Oxygen sensor#Automotive applications

The oxygen sensor ( λ sensor) is a sensor which measures the respective residual oxygen content in a combustion exhaust gas in order to set it, the ratio of combustion air to fuel can. The purpose of this rule is not necessarily in it to adjust the ratio of combustion air and fuel to a complete consumption of oxygen from the combustion air and fuel ( λ = 1). Instead of diesel engines, the exhaust gas recirculation rate is determined from the λ value in order to minimize NOx and soot. The oxygen sensor is the main sensor in the control loop lambda control for catalytic exhaust gas cleaning ( colloquially regulated catalytic converter). It uses two measuring principles: tension of a solid electrolyte ( Nernst ) and resistance change in a ceramic (resistance probe).

Lambda probes are used mainly in gasoline engines, but also in the exhaust gas control of pellet heaters, gas heaters and diesel engines.

The serial production began in 1976, when Bosch these delivered for the U.S. variants of the car models 240/260 Volvo.

Function of the Nernst / voltage jump probe

The Nernst (named after Walther Nernst ) uses zirconia ( zirconium (IV ) oxide ) as a membrane. This makes use of the property of the zirconium dioxide, at high temperature (approx. 650 ° C) to transport oxygen ions electrolytically, whereby a voltage is produced. This property determine zirconium - based oxygen sensors the difference in oxygen partial pressure (~ O2 concentration difference) of two different gases. A lambda probe in the side of the membrane is exposed to the exhaust gas flow, while the other side is located on a oxygen reference. At the lambda probe, the ambient air is used as a reference. This is brought by either opening directly on the probe or on a separate supply, creating a possible reference air poisoning by CO 2, CO, water, oil or fuel vapors is difficult. At a reference air poisoning of the oxygen content of the reference is reduced so that the probe voltage becomes smaller. For sensors with a pumped reference ( zirconia sensors with metallic sealed reference / MSRS ), no separate reference gas such as ambient air is required, the oxygen reference is here made ​​independently within the sensor, for this purpose, the membrane is used as a pump, and producing a substantially oxygen -free reference in sealed portion.

In some variants of the lambda probe zirconium comes as YSZ ceramics ( yttria stabilized zirconia ) is used, which among other things, the operating temperature is markedly reduced. At temperatures from about 300 ° C, the yttrium -doped zirconia membrane of the probe for negative oxygen ions is continuous. For all Nernstsonden it comes by the difference in concentration (or partial pressure ) to a diffusion of the oxygen ions, thus wander O2 - ions from the high concentration ( air) to low concentration ( exhaust ). The oxygen atoms can diffuse so as double negatively charged ions through the membrane of zirconium ceramic. The electrons necessary for the ionization of the oxygen atoms are supplied by the electrode electrically conductive. This can be between the inside and outside mounted platinum electrodes, an electrical voltage decrease, the probe voltage. This is transmitted via cable to the engine control unit. It is at λ 1 = 200 to 800 mV ( optimally at about 450 mV), in the range λ > 1 (lean mixture, too much air) at 200 mV where λ <1 (rich mixture, too much fuel ) via 800 mV. The voltage is described by the Nernst equation. In a very narrow transition region around λ = 1, between 200 and 800 mV, the so-called λ - window, the curve is extremely steep and non-linear. The voltage changes there in relation to the air -fuel ratio almost leaps and bounds, creating a continuous regulation of the gas mixture is difficult. Therefore, it is often replaced by a simple two-point regulator.

Function of the resistance jump probe

Much less frequently, the resistance jump probe is inserted. The sensor element is made of a semiconductive titanium dioxide ceramic. The charge carriers are provided by oxygen vacancies which act as donors, are available. When the ambient oxygen vacancies are occupied and reduce the number of free charge carriers. The oxygen ions carry here is not significantly to conductivity, but the oxygen reduces the number of free charge carriers. At high oxygen concentration, the sensor material has a high resistance. The electrical conductivity σ in the work area is described by an Arrhenius equation with an activation energy EA:

The signal is generated by a voltage divider comprising a fixed resistance.

A characteristic feature is the large reduction in electrical resistance coefficient between the narrow range from the rich (lambda 0.98 ) for lean mixture (lambda 1.02 ) to about 1/8 of the original value.

Use in engines

The probe is screwed in gasoline engines, usually in the exhaust manifold or the manifold just behind it. In vehicles with high legal requirements on emission control and self-diagnosis, multiple probes are used ( see Monitor probe) in V engines usually have a sensor on each cylinder bank, up to a sensor on each cylinder for selective cylinder control.

In modern gasoline engines with turbocharging the lambda probe is currently being built just behind the turbine.

Function

The correct oxygen ratio is an important parameter for control of the combustion and to allow the exhaust gas purification by the three-way catalyst. In the vehicle sector, the lambda sensor has prevailed due to the legal restriction of exhaust emissions in the U.S. first and then in Europe.

In the traditional gasoline engine, a so-called jump probe ( Nernst ) or λ = 1 probe for lambda measurement is used. The name " jump probe " is derived from this behavior of the probe signal at the transition between a rich mixture ( λ <1) and a lean mixture ( λ > 1). The signal of the lambda sensor makes these transitions a characteristic jump.

Construction

The first lambda probes were constructed as finger probes. The actual sensor element is shaped like a hat, with the exhaust outside and the reference air inside.

Increasingly, the sensors are constructed using planar made ​​of several layers, where the probe heater is already integrated.

The ceramic element (eg zirconia ZrO2) is surrounded by a so-called protective tube. It facilitates that the sensor element is maintained at the desired temperature, and prevents mechanical damage. For the gas to reach the protective tube is provided with holes.

Control

The oxygen sensor continuously compares the residual oxygen content in the exhaust gas with the atmospheric oxygen content, and forwards this value as an analog electrical signal to a control unit that generates, together with other parameters from a control signal for forming the mixture, which flows into the spark-ignition engine, in general in the adjustment of the injection quantity ( lambda regulation). In OBD vehicles, the function of the lambda control probe and monitor sensor must be monitored by the control unit. The monitoring is sporadic. The control unit monitors:

In case of malfunction of the control device, the MIL lamp ( check engine light ) is activated.

Defects

If a fault or a malfunction of the probe, it comes with the engines at an extremely increased consumption, since the mixing ratio usually can not be adjusted properly. In addition, the CO ² emissions increased.

Diesel engines and the so-called lean gasoline engines are not or only rarely operated in the λ - range one. In particular, the diesel engine is a classic lean concept, which always with an excess of air ( λ > 1 ) leaves (black smoking diesel engines are usually requires maintenance, defective, or in the injection quantities manipulated by so-called " chip tuning "). For the control of the diesel engine and the lean gasoline engines can not be used the λ = 1 probe, as their signal behavior in fats or in the lean can not be evaluated ( with reasonable effort ).

Broadband lambda probe

A variant of the simple oxygen sensor based on zirconium is the broadband lambda probe, which was presented in 1994 by Robert Bosch GmbH and is used for example in gasoline direct injection engines and diesel engines. Simple lambda sensors have their limitations, if you change eg the composition of the mixture in the gasoline engine from rich to lean and thereby measures the lambda voltage, it can be seen that there at λ = 1 an abrupt voltage drop of about 0.8 V to about 0, 2 V 's. Therefore, such sensors are only suitable for the measurement of the mixture composition in the range of λ = 1, and only there can also ( in the lambda window, the sensor area ≈ 0.98 to 1.02 ) are used for precise dosing of the injection quantity. For example, in gasoline direct injection engines of this range is not sufficient, because they are driven into the following three modes:

• Skim: λ > 1, the partial load range to reduce fuel consumption
• Balanced ( stoichiometric): λ = 1, in the full load range to optimize performance
• Bold: λ < 1, for the regeneration of the NOx catalyst

For these purposes, therefore, the broadband lambda probe was developed. It is suitable for lambda values ​​of 0.8 and higher. The construction of such a probe is much more complex. It is constructed of multiple layers in planar and has a built-in heater (black). Three parts are critical to the measurement principle:

• The diffusion channel (blue) passes through the pump cell between the exhaust gas and sample gas and
• The Nernst cell (green) between the sample gas and reference gas (air).

The oxygen content of the measurement gas in the measurement gap is on the one hand via the exhaust gas, which acts through a diffusion channel, and determines on the other hand be influenced by the current flow of the pump cell. By the pumping current depending on the polarity of oxygen is pumped from the exhaust side of the measuring gap in Zirkoniummembran and fed out therefrom. Here, the pump flow is controlled by an external controller so that the lambda value in the sample gas oxygen flow exactly balances through the diffusion channel and the sample gas is kept constant in the measuring gap at λ = 1. A lambda value of 1 is always given if the voltage at the Nernst cell is 0.45V. The pump current is pumping out at a rich mixture of oxygen ions in the sample gas into the measuring gap, with a lean mixture. About the sign and magnitude of this current, the exhaust gas lambda can be determined. The regulation of the current is carried by a separate controller chip in the engine control unit.

Broadband lambda probes are also essential components of NOx sensors for vehicles with NOx storage catalysts. The nitric oxide content is there determined indirectly by the resulting oxygen in the catalytic decomposition of nitrogen oxides.

Controller

Broadband lambda probes are also available independently of the engine control. This is an external controller (controller) is used, which takes over the control of the probe and the control of the probe heater and also passes on the values ​​of the sensor to an engine control unit or an indicating instrument. Usually individually programmable - - Output types are possible depending on the manufacturer are different. In general, the lambda value is converted into a voltage signal, which is then evaluated by the engine control unit. Most also have a function is integrated in the controller, the signal of a bistable sensor is simulated. Are used such controller mostly for motorsport applications and programmable motor controllers that have integrated not have its own controller for a broadband lambda probe. In tuned cars often come with two controller outputs are used. An output is connected to the engine control unit, simulating the standard ( jump) probe, to the second output, a display is connected, with which the driver can check the lambda value constantly.

Probe heating

Since the temperature on a cold engine is still far below 300 ° C, the probe and thus the control at cold start or only very slowly working. Therefore, almost all the modern probes are equipped with an electric heating element that brings the probe just after the cold start to the required temperature. This makes it possible to ensure emission already optimized operation in the warming-up phase of the engine. The optimum working temperature is λ = 1 probes 550-700 ° C. Broadband types are operated from 100 to 200 ° C hotter.

Electrical connection

To avoid faults and malfunctions in the delicate control by voltage fluctuations, today is no longer the common vehicle mass used as a negative lead for heating and probe voltage, but separate sensor connecting cable for signal and ground, leading directly to the electronic control unit.

Monitor probe

In recent gasoline engines, a second lambda probe, the so-called monitor probe used to monitor the function of the catalyst. The monitor sensor located behind the catalyst. The motor control device can thus compare the values ​​before and after the exhaust gas catalyst. With a fully functional catalyst, a certain proportion of the residual oxygen content of the exhaust gases is used, the respiratory poison carbon monoxide ( CO) and unburned hydrocarbons ( HC) to carbon dioxide (CO2 ) and water ( H2O) to convert. The monitor probe should therefore always measure a lower oxygen content than the control probe in a fully functional cat. The catalyst has lost its catalytic ability due to age, the same registers the monitor sensor rich-lean cycles as Vorkatsonde, depending on the degree of damage of the catalytic converter with greatly reduced voltage levels. The control unit detects this and can now put a message in the error memory and inform the driver via a warning message about a malfunction of the catalyst.

The monitor probe may be used in addition to the catalytic converter diagnosis also improve the accuracy of the first lambda control and to check the plausibility of the first probe in the context of self-diagnosis.

Use in the home heating

With the use of boilers, a lambda probe to measure the oxygen content of the exhaust gas and thus the boiler regulate an optimal mixture, so as to prevent an oversupply of cooling supply air or lack of oxygen arising carbon monoxide ( with unused residual calorific value ), which would be the heating system rob energy. The greater the distance between the flame and the probe is chosen, the more difficult it becomes to control because of the dead time then occurring. Therefore, it is important to mount the probe as close as possible to the combustion chamber. With an appropriate regime can be reached during continuous operation independent of external influences λ = 1.03. Since the voltage rise near λ = 1 is highly nonlinear and can lead to control oscillations, one has to be limited to voltages below 0.1V. In practice, wood heaters and gas heaters with Lambda probe in use. The signal of the lambda sensor is used to regulate the speed of the fan or the amount of fuel supply.