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, 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