Diesel engines have been an important part of transportation for decades, valued for their durability and efficiency. However, alongside these advantages came a significant drawback: during combustion, they produce nitrogen oxides (NOx), gases that contribute substantially to air pollution, smog formation, and acid rain. As environmental standards in Europe became increasingly stringent, particularly with the introduction of the Euro 4, 5, and 6 regulations, it became clear that traditional solutions were no longer sufficient.
In 2014, the automotive industry introduced Selective Catalytic Reduction (SCR) as one of the most effective technologies for reducing NOx emissions. In line with this development, the ISO 22241 standard defined Diesel Exhaust Fluid (DEF) for the treatment of diesel engine exhaust gases, the most widely known of which is AdBlue®.
So, what exactly is AdBlue® fluid?
The most common misconception is that AdBlue® is a diesel fuel additive. This can be dangerous, because AdBlue® is NOT a diesel fuel additive, and accidentally adding it to the fuel tank can cause very serious damage.
AdBlue® is a registered trademark for a fluid based on urea and water, which is injected into the exhaust gases to break down nitrogen oxides into nitrogen and water vapor—substances that are naturally present in the air.
It’s just urea and water, you can mix it yourself
AdBlue® consists of high-purity urea dissolved and suspended in deionized water in a precisely defined ratio. This ratio is not accidental. It ensures fluid stability, proper atomization, and reliable operation of the SCR system.
Because AdBlue® uses urea of exceptionally high purity, it should never be improvised by dissolving commercially available urea in demineralized water, as the quality is not the same and doing so may cause damage to your vehicle.
How does AdBlue® work?
Within the engine itself, the combustion of diesel fuel produces harmful nitrogen oxides. As the exhaust gases pass through the SCR system, AdBlue® is injected and, under the influence of heat, breaks down. Inside the catalyst, a chemical reaction then takes place in which NOx is converted into nitrogen and water vapor.
AdBlue® is harmful?
No. AdBlue® is a safe, non-flammable fluid with a mild ammonia-like odor, but like any technical fluid, it requires careful handling and proper storage. If it comes into contact with the skin, it may cause mild irritation and should be rinsed off with plenty of water. It can also damage vehicle paint, which is why it should be filled carefully.
It is recommended to store AdBlue® in its original container, in a cool, well-ventilated place and away from direct sunlight. This helps prevent water evaporation and urea crystallization, preserving the quality of the product.
But it does pollute… right?
On the contrary. SCR technology, in combination with AdBlue®, not only significantly reduces NOx emissions but can, under certain conditions, also contribute to lower fuel consumption, resulting in reduced CO₂ emissions. This is why AdBlue® is considered one of the key elements in reducing the environmental impact of diesel vehicles.
Can I add AdBlue® fluid myself?
Modern vehicles continuously monitor the level of AdBlue® and alert the driver in advance when a refill is required. Depending on the manufacturer, model, year of production, engine type, and other parameters, the vehicle’s system may respond differently if the AdBlue® tank runs empty—ranging from limiting engine speed, maximum speed, or power output, to preventing the engine from starting altogether.
To avoid confusion, manufacturers have designed a dedicated filling point with a blue cap that leads exclusively to the AdBlue® tank. This means AdBlue® can be topped up by the driver, provided the container is in good condition and care is taken not to pour it into the fuel tank. Of course, refilling can also be done at an authorized service center, but it should never be done at stations offering AdBlue® pumps for trucks and buses—although the fluid itself is the same, the filling pressure is too high and may damage the tank.
The tank capacity also varies, typically ranging between 8 and 25 liters. When the warning light appears, it is usually sufficient to add around 4 liters to the tank to switch off the alert and allow you to continue driving safely, although some models require the tank to be filled completely. It is important to avoid overfilling the tank, as this may cause damage.
What if the warning light stays on after I refill AdBlue®?
There are several reasons why the vehicle’s system may report a fault, and one of the most common is the presence of crystallization within the SCR system. Vehicles and work machines that operate on short routes or in demanding conditions where the system does not always reach its optimal operating temperature are particularly exposed to this risk. If the warning light remains on after refilling, it is recommended to visit a service center so the cause of the issue can be properly diagnosed.
How can crystallization be prevented?
The TotalEnergies research center has developed ClearNOx®, an enhanced DEF designed to prevent crystal formation even at lower operating temperatures. Field experience shows that vehicles used in “stop & go” driving, on short routes, or in rural environments experience significantly fewer SCR system blockages when using ClearNOx®. Fewer blockages mean fewer breakdowns, less downtime, fewer service hours, and lower unplanned maintenance costs.
ClearNOx® is compatible with all types of vehicles and is ready for use immediately. It is recommended as an advanced alternative to AdBlue® for users whose business operations and profitability depend on the reliability and availability of their vehicle fleets and work machinery.