Adblue is a fluid that suddenly appeared on the scene just over 20 years ago, as emission standards began to be felt in the world of diesel-powered vehicles.

It was a substance that few, if any, understood outside the world of automotive engineers and truck dealership technicians.

That has hardly changed since, although there is a broader appreciation that it is based on a solution of urea in water that needs to be added to a separate tank on the vehicle for feeding to the exhaust manifold, whereupon some sort of magic takes place to clean the gasses emitted by the engine.

As a brief explanation this is quite correct, yet there is a good deal more involved in ensuring that everything works as it should and emission standards are properly met.

The problem

There are two main constituents of diesel exhaust gas that are considered a health hazard, they are carbon particulate matter (PM) and nitrous oxides (NOx) both of which are the target of the ever-tightening emissions standards.

SCR unit on tractor — Adblue injection is now a standard part of all tractors, mid-sized and up

Control of both of these constituents are also interrelated. The hotter the combustion temperature of the engine, the less particulate matter is left in the exhaust gas, but the more NOx are created. This presents the engineers with a conundrum, with another variable to take into account that upsets the balance between the two.

That variable is engine efficiency and it increases in line with a rise in combustion temperature, but, as already noted, this will have a detrimental effect on NOx levels

The answers

There are three basic answers to this problem.

The first is to feed a portion of the exhaust gas back into the engine, which reduces the oxygen available for combustion and so limits peak combustion temperature. This technique is now falling out of favour.

The second approach is to run the engine at lower temperatures and catch the particulate matter exiting the engine with a diesel particulate filter (DPF). This is a tried and trusted method, although there can be problems with carbon build-up and ash over time.

A third method is to run the engine at higher temperatures and remove the NOx from the exhaust in a process known as selective catalytic reduction (SCR). This involves the use of ammonia (NH4) as a reducing agent, which is added to the exhaust in the form of urea.

The role of urea

Urea is an organic compound which can be thought of as containing two molecules of ammonia plus one carbon and one oxygen atom.

When it is fed into the exhaust gas, the urea molecule breaks down, releasing the ammonia which is then free to react with the NOx to produce nitrogen gas and water, a process that can remove 95% of NOx from the exhaust.

Adblue diagram — As adblue enters the exhaust stream it breaks down to produce two ammonia molecules which then combine with NOx to form straight nitrogen and water. Source: MTU

To ensure the process is completed, the mix of urea and exhaust gas is passed over a honeycomb structure supporting a catalyst, which may be one of several types, including oxides of tungsten and other rare metals.

It is at this stage that the NOx are neutralised and converted to diatomic nitrogen and water, both of which are natural constituents of the atmosphere.

Another reaction that may occur at this point is the production of sulphuric acid which can accelerate corrosion of the exhaust system. However, the latest standards for diesel fuel set the maximum limit for sulphur at 10ppm, so this is no longer considered an issue.

Adblue injection

To work correctly, it is essential that the urea is delivered in the precise quantity required to treat the the amount of NOx being produced by the engine.

This requires that the control of its addition is in accordance with the engine map, which will alter the rate according to variables such as engine speed, operating temperature, and load.

Urea in its natural form is a solid white compound, it therefore needs to be dissolved in water to enable it to be delivered to the exhaust system.

Adblue tanks are clearly marked with a blue cap. It must never be mixed with the diesel

The urea solution used, officially known as aqueous urea solution 32 (AUS 32), is a precisely measured 32.5% urea dissolved into 67.5% deionised water. For quality purposes, this solution should meet the ISO 22241 standard which sets out the limits for contamination and variance from this strength.

In the European context the fluid is known and sold as adblue, elsewhere it may be referred to as diesel engine fluid (DEF) which suggests that it is a fuel additive. It is not, and should never be mixed with diesel fuel.

Problems in the field

If adblue meets the ISO standard and is stored and dispensed correctly it should not cause any disruption to the working of a diesel engine.

Unfortunately, there have been a good number of reported incidences of engine failure due to issues with adblue systems. This has resulted in many of them being removed and the engine remapped to ensure it still runs without the system connected.

The blame for these problems is usually laid at the door of the engine manufacturers, yet if the fluid is not of the correct standard, has become contaminated, or is past its shelf life, then the adblue injectors can become clogged or the catalyst damaged, leading to the poor and inefficient performance that is remedied through removal.

Correct storage is vital

Being an organic substance urea is prone to degradation over time, so it is vital that that it is kept away from direct sunlight and preferably below 30°. The substance will freeze at -11°, but it will still be OK to use if thawed thoroughly before use.

It is also essential that it is not contaminated by water, other fluids, dust, or dirt as this degrades its purity and will eventually lead to performance issues, so funnels need to be clean and the air dust free when dispensing.

Being a non-hazardous material, small spillages can be mopped up although major leaks will contaminate groundwater, and bunded storage or catchment areas are advisable if large quantities are to be kept.

The problem

The answers

The role of urea

Adblue injection

Problems in the field

Correct storage is vital

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The importance of adblue explained