Non-Intrusive Piston Temperature Measurements using an Embedded Fiber Bragg Grating

1. Non-Intrusive Piston Temperature Measurements using an Embedded Fiber Bragg Grating λB=2neffΛ Timothy R. Pfeifer, Researcher & Jaal B. Ghandhi, Advisor • Fiber Bragg Grating Sensor • Fiber-based method • Relies on total internal reflection to propagate light through fiber core • Telecommunications industry has made components popular and inexpensive • Fiber Bragg grating acts as a band-stop filter on broadband light • The center wavelength that the FBG attenuates depends on the effective refractive index of the grating planes (neff), as well as the spacing between each plane (Λ) • Engine Optical System • Pitching assembly • VCSEL light source with FC fiber connection • Aspheric lens (f = 7.5 mm) to focus light through window to spot size of 400 μm • Collection assembly • Collimation package (f = 4.5 mm) to improve collection efficiency • Multimode fiber with 600 μm core to collect maximum amount of light • Motivation • Engine durability at high load is largely limited by piston strength • Tensile strength of aluminum alloys decreases severely as temperature increases • Engine heat transfer models require accurate piston temperature measurements • Magnitude and location of piston heat transfer are approximated • Experimental data to validate these approximations is scarce or invalid • Previous methods of measuring piston temperature pose problems • Attaching devices to piston increases reciprocating mass and alters heat transfer • Some methods require engine to be at rest for data acquisition Photodetector VCSEL Embedded FBG • Objectives • Investigate piston temperature using a recently developed fiber Bragg grating technique • Improve measurement system developed by Dennis Ward • Increase overall transmission efficiency • Increase Bragg dip attenuation Pitching assembly Collection assembly • Wavelength Agile Light Source • For Bragg dip identification all wavelengths must be scanned each revolution • VCSEL laser diode capable of scanning 3 nm range at speeds from 1 Hz to 100 kHz • Fiber Bragg Grating Properties • Fiber Bragg gratings exhibit both thermal and strain response • Strain response: ~ 500 με / nm • Physical elongation of the grating • Change in fiber index due to photoelastic effect • Thermal response: ~ 100 °C / nm • Thermal expansion of the fiber material • Refractive index dependence on temperature Reprinted from Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing • Engine Modifications for Optical Access • Holes are bored in engine block and sealed with sapphire windows and O-rings • FBG is embedded near piston surface perpendicular to wrist pin axis • The optical light path is completed once per revolution at BDC • Engine Experimental Data • Data taken for engine stationary, motoring and firing Stationary – 21.2 °C Motoring – 30.1 °C Firing – 135.5 °C Masters thesis work by Dennis M. Ward (2004) at the University of Wisconsin - Madison