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