Diesel Exhaust Fluid (DEF / AdBlue)

Definition

Diesel Exhaust Fluid (DEF) — marketed under the brand name AdBlue in Europe (a trademark of the German Association of the Automotive Industry, VDA) — is a liquid reducing agent used in Selective Catalytic Reduction (SCR) systems to reduce nitrogen oxide (NOx) emissions from diesel engines.

DEF is a precisely formulated solution of 32.5% high-purity urea in deionised water. It is stored in a separate tank in the vehicle (coloured blue, typically 5–25 litres capacity depending on vehicle size) and injected into the exhaust stream before the SCR catalyst. It is not added to diesel fuel and must not be confused with fuel additives.

How SCR and DEF work

The SCR chemical process converts toxic nitrogen oxides into harmless products:

1. The engine combustion produces NOx (nitrogen monoxide NO and nitrogen dioxide NO₂) as a byproduct of high-temperature, high-pressure combustion
2. DEF is injected into the hot exhaust stream upstream of the SCR catalyst
3. The urea in DEF decomposes to ammonia (NH₃) and isocyanic acid at exhaust temperatures above ~150°C
4. The ammonia reacts with NOx over the SCR catalyst: 4 NH₃ + 4 NO + O₂ → 4 N₂ + 6 H₂O
5. Output: harmless nitrogen gas (N₂) and water vapour (H₂O)

Modern systems achieve NOx reduction efficiencies of 85–95%, enabling diesel engines to meet Euro 6 (cars) and Euro VI (trucks) emission standards for NOx.

Which vehicles require DEF/AdBlue?

Not all Euro 6 diesels use SCR — some manufacturers (primarily VW Group vehicles post-Dieselgate) use high-pressure EGR (Exhaust Gas Recirculation) to achieve Euro 6 compliance without DEF. Check your vehicle's handbook.

DEF consumption and costs

DEF pricing at UK forecourts is approximately £1.00–1.30/litre for small volumes (1–10 litre containers). In bulk (IBC tanks, for fleet operators) the cost is approximately £0.35–0.55/litre. Supermarkets (Tesco, Asda) typically offer DEF at competitive consumer prices. AdBlue is also available in 10-litre containers at motor factors and agricultural merchants.

DEF quality and storage

DEF quality is critical — the urea concentration must be precisely 32.5% (±0.5%). Key handling notes:

• DEF must meet ISO 22241 standard; use only ISO 22241-certified AdBlue
• DEF freezes at −11°C (11°F). Vehicles have heated DEF tanks to prevent freezing; stored containers should be kept above freezing
• DEF degrades at high temperatures (>35°C sustained); store in a cool, dark location
• DEF shelf life is approximately 12–18 months at normal temperatures
• Never add diesel to the DEF tank or DEF to the diesel tank — either error causes severe and expensive damage to the SCR catalyst or fuel system

What is AdBlue and how does diesel exhaust fluid work?

What is AdBlue? AdBlue is the European trade name for diesel exhaust fluid (DEF) — a precisely formulated solution of 32.5% high-purity urea in deionised water. It is injected into the exhaust stream of diesel engines equipped with Selective Catalytic Reduction (SCR) systems to convert harmful nitrogen oxides (NOx) into harmless nitrogen gas and water vapour. DEF fluid is not a fuel additive — it goes into a separate tank (typically blue-capped) and must never be mixed with diesel. The UK government's fuels and lubricants guidance covers fuel quality standards including DEF requirements, and the European Commission's road transport emissions page explains the Euro 6 regulations that make SCR systems mandatory on most new diesel vehicles.

The cost of diesel exhaust fluid is modest — approximately GBP 1.00-1.30 per litre at UK forecourts, or GBP 0.35-0.55 per litre in bulk for fleet operators. A typical passenger car consumes 1.5-2 litres per 1,000 km, making the annual cost roughly GBP 20-35 for average driving. Running out triggers a dashboard warning and eventually a reduced-power "limp home" mode — a regulatory requirement to ensure the SCR system is always active when the engine runs. The U.S. EIA diesel fuel page provides broader context on how diesel emission standards have evolved globally.