Selective Catalytic Reduction
AdBlue, Urea, Diesel Exhaust Fluid (DEF), whatever you want to call it, they are all different names for an ammonia based liquid used in Selective Catalytic Reduction (SCR) Systems.
Even with combustion technology advancements, and problematic aids like EGR, manufacturers still struggled to meet low Nitrogen Oxides (NOx) emissions targets; so saw the increase in using SCR – an effective way of reducing NOx, with minimal side effects.
During engine loads producing high NOx (the system is not utilised at idle), AdBlue is sprayed through an exhaust mounted injector upstream of a SCR catalytic converter. All going well, the NOx will react with the AdBlue and precious metals in the SCR catalyst, breaking NOx into nitrogen and water.
The rate of AdBlue injected is calculated in a similar fashion to the regular fueling system, using various inputs such as MAF, MAP, throttle position, exhaust temperature etc. However the SCR dosing amount will also be adjusted in a closed loop system using an exhaust mounted NOx Sensor – similar to how a petrol vehicle adjusts fuel trims via oxygen sensor readings. This NOx sensor allows the system to fine tune and also account for situations like AdBlue that is past its recommended usage date.
Quality AdBlue is available from various sources, each going up in price, from the bowser, in aftermarket containers or direct from dealerships. As most dealerships are now including AdBlue refill as part of their servicing, I would recommend you keep some readily available when keeping these vehicles in your customer base.
When stored in a sealed container between 5°C and 20°C, AdBlue has a 2 year lifespan from the date of production. Higher temperatures or a vented container will see a shelf life of around 6 months. A good starting point for any SCR system efficiency fault is testing the remaining Urea content of the AdBlue, commonly done with a refractometer, readily available for low cost with a quick google search.
AdBlue is not flammable and poses little risk to humans, but as with any vehicle fluids, it is wise to wear gloves and glasses when working with it.
AdBlue freezes at -11°C, and for this reason you will typically see a heater In the AdBlue tank, but also the supply line to the injector wrapped in a heating coil, protected by a conduit-like outer sheathing on the line.
If AdBlue is accidentally put into the Diesel Tank, and the key has not been turned, drain the tank immediately and clean extensively with warm water. Let it dry completely. If it’s free of crystals, it can be used again.
If the car has been started, the entire fuel system is a complete write off and often an insurance claim is the best option to replace the entire system. The AdBlue will start to corrode many metals like steel, iron, brass, aluminium & zinc. Within 12-24hrs the ammonia also begins to crystallise, a function that will easily destroy fuel systems, but on the bright side makes finding any SCR system AdBlue leak extremely easy.
The AdBlue tank level in some models, such as many VW’s, can be a series of 4 resistors at staggered heights in the tank, rather than a typical float sensor. The driver will be given increasing intensity warning messages as the level gets low, with distance until the engine will not be allowed to start estimated based on the level of AdBlue usage per kilometre previous. AdBlue usage rate, depending on driving style and vehicle size, is typically around 1 Litre per 1000km.
Alternatively a separate error message or warning light will be used if there is a fault in the system, but just the same as running out of fluid, a kilometre countdown can be given until the engine will no longer be allowed to start until the fault is rectified.
Efficiency of the system is typically monitored via pre and post SCR catalyst NOx Sensors. These heated NOx sensors will often wait until exhaust temps exceed approximately 140°C before activating their heater to achieve approximately 800° and begin operation. This 140°C ‘dew point’ avoids damaging the sensor via any moisture accumulated on it.
NOx sensors can be expensive as they are typically sold complete with their own control module. They often use SENT protocol – communication via data packets – and as such efficiency of the system is generally best analysed via scan tool. True to logic, while the system is operating correctly, NOx Parts Per Million (PPM), should have a lower reading on the Post Catalyst NOx sensor compared to the Pre Catalyst NOx sensor while AdBlue is being injected. Many manufacturers’ scan tools now include a functional test which will run the system while analysing these pre and post signals and simply give a pass or fail result.
Later model SCR systems combine the DPF and SCR catalyst in one unit, and can even perform system efficiency evaluation using just one Pre Catalyst NOx sensor, with the aid of the low pressure EGR system exhaust flap as discussed in my Issue 61 article ‘Hybrid EGR’ (available via the www.tat.net.au search bar).
These clever single NOx sensor systems will periodically close the exhaust flap on decelerations over 4 seconds, sealing any exhaust from exiting. The exhaust gas is fed back through the engine via the EGR system, while AdBlue is being injected. As the exhaust gas passes through the engine (which is basically a non-combusting air-pump during deceleration), the Pre Catalyst NOx sensor can report the drop in NOx levels compared to the seconds before to determine if the SCR system is working correctly.
Furthermore, ‘Twin Dosing’ systems were recently introduced. As ideal NOx conversion rates in SCR occur between 220°C and 350°C, having just one injector and SCR catalyst at one point in the exhaust does not always give the optimum position for different engine loads. With Twin Dosing, the first AdBlue injector and SCR catalyst mounted close to the turbo outlet is used during low speed/low exhaust gas temperature loads. During high speed motorway driving, or fully loaded/towing, temperatures close to the turbo will well exceed 350°C, so the 1st AdBlue injector is no longer used, but a second AdBlue injector further down the exhaust stream in the ideal temperature zone is operated, utilising its own downstream SCR catalyst.