Effect of thermocouple size on the measurement of internal combustion engine exhaust gas temperature 2018-01-1765
Accurate measurement of exhaust gas temperature from internal combustion engines is essential for a large number of purposes including after-treatment systems, combustion modelling, and component durability. Discrepancy in such measurements could lead to inefficient hardware use or compromising the durability of engine components. Typically these measurements are made with thermocouples, which vary in size from 0.05 mm (for fast response applications) to a few millimetres. In this study, experimental testing supported by numerical simulations has been carried out in order to assess the performance of different size temperature sensors. Thus, the exhaust of a single cylinder diesel engine has been instrumented both with a fast-response probe (comprised of 0.002”, 0.005” and 0.01” thermocouples) and a 3 mm sheathed thermocouple and assessed under two speed/load conditions. The experimental results show that the measured time-average exhaust temperature is dependent on sensor size, with the smaller thermocouples indicating a lower apparent temperature for both speed/load conditions. Subject to operating conditions, measurement discrepancies of up to ~50 °C have been observed between the different thermocouples used. Thermocouple modelling supports the experimental trends and shows that conduction losses increase inversely proportional to junction size–an effect attributed to a reducing thermal inertia in conjunction with the unsteady exhaust gas behaviour. This conduction error is not typically considered in the literature for exhaust gas temperature measurement. Modelling results also show that radiative heat transfer is insignificant compared to the effect of conduction on the measurements. Finally, a new dynamic response thermocouple compensation method is presented, incorporating a correction for conduction error. Existing compensation techniques do not account for conduction errors thus this approach attempts to improve the estimation of true gas temperature. This new method improves the temperature prediction by ~30°C for parts of the engine cycle.
Nick Papaioannou, Felix Leach, Martin Davy
University of Oxford
International Powertrains, Fuels & Lubricants Meeting