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National Advisory Committee for Aeronautics, Technical Notes - A Method of Measuring Piston Temperatures

The present trend in aircraft engines toward higher
specific output requires, among other things, that more
adequate piston cooling be provided. Information on pis-
ton cooling is meager because of the lack of a practicable
method of making piston—temperature determinations at en—
gine speeds in use at present. References l and 2 present
piston—temperature data obtained by the procedure of stop-
ping the engine from an operating condition, inserting a
thermocouple through a spark-plug hole and into thermo-
couple holes in the piston, obtaining a temperaturéitime
curve and extrapolating it back to the time of stopping.
Reference 3 reports determinations of piston temperature
obtained by the use of fusible plugs.__Neither of.these
methods can be -considered satisfactory for the accurate
determination of piston temperatures during engine opera—
tion.

A few investigators have reported (references 4 to 7)’
the results of temperature determinations. by means of
thermocouples installed on the piston, covering a speed
range from 200 to 1,500 rpm made on oil and gasoline en— t
gines."In these tests, the circuit between the thermo-
couples and the indicating instrument was completed by
wires, which were supported by anmechanical linkage. This
system appeared to be quite limited as to speed of oper— . _. __h
ation and ease of applicability.

A method of determining temperatures of pistons_oper~ _fi _fl_
atinq at speeds up to 2,000 rpm was developed by F. Glen
Shoemaker of the Materiel Division of the Air Corps at
- Dayton. Ohio, in 1927. This method entailed the comple—
tion of the circuit of a piston thermocouple by spnings
that made Contact for.a short period at the bottom of the
-piston stroke. The thermal electromotive forces were meas-
ured by a potentiometer. This method was considered im~
practicable for continued operation at high speeds because
of the spring contacts.

As part of a program for the study of piston cooling
at high engine speeds, the N.A.C.A. undertook the devel— V
opment of a method of determining piston temperatures. {4
The development~consisted in,a modification of the Shoe- .
maker method with the spring~actuated contacts replaced by
pneumatically operated contacts to complete the thermo- _
couple circuit. This note presents a description of the .i
N.A.C.A. method and sample piston—temperature data ob— a
tained on a compression~i€nition and a spark—ignition en- _i
gins.