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What is AdBlue and how does it work?

If your car has a Selective Catalytic Reduction system, it'll drink AdBlue as well as diesel. We explain why and how to keep it topped up.

Audi SQ5 diesel SUV

What is AdBlue and how does it work?

Cars get through a lot of liquid, don’t they? Unless your vehicle is entirely electric, you’ll need to provide it with liquid fuel.

Then, of course, there’s engine oil. And coolant. And brake fluid. And antifreeze. Oh, and windscreen washer fluid.

Depending on the type of car you drive, there may well be another name to add to that list of essential liquids that your ride needs in order to operate – AdBlue.

What is AdBlue?

AdBlue is a liquid solution – an exhaust fluid that’s only needed if you drive a diesel car or van that’s fitted with a Selective Catalytic Reduction (SCR) system. This liquid is used to reduce the amount of air pollution that a diesel engine creates.

AdBlue on a diesel car

Why does my diesel car need AdBlue?

Put simply, it’s about the environment. Most major European cities, including the likes of London and Paris, are introducing new programmes and directives like the Ultra Low Emissions Zones. This is so they can ban any diesel-powered vehicles that don’t meet the Euro6 regulations in their centres at certain times and days.

Diesel exhaust fluids like AdBlue offer a simple two-ingredient solution that cuts particulate pollution significantly.

Keep your diesel car topped up with AdBlue – if it needs it – and you’ll comply with Euro6 regulations, meaning it can be driven in those cities.

You’ll know whether or not your car needs AdBlue or not, so don’t go worrying. There either is or there isn’t a noticeable separate tank next to your fuel tank.

If there isn’t one, relax. Your car doesn’t require the stuff. If, however, you do see a second screw cap – probably with ‘AdBlue’ written on it – you know you need it. If you’re still not entirely sure, check your vehicle’s handbook – it’ll say in there if you need to use AdBlue or not.

There’s really no need to panic if you do need it though. This exhaust fluid doesn’t need your constant attention. In fact, you can probably get away with one top-up between services and that’s it. Roughly every 6,000-8,000 miles, although you'll need to check yourself because each driver will consume AdBlue at a different rate – much like petrol/diesel.

Cars that have an SCR and AdBlue have gauges hooked up to dashboard icon lights, so you’ll soon know when you’re running low.

Any major manufacturer’s model of car or van that’s registered as new in Europe after September 2015 will likely use AdBlue.

You can practically guarantee any post-September 2015 used Jaguar, used Mercedes, used Audi, used BMW, used Peugeot, used Renault or used Citroen will use it.

Fuelling up a diesel car

How does AdBlue work?

We’ve mentioned Selective Catalytic Reduction already. Needing to cut down the number of pollutants in diesel engines, SCR is what’s called ‘an advanced active emissions control technology system’.

It injects a special liquid reductant agent (in this case, AdBlue) right through a catalyst that’s built into the exhaust stream of the engine.

Tiny amounts of the liquid are pumped into the exhaust gases.

At very high temperatures, AdBlue turns into carbon dioxide and ammonia. A chemical reaction occurs when the carbon dioxide and ammonia hit the harmful nitrogen oxides and nitrogen dioxides, converting them into harmless nitrogen and water.

Both are then emitted as steam out of the exhaust pipe and out into the world. Producing gases that are much safer than what would have been produced without it.

You can pour it into the tank yourself, it’s very easy. It’s perfectly safe if you spill any on your hands, but be careful not to spill any on your paintwork as it can damage it.

The average AdBlue tank has around a 12-litre capacity and you can buy it from garages, petrol stations, outlets with car sections, and, of course, online.

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