Effect of Thermocouple Size on the Measurement of Exhaust Gas Temperature in Internal Combustion Engines

Abstract

Accurate measurement of exhaust gas temperature in internal combustion engines is essential for a wide variety of monitoring and design purposes. Typically these measurements are made with thermocouples, which may vary in size from 0.05 mm (for fast response applications) to a few millimetres. In this work, the exhaust of a single cylinder diesel engine has been instrumented both with a fast-response probe (comprising of a 50.8 μm, 127 μm and a 254 μm thermocouple) and a standard 3 mm sheathed thermocouple in order to assess the performance of these sensors at two speed/load conditions. The experimental results show that the measured time-average exhaust temperature is dependent on the sensor size, with the smaller thermocouples indicating a lower average temperature for both speed/load conditions. Subject to operating conditions, measurement discrepancies of up to ~80 K have been observed between the different thermocouples used. Thermocouple modelling supports the experimental trends and shows that the effect of conduction is inversely proportional to the thermocouple junction size-an effect attributed to changes in the thermal inertia of the device. This conduction error is not typically considered in the literature for exhaust gas temperature measurement. Modelling results also show that radiative heat transfer is small compared to the effect of conduction on the measurements. Finally, a new dynamic response thermocouple compensation method is presented, in order to correct for the dynamic error induced by the thermocouples. This technique recovers the “true” gas temperature with a maximum error of ~1.5-2% in peak temperature depending on speed/load conditions.