What Are the Functions of the EGR Valve?by John S. Kepler
The exhaust gas recirculation valve (EGR) is perhaps one of the most misunderstood parts found under the hood of a modern automobile. Designed to help control pollution it, along with the PCV valve, has been instrumental in cleaning up the air in and around cities. But to understand what it does, you must first understand a few things about how fuel is burned inside the engine.
Basic Principles of Automotive Combustion
Gasoline burning engines operate by mixing fuel with air, compressing it inside the cylinder, and igniting it with the spark plug. Ideally gasoline burns oxygen. But air contains 70 percent nitrogen and has traces of other gases. Nitrogen is fairly inert and doesn't like to combine with the gasoline. But as combustion chamber temperatures climb, nitrogen is far more likely to combine with oxygen and form nitrous oxides, also known as NOx. Nitrous oxides are a major component of urban air pollution. Gasoline, too, burns best when combined with air at a ratio of 14.7 air to 1 fuel. But leaner combinations improve fuel economy. The problem is that when gasoline is burned lean it tends to knock. Knock greatly reduces thermal efficiency and can damage the engine if allowed to continue. While there are practical limits to how lean an engine can be forced to run, it can be improved somewhat with lower combustion chamber temperatures. By lowering combustion chamber temperatures, pollution is reduced and fuel economy improved. Performance will be sacrificed somewhat, but that is a necessary price for air quality.
Combustion Chamber Temperature
There are two ways to decrease combustion chamber temperatures. The first is to reduce compression ratio. Compression ratio is the amount of compression provided by the cylinder. But significantly reducing this factor to less than 8:1 or so drastically reduces both performance and efficiency. Another way to reduce combustion chamber temperature is to add something inert to the air-fuel charge. Something that won't burn. Fortunately every car and truck has a ready made supply of inert gas--the exhaust. Surprisingly, adding exhaust to the inlet air charge actually decreases maximum combustion chamber temperature. This seems counter-intuitive since the exhaust is hot. However, when it exits the cylinder it is cooler than the maximum combustion chamber temperature. So by pumping it back into the combustion chamber it will not burn again and absorbs heat.
The EGR Valve
EGR stands for Exhaust Gas Recirculation. The valve directs a small portion of exhaust gasses back into the inlet air charge and lowers the maximum temperature of the burning fuel. The valve makes sure that the EGR system is off at idle, where it would lead to erratic and loping idle, and at peak power, since adding exhaust gasses robs power from the engine.
Besides reducing pollution and lowering combustion chamber temperature, recirculating exhaust also has the effect of lowering pumping losses. Pumping losses are the work the engine must do to pump the inlet air past the throttle plate. Since EGR lowers power the throttle must be opened wider for a give desired power, meaning the throttle opening is wider so the engine doesn't have to work as hard to inhale the air. Moreover, with a lower combustion chamber temperature there is less heat loss to the metal walls of the cylinder, the piston, and the cylinder head so that more internal heat is retained to be converted into mechanical work.
History of EGR
The first EGR systems in the early 1970s operated strictly on manifold vacuum. They had a huge impact on performance, driveability and reliability. Many owners simply removed them and modified the carburetors to prevent running lean. Slightly later systems added electronic controls that improved both performance and reliability but problems continued and the systems, while mandated by Federal Law, were unpopular with drivers. Some foreign manufacturers opted to make their engines very small and, since they produced so little exhaust gas, were able to eliminate the EGR system completely. This move helped them gain a foothold in the American market. The system continued to evolve however, and combined with modern computer-controlled engine management systems, have no impact on driveability at all, and in fact provide a significant benefit both to controlling pollution and to improving fuel economy.