Low pressure EGR technology has great potential for reducing emissions of NOx from diesel engines.
There are two distinctively different types of EGR available. The, so called, High-Pressure EGR (Fig. 2) re-circulates exhaust between the two high pressure points - exhaust manifold and inlet manifold (downstream of an intercooler). The Low-Pressure EGR (Fig. 3) re-circulates exhaust between the two low pressure points - exhaust system tail pipe (downstream of a PM filter) and turbocharger inlet (downstream of an air filter).
When part of the exhaust gases is re-circulated to the intake air the oxygen content is decreased and the heat capacity is increased resulting in lower peak combustion temperature and less nitrogen monoxide formation.
Decrease in oxygen content causes increase in the amount of PM produced. Therefore, it is essential to control EGR rate at various engine operating conditions.
The differential pressure across the EGR line on a high-pressure EGR system is limited and achieving sufficient EGR flow rates is a challenge. As such, required differential pressures must be created artificially by altering the efficiency of a turbocharger or by introducing other sub systems.
The problem with achiving sufficient EGR flow rates does not exists on a low-pressure EGR system as other means of achieving required EGR rate are available without de-rating efficiency of a turbocharger. High-pressure EGR re-circulates unfiltered exhaust causing contamination of the engine itself and lubricating oil with soot. Contrarily, the low-pressure EGR re-circulates particulate matter free exhaust taken from downstream of the PM filter.