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Non-Intrusive Piston Temperature Measurements using an Embedded Fiber Bragg Grating. λ B =2n eff Λ. 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 - PowerPoint PPT Presentation
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University of Wisconsin Engine Research Center
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
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)
Non-Intrusive Piston Temperature Measurements using an Embedded Fiber Bragg Grating
Timothy R. Pfeifer, Researcher & Jaal B. Ghandhi, Advisor
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
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
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
λB=2neff
Λ
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
Engine Experimental Data
• Data taken for engine stationary, motoring and firing
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
Reprinted from Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing 200x10
-6
150
100
50
0
Po
we
r (W
)
20x10-6
151050Time (s)
Light Transmission through FBG1542.5
1542.0
1541.5
1541.0
1540.5
1540.0
Wa
vele
ng
th (
nm
)
20x10-6
151050Time (s)
VCSEL Wavelength
-18
-16
-14
-12
-10
Atte
nu
atio
n (
dB
)
1542.01541.51541.01540.5Wavelength (nm)
Correlated Light Transmission through FBG
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
VCSEL Photodetector
Pitching assembly Collection assembly
Embedded FBG