EGR in spark-ignited enginesIn diesel enginesEdit
In modern diesel engines, the EGR gas is cooled with a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, when there is otherwise a large excess of air) in controlling NOx emissions. Exhaust recirculated back into the cylinder can increase engine wear as carbon particulate wash past the rings and into the oil.
Since diesel engines are unthrottled, EGR does not lower throttling losses in the way that it does for SI engines. Exhaust gaslargely carbon dioxide and water vaporhas a higher specific heat than air, so it still serves to lower peak combustion temperatures. However, adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the power stroke. This reduces the amount of power that can be extracted by the piston. EGR also tends to reduce the amount of fuel burned in the power stroke. This is evident by the increase in particulate emissions that corresponds to an increase in EGR.
 Particulate matter (mainly carbon) that is not burned in the power stroke is wasted energy. Stricter regulations on particulate matter(PM) call for further emission controls to be introduced to compensate for the PM emissions introduced by EGR. The most common is a diesel particulate filterin the exhaust system which cleans the exhaust but reduces fuel efficiency. Since EGR increases the amount of PM that must be dealt with and reduces the exhaust gas temperatures and available oxygen these filters need to function properly to burn off soot, automakers have inject fuel and air directly into the exhaust system to keep these filters from plugging up.
By feeding the lower oxygen exhaust gas into the intake, diesel EGR systems lower combustion temperature, reducing emissions of NOx. This makes combustion less efficient, compromising economy and power. The normally "dry" intake system of a diesel engine is now subject to fouling from soot, unburned fuel and oil in the EGR bleed, which has little effect on airflow, however, when combined with oil vapor from a PCV system, can cause buildup of sticky tar in the intake manifold and valves. It can also cause problems with components such as swirl flaps, where fitted. Diesel EGR also increases soot production, though this was masked in the US by the simultaneous introduction of diesel particulate filters. EGR systems can also add abrasive contaminants and increase engine oil acidity, which in turn can reduce engine longevity.
Sent from my SM-N9005 using Ford OC mobile app
Looks like that was copied word-for-word out of wikipedia - wikipedia is just filled with peoples ideas, opinions,(like mine) not nessesarily facts
On a diesel engine, the EGR valve/ system allows burnt carbon/ soot back into the inlet, to contaminate and eventually choke up the inlet manifold etc over time, choking the engine, reducing the efficiency/ performance/ MPG(and increasing the smoke output) this is the trouble with fitting an EGR to a diesel (aka compression ignition/ not SI/ spark ignition, or petrol engined to you and me) engine in practice, whatever the theoretical benifits are