Power Train

Measurement of exhaust gas temperature and control of exhaust gas after-treatment systems

In order to enable sophisticated motor control circuits to reduce fuel consumption and CO2 emission, measurement of exhaust gas temperature is a key factor. The article describes in detail state-of-the art the sensor technology involved.

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In the automotive industry two main methods of measuring the exhaust gas temperature prevail:

-          Resistance Temperature Probes Pt200 (RTD)

-          Thermocouples

 Resistance Temperature Probes Pt200

Platinum as a precious metal exhibits an advantageous characteristic: its value of electrical resistance increases continuously as the surrounding temperature increases. For instance, one could imagine a long thin piece of wire, which at an ambient temperature of 0°C exhibits a resistance value of exactly 200 Ohm. If this wire is heated to 850° C, the resistance will show an increase to 774 Ohm. By applying resistance measurement, the vehicle's electronic system can easily determine the surrounding temperature at the measuring point. The trick now will be, to implement this wire into an exhaust gas temperature probe. This is achieved by using production methods similar to those of the semiconductor industry. In this case the basic material is a piece of substrate with a total surface of about 3 mm2. The substrate material is alumina ceramic, which even at high temperature levels ensures high insulation resistance. The substrate constitutes the carrier material for the development of the layer. Onto it a Platinum layer is applied under a high vacuum environment by means of processes such as “Sputter” or “PVD” (physical vapour deposition). This layer is the basis for creating the Platinum wire. From that cohesive type surface a meandering pattern is produced, which is so small and so delicate, that in fact it represents a multiple intricate uninterrupted conductor. This is achieved by processes such as laser meandering or photo-lithographics. In order to ensure that all probes have compatible resistance characteristics, each individual exhaust gas temperature probe is tested by means of a precision measurement setup and is individually laser-calibrated.

However, under exhaust gas environmental conditions, a structure such as the one described above would constitute a catalyst, which is undesired. Therefore, the platinum layer must be passivated. This in turn can be achieved by applying a high-melting temperature resistant glass layer or by covering the structure with another alumina ceramic plate. In the latter case the cutting edges will also be sealed with glass. Often, platinum connecting wires are used for establishing contact, which are affixed by bonding to pre-printed gold- or platinum pads.

Fig. 1: Pt200 resistance temperature probe: Build-up of a Pt200 resistance temperature probe for exhaust gas temperature measurement.

1- Cover and strain relief for the connecting wires by using alumina ceramics

2 - Platinum connecting wires

3 - Platinum or gold bond pads

4- Passivation from alumina ceramics or high-melting temperature resistant glass

5 - Platinum meander and laser trimming routes

6- alumina ceramics substrate

This resistance temperature detector is now implemented into an appropriate casing. For the casing two different designs have proven to best suit the purpose:

-          The open perforated housing

-          The fully enclosed and hermetically sealed housing.

“Open” Exhaust Gas Temperature Probe, type TS-200:

 In the case of the “open” housing design, the sensing element itself (RTD) is subjected to direct contact with the exhaust gas flow, as it is the case with an oxygen sensor or lambda probe. The TS-200 type of probe is a typical representative of this design and the same installation directives apply as it is the case with oxygen sensors. The tip of the probe must always point downwards to ensure that no condensate which might precipitate in cold nights can creep into the interior of the probe and finally lead there to corrosive effects.

Fig 2: Installation Instructions for “Open” Exhaust Gas Temperature Probes.

This type of temperature probe is used in great numbers by the automotive industry for systems such as particulate filter monitoring or for the protection of turbo-superchargers. In these cases the operating temperature range increases up to 950 °C and for short periods even beyond. Thus it is at the upper end of the possible measurement range for resistance temperature probes. It is an ideal product for the ambitious tuning of vehicles and with the broad temperature range it is particularly suited to protect components of retrofitted compressors and turbo superchargers.

Fig 3: Exhaust Gas Temperature Probe, type TS-200

 Pt200 Exhaust Gas Temperature Probe with fully enclosed and hermetically sealed housing - type HTS

 The HTS exhaust gas temperature probe is not subject to any installation restrictions, e.g. it can also be installed with the sensor tip pointing upwards. In particular, exhaust gas, which is heavily loaded with particles, cannot affect this type of sealed temperature probe, also sooting has no effect. As soot can oxidize only from 550°C upwards under an oxygen atmosphere, this sensor type is of particular interest for equipping diesel engines. Its main application by the automotive industry is the monitoring of particulate filters and the control of SCR-systems (SCR = Selective Catalytic Reduction) and exhaust gas recirculation.

This type of sensor is also particularly suited to the monitoring of cogeneration plants, which use alternative fuels such as palm oil, rapeseed oil or biogas. These applications rarely involve exhaust gas temperatures above 550°C.

The Test Bench Method: Thermocouples

A well-proven method for measuring exhaust gas temperature is the measurement by using thermocouples. At the same time, this is the oldest known method of electrical temperature measurement, which is based upon a discovery of the physicist Seebeck, which he published in 1823 in his papers. The so-called Seebeck-effect claims that between the two ends of an electrical conductor an electrical voltage is created if different temperatures exist between the two ends of the conductor. This happens independently of the temperature profile otherwise present along the conductor. The value and the polarity of the voltage depend of the materials used. For instance, in the case of Constantan the thermoelectric voltage is 

-3,4 mV/100°C, in the case of iron it is
+1,88 mV/100°C.

Fig. 4: The Seebeck effect: Between the left and right hand ends of the metallic conductor a thermoelectric voltage is created, independently of the question, whether an additional heat source located between the two ends of the conductor is providing a higher temperature or not.

However, in reality it is not possible, to place one end of a metallic conductor into the exhaust gas and the other end into the engine compartment and to just measure the voltage in between. Therefore a trick is used:

For instance a Nickel conductor with a thermoelectric voltage of -1,9mV/100°C is used to connect one end with another conductor of different material, e.g. Nickel-Chromium with a thermoelectric voltage of +6,7 mV/100°C. This will result in a thermocouple with 4,8 mV/100°C. Thus a higher signal gain is achieved as compared to the use of one type of material alone and no need exists to measure the thermal voltage at the “hot” junction as the measurement of the voltage at the “cold” junction is sufficient.

Fig 5.: Thermocouple – The temperature T1 is 200°C above the ambient temperature T3. In case two equal Ni-conductors are used, the thermoelectric voltage between the hot and the cold junction will be -3,8 mV. As this is true for both of the wires, both cold ends show the same electrical potential and thus no voltage can be measured. In the right hand illustration the upper wire has been replaced with a Ni-Cr wire, which has significantly different material properties. This material shows a thermoelectric voltage of 13,3 mV between the cold and the hot junction. As the bottom wire stays the same, both of the “cold” ends are now showing an electrical potential difference: This voltage difference can easily be measured.

A thermocouple can always only measure temperature differences. Therefore in the examples shown here, the ambient temperature T3 (ambient temperature) must be known, if the hot junction temperature T1 is to be determined. In our example this would be room temperature at 25°C plus a temperature difference of 200°C, which results in a measured temperature T1 of 225°C.

The measurement of the T3 temperature is usually done with resistance temperature probes of the Pt100, Pt1000 or NTC types. For use on test benches and in vehicle-testing, thermocouples Type K with green connector cables several meters long are now prevailing. However, for use in series-production vehicles a number of serious disadvantages have to be taken into consideration such as the following:

-          The cables cannot be shortened or elongated, nor can they be provided with electrical cable connectors at will since by such action a new thermocouple will be created.

-          The determination of the T3 temperature requires a complex electronic effort, so-called “cold-junction compensation” (CJC), for which no provision is made in usual engine control units.

-          Long thermocouple connector cables require a protection against electromagnetic (EMC) interference signals. Thus, the installation in the direct vicinity of spark coils must be avoided.

-          An integrated “cold-junction compensation” (CJC) requires more space during installation assembly

Based on the Thermocouple Technique: The DTS-V II Exhaust Gas Temperature Probe for Temperatures of up to 1100°C.

In order to eliminate the above disadvantages as far as possible, Delta-R Motorsensor has consequently enhanced its product DTS-V II. A metallic shielding by metal wire-mesh braid prevents to a great extent the negative effects of EMC-interference signals and it increases mechanical stability. An integrated electronic box contains the cold-junction compensation, which is effective up to an engine compartment temperature of 120°C, and which enhances the thermoelectric voltage up to 4,5 V. The three-conductor cable leading into the electronic box may be shortened or elongated at will or cable connectors may be added. The system is completely encapsulated by potting and it is usually installed directly in the engine compartment.

The supply voltage is +12V from the usual battery supply. In order to enable the required precision measurement, this voltage is reduced internally and stabilized to +5V. To improve response time, a mineral-insulated thermocouple with a diameter of 3 mm is used. It is mechanically stable up to 30 mm installation depth. For achieving the standard installation depth of 50 mm a supporting sleeve is used. In order to be prepared against aggressive contents of the exhaust gas, only stainless steels such as Inconel™ and 1.4571 are used. This type of exhaust gas temperature probes constitute the “high-end” of the product range and are mainly used in ambitious car-racing for monitoring the operation of turbo superchargers. The type DTS-V BHKW exhaust gas temperature probe has specifically been developed for monitoring the exhaust gas temperature in cogeneration plants. The mounting is achieved by means of an M6 swaging nut, which allows for choosing an individually selected installation depth. This in turn was possible by eliminating the supporting sleeve.

This design bears no risk, as in this type of application the exhaust gas temperatures are rather moderate, mostly below 700°C. The output signal of between 0.5 and 4.5 V can be fed into any programmable logic control (PLC) and switching values can be assigned to it individually.

Fig. 6: High-End Exhaust Gas Temperature Probe DTS-V II – Up to 1100°C, with Welding Socket SO-200 and Union Nut SW17 (Wrench Size – Metric)

Thermocouple Elements and Drift: The so-called “K-effect”

In case thermocouples type “K” are used on test benches for testing at high temperature gradients (e.g. exhaust manifold crack test), a temperature drift of up to 30°C can be noted. This effect is usually referred to as the “K-effect” and its reason are changes in the atomic structure of the metal.

The type-K thermocouple consists of one NiCr-leg and one pure-nickel leg. At high temperatures from about 600°C upwards, the NiCr-leg is responsible for partly indefinable changes of the thermoelectric voltage. This phenomenon is not known to many users and in order to understand it, we must look at the manufacturing procedures of high-grade thermo wires. In case a melt from a NiCr-alloy solidifies with a low cooling rate, a cubic atomic structure is formed. At temperature above 600°C, this type of alloy always results in this type of crystal structure.

The homogeneous crystal structure ensures a well reproducible thermoelectric voltage. If the cooling rate for this NiCr-thermo wire is kept below 100°C/h, a change in the atomic structure towards a short-range orientated condition (“U-condition”) occurs. In this case the previous arrangement of Cr-atoms on the corners and of Ni-atoms in the area centres is partially disturbed. This condition still results in reproducible thermoelectric voltages. However, this structure changes into an undefined state, when arbitrary mixed-type lattice structures develop due to higher cooling rates. As this effect can mostly be observed with Thermocouples of the “K”-type the phenomenon is called the “K-effect”.

With an exhaust gas temperature probe always the highest cooling- and heating rates must be anticipated.

Therefore, the K-effect had to be taken into consideration already during the concept-phase of the DTS-V II probe. Delta-R Motorsensor applies a technique, which totally cancels this effect without endangering the otherwise positive characteristics of this type of thermocouple. In this case small quantities of silicon are added to the alloy for both legs of the thermocouple, which leads to a near complete cancellation of the influences from the “U-Condition”. Thus, the “K-effect” cannot be proven anymore by way of measurement techniques. As a logical consequence, the DTS-V II exhaust gas temperature probe is built as type “N” with one leg from NiCrSi and a partner from NiSi material respectively.

Delta-R Motorsensor : Technical advice and the supply of exhaust gas temperature probes are provided from one source.

 With the models TS-200, HTS und DTS-V II Delta-R Motorsensor covers the whole spectrum of practicable exhaust gas temperature probes. For resistance temperature devices which have to be located in big diameter exhaust gas ducts, the EXT-versions have been developed, which provides for up to 100 mm installation depth.

 Of course, the related accessories equipment such as fittings, sealing plugs, welding sockets, display units, voltage diverter circuits, signal converters, etc. can also be offered. Upon customer request, so-called “Hot-Shake Tests” can be conducted to some extent. In these cases the test-items are subjected to defined vibration-tests under exhaust gas temperature conditions.

 Differential Pressure Sensors – Application in filter systems and cogeneration plants (CHP)

 The subject of particulate filters becomes more and more important in areas such as retrofitting of vehicles and in cogeneration plants. With some degree of experience the loading of filters can be determined relatively accurate by means of modelling techniques. If in case of a malfunction more soot is accumulated than predicted, the best solution to detect an early rise of the exhaust gas back pressure is by means of a differential pressure sensor. In addition, the ash content remaining in the filter-system after successful regeneration can be determined more precisely.

With cogeneration plants often the exhaust gas duct is followed by a heat exchanger. Due to the intense cooling-down of the exhaust gases in this area, considerable sooting effects can occur. The differential pressure sensor can serve for determined monitoring of this type of sooting in order to help to optimize the operating time between maintenance actions.

Especially for use in an exhaust gas environment, the pressure measuring range has been sized for a max. pressure of 100 kPa. The highest operating temperature is approximately 130°C. By means of correspondingly long tubing and hoses it can be ensured, that these parameters are not exceeded.

 This sensor is fed with a stabilised 5V direct current (DC) signal, as it is provided by engine control units.

Delta-R Motorsensor can provide customers with correct power supply units for 24V DC on-board networks, as well as for 230V AC-networks.

 Useable Under Changing Flow Conditions – The Air Mass Flow Sensor PB-LMS:

 The PB-LMS air mass flow sensor works on the principle of the hot-film anemometer. For this purpose two temperature sensors are used, one of those has a low nominal resistance and works as a heater element. Now, if the non-heated element for instance measures the air temperature as 15°C, the other element (RTD) gets loaded with an electrical current in a way as to achieve a temperature difference against the non-heated element of 40°C, i.e. its actual temperature then will be 55°C. In order to keep this 55°C constant, particular amperage is needed. If the engine now is started, the air mass which passes the heated element takes away energy from it, which thus tends to cool down. Making the element part of a Wheatstone-bridge type of circuit can be used to prevent that, or in other words, an increase in amperage will help to keep the temperature difference constant. As a consequence the required amperage is a measure for the air mass flow. This type of measurement is particularly suited to achieve precise adjustment of the fuel-air ratio under constant conditions.

However, in reality rpm-ranges exist, in particular under transient operating conditions, at which the accelerated air mass sees an intake valve which has already closed and thus gets reflected. This returning air flow hits the incoming airflow for the next cylinder and thus creates pulsations which are individual to that engine. In such case the air mass flow sensor would create implausible signals. Thanks to modern semiconductor technology an air mass flow sensor can be manufactured, where two hot-film anemometers are applied mirror-inverted on an ultra-thin foil. This helps to easily determine the returning airflow.

If the voltage signal from the second sensor is higher than the one from the first, the air flow is reversed. In this case a voltage signal between 0 and 1 V DC will be measured. In case the air flows undisturbed into the engine, a signal between 1 and 5 V DC will be measured, dependent on the actual air mass flow. In this way air mass flows up to 2500 kg/hr can be covered, depending on the cross section area of the air intake system.

This type of air mass flow sensor is currently used in many vehicles of the DAIMLER group. With this type of product Delta-R Motorsensor also serves vehicle tuning companies, cogeneration plants and the after sales business as well.

 Stefan Carstens is founder and CEO of Delta-R Motorsensor GmbH, Viernheim, Germany. He can be reached under info@motorsensor.de.

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