What is Diesel Exhaust Fluid?
DEF is a clear liquid that diesel engines with SCR use to clean their exhaust. Here's what's in it, how it works, and how to handle it.
01Composition
Two ingredients, both of them pure
DEF is a solution of high-purity urea in deionised water, at exactly 32.5% urea by weight. That ratio isn't arbitrary: it's the mix that freezes at the lowest temperature, and the one SCR systems are calibrated for.
Purity matters as much as the ratio. The minerals in ordinary water, or the additives in fertiliser-grade urea, can clog and poison an SCR system.
In every litre of DEF
By weight
32.5%
urea
- High-purity urea
- The active ingredient that reacts with nitrogen oxides.
- Deionised water
- Carries the urea into the exhaust without adding impurities.
02How it works
From the tank to clean exhaust
DEF never touches the fuel. It has its own tank, and it's sprayed into the exhaust after the engine.
Up to
90%
less NOx
-
1
Exhaust leaves the engine
Hot diesel exhaust carries nitrogen oxides (NOx), a major cause of smog.
-
2
DEF is sprayed in
A doser mists DEF into the exhaust, where the heat turns its urea into ammonia.
-
3
The SCR catalyst works
Inside the catalyst, ammonia reacts with the NOx and breaks it down.
-
4
Nitrogen and water come out
What leaves the tailpipe is mostly harmless nitrogen and water vapour.
Done properly, SCR with DEF can cut NOx emissions by up to 90%, and lets engines be tuned for better fuel economy and performance.
03Genuine matters
Cheap fluid is expensive
SCR sensors notice when the fluid isn't right. Diluted or impure DEF can set off warning lights, cut engine power and leave deposits that damage injectors and catalysts — repairs that cost far more than the fluid ever saved.
Using DEF made to ISO 22241 keeps your engine running as designed and within emission rules.
Storing and handling DEF
Keep containers sealed, in a cool place and out of direct sunlight.
DEF freezes at about −11 °C. Once thawed it's fine to use.
Never top it up with water, or add anything else to it.
Use clean equipment kept only for DEF: funnels, pumps and containers.
It's mildly corrosive to copper and brass, so store it in suitable containers.
Rinse spills off paintwork and skin with water.
04FAQ
Common questions about DEF
Is DEF the same as AdBlue®?
It's the same kind of fluid. AdBlue® is a trademark for urea solution made to the same standard; DEF and AUS 32 are other names for it. Any fluid made to ISO 22241 does the same job. (AdBlue® is a registered trademark of VDA.)
Can I use fertiliser urea mixed with water?
No. Fertiliser urea carries additives and impurities, and tap water carries minerals. Both damage SCR systems. Only automotive-grade urea and deionised water, mixed and tested to ISO 22241, are safe.
What happens if the DEF tank runs dry?
Most SCR vehicles warn you well in advance, then limit engine power until the tank is refilled. Check your vehicle's manual for exactly what yours does.
How much DEF will I use?
Usually a few litres for every 100 litres of diesel. The exact rate depends on the engine, the load and how it's driven.
Does DEF go off?
It keeps longest sealed and cool. Heat and direct sun shorten its life. If you're storing large volumes, ask us about the right set-up for your site.