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Thermocouples are commonly used to measure exhaust gas temperature during automotive engineering experiments. In most cases, the raw measurements are used directly as an absolute indication of the actual exhaust gas temperature. However, in reality, the signal from a TC is only an indication of its own tip temperature. The TC indicated tip temperature can deviate significantly from the actual gas temperature due to factors such as thermal capacitance of the tip itself, and heat transfer to the exhaust pipe wall through conduction and radiation. A model has been developed that calculates the effects of these factors to provide an estimate of the actual exhaust gas temperature. Experiments were performed to validate the model under both transient and steady state engine dynamometer conditions utilizing three popular sizes of TCs. Good correlation among predictions for various TC sizes confirms the model's accuracy.

Instrumentation of an exhaust system to measure surface temperature at multiple locations usually involves welding independent thermocouples to the surface of the system. This report describes a new type of thermocouple fabricated to measure temperature at a point or temperature difference between points on a metallic object utilizing the metal as one component of the new thermocouple. AISI 316 stainless steel is used in the current study to represent automotive exhaust pipe. The other component of the thermocouple is Nickel-Chromium (Chromel, Chromega), one of the two metals used in type K thermocouples, which are generally used for exhaust temperature measurements during emission tests. Use of the new thermocouple is contingent upon an accurate calibration of its response to changes in temperature.

Thermocouples (TCs) are commonly used to measure exhaust gas temperature during automotive engineering experiments. To enhance the durability of TCs in the harsh exhaust gas environment, in many cases larger tip TCs (such as 1/8″ diameter) are used rather than smaller TCs. However, the signal from a larger thermocouple can differ significantly from that of small TC due to thermal capacitance of the tip, heat transfer to the exhaust pipe wall via conduction and radiation, and convection with exhaust gas. A model has been developed that calculates the effects of these factors and provides an estimate, for TCs of different sizes, of exhaust gas temperature. Experiments were performed to validate the model under transient (FTP) engine dynamometer conditions utilizing three popular TC sizes (1/32″, 1/16″, and 1/8″). Good correlation was found among predictions for various TC sizes.