Originally Posted by Dan Stokes

I know a lot of you guys THINK you have "the truth" about Diesel emissions but I was an actual expert in the field, having worked 31 1/2 years for the EPA doing emissions testing with much of that applied to Diesels.

First of all, the EPA never "makes" any company install any particular technology. Standards are set based on health needs with the requirement (as set by Congress) to reduce emissions inventories - that is, the quantity of any given pollutant in the atmosphere. This is necessary as humans have an unfortunate habit of breathing air. Each company decides how they intend to achieve these reductions and they frequently choose similar technologies.

BTW - EPA will suggest to the industry at least one possible technology to achieve the required standard but the manufacturers are free to pick alternate technologies as long as they can demonstrate that they work and are durable.

Anyhow, as someone else noted the function of EGR is to lower Oxides of Nitrogen (NoX) emissions. NoX is a function of temperature, pressure, and time and Diesels are BAD about NoX production. Combustion chamber temps in Diesels are high (one of the reasons for our high fuel economy), pressures of course are much higher than in an Otto-cycle engine, and with our relatively slower crank and piston speeds the time for NoX to form is much longer. So, lots of NoX.

EGR, and especially cooled EGR, significantly lowers combustion chamber temps WITHOUT EFFECT ON OTHER EMISSIONS. I've done the testing using lab-grade instrumentation - this is no BS. Why folks think EGR raises CO emissions I have no clue. There is a SLIGHT impact on CO2 emissions which equals slightly lower fuel economy but not much.

The new Diesel aftertreatment systems use a catalyzed Diesel Particulate Filter (DPF) to remove particulate (which is what the general public gripes about - they write their Congresspeople) as well as reducing unburned hydrocarbons (HC) and carbon monoxide (CO) to levels often lower than the surrounding ambient air. Then a NoX trap ADSORBS (look it up) the NoX that remains after the EGR has done its job. The trap must be periodically cleaned (regenerated) and there are several ways to do this including adding DEF (a urea-based fluid) or simply driving the system rich for a short period of time. BTW - I'm named on the patent for the second method (just to establish my credibility). VW uses the fuel method on its lower HP engines and DEF on its higher output products. I have no idea why.

Anyhow, it may well be that over time the intake gets fouled as a result of EGR though I haven't personally experienced this with hundreds - maybe thousands - or hours of running these systems on the dyno. So clean the darn thing every 200K or so! But pulling the EGR will cost you something - FUEL ECONOMY! Lower combustion chamber temps lead to better FE and this is measured in lab conditions where you can actually tell the difference.

As a side note - fuel economy is VERY difficult to repeatedly measure. I could go on and on as to why but for this chat I'll just say that I won't respond to "my buddy pulled his EGR and got 2 (or 20) more MPG". You can't get more than a rough idea on the road - we've tried.

Dan