Measurement of combustion chamber surface temperatures is
necessary for many areas of engine research. These authors require
surface temperature data for a study on thermal barrier coatings
(TBCs) in an aluminum alloy spark-ignition (SI) engine. For many
years, the thermocouple has been the sensor most commonly used
to measure surface temperatures. However, there is no consensus
about the optimal thermocouple design or even general configuration
that best suits fast response engine applications.
The primary consideration when selecting or designing a surface
thermocouple is accuracy at measuring ”true” temperatures,
being the temperatures that would exist if the thermocouple was
not present. Indeed, this criteria embodies the experimenter’s age
old challenge of taking measurements without changing what is
being measured. For the surface thermocouple, this includes accurate
measurement of average temperatures and temperature
swings, while minimizing delays in response time. Additionally,
the thermocouple should be robust and simple to fabricate and install.
Finally, it is necessary that it fits within a researcher’s budget.
The accuracy issue was explored in 1989 by Assanis and Badillo
[1], who used Finite Element Analysis (FEA) to model a 0.452 mm
diameter coaxial thermocouple embedded in an aluminum piston.
A coaxial thermocouple is essentially an insulated wire of one thermocouple