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An optical telemetry technique has been developed for obtaining piston temperatures in reciprocating engines. The technique employs a transmitter mounted on the underside of the piston which transmits the temperature information as frequency modulated infrared pulses. This signal is picked up by a photodetector mounted in the crankcase which is wired to an external receiving unit. The transmitter consists of a thermocouple or thermistor, voltage-to-frequency converter, calibrating circuitry, and an infrared light-emitting-diode. The receiver consists of a phototransistor, frequency-to-voltage converter, calibrating circuitry and voltage readout. The transmitter is powered by lithium batteries. The signal has been successfully transmitted at speeds up to 4500 RPM in a 2-stroke engine and up to 5000 RPM in a 4-stroke engine. The accuracy of the infrared telemetry device using thermocouples is ±2.8°C (95% confidence level).