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.
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).
An informative and easy to find reference is
Heywood, John B., “Internal Combustion Engine Fundamentals,” McGraw Hill, 1988.
