## 1 Introduction
### The EGT as a Parameter Useful for Indirectly Measuring the Engine Performances
Thermal cameras and TCs are the two considered options, each having some advantages and drawbacks:* Thermal cameras usually need a proper preparation of the target object (in terms of surface reflectance, smoothness) to ensure a known value of its emissivity; but they have a very high cost if compared to thermocouples and can measure only superficial temperatures; among the advantages, there is the possibility to use them at a set distance from the studied object and to acquire the temperature of many points at the same time;
* TCs are inexpensive, small, robust, can be inserted in some difficult-access points of an engine, even, very close to the exhaust gas flow (using specific solutions, e.g., thermometric wells or \"thermowells\" to be screwed in dedicated threaded holes), and can be easily connected to a data logging device (output in voltage); on the other side, they need wiring, adaptors, thermometric wells (thus implying the creation of threaded holes on the exhaust pipe), and, finally, each thermocouple can measure the temperature in a single point only. For what concerns precision, studies show that there is good agreement between the two above-illustrated acquisition techniques [40]. However, for the present case, thermocouples were chosen. The main reason is the need to acquire the EGT also inside the exhaust pipe (at least to have a comparison for the temperatures acquired in other points). Furthermore, thermocouples are more suitable for possible future adoption in an engine monitoring system, thanks to their easier synchronization with other sensors, their robustness, and their cheapness. Nevertheless, the correct use of these sensors is not trivial, and some aspects must be evaluated carefully, as many studies demonstrated [41; 42; 43; 44]. Firstly, the right thermocouple-type must be chosen on the basis of the expected absolute value and variation range of the temperature to be measured. Secondly, if thermocouples are used to measure a temperature depending on the EGT (e.g., the surface temperature of the exhaust pipe), the position along the exhaust system must be individuated carefully. Indeed, in such a layout, the TC position is a key element in the measurement process and can highly affect the representativeness of temperature measurements for deducing the related engine performances. The longer the distance of the measuring point from the exhaust valves of an engine, the greater the heat losses due to many (and, sometimes, difficult-to-be-predicted) thermal phenomena. Apart from the position, another concern is about the installation layout and the system adopted to fix the thermocouples (superficial, welded, inside the pipe, with or without a thermometric well) because these choices may significantly affect the final measurement too. All those above-illustrated aspects determine different fluid dynamic and thermal conditions for the system as its whole, hence directly affect the heat exchange and this is the reason why so many other studies focus on these aspects [45; 46; 47; 48; 49]. Things to consider are, for example, the convective exchange coefficients, the emissivity, the thermal diffusivity, and other component properties. Issues to deal with are: the temporal delay of thermal waves due to thermal inertia (especially during non-stationary working conditions) and the presence of a pulsating flow that modifies the internal heat transfer coefficients, thus making the correct EGT estimation from outside the exhaust pipe even more challenging.