The eroding thermocouple is another style of thermocouple that
is frequently used for engine measurements. The design utilizes
two thin ribbons of thermocouple element, insulated and housed
within a metal sheath. The electrical connection between the two
elements is formed by abrading the surface. Yet despite the small
size of the thermocouple elements, FEA by Buttsworth [5] shows
the temperature at the junction is significantly higher than that
of the sheath during transient heat flux.
An alternative surface thermocouple (Fig. 1) was developed by
Heichal et al. [6] to measure temperatures of liquid droplets
impacting a metal surface. The steel substrate was used as one of
the thermocouple elements, and a 0.25 mm diameter constantan
wire was the second element. The wire was cemented in a
0.57 mm diameter hole through the steel, and the junction was
formed at the surface with an electrically conductive film.
As the substrate is one of the elements, the Heichal et al. design
has the potential to produce accurate true surface temperature
measurements. Additionally, it is simple enough to be fabricated
in a typical machine shop. However, it was found that modifications
were required to improve its robustness and accuracy for engine
use.
This paper describes the modifications made to the Heichal
et al. thermocouple to improve it for measuring temperatures of
aluminum combustion chamber surfaces. The process included
the use of FEA to optimize and evaluate the design. Ultimately,
the thermocouple was tested in an SI engine.