Effect of thermocouple design and engine exhaust pulsations on mean gas temperature accuracy

Abstract

Accurate measurement and modeling of mean exhaust gas temperature (EGT) in internal combustion engines (ICEs) is paramount to improve its emission and efficiency while maintaining component durability. Sheathed thermocouples provide a robust and cost-effective EGT measurement to the detriment of accuracy due to the junction’s heat balance. Literature indicates that exposing and reducing the thermocouple junction diameter improves the mean measured EGT accuracy. However, such designs can affect its interaction with the pipe wall, the conventional heat sink for sheathed thermocouples. Furthermore, characteristics of the pulsating exhaust gas flow govern junction heat convection and heat conduction linked to the exposed wire length-to-diameter (l/d) ratio. Therefore, the complex interaction between thermocouple design attributes and the pulsating exhaust flow requires isolating the effects to derive the measurement accuracy benefits from exposed junction thermocouples. This study utilizes Type-K thermocouples in sheathed (6 mm) and multiwire exposed junction (51-$254\mu m$) constructions downstream of a single-pipe exhaust of a heavy-duty diesel engine. Isolated engine speed and load sweeps provided distinct pulsating exhaust flow conditions measured using a Pitot tube for mass flux and the exposed junction heating rate to indicate the true EGT. The study highlights fundamental differences in the junction-to-pipe wall thermal interaction between sheathed and exposed thermocouples, motivating the need for distinct heat sinks and error correction before comparing measurements. Moreover, the nature of the pulsating exhaust flow conditionally enhances or undermines gains in mean EGT accuracy.