So Why Do We Have EGR?

By Glen Beanard, technical contributor

The Exhaust Gas Recirculation (EGR) system was created in the early 1970s for the same No. 1 ranking motivation that drives every other engine management system design - to reduce emissions. The EGR valve targets one specific pollutant, oxides of nitrogen (NOx).

Under normal combustion, the oxygen (O2) in the air plus hydrocarbons (HC) in the fuel combine into water (H2O); carbon dioxide (CO2) and the nitrogen remain unchanged. However, when things heat up in the combustion chambers to temperatures around 1,300° C or 2,500° F, all that changes. At those temperatures, oxygen and nitrogen start combining with each other and form NOx. This is a big problem because when the sunlight hits those NOx, they combine with hydrogen in the atmosphere. When that happens, we now have smog. So how do we combat that? By cooling the combustion chambers down to where normal combustion can take place.

There are several ways to lower the combustion chamber temps. Enrichening the fuel mixture, lowering the compression ratio, retarding the ignition timing, lowering the overall temperature of the engine, and reducing the amount of pure air can each lower the temperature in the cylinders. Problem is, all of those except for the last one will also reduce fuel economy. Also, each of those except for one will also raise the HC emissions. "Watering down" the intake air is the b...