EGR - Why it isn't really that evil

The EGR system often has a bad reputation. This in most cases is undeserved and in this article I'll try and explain what it does and why it gets the blame for things that aren't really its fault

 

EGR takes a controlled amount of exhasut gases and feeds them back into the air intake manifold (on some cars via a heat exchanger)

 

This has two main effects of reducing NOx emissions (not an MOT tested item), and decreasing the engine warm up time (which aids emissions and fuel economy)

 

So why does almost everyone recommend blanking it off?

Well two main reasons.

1. Hot exhaust gases entering the intake system mix with the incoming air from the turbo. Unfortunatly due to emissions reasons this air is contaminated by oil mist from the engines crank case breather system. I'm sure anyone who has done any cooking knows what happens when you get hot oil and very high temperatures together. You get carbonised oil which sticks to everything. As the hot gases encounter the oil mist they essentially partly burn the oil turning it into a right sticky old mess. The issue is that this sticky mess tends to stick to the walls of the inlet manifold and the head castings. This is rarely anything to worry about but it can build up to some pretty impressive levels without any obvious performance inducing problems. However for ultimate performance the carbon/tar mess would be better if it wasn't there but to effectively remove it it requires a strip down of the engine which most people aren't willing to do. Some people will clean out the inlet manifold but most won't go as far as cleaning out the valve pockets inside the engine. So EGR on its own doesn't create the buildup of tar, it is the combination of the oil mist and the EGR. Remove one of these and you'll not get any buildup.

2. Reports from owners saying that it has reduced their smoke levels. This is usually due to a couple of factors. The first is that the EGR is faulty and so isn't closing properly or is closing too slowly leading to too much exhaust gases being recirculated, i.e. the EGR is faulty to start with so it is hardly suprising that fixing/blanking it off reduces the smoke levels. The second is that owners who are getting too much smoke are those more likely to have made some performance mods. Why is that important, well the EGR system is set up against a "known" map. It is set in the ECU. If you start fiddling with how the turbo works, the pressure it produces, when it starts producing boost etc, the EGR map is then wrong. The EGR system quantity is highly reliant on the pressure difference between the exhaust manifold and the air intake. I.e. the pressure in the intake must be lower than that in the exhaust manifold in order for gas to flow in the right direction. If you suddenly increase the exhaust manifold pressure (as is often the case when you increase the fuelling into the engine) and the valve is open for the same length of time (as the EGR map doesn't know any different), then of course you get excess exhaust gases being recirculated and therefore more smoke. 

 

So you can see why EGR gets a bad reputation which isn't really deserved. If it is operating correctly and we didn't fiddle with our engines then it'd be perfectly ok and not cause us any issues!

 

Just as a side note the EGR system in the post '04 cars do actually monitor the airflow through the EGR, or more directly the airflow through the air intake to the turbo via the MAF sensor. When the EGR is open, the airflow through the MAF sensor drops and the ECU knows it is working properly. This means that if you try to blank off the EGR then the engine management light comes on as it detects a fault. The earlier cars do not have this error checking routine built into them.

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