To learn more, some useful links are:

Emissions regulations
US EPA
California Air Resources Board

Diesel information sources
Engine Manufacturers Society
US Society of Automotive engineers
Dieselnet
Cummins

Engine research Organisations
Ricardo engine research (UK)
AVL engine reseach (Austria)
Southwest Research (US)

Aftertreatment
Johnson Matthey (Aftertreatment)

This is a method of reducing NOx (oxides of Nitrogen) emissions and it is being employed by many manufacturers to meet current and future limits in Europe and the US. It works by adding exhaust gas back into the engine inlet manifold after the turbocharger. This has the effect of diluting the oxygen in the cylinder and reducing combustion temperatures, which has a direct effect on the amount of NOx produced.

The system requires the turbocharger to work harder and needs better control methods than used on current non-EGR engines. This is because to keep the system optimised, the ratio of fuel and air in the engine is much more important and it is the turbocharger that has the task of providing the air. The turbocharger also has to ensure the pressure difference between exhaust and inlet manifolds to drive the EGR is maintained.

EGR is attractive because it uses a readily available exhaust supply and requires a relatively small amount of extra equipment.