EGR is an abbreviation for Exhaust Gas Recirculation. This is used in cars mainly for two reasons i.e lower pollutant emission and higher efficiency.
As the term suggests, the exhaust gases released from the combustion chamber are taken in (by pressure methods), and then released back in the intake tube of the combustion chamber. This then proceeds to get mixed up with external air and fuel.
Air taken in from the atmosphere has a high percentage of nitrogen in it. The intense temperatures in the combustion chamber result in the formation of oxides of nitrogen (NOx), which are harmful to the environment. EGR helps in further combustion of these oxides to reduce the amount of oxide emitted.
The presence of NOx in the intake mixture reduces the amount of oxygen being burnt. The heat released during the combustion of oxygen is much more than the heat released by combustion of NOx. Thus, when NOx burns, the peak temperature of the combustion cylinder reduces.
This improves the efficiency too as the fuel combustion is aided by NOx combustion.
There is a valve located in the exhaust pipe which is controlled electronically. The valve opens up when the engine reaches ideal temperature.
Then, the exhaust travels through the tube to the intake chamber. Sometimes, this tube through which the exhaust passes is also cooled with the help of some coolant, so that the temperature of the input mixture reduces.
The valve closes when the car is idle or when the engine is stopped.
Most sensors depend on the temperature of the exhaust gases to control the valve.
Amount of NOx in exhaust gases is observed with increasing temperature, and this data is then programmed into the sensor. The computer is then programmed to open the valve when the optimum temperature (at which there is considerable amount of NOx ) is reached.
The system does not have the ability to react immediately to changes in exhaust and works more on some ground rules. Thus sometimes there is excess of NOx in the mixture, which causes uneven combustion.
In multi-cylinder engines, even distribution of this mixture is also important so that the energy released in each spark in the individual chambers is approximately equal. Because, the NOx gas is uncontrolled, there are pretty good chances of uneven distribution of NOx gases among the cylinders.