RE: What EGR Does...

["Storhok, Eric (E.M.)" <estorhok@domain.elided>]

A fellow digester asked me to explain what EGR does, so I'll do my best to keep it short.  Recall that I make the claim that is does 2 things- reduce NOx emission and increase fuel economy.
 
First the NOx.  Oxides of Nitrogen are formed at very high temperatures- 4000 F +, which are quite reasonable for peak combustion temps.  So, to keep the formation down, one could reduce the peak combustion temp.  Chemically, one very easy way to do this is to introduce the final product into the mixture before the reaction takes place.  You see, reaction rate is related to how much of "before" to how much "after" is around at any particular time.  So you can reduce the peak reaction rate by having more "after" at the start of the reaction.
 
With modern emissions equipment, NOx isn't too hard to convert back to N2 and O2 (which helps finish the CO/ HC reactions).  But around 1995, the NOx standard dropped rather significantly, so to help the conversion, I think Alfa added EGR to the 24V engine.
 
Now the fuel economy- the largest friction loss for spark ignition engine is the throttle, big enough that it can do negative work (the car will slow down quickly when you close the throttle).  So, to reach high fuel economy, not only do you want to run at optimum spark and fuel, you want to run as high a throttle position that you can.  Obviously, you can only go so far....  But what if, for a given amount of air and fuel in the combustion chamber, you added 15% of inert gas?  Of course, the ideal amount of energy will go down, but to get the correct air into the chamber, you need more throttle (as you displace it in the intake manifold with the EGR).  Well, the loss in power due to the inert gas is quite a bit less than the friction loss from opening the throttle (up to a point, of course).  So, with all the adjustments that can be done with modern computer controlled engines, most companies take advantage of this to add fuel economy by using EGR.
 
So, now review my earlier points about Alfa and EGR- I made the conclusion that Alfa managed to make the NOx numbers without EGR, and did not mind taking the fuel economy hit.  So they deleted it.
 
Any questions?

Eric Storhok
Core Emissions and Fuel Economy Team
Advanced Powertrain Engineering
(313)33x75011
MD 35, FPC-B
Ford Motor Company

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