Exhaust Gas Temperature Sensors

Stefan Carstens

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• Introduction
• Resistance Temperature Detectors (RTD)
• Thermocouples
• Future Trends

Introduction

In modern internal combustion engines, the knowledge of the exhaust gas temperature (EGT) is necessary for the management and diagnosis of the exhaust gas aftertreatment system, as well as for the protection of components that may be sensitive to thermal overloads.

In the diesel engine, aftertreatment components that are often actively managed based on exhaust gas temperature include diesel particulate filters (DPF) and NOx reduction catalysts such as SCR catalysts and NOx adsorber catalysts (NAC/LNT). In stoichiometric petrol engines, the engine management strategy depends on the temperature of the three-way catalyst (TWC). Examples of components that may require protection from thermal overload include turbochargers, EGR system components, as well as emission control catalysts.

Classification. In engine applications, two main measurement methods have been used for the determination of exhaust gas temperatures [3390]:

• Resistance temperature sensors
• Thermocouples

In resistance temperature sensors, an electrical resistance is generated which correlates with the sensor temperature. Here we differentiate between positive and negative temperature coefficients. In the case of negative temperature coefficients, ohmic resistances decrease as the temperature increases. Therefore, these sensors are referred to as NTC (negative temperature coefficient). Due to the fact that in a hot temperature range, their resistance falls to a fraction of the cold resistance, they are also referred to as “hot conductors”. Another alternative designation is “thermistor”, a portmanteau of “thermal resistor”.

In contrast to this, there are temperature sensors with increasing resistance values at simultaneously increasing temperatures [3393]. Some of these are so-called PTC thermal resistors (positive temperature coefficient), mainly based on semiconductors, polycrystalline ceramics (e.g., barium titanate) which, on exceeding a limit temperature, abruptly increase their resistance and therefore are ideal for use as inrush current limiters and switch-off temperature limiters. These are also often referred to as thermistors. On the other temperature sensors in this category, platinum elements are used. These are known in industrial measuring technology as platinum resistance temperature detectors (PRTD), designated as Pt100 through Pt1000 in the DIN EC 60751 standard.

KTY is a trade name for PTC temperature measurement resistors based on semi-conductors. These temperature resistors based on silicon have a characteristic curve which is comparable to that of platinum elements. Their application temperature range reaches up to 300°C, which is why these construction elements are not of interest for use in exhaust gas flows, and are not further considered in the following text. The same applies for PTC thermistors, which can only be used up to around 150°C.

In thermocouples, the other major class of EGT sensors, the temperature measurement is based on the Seeback effect where a voltage is generated across the junction of two conductors of different metals in the presence of a temperature gradient. Different types of thermocouples exist, depending on the combination of metals that is used.

The classification of resistive and thermocouple-based temperature sensors is summarized in Table 1. As apparent from the table, sensor technologies suitable for exhaust gas applications include NTC thermistors, Pt resistance sensors, and Type N thermocouples.

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