What does EGR mean? EGR stands for exhaust gas recirculation. This is a vehicle emissions control strategy used in both gasoline and diesel engines which uses a valve (referred to simply as an EGR valve) to meter a portion of exhaust gas back into the fresh air intake of the engine. Diesel engines often use an additional EGR cooler (coolant heat exchanger) to bring down the exhaust gas temperatures as well. Since air is un-throttled in diesel engines, they can tolerate much higher rates of exhaust gas recirculation.
These systems have been around for decades with some of the early ones being problematic and crude by today’s standards. They were known to cause erratic idle, rough starting, misfire issues and other drivability problems. Carbon build-up is still likely to be the largest issue most will face, with EGR valve replacement often being the solution. Most modern versions would be either directly electronically controlled or via a vacuum line and a solenoid. Either would lend no clues as to its existence.
In an era of ever growing efficiency, todays smaller displacement engines are under stress to produce more power, better fuel economy and lower tailpipe emissions than ever before. This can often mean high combustion temperatures in the cylinders that produce Nitrogen Oxides or NOx. This is a gas which is a known contributor to air pollution and smog.
As the exhaust gases that are exiting the engine have already been through the combustion processes once, they are essentially inert. Recirculation of some of these gases can displace oxygen in the combustion chamber, which helps to cool combustion and inhibit the production of NOx. This can also have other benefits which may increase overall efficiency and reduce fuel consumption. The downside could also be a reduction in peak power output, which is why such systems are often not employed at high loads.
The EGR system is only part of the overall emissions system and, by itself, may not always be sufficient to keep NOx at the appropriate levels. Some diesel engines employ various NOx traps or selective catalyst reduction (SCR) as an after-treatment of the exhaust. SCR employs an exhaust fluid to break down the NOx.