[Olympiadis]

Some notes/info about EGR.

The EGR system was neither designed for nor intended to reduce pumping loss.
Most pumping loss due to the engine generating pressure drop is created at the valve and not at the throttle. In fact reversion reduces a significant portion of the pressure drop inside a shared intake manifold plenum.
It is also fact that the engine must physically move more total mass in and out of the cylinder during EGR operation.


Further, EGR contaminates/dilutes the intake charge, and by design reduces burn speed, and therefor efficiency in the chamber. This reduction in burn speed is normally accompanied by an increase in spark advance during EGR operation. The need for starting the spark earlier signifies the loss of burn efficiency due to lower peak cylinder pressures around TDC. At the same time there is increased cylinder pressure earlier in the cycle before TDC, which in turn requires more energy to complete the compression stroke. This situation is less efficient than a quicker burning intake charge with the spark started later in the cycle. In driving situations where more engine power is required, EGR reduces both engine efficiency and vehicle fuel efficiency. This wouldn't be so bad except for the fact that the EGR algorithm is factory programmed to deliver the EGR charge during times of needed power production. In fact up to a point, the more power that is asked of the engine, the more EGR is delivered. This is no accident in that asking the engine to produce power increases the VE, cylinder pressure, burn speed, and BSFC of the engine, but as a side effect also creates conditions for increased NOX production.

The EGR system was neither designed nor intended to increase fuel economy. It (EGR) was designed to reduce NOX production as efficiently as possible, - to mean being effective and having as minimal effect on overall fuel economy as possible. The design engineers did a fairly good job of this within the constraints of keeping the system relatively cheap and simple. Their result is a system that does not negatively affect fuel economy or power for a fairly wide range of engine operation. This qualifier is not to be confused with "the entire range of engine operation", nor is it meant to infer that the operation of the EGR system can't result in an overall loss in fuel economy (as factory programmed). It also doesn't mean that the functioning EGR system (even when functioning properly) can't result in several other negative effects to the engine that increase the overall cost of vehicle operation. I recondition cylinder heads in my shop, but I'm not going to go into details along the lines of the negative effects of EGR. Basically the effects are similar to other conditions that cause inefficient/slower burn in the chambers.

A slower burn in the chamber means that less energy from the fuel can be converted into motion energy, which often results in more heat rejected into the cooling system. One can test and measure this effect by reducing your spark advance and monitoring coolant temp and EGT. EGR operation includes an increase in spark advance that mitigates this heat rejection problem during the BTDC phase of the burn cycle, but increases resistance to the upward moving piston by doing so.

There are points of diminishing returns for the amount of EGR delivered in respect to reduced engine efficiency and reduced thermal efficiency. Out of all the factory calibrations that I've seen, 35% is the highest amount of EGR delivery applied. This isn't to say that you can't get away with more, or change the conditions so that you can get away with significantly more. How to apply this possibility is the question. I'll try to offer a helpful suggestion.

In that it reduces engine power, EGR is similar to a modification like the WAI - warm/hot air induction. If you study the WAI performance closely you will see that a reduction in engine power can improve your vehicle fuel efficiency during times when you can get away with less power production from the engine. If you need the power, then a modification like this is going to hurt your vehicle fuel efficiency. Most people don't realize this because they use a mix of driving conditions during their testing. I have thoroughly tested both EGR and WAI back in the early 1990's.

My results are as follows:

WAI tested on wet-flow (TBI) systems:
unloaded driving +9.5% MPG
loaded driving -16.6% MPG

WAI tested on dry-flow (MPFI) systems:
unloaded driving +1.1% MPG
loaded driving -8.3% MPG

WAI overall mixed conditions with wet-flow systems (TBI) +7.3%

EGR tested on wet-flow (TBI) systems:
unloaded driving +3.9%
loaded driving -1.6%

EGR overall mixed conditions with wet-flow (TBI) -1.5%

In tests of a dozen different vehicles I have always seen a small reduction in overall fuel mileage with the use of EGR. In testing WAI in about the same amount of different vehicles I always see between 0% and around 10% overall gain in MPG depending on the fueling system (wet or dry), and depending on how over-powered or under-powered the vehicle/engine combination is.

Since EGR and WAI have a somewhat similar effect, then why the big discrepancy in MPG during mixed driving conditions between the two strategies? Well, that is explained by the factory ECM calibrations and logic that control EGR operation. Instead of delivering the EGR to the engine during times of unloaded driving, the logic does the opposite. As explained earlier, the need to reduce NOX comes at a time when engine efficiency would normally be high - when you are asking power of the engine. Therefor the factory algorithm delivers the EGR during partial loading conditions when you are accelerating (not WOT or PE mode), or cruising steadily at a moderate load. When you are at very light loads or in DECEL conditions, most algorithms stop the flow of EGR. My test results of EGR stated above were not from hypermiling, so naturally there is more to be gained and less to be lost from EGR if you hypermile - due to the lower % engine loading time.

With hypermiling, the best I've seen personally with EGR vs no-EGR is to break even on the overall MPG. That's not to say that some vehicle combinations and/or calibrations do not result in increased MPG with the use of EGR. I can see how that could very well happen given certain conditions. I've just not experienced them myself. I'm certainly not discouraging you from trying to improve MPG using an EGR system.

Based on this information, my suggestion is to basically reverse the condition thresholds for EGR operation so that you get less or none during engine loading, and more during light load/DECEL. At light load and steady throttle I don't see why you couldn't get away with 50% EGR or more, and Scangauge + datalogging should show you the point where you lose any MPG advantage that you may find. I'm very interested to see the results.

If you don't mind I will post some calibration/algorithm data, and some screen-shots of the EGR calibrations from my 1991 Chevy p/u. They may give you some insight into the system strategy that the factory set up. It will be the stock calibrations shown to avoid confusion. I modified the EGR algorithm on my truck to run the electric cooling fan instead of the EGR.