Exhaust Gas Recirculation

 By Brandon Baldwin

              Back in the 1970’s EGR  (Exhaust Gas Recirculation)  became a  mandatory  emissions  reduction system.  It  was found  that combustion temperatures above 2500 degrees Fahrenheit produced oxides of nitrogen.  That’s where the oxygen in the combustion chamber combines with the nitrogen in the air (which is air is mostly made of) to create NOx.  The “x” can be various numbers of attached oxygen atoms.  Places such as LA have this as a regular pollutant we can see as smog.  

To  reduce  NOx,  research  found  that,  by  introducing an inert gas,  we  can  lower  combustion temperatures, and therefore, reduce NOx.  Tanks of inert gases were tried, but that’s expensive.  CO2 (Carbon Dioxide) is an inert gas, and happens to be one of the byproducts of combustion anyway.  Hence, we  introduce a little  exhaust  into  the  intake  stream  to  reduce  combustion  temperatures.  This is only introduced a little at a time when we have a load on the engine.  The greatest amount of EGR flow occurs at cruising speeds.  

Symptoms  of  an  EGR  valve  stuck  open  are  a  poor  idle,  which  seems  like  a  rough  running engine.  But, the roughness goes away with increased rpm or load.  Symptoms of an EGR valve stuck closed will usually create a hotter running condition at cruising speeds, which may lead to a “pinging” sound which is pre-detonation or also called pre-combustion.  Let this go long enough, and small pits will occur in the top of the piston, or even piston cracking.  In the late 1980’s some engines began to have knock sensors to detect this condition so that we might stop this mechanical failure.  

Speaking of knock sensors, here is precisely why you shouldn’t remove an EGR system.  When a knock sensor detects pre-detonation, it sends a voltage to the PCM (Powertrain Control Module).  The result is to retard ignition timing until the “pinging” or knock stops happening.  Do you know what that does to your power?  It’s reduced too.  And, remember how I said in the beginning that EGR is mostly used at cruising speeds? It’s NOT used at idle and WOT )Wide Open Throttle.  WOT is where you would be looking for the most power anyway.  And, each manufacturer purposely runs the engine a little rich (meaning using a little more fuel than needed) at both idle and at WOT.  Running at little rich at idle makes the engine runs a little smoother, because a customer notices mostly at idle when the number of combustion events are farther apart.  And, at WOT we run richer because there is a little more power that way.  The other benefit to a richer condition at WOT is that a richer condition is also a cooler one.  No need for EGR then.  But, that doesn’t mean we should start running rich in all conditions.  You don’t need it, nor would you want it if you saw the Ask-A-Tech article on not letting your vehicle sit there to “warm-up” (one of my longest and most informative articles in the newsletter).  

New engines with Variable Valve Timing (VVT) on the exhaust cam will not have EGR.  On those, we close the exhaust valve early in some cases to retain the “inert gas” or late in other cases, to pull the CO2 back in.  When  electric  cars  become  the  norm, we won’t have to deal with EGR or EVAP, or any of the sensors associated with air, fuel, or emissions.   We  will  look  at  EMF  instead,  but  even  that  is  getting better.  Until next time, happy travels.

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