Lambda probe
The lambda probe (λ probe) compares the residual oxygen content in the exhaust gas with the oxygen content of a reference, usually the current atmospheric air. From this, the combustion air ratio λ (ratio of combustion air to fuel) can be determined and thus adjusted. Two measuring principles are used: voltage of a solid electrolyte (Nernst probe) and change in resistance of a ceramic (resistance probe).
The lambda probe is the main sensor in the lambda control loop for catalytic exhaust gas cleaning (colloquially: regulated catalytic converter). The aim is to minimize the emission of pollutants such as nitrogen oxides , hydrocarbons and soot .
Lambda sensors are mainly used in gasoline engines , but also in the exhaust gas control of wood chip heating systems , biomass heating systems , pellet heating systems , gas boilers and diesel engines.
Series production began in 1976 after seven years of research, when Bosch supplied these for the USA variants of Volvo's 240/260 passenger car models .
Function of the Nernst probe / voltage jump probe
The Nernst probe (named after Walther Nernst ) uses zirconium dioxide ( zirconium (IV) oxide ) as a membrane . The property of zirconium dioxide is used to be able to transport oxygen ions electrolytically at temperatures from approx. 350 ° C, which creates a voltage between the external electrodes. With this property, zirconium-based oxygen sensors determine the difference in the oxygen partial pressure (~ O 2 concentration difference) of two different gases. With the lambda probe, one side of the membrane is exposed to the exhaust gas flow, while the other side is on an oxygen reference. The ambient air is used as reference for the lambda probe. This is either brought directly to the probe through an opening or via a separate supply line, which makes possible reference air poisoning by CO 2 , CO, water, oil or fuel vapors more difficult. In the case of reference air poisoning, the oxygen content of the reference is reduced, which reduces the probe voltage. Sensors with a pumped reference do not require a separate reference gas such as ambient air, which can be poisoned. The oxygen reference is produced independently in the sensor. For this purpose, a current is passed through the membrane and oxygen is pumped out of the exhaust gas. This creates a reference of pure oxygen on the inner electrode.
In some variants of the lambda probe, zirconium is also used as YSZ ceramic (Yttria-stabilized zirconia), which among other things significantly reduces the operating temperature. Even at temperatures from around 300 ° C, the yttrium - doped zirconium dioxide membrane of the probe becomes permeable to negative oxygen ions. In all Nernst probes, the concentration difference (or partial pressure difference) leads to ion diffusion of the oxygen, consequently O 2− ions migrate from the high concentration (air) to the low concentration (exhaust gas). The oxygen atoms can diffuse through the zirconium ceramic membrane as doubly negatively charged ions. The electrons required to ionize the oxygen atoms are supplied by the electrically conductive electrodes. This enables an electrical voltage to be picked up between the platinum electrodes attached inside and outside , the probe voltage . This is forwarded to the engine control unit via cable. With λ = 1 it is between 200 and 800 mV (optimally around 450 mV), in the range with λ> 1 (lean mixture, too much air) below 200 mV, with λ <1 (rich mixture, too much fuel) above 800 mV. The voltage is described by the Nernst equation . In a very narrow transition area around λ = 1, between 200 and 800 mV, the so-called λ window, the characteristic curve is extremely steep and non-linear. The voltage there changes almost abruptly as a function of the air-fuel ratio, which on the one hand enables precise, but on the other hand, no continuous regulation of the gas mixture at the operating point . This is why this is often implemented using a simple two-position controller .
Function of the resistance jump probe
The resistance jump probe is used much less frequently. The sensor element consists of a semiconducting titanium dioxide ceramic. The charge carriers are made available by oxygen vacancies that act as donors . With surrounding oxygen, the defects are filled and reduce the number of free charge carriers. The oxygen ions do not contribute significantly to the conductivity, but the oxygen reduces the number of free charge carriers. If the oxygen concentration is high, the sensor material has a high resistance. The electrical conductivity σ in the working range is described by an Arrhenius equation with an activation energy E A :
The signal is generated by a voltage divider with a fixed resistor .
A characteristic feature is the large reduction in the electrical resistance coefficient between the narrow range from the rich (lambda 0.98) to the lean mixture (lambda 1.02) to about 1/8 of the original value.
Use in engines
In gasoline engines, the probe is usually screwed into the exhaust manifold or the manifold just behind it. In vehicles with high legal requirements for exhaust gas cleaning and self-diagnosis, several probes are used (see monitor probe ), in V-engines usually one probe per cylinder bank , up to one probe per cylinder for selective cylinder control.
function
The correct lambda ratio is an important parameter for controlling the combustion and for enabling exhaust gas cleaning by the three-way catalytic converter. In the vehicle sector, the lambda probe first established itself in the USA and subsequently also in Europe due to legal restrictions on exhaust emissions.
In the classic Otto engine, a so-called jump probe (Nernst probe) or λ = 1 probe is used for lambda measurement. The name "jump probe" is derived from the behavior of the probe signal during the transition between a rich mixture (λ <1) and a lean mixture (λ> 1). The signal from the lambda probe makes a characteristic jump at these transitions.
construction
The first lambda probes were built as finger probes . The actual sensor element is shaped like a cap, with the exhaust gas outside and the reference air inside.
The sensors are increasingly being built up in planar technology from several layers in which the probe heating is already integrated for a faster start of the regulated operation.
The ceramic element (e.g. made of zirconium dioxide, ZrO 2 ) is surrounded by a so-called protective tube. It makes it easier to keep the sensor element at the desired temperature and prevents mechanical damage. The protective tube is provided with holes for gas access.
regulation
The lambda probe constantly compares the residual oxygen content in the exhaust gas with the oxygen content in the air and sends this value as an analog electrical signal to a control unit , which, together with other parameters, generates a control signal for mixture formation, which in gasoline engines generally results in the adjustment of the injection quantity ( lambda control ). In OBD vehicles, the function of the regulating lambda probe and monitor probe must be monitored by the control unit. Monitoring takes place sporadically. The control unit monitors:
- the voltage swing (max. 300 mV control probe)
- the amplitude
- the control frequency
- Interruption of the heating coil
- Ground connection
In the event of a malfunction, the control unit activates the MIL lamp ( engine control lamp ).
Regulation with lambda model
A continuous lambda signal is required for engine control. If there is an air mass signal in the engine electronics, the lambda value can be calculated from this and the injection quantity. However, such lambda models are only partially accurate. Satisfactory control is possible if the model is combined with a jump probe and the drift is corrected for every jump (λ = 1).
Defects
In the event of a malfunction or a defect in the probe, the motors will consume an extremely high amount of fuel, since the mixing ratio can usually no longer be adjusted correctly. In addition, the CO 2 emissions increase.
Diesel engines and the so-called lean Otto engines are not or only rarely operated in the λ range one. The diesel engine, in particular, is a classic lean concept that always runs with excess air (λ> 1) (black-smoking diesel engines usually require maintenance, are defective or the injection quantities have been manipulated by chip tuning ). The λ = 1 probe cannot be used to regulate the diesel engine and the lean Otto engine, as its signal behavior in the rich or lean cannot be evaluated (with reasonable effort).
Broadband lambda probe
A broadband lambda probe, which is a variant of the simple lambda probe based on zirconium that was introduced by Robert Bosch GmbH in 1994 , is used in gasoline direct injection and diesel engines . Simple lambda probes have their limits when z. If, for example, the mixture composition in the gasoline engine changes from rich to lean and the lambda voltage is measured, it can be seen that at λ = 1 there is an abrupt voltage drop from approx. 0.8 V to approx. 0.2 V. Such probes are therefore only suitable for measuring the mixture composition in the value range around λ = 1 and can only be used there (in the lambda window of the sensor ≈ range 0.98 and 1.02) for precise metering of the injection quantity. For example, with gasoline direct injection engines, this measuring range is not sufficient because they are operated in the following three operating modes:
- Lean : λ> 1, in the partial load range to reduce consumption
- Balanced ( stoichiometric ): λ = 1, in the full load range for performance optimization
- Fat : λ <1, for regeneration of the NO x catalytic converter
The broadband lambda probe was therefore developed for these purposes. It is suitable for lambda values of 0.8 and higher. The construction of such a probe is much more complex. It is made up of several layers using planar technology and has an integrated heater (black). Three parts are decisive for the measuring principle:
- the pump cell (pink) between the exhaust gas and the measuring gap / measuring gas,
- the diffusion channel (blue) leads through the pump cell between exhaust gas and sample gas and
- the Nernst cell (green) between sample gas and reference gas (air).
The oxygen content of the measurement gas in the measurement gap is determined on the one hand by the exhaust gas, which acts through a diffusion channel, and on the other hand is influenced by the current flow of the pump cell. Depending on the polarity, the pump current pumps oxygen from the exhaust side of the zirconium membrane into or out of the measuring gap. The pumping current is regulated by an external controller in such a way that the lambda value in the measurement gas precisely compensates for the oxygen flow through the diffusion channel and keeps the measurement gas in the measurement gap constant at λ = 1. A lambda value of 1 is always given when the voltage on the Nernst cell is 0.45 V. In the case of a rich mixture, the pump current pumps oxygen ions into the measuring gas in the measuring gap, in the case of a lean mixture out. The exhaust gas lambda can be determined via the sign and the size of this current. The current is regulated by its own control chip in the engine control unit .
Broadband lambda sensors are also essential components of NO x sensors in vehicles with NO x storage catalytic converters . The nitrogen oxide content is determined there indirectly by the oxygen produced during the catalytic breakdown of nitrogen oxides.
Controller
Broadband lambda probes are also available independently of the motor control. An external controller (control) is used here, which controls the probe and regulates the probe heating and also forwards the values of the probe to a motor control unit or a display instrument. Depending on the manufacturer, different - mostly individually programmable - output types are possible. As a rule, the lambda value is converted into a voltage signal, which is then evaluated by the engine control unit. Usually a function is also integrated in which the controller simulates the signal of a jump probe. Such controllers are mostly used for motorsport applications and for freely programmable engine controls that do not have an integrated controller for a broadband lambda probe. Controllers with two outputs are often used in tuned vehicles. One output is connected to the engine control unit to simulate the standard (jump) probe; a display is connected to the second output, with which the driver can constantly check the lambda value.
Probe heating
Since the temperature is far below 300 ° C when the engine is cold, the probe and thus the control do not work or only work very slowly during a cold start . That is why almost all newer probes are equipped with an electrical heating element, which brings the probe to the required temperature shortly after a cold start. This makes it possible to ensure emission-optimized operation as early as the engine's warm-up phase. The optimal working temperature for λ = 1 probes is between 550 and 700 ° C. Broadband types are operated 100 to 200 ° C hotter.
Electrical connection
In order to avoid malfunctions and malfunctions in the sensitive control system due to voltage fluctuations, the common vehicle ground is no longer used as a negative line for heating and probe voltage, but rather separate sensor connection cables for signal and ground that lead directly to the electronic control unit.
Monitor probe
For engines with on-board diagnosis , i.e. in the USA from 1988, in the EU for gasoline engines from 2000, a second lambda probe must be used to monitor the function of the catalytic converter . The monitor probe is located behind the catalytic converter and is a jump probe in the three-way catalytic converter. The engine control unit can thus compare the oxygen content of the exhaust gas before and after the catalytic converter. Different measurement methods based on the oxygen storage or the conversion of the catalytic converter are used.
A fully functional catalytic converter has a strong damping and delay in the fluctuations in front of the catalytic converter. The control unit records these values and uses them to calculate a quality value for the catalytic converter. If this falls below the minimum value, a corresponding message is stored in the fault memory and the driver is informed of the malfunction by means of the engine control lamp .
In addition to the catalytic converter diagnosis , the monitor probe is also used to improve the accuracy of the first lambda control and to check the plausibility of the first probe as part of the self-diagnosis .
Use in domestic heating technology
When using boilers , a lambda probe can measure the oxygen content of the flue gas and thus regulate an optimal mixture in the boiler in order to prevent an oversupply of cooling supply air or carbon monoxide (with unused residual calorific value) resulting from a lack of oxygen , which would rob the heating system of energy. The greater the distance between the flame and the probe, the more difficult the control becomes because of the dead time that then occurs . It is therefore important to mount the probe as close as possible to the combustion chamber. With a suitable control system, λ = 1.03 can be achieved in continuous operation regardless of external influences. As a rule, devices in Europe are set with a λ> 1.3 in order to comply with the pollutant emissions of the standards DIN EN 15502 boilers for gaseous fuels and DIN EN 676 forced-draft burners for gaseous fuels. The reduction in emissions results from the increased cooling of the combustion gases by the air supplied (reduction of thermal nitrogen oxides ). The proportion of unburned hydrocarbons and carbon monoxide is also reduced, since more oxygen molecules are present in the combustion mixture and the probability of a complete reaction of fuel and oxygen to form carbon dioxide and water increases.
Since the voltage increase in the vicinity of λ = 1 is strongly non-linear and can lead to control oscillations, voltage values below 0.1 V are limited. In practice, wood heating systems and gas heating systems with lambda sensors are used. The signal from the lambda probe is used to regulate the speed of the fan or the amount of fuel supplied.
Use in ovens
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Household ovens have been available since 2014, which can recognize the state of cooking by determining the amount of moisture released by the food. Since it is not possible to measure the air humidity with conventional sensors due to the temperatures in an oven, a lambda probe based on zirconium oxide is used indirectly to measure the air humidity. The oxygen content in the cooking space - measurable by the lambda probe - is reduced by the water vapor produced during the cooking process. By evaluating the signals from the lambda probe, algorithms enable conclusions to be drawn about the humidity in the oven. The oven can control the temperature accordingly and determine when a dish is ready. The technology makes it possible, for example, to bake a cake without setting the temperature and time, the device recognizes when the baking process is complete and informs the user about it.
Since the function of a lambda probe can be irreversibly impaired by silicone, silicone baking molds must not be used in ovens equipped in this way. Likewise, the oven door must not be opened during a sensor-controlled cooking process, as the atmosphere in the cooking space is falsified by the ingress of ambient air.
literature
- Hans Jörg Leyhausen: The master's examination in the automotive trade part 1. 12th edition, Vogel Buchverlag, Würzburg, 1991, ISBN 3-8023-0857-3 .
- Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6 .
- Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition, Verlag Gehlen, Bad Homburg vor der Höhe, 2000, ISBN 3-441-92250-6 .
- Robert Bosch GmbH (ed.); Konrad Reif (author), Karl-Heinz Dietsche (author) and 160 other authors: Kraftfahrtechnisches Taschenbuch . 27th, revised and expanded edition, Vieweg + Teubner, Wiesbaden 2011, ISBN 978-3-8348-1440-1 .
See also
Individual evidence
- ↑ Biokompakt Heiztechnik GmbH (Ed.): Control engineering | Lamda microcomputer control . ( biokompakt.com [accessed November 17, 2016]).
- ↑ Control engineering | Lamda microcomputer control . In: Biokompakt GmbH . ( biokompakt.com [accessed November 17, 2016]).
- ↑ Elektronikpraxis , No. 3/2016, Vogel Business Media (ed.), Würzburg 2016
- ^ Robert Bosch GmbH: 30 Years of Bosch Lambda Probe ( Memento from November 21, 2008 in the Internet Archive ) . Press release 5205, February 2006.
- ↑ Richard van Basshuysen (Ed.): Otto engine with direct injection - process, systems, development, potential . 3. Edition. Springer Vieweg, Wiesbaden 2013, p. 231 f ., doi : 10.1007 / 978-3-658-01408-7 ( springer.com [accessed on August 16, 2016]).
- ^ Robert Bosch GmbH (ed.): Kraftfahrtechnisches Taschenbuch . 25th edition. Vieweg, 2003, ISBN 3-528-23876-3 , pp. 584 f .
- ↑ Patent application DE102012210749A1 : Cooking device with sensor for cooking space. Registered on June 25, 2012 , published on January 2, 2014 , applicant: BSH Bosch und Siemens Hausgeräte GmbH; Robert Bosch GmbH, inventor: Dr. Lothar Diehl; Dr. Christoph Peters; Harald Pfersch; Hans-Jürgen Bauer; Dr. Frank Stanglmeier.