I am seeking a little insight on this matter, Many people swear that egr can only take away, many others swear it can add.


  Just for a little clairification I will base this assumption on a modern, normally aspirated 4 stroke ice running on 87 octane unleaded pump gas with most modern sensors, actuators and control systems.


  #1 - the way that I understand it could add, -

I'm sure that you have heard of MBT (minimum spark timing
for best torque). MBT yeilds the maximum work for a given
a/f mixture unless it is limited by engine knock or
emission restriction. MBT occurs when the maximum pressure
acceleration point (rapid burning period) is located at
TDC. If it happens too soon it will work against the rising
piston, too late and it will occur at a larger cylinder
volume and will result in lower combustion efficiency.

Now imagine that my generic engine is at a specific load
and speed where my 87 octane fuel cannot be fired at MBT
because the engine will knock, you may agree that my fuel
is not being fully utilized because the maximum pressure
acceleration point is now being forced too far away from
TDC to help avoid engine knock.

EGR enriched fuel mixtures burn at a slower rate, this
means that it will take a longer time for the flame to
propagate and the maximum pressure acceleration point will
occur later.

Now getting back to my generic engine that cannot be fired
at MBT because the load and speed of the engine will not
allow it without knock on pump 87 octane gas. I'm going to
add (yes add) some exhaust gas effectivly reducing with an
inert gas the effective engine displacement or volumetric
efficiency (of the potent A/F ratio) slightly. increasing
the burn time of the mixture, in turn allowing the ignition
timing to be advanced more accuratley towards MBT for the
fuel and air that CAN BURN without causing knock or
elevated combustion temperatures associated with NOx.

So the fuel that is being burned is being utilized properly
for that specific load and speed, and is not being held
back ( or forward depending on how you look at it) because
it may cause knock. When you are using the fuel at the
right moment in the combustion process for that specific
condition you are utilizing it better and thus getting
better fuel economy.

  #2, pumping losses, ???  a little help in this department would be much appreciated.    - as far as I know this is best reduced by super or turbocharging, It seems as though the small amount of egr contribution would not be enough to cause measurable gains in fuel economy.


  #3, Less thermal loss due to lower combustion temperature.   - this seems apparent.

A throttled engine has to suck air past the throttle when running at part load. Obviously there is an efficiency gain if we can avoid doing that, and that you cannot make the same efficiency gain twice.

So I would say there are gains to be made by filling the cylinders by having some recycled exhaust gas in there, but that one should beware that other engine improvements (like variable valve timing to achieve intake valve throttling, dicussed elsewhere) can not give you the same savings again.

You then get into the grey areas of whether things are engine improvements, or cost reductions, or have side benefits (like emissions) when implementing combinations of the fuel saving technologies; or when swapping an improvement made one way for the same improvement made another way.

A problem with writing computer programs is that they are rarely right first time, never do enough, leave the author open to criticism of programing style and accuracy of content, etc, even if they are just quick off-the-cuff little tests. So I'm am a bit reluctant to make a fool of myself here, but here goes.

I'm just going to see what sort of improvement might be possible to suction losses with EGR. Start with some definitions - too many - but just a copy paste from a quick test of something else.

Add some assumptions for an engine. It's easier to think with a particular engine size, even if the same results could be evaluated more theoretically without assuming a particular displacement, or RPM.

Then do some Noddy calculations where I hope I don't embarass myself by rushing and making some very stupid mistake that invalidates all conclusions.

Run program (with some messy PRINT statements added), a bit like this, but actually worse since they print to a file, so I can show the results.

(Hopefully that would give somebody enough code to have some sort of executable program, but testing it is not a priority as I doubt too many are interested.)

And (with more PRINT statements to a file) the output is then like this

So if you can gently suck in air and recycled exhaust gas instead of struggling to suck clean air against a partial vaccuum there is a real fuel saving to be made. I'm not suggesting you could make an 8% fuel saving. I'm suggesting there is a maximum of about 8% available (for given assumptions) and that if EGR is even 1/4 efficient at helping to make that fuel saving, then you gain a couple of percent in fuel economy when cruising along, and it is a gain worth having - though its worth saying again: you cannot make the same saving twice.

#4 (#1 revisited!)
You mention the dilution effects of EGR and its effect of slowing down combustion. Just to throw a spanner in the works, there can also be some speeding up of combustion with EGR due to the presence of particles from previous the combustion which form intermediate steps in the combustion process. I believe some HCCI engines have their fuel's autoigintion time set by controlling the amount of exhaust gas that is recycled, with more EGR giving shorter ignition times since particles participating in important reactions are already present.

So with the same amount of air and fuel as in the throttled case, but with what I'll stupidly call 'the air gap' filled with recycled exhaust gas, combustion should be quicker, enabling you to spark closer to TDC and better approximate ideal instananeous combustion. (Well, ideal  in some ways, even if it would blow your engine apart in practice.)

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