Why AdBlue is the new green when it comes to diesel emissions... but how does it work?

AdBlue, made from a mixture of water and urea, is injected in a diesel vehicle's exhaust sysem to lo

AdBlue, made from a mixture of water and urea, is injected in a diesel vehicle's exhaust sysem to lower its emissions to bring it in line with new Euro 6 standards. - Credit: PA

AdBlue is used to lower diesel car emissions, but few people know how it works. Jack Evans looks at what it does and why.

For a little while now, certain manufacturers have been using AdBlue as a way of lowering exhaust emissions in diesel engines. However, few people know exactly what it is, or how it works. We're going to investigate and see what's what.

You might have noticed while filling up your diesel car a small blue filler cap, sitting alongside the regular fuel filler cap. This leads to the AdBlue tank, which works in a similar way to a regular fuel tank. However, unlike diesel it, it doesn't run to the engine.

How does AdBlue work?


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Made from a mixture of water and urea, AdBlue is put directly into a specially-modified area of the car's exhaust sysem. By doing so, this starts of a chemical reaction which removes much of the polluting nitrogen oxide (NOx) elements of the exhaust gas, and converts them into water. This not only gives cars better emissions figures, but allows them to pass more stringent standards which heralded Euro 6 emission regulations.

This process is known as Selective Catalytic Reduction, or SCR, and is only used in diesel engines. In a similar way to petrol or diesel, the more economically you drive the less AdBlue is used.

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Which cars need AdBlue?

The integration of AdBlue into diesel engines has been a relatively recent development. As such, you won't find it on all oil-burners, though if it's a newer vehicle there's a good chance of it using AdBlue.

The additive makes no difference to how the car drives or responds, so it may not be initially clear whether or not it uses it. Some fillers are located under the fuel filler cap next to the diesel tank filler, though some may be placed in some hard-to-find areas such as beneath the boot floor or under floor mats.

How do I know when it needs topping up?

Most cars will have in-built warning sensors to alert you as to when the level of AdBlue is too low. For average-sized cars, the tank will be around 12 litres, and will need filling up after around 4,000 to 6,000 miles.

Because it is introduced to the car's exhaust system in small amounts, there shouldn't be much need to top up too often – just when the car signals for you to do so.

How do I fill up the AdBlue?

Some manufacturers will request that you take the vehicle in to service centres to have its levels of AdBlue topped up, though you can quite easily do it at home. In a similar way to washer fluid, the AdBlue should be poured in through the relevant filler nozzle. In some vehicles, a minimum of a three to four litres will need to be added to remove the warning message from the car's dashboard.

The whole amount of AdBlue in a bottle needs to be used in one go, as leaving a small amount standing for some time will affect its chemical make-up and make it less effective.

How much does it cost?

If you're planning on having your dealer service your car, it's a good idea to first check how much they charge for topping up AdBlue levels, as it could be more cost-effective to do it yourself. Dealers will sell it to you for around £1.50 a litre, although you'll also be able to find it in some high street retailers and online marketplaces.

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