Hi Bryan,
Lets stick to diesels and forget petrol's for the moment.

Diesels have historically always run on excess air, they are not throttled and therefore in theory the amount of air drawn in on the intake stoke is matched to the cylinder capacity.

The speed of the diesel engine is controlled by the amount of fuel fed into the cylinder, under idle conditions a very small amount of fuel is injected into the cylinder, with more being fed in as more speed/load is needed.

The excess air and the varying fuel quantity's result in a constantly changing AFR (Air Fuel Ratio)

It is this way of working that generates NOx in the diesel engine. It has always produced NOx under certain operating conditions but more so with new technologies. It is only in the last 10 years or so that NOx has become an issue for the VM's and this is because of legislation.

So we have to look at what NOx is and how it is produced.
Nitrogen oxides are produced during the combustion process, especially at high temperatures and where an excess of Oxygen is present (the atoms weld themselves together above a temperature threshold)

EGR was brought in to reduce NOx and the way it does it is by reducing the oxygen content of the fresh charge in the cylinder.

This is achieved by partly filling the cylinder with an inert gas(exhaust gases) and thus reducing the "excess" oxygen to more acceptable levels. This also helps to reduce the internal cylinder temperature.

As a rule it is used at idle and part load on a modern diesel engine. HTH


Regards Mark