EGR = Exhaust Gas Recirculation
A widely adopted route to reduce NOx emissions is Exhaust Gas Recirculation (EGR). This involves recirculating a controllable proportion of the engine’s exhaust back into the intake air. A valve is usually used to control the flow of gas, and the valve may be closed completely if required.
The substitution of burnt gas (which takes no further part in combustion) for oxygen rich air reduces the proportion of the cylinder contents available for combustion. This causes a correspondingly lower heat release and peak cylinder temperature, and reduces the formation of NOx. The presence of an inert gas in the cylinder further limits the peak temperature (more than throttling alone in a spark ignition engine).
The gas to be recirculated may also be passed through an EGR cooler, which is usually of the air/water type. This reduces the temperature of the gas, which reduces the cylinder charge temperature when EGR is employed. This has two benefits- the reduction of charge temperature results in lower peak temperature, and the greater density of cooled EGR gas allows a higher proportion of EGR to be used. On a diesel engine the recirculated fraction may be as high as 50% under some operating conditions.
Modern engines recirculate some of their exhaust gases back into the engine with the aim of reducing emissions.
This is the is the main benefit of the EGR system. The downside is that exhaust fumes containing carbon deposits are constantly being recirculated through the engine through time causing a build up of carbon deposits in the intake manifold etc.
These deposits sometimes contribute to the failure of the EGR system itself. Replacement EGR valves can be expensive and in the case of high mileage engines the continual carbon build up can eventually lead to total engine failure.
Removal of the EGR system by banking off the exhaust input and reprogramming of the engine managment ECU results in lowered engine temperatures, improved throttle response and economy and also can lengthen engine life by reducing oil contamination and carbon deposits.