NOx (nitrogen oxides: NO and NO2) are polluting gases formed in the combustion chamber when high temperatures (>1500°C) oxidize atmospheric nitrogen. They are the main environmental problem of diesel engines.
Effects
- Urban smog
- Acid rain
- Respiratory problems (asthma, COPD)
- Destruction of the tropospheric ozone layer
NOx reduction
Three main technologies:
- EGR: reduces combustion temperature
- SCR + AdBlue: selective catalytic converter
- LNT (Lean NOx Trap): less used in Europe
NOx and chiptuning
Well done Stage 1 DOES NOT significantly increase NOx (combustion optimization). EGR off / AdBlue off instead BLOCKS the reduction → NOx outside the legal limits → illegal on the road.
How they are formed
Nitrogen oxides arise when high combustion temperatures (over 1,500 °C) cause nitrogen and oxygen in the air to react. There is a technical compromise: parameters that reduce consumption and particulate matter tend to raise temperatures and therefore NOx, and vice versa. This is why modern diesels combine multiple strategies.
Measurement and control
Dedicated NOx sensors, placed before and after the SCR catalyst, allow the ECU to dose the AdBlue and verify the effectiveness of the reduction. In remapping, a well-executed Stage 1 optimizes combustion without significantly increasing NOx; on the contrary, deactivating EGR or SCR removes the reduction and takes emissions out of limits, making the modification illegal on the road.
Why they matter in legal tuning
The key point is to distinguish optimization and tampering: a well-made Stage 1 improves combustion without increasing NOx appreciably and remains compatible with emission systems. Deactivating EGR or SCR, however, eliminates the reduction and takes NOx out of limits, making the vehicle non-compliant and unusable on the road. It is the difference between sustainable tuning and illegal intervention.
In short, NOx are the test of the difference between legal tuning and tampering: optimizing yes, deactivating the abatement systems no, because it means going outside the limits and losing the approval for road use.