Exhaust Gas RECIRCULATION System

If small volumes of fuel are injected into a combustion chamber full of pure air, the effect is to create a very lean mixture. This burns at a high temperature, which in turn causes the excess oxygen in the mixture to combine with the naturally occurring nitrogen in the air to create Oxides of Nitrogen (NOX), a noxious class of pollutants associated with acid rain. This is a particular problem for diesel engines at low to medium loads. Exhaust gas is blended into the intake air charge to create the cylinder charge. As the exhaust gas effectively contains no oxygen, it prevents the formation of a very lean mixture, so lowering combustion temperatures and minimizing the formation of NOX.

At low engine speeds and loads, over 50 percent of the cylinder charge can be made up of recycled exhaust gas. This is routed from the exhaust manifolds through the Exhaust Gas Recirculation (EGR) coolers before being supplied to the electric throttle. The volume of exhaust gas added to the intake charge is regulated by the EGR valves, actuated according to precise engine speed and load maps by the Powertrain Control Module (PCM).