SCR catalytic converter and AdBlue:
In the exhaust of certain modern diesel engines there is a “Selective Catalytic Reduction” catalytic converter. Together with the AdBlue dosing system, this provides exhaust aftertreatment. The aim, just like with the EGR, is to reduce the emissions of NOx (nitrogen oxides). NOx is formed at high combustion temperatures.
The operation is as follows: the NOx in the exhaust gases is stored in the SCR catalytic converter. The SCR catalytic converter is placed in the exhaust behind the diesel particulate filter. When a certain amount of NOx has been stored in the catalytic converter, AdBlue is injected into the exhaust by an injector. Immediately after injection, the hydrolysis process begins; this is a splitting of a chemical compound by a reaction with water. During this hydrolysis process, the AdBlue is converted into ammonia and carbon dioxide. The ammonia and carbon dioxide then enter the SCR catalytic converter.
In the SCR catalytic converter, the ammonia reacts with the NOx (nitrogen oxides). The harmful substances in NOx are converted into the harmless substance nitrogen (N2) and water (H2O). Before the nitrogen and water enter the outside air through the exhaust, the NOx sensor will measure how much NOx is still present in the exhaust gases. This is described in the next paragraph.
By using AdBlue, diesel engines can comply with the Euro 5 and 6 standards.
The AdBlue is stored in a separate tank elsewhere in the vehicle. This can be either in the engine compartment or near the fuel tank.
The average consumption of AdBlue is approximately 3 to 5% of diesel consumption. Therefore, AdBlue needs to be topped up less frequently than refuelling is required.
When the AdBlue tank is almost empty, a warning light or message will appear on the dashboard. This happens several thousand kilometres before the AdBlue tank is actually empty, so that the driver has time to refill the system. If the driver ignores the warning and the tank runs completely empty, there is a chance that the engine electronics will prevent the engine from being started. When there is no AdBlue left in the tank, the vehicle no longer meets the environmental requirements. As soon as the system recognises that there is AdBlue in the tank again, the engine can be started.
Injecting AdBlue:
From the storage tank, the AdBlue is pumped by a pump to the AdBlue injector at a pressure of about 5 bar. This high pressure is needed to ensure that the AdBlue atomises properly into the exhaust gas. The injector is controlled by the engine electronics using a PWM signal.
The engine electronics determine the amount of AdBlue to be injected. The dosing must be as accurate as possible. The risks of incorrect dosing are as follows:
- injecting too little: not all NOx is converted.
- injecting too much: emission of harmful ammonia because the chemical reaction in the SCR catalytic converter has not sufficiently taken place.
The NOx sensor behind the SCR catalytic converter measures the NOx present. When the amount of NOx is too high, the engine management system will ensure that a bit more AdBlue is injected. Nowadays an ammonia sensor is also used. The excess ammonia is measured by this sensor, so that the amount of AdBlue to be injected is reduced.
Refilling AdBlue:
AdBlue must be refilled periodically. As described in the previous paragraph, you cannot wait too long to refill when the AdBlue tank is almost empty. The driver is responsible for checking the fluid level and topping it up.
In passenger cars, the filler openings can be located in several places. In exceptional cases, the AdBlue filler opening is located behind the rear bumper, but usually it is located behind the fuel flap next to the fuel cap, or in the engine compartment. The filler opening can be recognised by the blue screw cap.
AdBlue is always supplied in a sealed bottle. Contact between AdBlue and the outside air should be prevented as much as possible. There is moisture in the outside air. When AdBlue comes into contact with moisture, it can start to crystallise. That is why a threaded connection is possible between the bottle and the filler opening, as can be seen in the image.
The bottle must be screwed onto the vehicle’s filler opening. By pressing the bottle against the spring force of the screw cap, the AdBlue slowly flows through the filler opening into the vehicle’s tank. You can top up until the bottle no longer empties; then the AdBlue level in the vehicle’s reservoir is at maximum.
