Good Read on EGR, how it helps and why it does not hurt!

[EagleMark]

Here's some good information from Detroit Engineer!


Increased Efficiency Through EGR?

I enjoy reading your column and just
wanted to pass on some additional info on a question I saw in the May issue
entitled "Pinging Imp". While I dont disagree with your response, there is
something involving the EGR and spark knock that is far more influential than
any impact the slightly lean condition due to lack of EGR flow would cause. EGR
is added to the intake charge as you describe to lower the combustion temps to
reduce the formation of NOx. Older systems that just had EGR tacked on without
any electronic controls caused driveability and fuel economy penalties with EGR
and gave EGR a bad rap. Later systems, like the one on the Impala SS described,
actually used EGR to improve economy by adding a lot of spark advance when EGR
is enabled. A very rough rule of thumb is that for every single percent increase
in EGR, the spark calibration is increased 2 degrees to offset the slower burn
due to the dilution effect of EGR on the charge. So, if an engine is running 10
percent external EGR thru the EGR valve (http://motors.shop.ebay.com/Car-Truck-Parts-/6030/i.html?_nkw=EGR+Valve), then the spark
is increased 20 degrees roughly to compensate. The engine will actually be more
efficient like this than without EGR, as the EGR is "throttling" the engine and
reducing the pumping losses slightly by reducing the intake vacuum. It works-
trust me. In any case, there are multiple spark calibrations and tables in the
OEM PCM for "EGR on" and "EGR off" spark operation. So, if the EGR is supposed
to be on, the spark advance is increased dramatically over the "normal" "EGR
off" spark levels. If the EGR is not flowing because of a failed valve,
restricted EGR feed port, loss of vacuum signal, or so on, the engine will
likely detonate or spark-knock heavily due to the fact that the spark advance is
being advanced considerably by the PCM, although there is no accompanying EGR
flow to dilute the charge and slow the burn rate. That is why the Impala SS is
likely detonating, not just because of the slight lean condition caused by the
EGR fuel compensation and lack of EGR. At part-throttle, the closed -loop
control will pretty much correct for the fueling difference anyway, so I suspect
that the reason the detonation is there is because of the extra EGR spark
advance in the calibration without the accompanying EGR flow.
Al
Cline
General Motors Powertrain
High Performance Vehicle Operations

Pontiac Engineering Center
Pontiac, MI

[93V8S10]

Very good info, Thanks!

[EagleMark]

I knew about this but wish I would have read this article a few weeks ago... I had disabled my EGR on my LT1 in the bin and checked fuel mileage for that tank, lost 5 MPG.

Always knew the timing tables are different with/without EGR but I did not adjust them for that tank, so on the LT1 PCM you need to do more than just disable EGR, the way it looked my timing would be intact without, I checked and got no knock counts so thought I was good to go... Ha!...

I always try to warn people when deleting an EGR that it may leave timing for EGR enabled which would lead to MAJOR ping/knock.detonation!

[d0nk3y]

While I agree with most of what was said above, the increase in fuel economy comes more from the reduction in pumping losses than with an increase in spark advance. I cannot share any "Actual" data, but if you look at a spark sweep of an engine at one speed & load point, you would see that torque increases with spark advance.

To a point. As you approach MBT (Minimum spark for Best Torque), you get less and less torque from added spark advance until you actually start losing torque from advancing the ignition. At most operating conditions, you are never 20 degrees retarded from MBT - so advancing spark during EGR is only beneficial up to reaching your MBT timing.

The larger contributor is something he briefly touched upon:Reduction of Pumping Losses. Forgive me if I can't explain this simply - I am a horrible "explainer of things" sometimes... Exhaust gas can be treated as an "inert" gas, i.e. - whatever percentage of exhaust gas you have in the cylinder - you can count on that fraction to NOT support combustion.
So, what happens if we're cruising along with NO EGR, say 60 mph on a flat road, no wind: We're holding the accelerator pedal down as far as needed to maintain 60 mph. This pedal position will equate to some angle of throttle opening. Let's say you're running 28 degrees advance at this point (knock limited).
Now, turn EGR ON, let's say to add 20% dilution:
1. You can add spark as noted in the article above, but adding more than ~ 10 degrees is going to put you over MBT. Again, some of the advance is added to offset the slower burn, but most is added to take advantage of the dilution / lower combustion temps.
2. Now that EGR is replacing 20% of your trapped cylinder volume, you have roughly 20% less power available; if you're trying to maintain 60 mpg in the same vehicle, you'll need to press on the accelerator further / open the throttle more to produce the same power level you had before (to maintain the 60 mph). Lucky for us, we're in closed loop, and our fuel control adjusts accordingly and we don't add fuel just based on TPS voltage.
The one thing that is definitely different is that we now use less fuel because we're running with less throttling (pumping losses). This is the same reasoning behind why freer flowing intakes and exhausts make more power.

At any point in time in the operation of an internal combustion engine, you have a very "simple" equation:

Power(net) = Power(gross) - Power(losses), where your gross power comes from the combustion efficiency & energy from the fuel burned & your losses come from things such as:
Intake restrictions
Exhaust restrictions
Accessory drive losses
Frictional losses
Tribology (oil lubricity/pumping/shear) losses

Anyway, long story short: EGR is your friend! :)