modified motoring experiment proved that light coupling was occurring for each engine rotation, and analysis determined the piston temperature to be approximately 56 �C. Due to substantial noise in the photodetector data, it was difficult to qualitatively identify the Bragg wavelength without making assumptions about the expected Bragg dip location.