What is diesel exhaust fluid?

What is diesel exhaust fluid?

05 November 2019 | Fred Cardoso | 5 min read

What is Diesel exhaust fluid aka AdBlue?

Diesel vehicle enthusiasts, long-haul truckers and fleet managers collectively had a tough moment back in 2010 when the Environmental Protection Agency mandated the use of selective catalytic reduction (SCR) in diesel engines.

Why? Because the thing that makes SCR work is a consumable fluid called diesel exhaust fluid (DEF)  – also known as AdBlue® – and owners of diesel vehicles were going to have to add it to their vehicles beside the normal fuel. Nobody likes paying more money for something that is nevertheless inconvenient.

The reality of AdBlue® and SCR turned out to be not that bad, and actually – despite the added cost of the fluid itself – the increased fuel efficiency and reduced emissions made the hassle of topping up an extra tank of liquid once in a while kind of worth it.

How exactly does SCR work and what role does AdBlue® play in making that happen?

First of all, selective catalyst reduction is not a new technology – despite only having been mandated by the EPA in the last decade or so – it has been around for nearly half a century and was firstly used in the power generation industry to reduce oxides of nitrogen from coal-fired power plants.

These oxides of nitrogen – nitrogen monoxide and nitrogen dioxide – are the big problem with diesel combustion engines and were the ones that gave Volkswagen group so much trouble around cheating on the emission tests.

In an SCR-equipped vehicle, the exhaust gases from the engine are routed first through a particulate filter to catch all the soot and ash generated from burning what is a relatively impure fuel. That takes care of the "rolling coal" aspect of old diesel engines that made them relatively unpopular in the world around the 80s.

From the particulate filter, the exhaust gas travels past a nozzle which sprays diesel exhaust fluid into the stream of gases. AdBlue® is made from deionized water and a very pure form of urea. Yes, urea is found in urine – don't laugh please – but this is a refined form of the compound and is mostly used in the agricultural industry as a component of fertilizer.

The hot exhaust gas and AdBlue® then enter the catalytic converter where they react with a variety of metallic compounds to convert nitrogen dioxide and monoxide into nitrogen and water. Nitrogen is the primary component of the air we breathe and is harmless to the environment; water is, well, water.

This is obviously a super simplified version of how selective catalytic reduction works, but it is not unlike the way your petrol-powered car's catalytic converter works, aside from the extra step of injecting urea into the exhaust stream. Most modern diesel engines use SCR in combination with exhaust gas recirculation to reduce emissions.

The following video can give you a better visual explanation of how this process works.

Exhaust gas recirculation or EGR is a common process that is used in nearly all modern internal combustion engines (ICE) to reduce the amount of unburnt fuel in a vehicle's exhaust gases. The downside to EGR is that it can negatively affect vehicle performance and fuel economy, plus it adds another complex system to an already complex machine.

As a response to the weaknesses of EGR, some companies are removing that system from their engines and using slightly more AdBlue® to treat their exhaust gases, thus achieving similar results without the sacrifices in performance and economy.

All of this may sound good but not everyone is convinced that SCR and AdBlue® are good things. People might think that probably they will have to fill up AdBlue® all the time and not counting the extra cost to buy it in the first place. But it is not that bad really!

A typical full tank of AdBlue® will need to be refilled approximately every time you change your oil (this will depend of course on driving habits, road and climate circumstances, where it might need top-ups more often). Besides that, it is mostly water with added urea, so it is not going to rip your wallet. A 10-litre pack of AdBlue® or BlueDEF – as opposed to what dealership garages might say and try to sell – will cost online and in most shops under £10.

Understanding this increasingly visible emissions control system is becoming more and more critical as worldwide manufacturers begin to offer more diesel models in traditionally gasoline-dominant segments. 

Every manufacturer is either already offering or plans to offer smaller-displacement lighter-duty diesel engines in their diesel vehicle ranges. All of these will have AdBlue® tanks and SCR systems.

Where AdBlue® really becomes critical is in big diesel engines – where vehicles do millions of miles over their lifespan – and their diesel engines go through a lot of fuel will therefore also go through a lot of DEF as consequence, so AdBlue® is also sold at the fuel stations for extra convenience. 

SCR technology is also coming to the world of marine diesel. The International Maritime Organization (IMO) first introduced mandates limiting the amount of NOx emissions in 2000 and has since then been tightening those regulations further. With some marine diesel engines easily being the size of a house, their capacity for pollution is immense, so again, SCR and AdBlue® can go a long way towards cleaning the emissions of those engines.

Charles Culverhouse, CEO of Old World Industries – makers of BlueDEF and Peak automotive chemicals – said in an interview "SCR is a technology that exists right now and is being employed all over the world to increase fuel efficiency and reduce NOx emissions."; "DEF works and its made from commonly available ingredients that are already being produced in vast amounts for the agricultural industry. The infrastructure is already in place."

That is an important thing to remember. The world is not going to abandon diesel anytime soon. We depend on diesel-powered vehicles – be they vans, trucks, trains or boats – to move our goods and ourselves around the world.

While traditional diesel fuel may not be a great long-term solution for the planet, the SCR technology and the advent of more cost-effective biofuels mean that until we are ready to abandon internal combustion entirely, we are keeping things relatively clean.

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