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Abstract: Automotive NOx sensors are mostly of the amperometric type, with two or three electrochemical cells in surrounding chambers. The very first cell electrochemically pumps O2 out of the sample so it does no interfere v the NOx measure up in the 2nd cell. Advertising sensors, accessible from several suppliers, are offered for the regulate of NOx adsorber and also SCR aftertreatment. NH3 sensors have been likewise developed for usage in SCR systems.

NOx Sensor Applications

The breakthrough of exhaust gas NOx sensors started in the 1990s. Advertisement sensors were an initial introduced in the at an early stage 2000s top top lean-burn, stratified fee gasoline passenger cars v NOx adsorbers, followed by diesel cars with NOx adsorbers and light- and heavy-duty diesel engines through urea-SCR aftertreatment.

The first generation of NOx sensors was arisen by NTK, likewise known as NGK/NTK or NGK Spark Plug (not to be puzzled with NGK Ceramics) in Japan, and was first used in 2001 in the Volkswagen Lupo 1.4 FSI. Eventually, all stratified charge gasoline engines in the Volkswagen team (1.4, 1.6 and 2.0 L) to be equipped with NOx sensors. Other OEMs, consisting of Daimler and also BMW, additionally put huge numbers of petrol engines v charge stratification ~ above the roads. ~ a couple of years, however, the use of stratified fee engines and also the associated market for NOx sensors began to decline, due to the lower than expected CO2 emissions benefits and the high price of NOx adsorber aftertreatment. Volkswagen bid farewell come stratified charge engines in 2006, and also BMW followed suit five years later. Only Daimler has ongoing to use spray-guided stratified charging in your M270/M274 engine family.

Another area that NOx sensor application has opened with the advent of NOx adsorber catalysts ~ above light-duty diesel engines. Some of the first applications consisted of the Toyota DPNR system, released in 2003, and the diesel engine Renault Espace model. The modern technology was widely embraced on diesel cars—primarily in Europe, but also in the US and other markets—including models from Volkswagen, BMW, and also Daimler. These vehicles were typically equipped through a NOx sensor after ~ the NOx storage catalytic converter.

The many recent area of NOx sensor application room urea-SCR equipment for light- and also heavy-duty diesel engines. To accomplish various OBD (on-board diagnostics) requirements, SCR systems commonly use a NOx sensor downstream the the SCR catalyst. If extreme NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction will it is in triggered, as NOx sensors space sensitive to both gases. Depending upon the SCR control strategy, another NOx sensor may be mounted in former of the SCR catalytic converter. If two sensors are installed, the conversion price of the SCR catalytic converter have the right to be easily determined.

Further development of NOx sensors is thrust by future heavy-duty engine emission requirements such as those gift proposed by CARB and the us EPA for 2027. The NOx boundaries may be lower to values as low as 0.015 g/bhp-hr, when the durability and also useful life requirements might be extended up come 850,000 miles (1,360,000 km) and 18 years. Enhanced sensor performance would certainly not just be compelled for potential changes to OBD thresholds but likewise for in-use emissions security that is gift proposed as an alternate to the an ext conventional trust demonstrations. NOx sensor modern technology would need to construct further to be able to monitor emissions at short NOx levels, end the totality duty cycle of heavy-duty vehicle operations, and also over your entire helpful life.

The most typical in-situ NOx measurement technology relies top top yttrium-stabilized ZrO2 (YSZ) electrochemical sensors <984>, comparable in construction and operating principle to broadband oxygen sensors. Advertising sensors are accessible from Continental/NGK <3737> and Bosch <3740>, while others such as Denso have sensor advancement programs <3739><3738><4158>. The YSZ sensors are discussed in information in the complying with sections.

The two last sections of this write-up cover, respectively, brand-new NOx sensor advances and ammonia sensors. The last technology, based upon the exact same YSZ electrochemical system, has been commercialized in some SCR applications, yet its use stays limited.

Principle the Operation


Commercial NOx sensors because that automotive applications are mostly YSZ electrochemical sensors the the amperometric type. Figure 1 illustrates the straightforward operating principle. The sensor supplies two or 3 electrochemical cells in surrounding chambers. The first cell electrochemically pumps O2 the end of the sample so the does not interfere through the NOx measure up in the 2nd cell. The need to remove O2 allows this kind of NOx sensor to offer a twin purpose; that can also detect exhaust O2 level.

" />Figure 1. Schematic representation of one amperometric NOx sensor

The O2 in the very first cell is reduced and also the result O ions space pumped through the zirconia electrolyte by using a predisposition of about -200 mV to -400 mV. The pumping current is proportional come the O2 concentration. The staying gases diffuse into the second cell where a to reduce catalyst reasons NOx to decompose right into N2 and O2. As with the very first cell, a bias of -400 mV applied to the electrode dissociates the resulting O2 which is climate pumped the end of the cell; the pumping current of the second cell is proportional come the amount of oxygen native the NOx decomposition. An additional electrochemical cell have the right to be used as a Nernstian lambda sensor to help control the NOx sensing cell <3741>.

All HC and CO in the exhaust gas should be oxidized before the NOx sensing cabinet to protect against interference. Also, any NO2 in the sample have to be converted to NO prior to NOx sensing to ensure the sensor calculation is proportional to the lot of NOx.

Solid Zirconia Electrolyte

A variety of zirconia formulations doped with steel oxides have been investigated for use in oxygen (λ, lambda), as well as NOx sensors. Products that have been tested include Fe2O3, Co3O4, NiO, CuO, ZnO, CeO2, La2O3, Y2O3, and mixtures the zeolites, aluminum and also silicates <3892><3894><3893>. Number of chemical aspects were also selected as potential electrode materials, consisting of platinum, rhodium and also palladium.

The system that has actually been most widely adopted and offered in virtually all commercial NOx and lambda sensors is based upon solid state yttrium-stabilized zirconia electrolyte (the same product that was provided in the Nernst lamp). A vital property the the YSZ ceramics is that is high conductivity because that O2 ions at elevated temperatures. The stabilization v yttrium has two benefits: (1) the impedes ZrO2 step transformation, which rises the mechanical stamin of the material, and also (2) it boosts the oxygen ion conductivity of zirconia.

Zirconium oxide ceramics can have among three crystalline phases, relying on the temperature <3891>:

Monoclinic crystal structure at room temperaturesTetragonal crystal framework from 1,170°CCubic crystal structure from 2,370°C

The cubic crystal structure screens a specifically regular plan of elements, and is characterized by high oxygen ion conductivity. V the addition of metal oxides, the high temperature crystal structures can remain secure at reduced temperatures. By adding sufficient amounts of yttrium oxide (Y2O3) in a sintering procedure at roughly 1,000°C, that is possible to cubically stabilize zirconium oxide.

If the yttrium oxide quantities are as well low, mixed crystals form, consist of of the monoclinic and cubic phase. These partially stabilized zirconium oxide (PSZ) materials feature a express resistance to heat fluctuations.

Two types of YSZ ceramics, 4YSZ and also 8YSZ, are the basis of practically all lambda and nitrogen oxide sensors. These designations show the level of doping with yttrium oxide, together follows:

4YSZ—partially stabilized ZrO2 doped v 4 mol% of Y2O38YSZ—fully stabilized ZrO2 doped v 8 mol% the Y2O3

When zirconia is stabilized with yttrium oxide, the Y3+ ions replace Zr4+ in the atomic lattice. This way, two Y3+ ion generate one oxygen gap. This gaps room utilized because that the deliver of oxygen.

The preferably oxygen ion conductivity is observed within the temperature range from 800°C come 1,200°C. Unfortunately, at these temperatures a separation additionally occurs right into Y-lean and Y-rich areas. This process is irreversible and also results in a significant reduction in oxygen conductivity. At 950°C, O2 conductivity have the right to be reduced by as lot as 40% ~ 2,500 hours <3891>. This is the factor why lambda and NOx probes may not be based on temperatures over approximately 930°C. Nitrogen oxide sensors through Continental, because that example, room operated in ~ 800°C <2827>.

Oxygen Pump Cells

If a dividing wall surface made the YSZ ceramics is placed between two chambers with various oxygen partial pressure, naught will happen at room temperature. However, as soon as the temperature the the ceramic wall is increased to about 600°C, oxygen ions can move v the gaps in the decision lattice. An alignment bring away place, wherein the chamber v the higher partial push pushes oxygen ions v the wall surface to the chamber v the lower pressure.

If both surfaces of the dividing wall are fitted with an electrode, that is possible to verify the movement of ions with voltage measurement. And this is specifically what happens in the binary (switching) lambda sensor. The reduction of oxygen to O2- the occurs in the chamber of a higher O2 pressure is explained by Equation (1):

O2 + 4e- = 2O2-(1)

and the sensor voltage is provided by the Nernst equation:

Us = (RT/4F) ln(pref / pexh)(2)

where:Us - sensor signal, VT - temperature, Kp - partial pressure of oxygenR - gas consistent = 8.314 J/molF - Faraday consistent = 96,485 sA/mol

The chart in figure 2 gift the chamber with high oxygen partial press as the blue-colored area, and also the chamber through low oxygen partial pressure as the gray area. If the brown-colored ceramic is heated come 600°C, the micro-porous platinum electrodes presented in yellow will generate roughly 1V.

number 2. Schematic that a hard zirconia electrolyte cell

Passive Cells. The chamber v the high partial press of oxygen is the recommendation air duct. Affluent exhaust gas (λ 1), the oxygen partial pressure difference relative come the reference air is low and also a signal of only 0.1V or much less is measured. In ~ λ = 1, the signal voltage is around 0.4-0.5V, depending on the manufacturer and also probe model. The voltage-lambda characteristic is nearly stepwise, enabling the sensor to distinguish between two lambda values—rich and also lean—hence the ax “binary” lambda sensor.

In such operation—representative of a binary lambda probe—the generated voltage correlates v the autumn in oxygen partial pressure. The passive YSZ ceramics cell is additionally called the potentiometric or Nernst cell.

Active Cells. it is also feasible to proactively operate the probes, together is the case in broadband (linear) oxygen sensors and in the amperometric cells in NOx sensors. In active operation, no voltage is picked up top top the electrodes, but rather the electrodes are connected to a power source. In such energetic cells—referred to as “pump cells”—it is possible to “pump” oxygen ions from the oxygen-lean come the oxygen-rich next by reversing the polarity. The pumping existing provides a measure of oxygen concentration. The current-lambda characteristic is linear, which makes it feasible to measure O2 concentrations at miscellaneous air-to-fuel ratios.

NOx sensors encompass at the very least two oxygen pump cells (Figure 1)—one to remove excess oxygen native the exhaust gas, and also another to measure the concentration that oxygen released from the decomposition of NOx.

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We appreciate the assistance from Dirk Bleicker that Carit Automotive GmbH, who detailed information and images ~ above dosimeter-based NOx sensors.