How does the Adblue SCR DEF system work? Explained Diesel Exhaust Fluid Selective Catalyst Reduction

The Adblue or Selective Catalyst Reduction (SCR) system is a solution for the exhaust gas aftertreatment used to reduce the NOx within the exhaust gas emissions.

For this purpose a specified amount of (Adblue) or Diesel Exhaust Fluid (DEF) is injected into the gas inlets of the Diesel Particulate Filter (DPF) and the SCR catalytic converter. The injected DEF into the exhaust system is converted to ammonia (NH3) and carbon dioxide (CO2). The resulting ammonia (NH3) is used within a special catalyst in the exhaust stream. The resulting reaction converts the unwanted NOx into harmless nitrogen (N2) and water (H2O) vapor.

The SCR system is equipped with LNT catalytic converter and DPF assembly is located in the downpipe from the turbocharger. An additional SCR catalytic converter is located in the exhaust center section.

The DEF is a pure, odorless, colorless, synthetically manufactured, aqueous solution. The DEF is used for the aftertreatment of exhaust gases in the DPF and the SCR catalytic converter.

The SCR catalytic converter can be contaminated by low quantities of metals and thus the quality of the DEF is held to closely controlled standards. The DEF cannot be substituted or diluted with any other fluid.

The diesel exhaust fluid is not categorized as a dangerous substance, because
· The DEF is non-flammable
· The DEF is non-toxic
· There is no danger in the event of spills.

The DEF can be stored on board vehicles, despite the limitation that it crystallizes at temperatures below -11°C (12°F).

In Europe, DEF is also known as AdBlue®, the fluid is specified by ISO22241.

The DEF tank is blow moulded from High Density Polyethylene (HDPE). The DEF tank unit contains the DEF module. The DEF tank includes a 12% unusable volume to protect the internal components because of fluid expansion under freezing conditions.

If the ambient temperature falls below -11°C (12°F), the DEF will freeze in the DEF tank and this will provide difficulty with the refill procedure. In order to thaw the DEF in the DEF tank, place the vehicle in a warm place for 2 hours before refilling the DEF.

The DEF level sensor is an ultrasonic device, which sends the DEF level value to the Powertrain Control Module (PCM) through Pulse Width Modulated (PWM) signals. The ultrasonic cone angle is 10° and the readings are most accurate when the vehicle is stationary and on level ground.

The heater element is a Positive Temperature Coefficient (PTC) type heater, which provides safe operation to the system. Increased heater element temperature results in decreased current drawn from the DEF heater control module, which actuates the power supply of the heater element. Under normal operation the maximum current is 6A.

The DEF injection pump is an assembly of 2 solenoid pumps, located in the DEF tank module. The pressure pump provides 6.5 bar (94.3 psi) operating pressure. The 6.5 bar (94.3 psi) pressure is required to maintain the complete DEF atomization in the exhaust gas. The purge pump purges the DEF from the DEF line at engine shutdown to prevent freezing in the DEF injector at low temperatures. The PCM controls the ground connection of the pumps individually.

The DEF line provides hydraulic connection between the DEF injection pump and the DEF injector. The DEF line is manufactured from a plastic material, which is specifically designed for use with DEF. A copper based resistor wire is installed within the DEF line with an electrical connector. The resistor wire enables electrical heating of the DEF at low-ambient temperatures.

A pre DPF DEF injector is located on the top of the DPF. The DEF is injected vertically to the exhaust gas flow direction. The mixer plate and the high exhaust gas speed provide the DEF is mixed well and distributed evenly within the exhaust gas. The DEF injector consists of an injector and a liquid cooled housing. The liquid cooled housing connects to the engine cooling system to protects the injector from overheating.

The DEF injectors works at high pressures to obtain the complete atomization of the injected DEF. This provides the SCR catalytic converter is working to its optimum performance. The PCM controls the DEF injector with PWM signals.

The DEF heater control module has hardwired connections to the DEF tank module and the DEF line heater element. If the ambient air temperature is below -7°C (19.4°F) the PCM switches on the DEF heater control module through the private CAN bus.

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[sinegra]

This is so far the most clear explanation I've found on the internet, thank you!

[andrewwilson]

Super video. Well explained. Helps me understand how the system works and what i need to look fur when im testing and resetting it. Thank you.

[ronaldlloyd]

Excellent video. Your video describes more of the components that make the def system rather than a lot of colorful dots changing colours.

[Mal_Outdoors]

Thank you for such a thorough explanation. It's even more complicated than I thought!

[swarajjoye]

Jesus, that was heavy but very interesting.
Will have to watch this over and over with many cups of coffee

[peteyourdoom]

Really good explanation. Have a new (old) Diesel car and the Adblue is the new thing for me, as well as the NOX sensor (which I've just had replaced). Found the filler for it in the back of the car and wondered what AdBlue was and how it works.

[nickayivor]

KNOWLEDGEABLE GarageTech
Thank you very helpful information video start to finish loved it
From Nick Ayivor from London England UK 🇬🇧

[noelf]

Good video. This is the most informative I've found on Adblue and how it works, including the actual reactions that happen.

[Legolize]

Well done, that’s a great explanation!

[xjonasxv]

Best video on the FULL explanation of adblue. Thank you so much. Inwas wondering if the adblue tank was crystallised because the line was empty. Now i know that it is pumped back in the tank.

[allan]

Imagine the power the engine could make with a strait through system !

[antohosm]

Great video! So clear and of much help to me with regards to my TDi A4. I hold a Chemistry Ph.D. but my automotive knowledge needs much improvement.

[EddieDaOnly]

Thanks for explaining I just filled up adblue in my Audi TDI

I can't wait to delete.

[markm]

Excellent video- just bought a car with this system. Always good to know how these things work.

[fastlanetuningschweiz]

Excellent video, well prepared and very clear. Thankyou.

[csengo]

Excellent excellent explanation.

This system ads so much complexity to diesel engines. Don't think electric cars are not complex. Their complexity is different, but still overly complex. Petrol is still the best way to go.

[chuckmaddison]

The best thing with Adblue is the amount of damage it can do if you put in the fuel. It's also good for making crystals

[bbagain.]

You state that 30% urea is used for its freezing point, does that mean if cold was not a concern would a higher % mean less adblue volume needed to be injected for the same results?

[foppo]

My son in laws car broke down the other day Ad Blue problem.It was repaired by Peugeot he had problems before.The AA man said to him I can't do anything with this car has to go back to the dealership.Drove him and the car to a garage.He just paid over a thousand pounds for repairs.So another thing to go horrible wrong with a modern diesel arn't we happy.

[ashishkaradage]

Could you please explain when adblie in injected at what temp threshold or based on NOx levels?