Purpose of an EGR
An
informative and easy to find reference is
Heywood, John B., "Internal Combustion Engine Fundamentals," McGraw Hill, 1988.
In short, the EGR system is a useful engine control component, which should retained unless new engine control parameters are set (i.e. reprogram the computer).
There has been some
controversy regarding the function and utility of the EGR system in our cars.
This post is an effort to clarify these debates by giving factual info upon
which to rely.
High combustion chamber peak temperatures (the really short duration high
temperatures near the end of the combustion process) cause oxygen and nitrogen
to combine chemically and form oxides of nitrogen. To combat this, the EGR
system injects a carefully controlled (by the ECU) amount of oxygen deficient
(inert) gas. This exhaust gas serves to cool the combustion process (to below
the threshold for NOx formation), and also slow combustion (for this reason,
advanced timing curves are designed into these systems).
Contrary to popular belief, EGR actually increases the efficiency of gasoline
engines via several mechanisms:
Reduced throttling losses. The addition of inert exhaust gas into the intake
system means that for a given power output, the throttle plate must be opened
further, resulting in increased inlet manifold pressure and reduced throttling
losses.
Reduced heat rejection. Lowered peak combustion temperatures not only reduces
NOx formation, it also reduces the loss of thermal energy to combustion chamber
surfaces, leaving more available for conversion to mechanical work during the
expansion stroke.
Reduced chemical dissociation. The lower peak temperatures result in more of the
released energy remaining as sensible energy near TDC, rather than being bound
up (early in the expansion stroke) in the dissociation of combustion products.
This effect is relatively minor compared to the first two.
EGR is typically not employed at high loads because it would reduce peak power
output, and it is not employed at idle (low-speed, zero load) because it would
cause unstable combustion, resulting in rough idle. The EGR operates from a
ported vacuum source and thus is not even capable of actuation (opening)at high
vacuum closed throttle conditions.
Early EGR systems were unsophisticated affairs utilizing manifold vacuum as the
only input to an on/off EGR valve; reduced performance and/or drivability were
common side-effects. However, modern systems utilizing electronic engine control
computers, multiple control inputs, and servo-driven EGR valves typically
improve performance/efficiency with no impact on drivability. In the past, a
meaningful fraction of car owners disconnected their EGR systems. Some still do
either because they mistakenly believe EGR reduces power output or causes a
build-up in the intake manifold in diesel engines, or because they feel the
environmental intentions of EGR are misguided. Disconnecting an EGR system is
usually as simple as unplugging an electrically-operated valve or inserting a
ball bearing into the vacuum line in a vacuum-operated EGR valve. In all cases,
the EGR system will need to be operating normally in order to pass emissions
tests.
Because EGR operates under part throttle cruising conditions, it lowers the
operating temperatures of the combustion chamber. The ECU is calibrated taking
in to account the mass flow of the EGR under these conditions, so the engine can
possibly run lean without it. The combination of higher CC temperatures and a
slight lean condition greatly increases the chance of preignition at transient
part throttle conditions, and also potentially at full throttle conditions (due
to higher CC surface temperatures). There are no well demonstrated drawbacks to
a modern, properly operating EGR system.