EGR - What is it? And is it really evil?

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EGR - What is it? And is it really evil?

Post by E_T_V » Mon Apr 27, 2009 11:58 pm

EGR = Exhaust Gas Recirculation

Basically under certain conditions the engine re-breathes a portion of the exhaust gas in order to reduce combustion temperatures and reduce NOx emissions.

The EGR system also decreases warm up times as when the engine is cold recycling exhaust gases increases the cylinder temperatures and so speeds up the warm up cycle improving fuel economy. There are also arguements that reburning these hot exhaust gases also improves economy at low throttle openings.

Ok so why does pretty much everyone recommend bypassing/removing it. Well people say it clogs up the air intake.
To some degree this is true but it is not soley due to the EGR system so it shouldn't get all the blame.
The sump has to hve some ventilation to remove excess air and combustion gases from the engine. On old engines this vent used to be to the atmosphere. This usually meant a discharge of oily fumes to the atmosphere which is now not allowed due to emissions regulations. So pretty much all engines discharge this crank case fume into the air intake for the engine so effectively it burns any oily mist.

So you've got an oily mist being sprayed into the air intake. Is this bad? Well it isn't wonderful but it isn't terrible. Ok now you add the recycled exhaust gases at many hundreds of degrees celcius. What happens, well the oil carbonises. If you look at the bottom of a students grill/oven, then that is the result of carbonised oil and fat. Its a black sticky carbon tar type mess and it sticks to everything. THIS is what causes the air intake to become blocked. Not the EGR, not the crank case ventilation, but the combination of them both. Remove either and you'll prevent the problem.

Is it a problem? Well yes and no. Yes as it deposits crud into the air intake. No as the deposits only really settle in the low velocity areas of the air intake which are less vital than the high velocity ones.

So what does that mean? Well for ultimate performance then the deposits should be avoided. However for normal cars then performance doesn't seem to suffer even with large amounts of buildup present.

For example this car performed perfectly well when out on the road. As you can see the buildup was HUGE.


Equally my car at 200k miles with EGR enabled makes about 125-130bhp.

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