A process in which some exhaust gas is sent to the intake manifold to be mixed in with incoming air, used in internal combustion engines.

The idea is that by replacing some of the highly volatile fuel/air mixture with inert exhaust gas, the resulting combustion is slower and cooler, which causes more complete combustion and consequently lower harmful emissions.

The EGR Valve regulates the amount of exhaust gasses that are mixed back in.

The EGR Valve also reintroduces a larger portion of gases during the engine's first few moments running, when a necessarily rich mixture is used to get things going. The unburnt fuel in these first few moments of an engine's run can be a large percentage of its polluting output, especially if it's a modern engine in good repair. Redirecting fuel-rich exhaust gases back into the manifold does (at least) two things: one, it burns a greater percentage of the rich fuel mixture, and two, it ensures that warmer air is sent into the cold engine so as to speed up the warmup cycle.

One of the most misunderstood and falsely maligned emision controls is the EGR. Removing, disabling, or disconnecting it is an ignorant thing to do. Unless you replace your carburetor with a non-EGR model or the mid-range jets in your current one, you'll only succeed in wasting gas and burning up your engine. If you do compensate for removing it, you'll still waste gas, but you won't burn up your engine. Contrary to popular opinion, the EGR system does not make your car slower, burn unburnt fuel in the exhaust, or operate at wide open throttle. It improves efficiency and prolongs engine life. Technically, it doesn't even reduce emissions.

If your car came with an EGR, it runs very lean at part-throttle. This is to eliminate unburnt fuel* and improve gas mileage. Lean combustion is excessively violent and hot. If lean enough the pistons will be used for fuel. Inert exhaust gas is introduced to buffer the lean combustion, reduce heat, and prevent your pistons from becoming fuel. In addition, ignition timing is usually advanced under part throttle, even more so in engines with an EGR. The EGR helps prevent detonation, or the ignition is further advanced to compensate for the EGR, depending on how you look at it. 

In short, don't mess with your EGR, except to make sure it works properly! If someone recommends disconnecting it, they don't know what they're talking about; you may want to consider not listening to any of their performance recommendations.

* The amount of exhaust that is recirculated is minuscule, compared to the rest of the exhaust. Unburnt fuel is eliminated by running lean at part-throttle and the EGR only operates at part-throttle. If the mixture was rich enough to leave unburnt fuel in the exhaust, the EGR would be pointless. That's what the air-pump and catalytic converter are for.

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