The Euro 6 emission standard, introduced in September 2014 for new type approvals and September 2015 for all new registrations, represents a significant tightening of permissible pollutant limits for vehicles sold in the European Union.
What Changed with Euro 6?
Compared to Euro 5, Euro 6 reduced the permitted NOx emissions for diesel engines from 180 mg/km to just 80 mg/km — a reduction of 55%. Particulate matter (PM) limits were also reduced from 5 mg/km to 4.5 mg/km. For petrol engines, NOx limits remained at 60 mg/km, but particulate limits were introduced for direct injection engines.
Key Technologies Required for Euro 6 Compliance
Diesel Particulate Filter (DPF)
While DPFs were already common on Euro 5 diesels, Euro 6 made them essentially mandatory. Modern DPFs achieve over 99% filtration efficiency. The DPF captures soot particles and periodically burns them off through a process called regeneration, where exhaust temperatures are raised to approximately 600°C to oxidise accumulated soot into CO₂.
Selective Catalytic Reduction (SCR)
SCR is the primary technology used to reduce NOx emissions in diesel engines. It works by injecting a urea-based solution called AdBlue (Diesel Exhaust Fluid / AUS 32) into the exhaust stream. The ammonia from AdBlue reacts with NOx over a catalyst to form harmless nitrogen (N₂) and water (H₂O). The chemical reaction:
4NO + 2NH₃ + O₂ → 3N₂ + 6H₂O
6NO₂ + 8NH₃ → 7N₂ + 12H₂O
Exhaust Gas Recirculation (EGR)
EGR recirculates a portion of exhaust gas back into the engine intake, reducing combustion temperatures and thereby limiting NOx formation at the source. Euro 6 engines typically use both low-pressure and high-pressure EGR circuits for optimal NOx reduction across all operating conditions.
NOx Sensors
Modern vehicles use at least two NOx sensors (upstream and downstream of the SCR catalyst) to monitor conversion efficiency. These sensors provide real-time feedback to the ECU, enabling precise AdBlue dosing. Failure of a NOx sensor almost always triggers a warning light and can lead to limp mode.
Oxidation Catalyst (DOC)
The Diesel Oxidation Catalyst sits upstream of the DPF and converts carbon monoxide (CO) and unburned hydrocarbons (HC) into CO₂ and H₂O. The DOC also generates heat and nitrogen dioxide (NO₂) needed for passive DPF regeneration.
Real-World Driving Emissions (RDE)
A key evolution under Euro 6d-TEMP (2017) and Euro 6d (2020) was the introduction of Real Driving Emissions (RDE) testing. Vehicles must now meet emissions limits not only in the laboratory but also under real-world driving conditions, measured using Portable Emissions Measurement Systems (PEMS). This closed the gap between lab test results and actual on-road emissions.
Maintenance for Euro 6 Compliance
- AdBlue Top-Ups — Ensure AdBlue levels are maintained. Running empty can cause the vehicle to restrict speed to 20 km/h in some models.
- DPF Regenerations — Regular longer journeys help the DPF complete regeneration cycles. Short trips prevent regeneration and lead to clogging.
- Quality Engine Oil — Use low-SAPS (Sulphated Ash, Phosphorus and Sulphur) oils specifically formulated for DPF-equipped vehicles (e.g., BMW LL-04, Mercedes 229.51/229.52).
- EGR System Cleaning — EGR valves and coolers on Euro 6 engines can become clogged with carbon deposits. Periodic cleaning (every 60,000–80,000 km) helps maintain performance.
- Timely Sensor Replacement — NOx sensors, lambda sensors, and differential pressure sensors have finite service lives. Replace them at the first sign of trouble.
Euro 7 — What’s Coming?
The proposed Euro 7 standard aims to further tighten emission limits, introduce stricter durability requirements (200,000 km or 10 years), and regulate particulate emissions from brakes and tyres. While the timeline remains under discussion, the trend is clear: emissions systems will only become more sophisticated.
At NOXWX, we stock a comprehensive range of emission system components to keep your Euro 6 vehicle compliant and performing at its best.