An important part of reducing the emissions of diesel combustion engines depends on knowing the temperature of the exhaust gases - let’s take a closer look at this topic:
Starting in the early 1990s, legislators around the world began to limit the amount of pollutants emitted by motor vehicles to protect the environment. While CO2 and other emissions from petrol-engines is a concern, a more significant focus has clearly been on the diesel engine.
The result - despite all the negative headlines - is, that today's diesel vehicles are significantly cleaner than 10 or 15 years ago. There are a lot of improvements but importantly amongst these are temperature sensors, measuring directly in the exhaust pipe. This is a very demanding environment and, since the introduction of the EURO 5 emission standard in 2008 that demands diesel particulate filters, TT Electronics has been actively involved in meeting this requirement.
Diesel particulate filters need temperature sensing for the regeneration process
To be effective, diesel particulate filters must be regularly regenerated during operation. Regeneration is an internal burning process, which starts at about 500 °C, and takes place without any involvement from the driver. Due to exothermicity, the release of heat during a chemical reaction, temperatures in the range of 700 to 800 °C can be achieved, which allow accumulated soot to be burnt off. In addition to pressure monitoring, which will be discussed in a later blog, temperature detection plays an important role in this process.
TT Electronics’ engineering involvement in such applications began in 2005. At that time, TT Electronics was able to purchase a license for a very robust temperature sensor setup. Subsequently based on the company’s long term experience with temperature sensors and with significant further engineering investment, TT Electronics has refined its exhaust sensor design and made it ready for high volume production.
PT 200 / Thermocouple Sensors
Robust design is key for the harsh environment in the exhaust pipe of diesel engines
The outcome is a very robust temperature sensor, based on a passive resistance measuring element that responds with different resistance values at different temperatures. Platinum is used as the resistance material as this element is characterised by a nominal resistance of 200 Ohm at 0 °C (PT-200).
The most important advantage of this high-temperature sensor is its robust design. The measuring element is embedded in a monolithically closed tube made of a special stainless steel using a particular ceramic powder mixture. The bubble-free filling of the sensor tip ensures that motor vibrations do not pose a threat to the sensor's service life. In addition, the sensor can be bent from 0 to 120°. Specially developed seals guarantee a long service life even under the harshest environmental conditions in and around the exhaust pipe.
The next level of high temperature design is up to 1200°C
The experience gained in the last few years has enabled TT Electronics to develop a new generation of high-temperature sensors. The aim of this development was to make a sensor technology suitable for use in vehicles that require temperature measurement up to 1200°C, which is the case in petrol engines.
Robust sensor design
TT Electronics’ sensors incorporate N-type thermocouples to send continuous measurement signals to the vehicle’s control electronics. Thermocouples use the physical effect of thermoelectricity. A pair of metallic conductors of different materials are joined at one end. Due to the temperature gradient between the measuring point (hot side) and the plug side (cold side) an electric voltage is generated (see Wikipedia). This electrical voltage is proportional to the temperature change on the hot side and can thus be processed as a sensor signal. An electronic unit, which is integrated into the plug housing, provides the necessary signal processing.
Thermocouples have been used for many years in industrial electronics for temperature measurement. TT Electronics’ sensor has been specially adapted to the requirements of automotive applications and only use welded or soldered connections to avoid signal interruption. Protective housings at the sensor tip are made exclusively from high-temperature-resistant stainless steel, using deep-draw technology. The sensors can be attached to the exhaust pipe with a union nut or hollow screw of various dimensions. Cable shields are used to allow high temperature resistance measurements in the vicinity of exhaust pipes.
Various electronic interfaces are available
The electrical signal, as already mentioned, is processed electronically. Users can choose between different digital interfaces. To date, TT Electronics has implemented PWM, SENT and CAN (according to SAE J1939). Like the entire sensor, the electronics unit offers protection class IP6K9K (with its mating connector).
This sensor is suitable for temperature measurements in the range -40 to 1200 ° C. With these characteristics, the sensor can be used at any point in the exhaust pipe of petrol engines. Manufacturers of turbo-charged vehicle engines can also use this sensor in the exhaust manifold (before the turbocharger) to warn against excessive temperatures, which can be dangerous for the turbocharger.
Double high temperature sensors will be required for next generation of petrol engines
With the introduction of the latest emission standard (EURO 6c), direct-injection petrol engines must also be equipped with particulate filters for the first time. These use slightly different regeneration processes that, while similar to the particulate filters in diesel engines, need to operate at higher temperatures and require temperature measurements at two points.
Precisely for this application, TT Electronics has developed a thermocouple sensor as a double module that combine two sensors with one electronics unit. Apart from cost benefits, this allows the sensors to be matched with tolerances of +/- 1°C.