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MKW Motors explains Selective Catalytic Reduction (Part 2 of 3)

Reducing agent

Design
The NOx sender consists of two chambers, two pump cells, several electrodes and a heater. The sensor element consists of zirconium dioxide. This substance has the characteristic that, when electrical voltage is applied, the negative oxygen ions migrate from the negative electrode to the positive electrode.

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Function of the NOx sender
The manner in which the NOx sender functions is based on oxygen measurement and can be determined from a broadband lambda probe.

Function of the first chamber
Part of the exhaust gas flows into the 1st chamber. In the 1st chamber, the oxygen concentration is reduced to enable the low nitrogen oxide content in the exhaust gas to be measured. Due to different oxygen contents in the exhaust gas and the reference cell, an electrical voltage can be measured at the electrodes. The control unit for NOx sender 2 regulates this voltage to a constant value. This value corresponds to an air-fuel ratio of lambda ~~ 1. In this case, the pump cell pumps oxygen out or in, therefore regulating the oxygen concentration in the 1st chamber to a specific value.

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Function of the second chamber
The exhaust gas flows from the 1st into the 2nd Chamber. The NOx molecules in the exhaust gas are separated at a special electrode into N2 and O2.
As the inner and outer electrodes are regulated to an even voltage of 400 mV, the oxygen ions migrate from the inner to the outer electrode. The oxygen pump current which flows is a measure of the nitrogen oxide content in the 2nd chamber. As the oxygen pump current is proportionate to the nitrogen oxide content in the exhaust gas, it can be used to determine the nitrogen oxide content.

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Effects of signal failure
In the event of signal failure, a fault is entered in the engine control unit’s fault memory; the exhaust emissions warning lamp (MIL) and the AdBlue® warning display for system faults in the dash panel insert display are switched on.

The NOx sender is only switched on, together with its heater, when it is ensured that no condensation is able to destroy the sensor ceramic. To achieve this, the temperature in the exhaust system must be so high that the dew point temperature of water is exceeded and that no further condensed water can be found in the sensor.

AdBlue® reducing agent
The ammonia required to reduce the nitrogen oxides is not used in its pure form, but in the form of a urea solution. In its pure form, ammonia acts to irritate the skin and mucous membranes, and additionally has a penetrating odour. A fluid, which is universally referred to under its brand name, AdBlue®, throughout the automotive industry, is used as the reducing agent for the SCR system.
AdBlue® is a very pure, transparent 32.5 % solution of urea in water. It is manufactured synthetically.

pic4Freezing point of AdBlue®
AdBlue® has a urea content of 32.5 % , because the reducing agent has the lowest freezing point of –11 °C at this mixture ratio. Deviation from the mixture ratio due to an excessively high urea or water content causes the freezing point of AdBlue® to be raised.

Characteristics of AdBlue®
● AdBlue® freezes at temperatures below –11 °C.
● AdBlue® breaks down at high temperatures (approx. 70 °C – 80 °C) . This results in the formation of ammonia and therefore possible odour pollution.
● Contamination with foreign substances and bacteria may render AdBlue® unusable.
● Leaked and crystallised urea causes white flecks. These flecks must be removed with water and a brush (as soon as possible).
● AdBlue® has a high creep capability. Electrical components and connectors must be protected against AdBlue® penetration.

Notes on handling AdBlue®:
● Only use AdBlue® from original containers according to the manufacturer’s approved standard.
● Drained AdBlue® must not be reused in order to avoid contamination.
● The reducing agent tank must only be filled using the containers and adapters approved by the manufacturer.
● The reducing agent can irritate the skin, eyes and respiratory organs. If this fluid comes into contact with skin, it should be immediately washed off with plenty of water. If necessary, seek medical advice.

Reducing agent tank system

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● Reducing agent delivery module – the delivery module contains the sensors and actuators for delivering the reducing agent in the reducing agent tank system.
● Reducing agent level evaluation unit – the evaluation unit measures the level in the reducing agent tank.
● Control unit for reducing agent heater – the control unit actuates the reducing agent tank system heaters.
● Polypropylene foam insulation – the insulation serves to protect the reducing agent from high and low ambient temperatures.
● Breather and ventilation system – the reducing agent tank system is a system which is virtually sealed from the outside. This is achieved by means of a sintered material located in the breather and ventilation system connections.
Reducing agent tank pressure compensation therefore always takes place over a long period of time.

When filling the reducing agent tank, it must be ensured that a sufficiently large expansion volume remains for the reducing agent in the tank. Only ever fill the reducing agent tank with the intended containers and filling systems. This guarantees the correct filling speed and prevents overfilling the tank. In addition, all gases from the tank are trapped in the containers and do not therefore enter the atmosphere.
Never use a funnel or the like to fill the reducing agent tank, and do not independently pour reducing agent into a refill bottle for filling the tank!

Delivery module

Schematic layout of the reducing agent tank system

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Reducing agent delivery module
The reducing agent delivery module is secured with locking lugs in the tank and a locking ring on the upper side of the tank. The following components are integrated into the delivery module:

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Heater pot

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The reducing agent is sucked out of what is called the heater pot via a suction lance and a filter by the reducing agent pump. The filter is intended to avoid damage to the SCR system caused by particles of dirt in the reducing agent. A heater in the heater pot ensures that SCR operation is possible even at low exterior temperatures. The reducing agent flowing back from the pump drips back into the heater pot on the outer side of the suction lance.
The reducing agent enters the heater pot from the tank via splash slots. At low temperatures, the splashing motion of the reducing agent from the heater pot thaws out the frozen reducing agent in the tank.

Pump for reducing agent
The pump for reducing agent is a diaphragm pump. It is driven by a brushless direct current motor. The pump for reducing agent is integrated into the delivery module housing and is actuated by the engine control unit.

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Tasks
The pump for reducing agent’s tasks differ depending on the position to which the reversing valve is switched:
● When the ignition is switched on and the operating conditions for the SCR system are met, the pump delivers the reducing agent from the tank to the injector at a pressure of approx. 5 bar.
● When the diesel engine is switched off, it pumps the reducing agent out of the delivery line from the injector and back into the tank.

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How it works
The engine control unit actuates the motor with a pulse-width-modulated signal. The motor drives the diaphragm pump via a connecting rod. When the SCR system is active, the reducing agent is sucked out of the tank via the diaphragm pump and is pumped into the delivery line.

Effects in the event of failure
If the pump for reducing agent fails, the SCR system is unable to function. The exhaust emissions warning lamp (MIL) and the AdBlue® warning display for system faults in the dash panel insert display are switched on.

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Pressure sender for reducing agent metering system
The pressure sender for reducing agent metering system is screwed into the delivery module. It determines the current for the reducing agent delivery pressure and transmits a voltage signal to the engine control unit.

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Signal use
Based on the signals, the engine control unit calculates the current reducing agent pressure in the line to the injector. This enables the engine control unit to regulate the pump motor’s speed and therefore adapt the pump’s required delivery rate.

pic13Effects of signal failure
If the signal from the pressure sender for reducing agent metering system fails, reducing agent metering is switched off. The SCR system is unable to function. The exhaust emissions warning lamp (MIL) and the AdBlue® warning display for system faults in the dash panel insert display are switched on.

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Published on: May 6, 2015

Filled Under: Technical Articles

Web site: http://mkwmotors.boschauto.co.uk/

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