If your car has this blue cap next to the fuel cap, you should know this so you don't end up stranded

Your diesel-engined car may use AdBlue, the additive needed for some particulate filters to work properly. Or perhaps you’re thinking of buying a diesel that uses this system. You may even have a diesel and not know whether it works with AdBlue. Either way, this AdBlue guide is for you. Above all, to extend the life of the engine, to avoid faults in the particulate filter, to carry out maintenance properly… and to avoid being stranded!

What is AdBlue?

AdBlue is an additive that some diesel-engined vehicles require when fitted with a particulate filter to meet anti-pollution regulations. Some would not comply without this technology; others began using it to get ahead of new standards due to come into force later. It is used in diesel passenger cars as well as trucks, buses and special machinery.

Basically, AdBlue is a chemical product consisting of a solution of demineralised water and urea. The proportion is usually 67.5% urea and 32.5% water. It appears as a colourless, odourless liquid, and its characteristics are regulated at European level by a specific standard (originally DIN 70070, now replaced by ISO 22241). For complete data, AdBlue has a melting point of -11 degrees Celsius and a pH of approximately 9.5. It is not flammable, nor explosive, and is not considered dangerous to people or the environment. Although it is not toxic, it is advisable to wear gloves when handling it, since it is corrosive.

What is AdBlue for and how does it work?

AdBlue is used to reduce the nitrogen oxides emitted through the exhaust of an engine. These nitrogen oxides form when the nitrogen in the air oxidises during combustion due to high temperatures.

To eliminate nitrogen oxides (NOx) and other harmful elements present in exhaust gases, some diesel engines use a special emissions-reduction system. Normally, the system consists of a DeNOx oxidation catalyst plus a DPF particulate filter. When passing through the emissions system and undergoing what is known as selective catalytic reduction (SCR), NOx is transformed into water and nitrogen, which are not harmful, and can be released into the atmosphere.

What AdBlue does is that, when injected and brought into contact with the high-temperature exhaust gases, it reacts, releasing ammonia, which acts as an “accelerator” for the reaction taking place in the DeNOx catalyst. This facilitates the transformation of nitrogen oxides into water and free nitrogen.

Which cars need AdBlue?

Not all cars need this additive. It began to be used in some saloons and SUVs with powerful diesel engines under the Euro 5 standard in 2014, although it had been used earlier in trucks and industrial vehicles. With the Euro 6 standard, which came into force on 1 September 2015, NOx levels for diesels were reduced from 180 mg/km to just 80 mg/km. And in September 2018, the Euro 6C standard came into force for newly registered cars.

Petrol-engined cars emit less NOx and have no difficulty meeting these emission levels. In diesels, smaller models generally do not need AdBlue; it is necessary in engines of a certain displacement and power. Although some brands use SCR systems with this additive even for low-displacement engines, such as the TDI engines of many models in the group Volkswagen. Other brands such as Mazda have developed diesel engines capable of meeting the emission levels set by the Euro 7 standard without needing AdBlue.

How can I tell if my car uses AdBlue?

If in doubt, the best option is to consult the dealership or workshop. The car’s manual can also help you locate where the AdBlue tank and filler neck would be, and try to find it on your car. If next to the fuel tank cap you have a second, smaller filler neck – usually with a blue cap – or if you have such an opening somewhere in the boot, then your car uses AdBlue.

Many users only find out that their car needs this additive when the corresponding warning light appears on the instrument cluster for the first time, when the tank is nearly empty. Depending on the vehicle, the AdBlue tank capacity can vary between 10 and 30 litres for a passenger car, while an industrial vehicle may have a capacity of up to 200 litres. . Some brands, such as Audi, even offer a choice of two different tank sizes for AdBlue on certain models.

How much AdBlue does a diesel use?

It depends greatly on the engine type and driving style, as the engine control unit adjusts the necessary parameters to neutralise the nitrogen oxides generated. In intensive urban use, for example, AdBlue consumption will be higher. As a reference, additive consumption in an average passenger car may be between 1.5 and 2 litres every 1,000 km.

In general, manufacturers state that the AdBlue tank usually lasts more than enough for the driver not to have to worry between service intervals, and that it is at the workshop where the AdBlue level is checked and the additive topped up between one service and the next. That can be between 15,000 and 20,000 km, depending on the car. Unfortunately, this is not always the case, and it is not uncommon to find that our car is using up its AdBlue before we are due at the workshop.