Exhaust Gas Recirculation: Description and Operation

EXHAUST GAS RECIRCULATION





Exhaust gas recirculation is used to control part-throttle exhaust emissions - EGR is not necessary at idle and is not used at full throttle so that full engine power can be produced.

During recirculation, a small amount of exhaust gas enters the intake manifold - not to be reburned but to "take up space" and reduce the useful volume of fuel/air mixture. Less fuel/air volume means lower combustion chamber temperatures, which reduce the formation of oxides of nitrogen.

Three conditions produce NOX: a lean fuel/air mixture, pressure, and heat. Modern engines must use a lean fuel/mixture to reduce CO/HC emissions. Even with low compression ratios to reduce chamber pressures, most engines still require part-throttle EGR to further reduce combustion temperatures.

There are many types of mechanical, electrical, and vacuum-operated EGR valves. VW and Audi cars use a vacuum unit controlled by a connection near the throttle plate where vacuum is only present at part-throttle.