Three quarters of the atmosphere consists of nitrogen, and while it is generally inert, at high temperatures and pressures it turns into potentially harmful nitrogen oxides (NOx - see sidebar overleaf). International emissions standards have legislated for lower NOx since the early nineties, with the Euro I limit as 8.0g/kWh. EGR was able to deal with this, but the real shift came in 2006 with the introduction of Euro IV: this limited NOx to 3.5g/kWh, and also drastically tightened hydrocarbon (HC) and particulates (PM) limits.
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