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  • SCR catalytic converter and AdBlue
  • Injecting the AdBlue
  • Top up AdBlue

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 ensures an exhaust gas after-treatment. The aim is, as with the EGR, around the emissions of NOx (nitrogen oxides). NOx is formed at high combustion temperatures.

It works as follows: the NOx in the exhaust gases is stored in the SCR catalytic converter. The SCR catalytic converter is located behind the particulate filter in the exhaust. As soon as a certain amount of NOx is stored in the catalytic converter, AdBlue is injected into the exhaust with an injector. The hydrolysis process starts immediately after the injection; 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 dioxides. Then the ammonia and carbon dioxides end up in 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 are released into the atmosphere through the exhaust, the NOx sensor measure how much NOx is still present in the exhaust gases. This is described in the next section.

The use of AdBlue enables diesel engines to comply with the standards of Euro 5 and 6.

The AdBlue is located 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 the diesel consumption. This means that AdBlue needs to be topped up less often than it needs to be refueled.
When the AdBlue tank is almost empty, an indicator light or message will appear on the dashboard. This happens several thousand kilometers before the AdBlue tank is actually empty, giving the driver time to top up the system. If the driver ignores the message and the tank is completely empty, there is a chance that the engine electronics will prevent the engine from being started. When there is no longer any AdBlue in the tank, the vehicle no longer meets the environmental requirements. As soon as the system detects that there is again AdBlue in the tank, the engine can be started again.

Injecting the AdBlue:
From the storage tank, the AdBlue is pumped by a pump under a pressure of approximately 5 bar to the AdBlue injector. This high pressure is necessary to allow the AdBlue to be properly atomized in the exhaust gas. The injector is controlled by means of a PWM signal via the engine electronics.

The engine electronics determine the amount of AdBlue to be injected. The dosage should be as accurate as possible. The dangers of incorrect dosage are as follows:

  • Inject too little: not all NOx is converted.
  • over-injection: emission of harmful ammonia because the chemical reaction in the SCR catalytic converter is insufficient.

De NOx sensor behind the SCR catalytic converter measures the NOx present. If the amount of NOx is too high, the engine management system will ensure that more AdBlue is injected. Nowadays an ammonia sensor is also used. The excess of ammonia is measured by this sensor, so that the amount of AdBlue to be injected is reduced.

Top up AdBlue:
AdBlue must be topped up periodically. As described in the previous section, you should not wait too long to top up if the AdBlue tank is almost empty. The driver is responsible for checking the fluid level and refilling.

In passenger cars, the filler holes can be located in a number of places. In exceptional cases, the AdBlue filling opening is located behind the rear bumper, but usually it is located behind the fuel filler flap next to the fuel filler cap, or in the engine compartment. The filling opening can be recognized by the blue screw cap.

AdBlue is always in a closed bottle. It should be prevented as much as possible that the AdBlue comes into contact with the outside air. There is moisture in the outside air. When the AdBlue comes into contact with moisture, it can crystallize. Therefore, a screw connection is possible between the bottle and the filling opening, as shown in the picture.
The bottle should be screwed onto the filling opening of the car. By pressing the bottle against the spring force of the screw cap, the AdBlue slowly flows through the filling opening into the tank of the car. Just as much can be refilled until the bottle stops emptying; then the AdBlue level in the tank of the car is at its maximum.