Some Metrological Observations on the Use of the Exhaust Gas Temperature for the Indirect Measurement of the Torque in Agricultural Engines

The temperature of the exhaust gases can easily be correlated to the torque that an internal combustion engine is instantaneously delivering. Therefore, the temperature measurement is a concrete and viable possibility for indirectly quantifying the performance of agricultural machinery, even in terms of retrofitting, if the considered farm tractor is not natively equipped with any electronic system to interface with. The positioning of a thermocouple along the exhaust line must, however, be accompanied by a series of technical considerations and preliminary trials to test its main metrological characteristics (firstly: its "time constant") and, hence, its overall suitability. The experimental campaign carried out in this study involved the characterization of a K-type thermocouple in water (at the temperature of melting ice and at the boiling point) and, then, in an installation close to the one which could be used for an endothermic engine performance monitoring system, i.e., along the exhaust line. Both the thermowell and the installation setup have a non-negligible influence on the overall time constant of the sensor: the former increases the time constant by 12.8 times, the latter by a factor of 42.5 with reference to the thermocouple alone. This evidence has interesting implications for possible installations of a monitoring system, starting with the possibility of using more conductive metals for the thermowell or even installing the thermocouple without any thermowell to purse the maximum possible quickness of response for this sensor.