Selective catalytic reduction (SCR is emissions control technology that has been used on heavy vehicles for quite some time and is now increasingly common on diesel passenger cars, SUVs and four-wheel drives.

Consequently, Adblue fluid dispensers very similar to and easily mistaken for fuel bowsers are often to be found on service station forecourts, possibly next to the normal petrol and diesel bowsers.

Adblue, or diesel exhaust fluid (DEF), is an almost colourless solution of urea and de-ionised water that is stored in a separate tank to the diesel fuel on vehicles that use SCR technology.

A very small but precisely metered amount of the fluid is automatically injected into the exhaust system upstream of a special-design catalytic convertor.

In the convertor it decomposes to form ammonia and a further reaction converts the NOₓ (oxides of nitrogen, an exhaust pollutant) into nitrogen and water.

The convertor is also designed to deal with hydrocarbons and carbon monoxide in the exhaust while a diesel particulate filter removes soot.

The Adblue fluid is a ‘consumable’ and will need occasional topping up (often as part of a service), though how often will depend on the vehicle, driving conditions and tank size.

The car won’t start if the system is ‘dry’ and won’t be fooled into working if it runs out by filling with water or anything else.  

Adblue is not a fuel additive, is only required for vehicles with SCR systems and most definitely must not be put into the fuel tank (intentionally or by accident).

It will cause expensive-to-fix fuel system damage, leading to very blue language.