THE ABCs of EGR

Unless your vehicle fails an exhaust emissions test due to a high NOx reading, you probably haven't needed to know much about your vehicle's EGR (Exhaust Gas Recirculation) system. That's fine—you have enough to worry about already, right? Still, it's a vital component so it's important to understand how it works, which might help when making repairs to restore its function.


In this first of a 2-part series, we'll provide some background and overview of the EGR system, including the reason for its existence. And in our second part, we'll dig into some specifics on maintenance and repair.

Purpose
The EGR system made its debut on pollution-controlled motor vehicles in the U.S. in the early 1970s, preceding the catalytic converter by a couple of years. Its sole function was to reduce NOx emissions by recirculating exhaust gas into the engine's intake system during above-idle driving conditions.


NOx is often referred to as nitrous oxide, but correctly known as oxides of nitrogen when produced as an exhaust byproduct of the internal combustion process. The higher the combustion temperatures go, the more NOx emissions are produced.


Other factors that affect NOx production include combustion pressure (compression ratio), ignition timing, fuel mixture, intake air temperature, and coolant temperature. Of course, variations in these conditions also affect other exhaust by-products, such as carbon monoxide and hydrocarbon emissions, as well as overall engine performance and fuel economy. Balancing these factors to achieve optimal results in the areas of engine performance, emissions control, and fuel economy has been an engineering challenge, to say the least.