NonIntrusive Piston Temperature Measurements using an Embedded Fiber Bragg Grating

1
Non-Intrusive Piston Temperature Measurements
using an Embedded Fiber Bragg Grating
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 (?)

• 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

VCSEL
Photodetector
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 µe / 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