How Many Many Ppm of Nox Sensor Should Read

NOx Sensors

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Abstract: Automotive NOx sensors are primarily of the amperometric type, with ii or 3 electrochemical cells in side by side chambers. The outset prison cell electrochemically pumps O_two out of the sample so it does not interfere with the NOx measurement in the second cell. Commercial sensors, available from several suppliers, are used for the command of NOx adsorber and SCR aftertreatment. NH₃ sensors take been also developed for apply in SCR systems.

• NOx Sensor Applications
• Principle of Operation
• Commercial NOx Sensors
• Issues with YSZ Ceramics Sensors
• NOx Sensor Developments
• NH_three Sensors

NOx Sensor Applications

The development of exhaust gas NOx sensors started in the 1990s. Commercial sensors were first introduced in the early 2000s on lean-burn, stratified charge gasoline rider cars with NOx adsorbers, followed by diesel fuel cars with NOx adsorbers and lite- and heavy-duty diesel engines with urea-SCR aftertreatment.

The first generation of NOx sensors was developed by NTK, also known as NGK/NTK or NGK Spark Plug (not to be confused with NGK Ceramics) in Japan, and was first used in 2001 in the Volkswagen Lupo 1.iv FSI. Somewhen, all stratified accuse gasoline engines in the Volkswagen Grouping (ane.four, 1.vi and 2.0 50) were equipped with NOx sensors. Other OEMs, including Daimler and BMW, as well put large numbers of gasoline engines with charge stratification onto the roads. After a few years, however, the employ of stratified accuse engines and the associated market for NOx sensors started to decline, due to the lower than expected CO₂ emission benefits and the high cost of NOx adsorber aftertreatment. Volkswagen bid goodbye to stratified charge engines in 2006, and BMW followed conform five years afterward. Just Daimler has continued to use spray-guided stratified charging in their M270/M274 engine family.

Another area of NOx sensor awarding has opened with the introduction of NOx adsorber catalysts on lite-duty diesel engines. Some of the start applications included the Toyota DPNR arrangement, launched in 2003, and the diesel engine Renault Espace model. The applied science was widely adopted on diesel fuel cars—primarily in Europe, just also in the United states of america and other markets—including models from Volkswagen, BMW, and Daimler. These vehicles were typically equipped with a NOx sensor after the NOx storage catalytic converter.

The nearly recent area of NOx sensor awarding are urea-SCR systems for calorie-free- and heavy-duty diesel engines. To satisfy various OBD (on-board diagnostics) requirements, SCR systems typically utilize a NOx sensor downstream of the SCR goad. If excessive NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction will be triggered, as NOx sensors are sensitive to both gases. Depending on the SCR control strategy, some other NOx sensor may be installed in front of the SCR catalytic converter. If two sensors are installed, the conversion rate of the SCR catalytic converter can be easily adamant.

Further development of NOx sensors is driven by future heavy-duty engine emission standards such every bit those beingness proposed by CARB and the US EPA for 2027. The NOx limits may be lowered to values as depression as 0.015 g/bhp-hr, while the durability and useful life requirements could be extended up to 850,000 miles (1,360,000 km) and 18 years. Improved sensor performance would not just exist required for potential changes to OBD thresholds simply also for in-utilise emissions monitoring that is existence proposed every bit an culling to the more conventional immovability demonstrations. NOx sensor technology would need to develop further to be able to monitor emissions at low NOx levels, over the whole duty cycle of heavy-duty vehicle operations, and over their entire useful life.

The most mutual in-situ NOx measurement technology relies on yttrium-stabilized ZrO₂ (YSZ) electrochemical sensors [984], similar in construction and operating principle to broadband oxygen sensors. Commercial sensors are available from Continental/NGK [3737] and Bosch [3740], while others such every bit Denso have sensor development programs [3739] [3738] [4158]. The YSZ sensors are discussed in detail in the following sections.

The two final sections of this commodity encompass, respectively, new NOx sensor developments and ammonia sensors. The latter technology, based on the same YSZ electrochemical system, has been commercialized in some SCR applications, but its employ remains limited.

Principle of Functioning

Overview

Commercial NOx sensors for automotive applications are primarily YSZ electrochemical sensors of the amperometric blazon. Figure 1 illustrates the bones operating principle. The sensor uses two or iii electrochemical cells in adjacent chambers. The offset prison cell electrochemically pumps O₂ out of the sample and so it does non interfere with the NOx measurement in the 2d cell. The need to remove O₂ allows this type of NOx sensor to serve a dual purpose; information technology tin also detect exhaust O₂ level.

Effigy one. Schematic representation of an amperometric NOx sensor

The O₂ in the commencement cell is reduced and the resulting O ions are pumped through the zirconia electrolyte by applying a bias of approximately -200 mV to -400 mV. The pumping electric current is proportional to the O_ii concentration. The remaining gases diffuse into the 2d cell where a reducing catalyst causes NOx to decompose into Due north₂ and O_two. As with the first cell, a bias of -400 mV applied to the electrode dissociates the resulting O₂ which is then pumped out of the cell; the pumping current of the 2d cell is proportional to the amount of oxygen from the NOx decomposition. An additional electrochemical jail cell can be used every bit a Nernstian lambda sensor to help command the NOx sensing cell [3741].

All HC and CO in the exhaust gas should be oxidized before the NOx sensing cell to avert interference. Also, whatever NO_two in the sample should exist converted to NO prior to NOx sensing to ensure the sensor output is proportional to the amount of NOx.

Solid Zirconia Electrolyte

A number of zirconia formulations doped with metal oxides have been investigated for use in oxygen (λ, lambda), as well as NOx sensors. Materials that take been tested include Atomic number 26₂O₃, Co_threeO_four, NiO, CuO, ZnO, CeO_ii, La_iiO₃, Y_iiO_three, as well as mixtures of zeolites, aluminum and silicates [3892] [3894] [3893]. Several chemical elements were too selected every bit potential electrode materials, including platinum, rhodium and palladium.

The organisation that has been most widely adopted and used in almost all commercial NOx and lambda sensors is based on solid state yttrium-stabilized zirconia electrolyte (the same cloth that was used in the Nernst lamp). A cardinal property of the YSZ ceramics is its loftier electrical conductivity for O₂ ions at elevated temperatures. The stabilization with yttrium has two benefits: (one) it impedes ZrO₂ phase transformation, which increases the mechanical strength of the material, and (2) it enhances the oxygen ion conductivity of zirconia.

Zirconium oxide ceramics can have one of iii crystalline phases, depending on the temperature [3891]:

• Monoclinic crystal construction at room temperatures
• Tetragonal crystal structure from 1,170°C
• Cubic crystal construction from 2,370°C

The cubic crystal construction displays a particularly regular arrangement of elements, and is characterized by high oxygen ion conductivity. Through the improver of metal oxides, the loftier temperature crystal structures can remain stable at lower temperatures. By adding sufficient quantities of yttrium oxide (Y₂O₃) in a sintering process at approximately 1,000°C, it is possible to cubically stabilize zirconium oxide.

If the yttrium oxide quantities are also low, mixed crystals form, consisting of the monoclinic and cubic phase. These partially stabilized zirconium oxide (PSZ) materials feature a pronounced resistance to thermal fluctuations.

Two types of YSZ ceramics, 4YSZ and 8YSZ, are the basis of almost all lambda and nitrogen oxide sensors. These designations indicate the level of doping with yttrium oxide, every bit follows:

• 4YSZ—partially stabilized ZrO₂ doped with 4 mol% of Y_twoO_iii
• 8YSZ—fully stabilized ZrO_two doped with 8 mol% of Y_twoO₃

When zirconia is stabilized with yttrium oxide, the Yⁱⁱⁱ⁺ ions supplant Zr^four+ in the atomic lattice. This way, ii Y³⁺ ions generate 1 oxygen gap. These gaps are utilized for the send of oxygen.

The maximum oxygen ion conductivity is observed inside the temperature range from 800°C to 1,200°C. Unfortunately, at these temperatures a separation also occurs into Y-lean and Y-rich areas. This process is irreversible and results in a severe reduction in oxygen conductivity. At 950°C, O₂ electrical conductivity can be reduced by every bit much equally 40% after ii,500 hours [3891]. This is the reason why lambda and NOx probes may not be subjected to temperatures above approximately 930°C. Nitrogen oxide sensors by Continental, for instance, are operated at 800°C [2827].

Oxygen Pump Cells

If a dividing wall made of YSZ ceramics is placed between two chambers with unlike oxygen partial pressure, nothing will happen at room temperature. However, when the temperature of the ceramic wall is increased to approximately 600°C, oxygen ions can move through the gaps in the crystal lattice. An alignment takes place, where the chamber with the higher fractional pressure pushes oxygen ions through the wall to the chamber with the lower pressure.

If both surfaces of the dividing wall are fitted with an electrode, it is possible to verify the move of ions through voltage measurement. And this is precisely what happens in the binary (switching) lambda sensor. The reduction of oxygen to O^2- that occurs in the bedroom of a higher O₂ pressure is described by Equation (one):

O_ii + 4e- = 2O^2- (1)

and the sensor voltage is given by the Nernst equation:

U_s = (RT/4F) ln(p_ref / p_exh) (ii)

where:
U_s - sensor indicate, V
T - temperature, K
p - partial pressure of oxygen
R - gas constant = 8.314 J/mol
F - Faraday constant = 96,485 sA/mol

The diagram in Effigy ii presents the chamber with high oxygen partial pressure equally the blue-colored area, and the sleeping accommodation with depression oxygen partial pressure level equally the grey area. If the chocolate-brown-colored ceramic is heated to 600°C, the micro-porous platinum electrodes presented in yellowish volition generate approximately 1V.

Figure 2. Schematic of a solid zirconia electrolyte cell

Passive Cells. The sleeping accommodation with the loftier partial pressure of oxygen is the reference air duct. Rich exhaust gas (λ < 1) has a low oxygen content. If the zirconium oxide ceramics is heated using a heating element to approximately 600°C, oxygen ions move from the reference air duct through the ceramic wall onto the frazzle gas side and about one volt signal voltage is generated. In the case of lean exhaust gas (λ > 1), the oxygen partial pressure deviation relative to the reference air is low and a signal of but 0.1V or less is measured. At λ = ane, the signal voltage is approximately 0.iv-0.5V, depending on the manufacturer and probe model. The voltage-lambda characteristic is almost stepwise, allowing the sensor to distinguish betwixt two lambda values—rich and lean—hence the term "binary" lambda sensor.

In such operation—representative of a binary lambda probe—the generated voltage correlates with the drib in oxygen fractional pressure. The passive YSZ ceramics cell is likewise chosen the potentiometric or Nernst prison cell.

Active Cells. It is likewise possible to actively operate the probes, as is the example in broadband (linear) oxygen sensors and in the amperometric cells in NOx sensors. In active operation, no voltage is picked up on the electrodes, but rather the electrodes are connected to a power source. In such active cells—referred to as "pump cells"—it is possible to "pump" oxygen ions from the oxygen-lean to the oxygen-rich side by reversing the polarity. The pumping electric current provides a measure of oxygen concentration. The current-lambda characteristic is linear, which makes it possible to measure O₂ concentrations at various air-to-fuel ratios.

NOx sensors include at to the lowest degree two oxygen pump cells (Figure 1)—one to remove excess oxygen from the exhaust gas, and another to measure the concentration of oxygen released from the decomposition of NOx.

Acknowledgements

We appreciate the help from Dirk Bleicker of Carit Automotive GmbH, who provided information and images on dosimeter-based NOx sensors.

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