Selective catalytic reduction, or SCR, is how modern diesel engines meet Euro V and Euro VI limits. It injects a urea solution into the exhaust stream, where it converts nitrogen oxides into nitrogen and water. That solution is diesel exhaust fluid — also sold as AdBlue, DEF or AUS 32, depending on the market.
What DEF actually is
A solution of 32.5 % high-purity urea in deionised water. Nothing else. No additives, no dye, no fragrance. The concentration is fixed by ISO 22241 precisely because SCR systems meter it by volume and the dosing strategy assumes that exact strength.
How much DEF will your fleet use?
Consumption tracks diesel consumption rather than distance, because dosing is proportional to the nitrogen oxides being produced. Typical ratios:
| Application | Typical DEF as share of diesel volume |
|---|---|
| Long-haul Euro VI truck | 4 – 6 % |
| Euro V truck | 3 – 5 % |
| Urban delivery / stop-start duty | 5 – 7 % |
| Intercity bus | 4 – 6 % |
| Off-road and construction diesel | 3 – 5 % |
Practical translation: a truck burning 350 litres of diesel per 1,000 km will use roughly 15 – 20 litres of DEF over the same distance. Plan replenishment on that basis, not on an assumed fixed interval.
Open the DEF calculator
Enter your fleet's diesel or distance figures and get the DEF volume and drum count you need.
Storage: where most quality problems come from
- Keep it between 0 °C and 30 °C. Above about 30 °C urea degrades and releases ammonia, so the strength drifts below specification. Below − 11 °C it freezes — freezing itself does not ruin it, but thawing it badly can.
- Keep it out of direct sunlight. Light and heat accelerate decomposition.
- Keep containers sealed. Urea absorbs moisture from humid air, which dilutes the solution.
- Never store it in a galvanised, zinc-plated or bare aluminium container. Urea attacks zinc and copper alloys and picks up metal ions that poison the SCR catalyst.
- Watch shelf life. Under good conditions expect 12 – 18 months; anything past that should be tested for concentration before use.
The mistakes that cause SCR faults
- Diluting DEF with water, or 'stretching' a drum between vehicles. The SCR system detects it and the engine derates.
- Filling the DEF tank from a dirty or previously-used container that held diesel, coolant or a cleaning chemical.
- Using agricultural-grade urea instead of automotive-grade. The biuret and metal content differ, and both damage the catalyst.
- Storing drums in the sun or in a hot container over a long sea freight period without checking concentration on arrival.
- Ignoring crystallisation on the injector. White crust around the dosing module is a warning that dosing quality is off.