Why DEF is essential
Diesel engines produce nitrogen oxides (NOx), gases that cause smog and harm the lungs. BS6 vehicles use a Selective Catalytic Reduction (SCR) system to break NOx down, and that system only works with Diesel Exhaust Fluid.
No DEF, no SCR. And without SCR, a BS6 diesel cannot meet the emission norms it was built for.
- THE PROBLEM
- NOx gases
- Released by every diesel engine. They form smog and irritate the airways.
- THE RULE
- BS6 norms
- In force across India since April 2020, with a far lower NOx limit for diesel engines than BS IV.
- THE SOLUTION
- SCR + DEF
- DEF lets the SCR system turn NOx into nitrogen and water vapour.
From tank to clean exhaust
Injected into the exhaust
A doser sprays a measured amount of DEF into the hot exhaust stream.
Turns into ammonia
The heat breaks the urea in DEF down into ammonia.
Reacts in the catalyst
Inside the SCR catalyst, ammonia reacts with the NOx.
Nitrogen and water out
What leaves the tailpipe is mostly harmless nitrogen and water vapour.
NOx+Ammonia (from DEF)Nitrogen+Water vapour
What DEF does for your vehicle
Beyond cleaner air, keeping the DEF tank topped up keeps a BS6 vehicle running the way it should.
Stays within BS6 norms
The SCR system needs DEF to keep NOx within the legal limit.
Full engine power
Most BS6 vehicles cut power when DEF runs out. A full tank avoids that.
Efficient engine tuning
With SCR cleaning up NOx, engines can be tuned for power and fuel efficiency.
Cleaner air
Less NOx on the road means less smog in the towns and cities it passes through.
What people get wrong about DEF
| Myth | Fact |
|---|---|
| DEF is a fuel additive. | FactIt never goes in the diesel tank. DEF has its own tank, usually with a blue cap. |
| Water works just as well. | FactWater has no urea, so NOx is not treated. The sensor flags it and the system can be damaged. |
| Any urea mixed with water will do. | FactFertiliser-grade urea carries impurities that damage the catalyst. DEF must meet ISO 22241. |
| DEF is dangerous to handle. | FactIt is non-flammable and safe to handle. Wipe up spills, as dried DEF leaves white crystals on paint and metal. |
| Frozen DEF is ruined. | FactDEF freezes at about −11 °C and works normally once thawed. Vehicles have heaters for the tank. |