EGR and how it adds to fuel economy??? 1

[Tech50]

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


Thank you for the time, And any contribution to my knowlege.

 

[crysta1c1ear]

#2
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.

      REM Written in BBC BASIC - will run on free demo version from here:-
      REM [URL unfurl="true"]http://www.cix.co.uk/~rrussell/products/bbcwin/bbcwdemo.exe[/URL]
      
      percent=1/100
      
      kg=1 : m=1 : s=1
      
      gram=kg/1000
      
      cm=m/100 : km=1000
      m2=m*m
      m3=m*m*m : cm3=cm*cm*cm : litre=m3/1000
      
      minute=60*s
      hour=60*minute
      
      lb=0.45359237*kg
      
      inch=2.54*cm : foot=12*inch : yard=3*foot : chain=22*yard : furlong=10*chain : mile=8*furlong
      in3=inch*inch*inch : USgallon=231*in3 : barrel=42*USgallon
      waterdensity=1*kg/litre : UKgallon=10*lb/waterdensity
      
      mph = mile/hour
      kmh = km/hour
      
      N=kg*m/s/s : Pascal=N/m2
      atm=101325*Pascal
      
      J=N*m : MJ=1000*1000*J
      kW=1000
      kWh = kW * hour
      UKmpg = mile/UKgallon
      USmpg = mile/USgallon
      
      
      g=9.81 * m/s/s
      hp = 33000*lb*g*foot/minute

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.

      enginespeed = 2000/minute
      displacement = 1*litre
      pressuredrop = (1/2)*atm
      AFR = 14.7
      fuelenergydensity = 43 * MJ/kg
      airdensity = 1.2 * kg/m3
      engineefficiency = 35 * percent

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.

      onecylinderfiringrate = enginespeed/2
      
      suckingenergy = displacement * pressuredrop
      suckingpower = suckingenergy * onecylinderfiringrate
      
      volairflow = onecylinderfiringrate * displacement
      intakepressure = atm - pressuredrop
      massairflow = (intakepressure/atm) * airdensity * volairflow
      
      fuelmassflow = massairflow / AFR
      fuelpower = fuelenergydensity * fuelmassflow
      
      enginepower = fuelpower * engineefficiency
      
      EGRsuckingefficiencygainlimit = suckingpower/enginepower

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.

      PRINT "Sucking Power is ", suckingpower/hp, " horsepower"
      PRINT "Sucking Power is ", suckingpower/kW, " kiloWatts"
      PRINT "Volume Airflow is ", volairflow/(litre/s), " litres per second"
      PRINT "Mass Airflow is ", massairflow/(gram/s), " grams per second"
      PRINT "Mass Airflow is ", massairflow/(lb/minute), " pounds per minute"
      PRINT "Fuel massflow is ", fuelmassflow/(gram/s), " grams per second"
      PRINT "Fuel power ", fuelpower/kW, " kiloWatts"
      PRINT "Fuel power ", fuelpower/hp, " horsepower"
      PRINT "Engine power ", enginepower/kW, " kiloWatts"
      PRINT "Engine power ", enginepower/hp, " horsepower"

      PRINT "Vehicle efficiency gain limit due to EGR affecting piston suction losses is "
      PRINT EGRsuckingefficiencygainlimit / percent, " percent"

(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

Assumptions
-----------
Engine Speed is 2000 RPM
Displacement is 1 litres
Pressure Drop is 0.5 atmospheres
Air Fuel Ratio is 14.7 (dimensionless)
Fuel Energy Density is 43 MJ/kg
Air Density is 1.2 kg/m3
Engine Efficiency is 35 percent

Rough Calculations
------------------
Sucking Power is    1.13193885 horsepower
Sucking Power is    0.844375 kiloWatts
Volume Airflow is   16.6666667 litres per second
Mass Airflow is     10 grams per second
Mass Airflow is     1.32277357 pounds per minute
Fuel massflow is    0.680272109 grams per second
Fuel power          29.2517007 kiloWatts
Fuel power          39.2137812 horsepower
Engine power        10.2380952 kilowatts
Engine power        13.7248234 horsepower

Conclusion
----------
Vehicle efficiency gain limit due to EGR affecting piston suction losses is 
8.24738372 percent

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.)

 

[Tech50]

Thank you crysta1c1ear,

Any input on this topic is greatly appreciated. I want to also let you know that before today, I had not written any code. This alone has been fun for me to try out, although I had a few bugs :-( , it did work out in the end .

I was wondering if you thought mechanical friction type losses and the possibility of more cylinders could be added. Nothing too exact, just roughly and simple. If it is too much of a pain, I understand and thank you for your help this far.

A bit of a side note: after figuring x amount for pumping loss gain back with EGR, x amount gain for increased thermal efficiency, and x amount gain for fuel knock stability. What would you think an overall gain could be and what would you think could offset this gain. Pro's & Con's.

Part of the reason I am doing this is I have heard many contradictions on this topic, some say egr can only take away & others say it can add. I know a few autocrossers that swear "pinching the EGR tube gives it more power and better gas mileage" and many others that were getting crappy gas mileage with a "plugged" EGR passage.

Thanks for the time,
Tech50

 

[carnage1]

Early egr systems were notorious for cutting power. the reason was that they put too much in so you ended up with a flat spot on acceleration or they became stuck open and decreased the o2 available for burning at WOT and messed up your idle mixtures. (EGR should be fully closed at WOT and idle on most engines) A propperly designed EGR system in good working order will give you a much more efficiant engine at part throttle loads.

 

[mattsooty]

Actually, EGR will not flow at WOT so the EGR valve can be wide open and it will not make a difference. This is due to the small DeltaP between intake & exhaust manifolds prohibiting flow.

In a diesel engine a venturi is used to promote flow but in a gasoline engine this venturi will not be present due to the fact that the engine is throttled at part load.

MS

 

[Rob45]

"Actually, EGR will not flow at WOT so the EGR valve can be wide open and it will not make a difference. This is due to the small DeltaP between intake & exhaust manifolds prohibiting flow."

That seems a pretty remarkable statement: any naturally-aspirated engine I've ever seen still has pressure below atmospheric at WOT; under load, they will always have a pressure significantly above atmospheric in the exhaust manifold: 12 - 20 in. Hg backpressure (or more) for real-world vehicles with catalytic converters, mufflers, etc.
The result is something other than what I'd call a "small" pressure differential.

Of course, Iv'e been wrong before...

 

[Azmio]

Rob

I think you're right, exhaust pressure say at the port or manifold is always bigger than the intake manifold pressure. The exhaust pressure is at its peak at maximum torque. With pressure difference and no EGR valve, there will be movement of EGR flow to the plenum or manifold.

I agree that EGR must still be regulated and that's why we need the EGR valve. Even when the effect of putting too much EGR may not cause drivability problem, i suspect that too much EGR will cause power down at WOT.

 

[NormPeterson]

As described in the Factory Service Manual, the EGR logic for one of my cars is

(1) Idle/Deceleration/High Speed/Heavy Load/Cold - No EGR
(2) Acceleration/Warming up - Reduced EGR gas amount
(3) Other - EGR gas supply controlled using EGR position sensor signal

Now suppose that the EGR gas flow were to be reduced below that required by the ECU's definition for (2). Part throttle acceleration could be expected to be slightly stronger, as the engine would be "seeing" conditions closer to "High Speed/Heavy Load". Fuel economy might then be a few percent poorer. WOT acceleration or acceleration that falls within the ECU's definition of "Heavy Load" should remain unaffected with an otherwise properly functioning EGR system.

Norm

 

[globi5]

If the amount of recycled exhaust gas is high and hot enough it can provoke auto ignition (detonation) and therefore result in a faster initial burn: Honda has shown this on its 2 cycle engines with activated radical combustion.

http://www.motorcycle.com/mo/mchonda/exp2_tech.html

If valve timing was more flexible this should be doable on a 4 cycle engine as well.

I just noticed that crysta1c1ear already mentioned following:
"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."

However I would believe that it is the heat of the exhaust particles causing a faster burn and not the actual exhaust particles, since less oxygen should usually lead to a cooler and slower burn unless there's more initial heat to start with. Or does for instance carbon monoxide burn significantly faster/hotter than gasoline?

 

[OP800]

In reference to the original post, in automotive use the EGR application is highly driven by the EPA requirments.
The EGR is usually invoked during low load, low throttle cruise conditions detected by the engine's control system.
When the routine is called, the fuel is cut back and ignition timing advanced.
This is where the fuel mileage gain occurrs.
As the inital intent of EGR is to reduce peak combustion temps. the increase in fuel milage is the side benifit.
On some of the latest engine designs with variable cam timing, there is/will be an attempt to optimize/control combustion events so EGR, as we know it now, may be eliminated.

 

[DanEE]

The Toyota 3.4L V6 (used in 4Runners since 1998) no longer has a discrete EGR system.

It's a DOHC 4 valve/cylinder, pent roof cylinder head and fixed cam timing..

In God we trust. All others state the facts and cite references.

 

[OP800]

That is correct. The smaller motors often are clean enough not need EGR application.
This is a point that needs to be considered, the displacment and application.
I have two vehichles with the same engine, two years apart in age. One has EGR one does not. Both are OHV v6 2.9L with fixed cam timing.

 

[Azmio]

not to forget that engine equipped with variable valve timing is capable of regulating the amount of EGR going into the combustion chamber without the need for external EGR valve.

EGR is almost a must in lowering NOx and I am yet to see an engine passing SULEV or Euro 5 without some sort of mechanism to regulate EGR into combustion chamber.

However, it is still possible to rely on the fixed cam timing to have certain valve overlap for internal EGR provided that the engine rpm range is small. Still, the best way to regulate EGR flow is through the use of external EGR valve or VVT.

 

[cleverlever]

Patent 3,714,932 has some great information on the virtues of internal EGR derived from VVT and external EGR.
In a nutshell Internal EGR is vastly superior to external EGR in accelerating engine warm up. Internal EGR is superior to external EGR in reducing hydrocarbon emissions because the hydrocarbon emissions are more concentrated in the last part of the exhaust stroke. Internal EGR is better for uniform distribution of EGR because its regulation is confined to each cylinder rather than diffused in the intake manifold

Someone with better computer skills than my self might consider posting some illustrations from the patent. Makes you wonder why we ever did external EGR at all..

 

[crysta1c1ear]

http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=US3714932&F=0&QPN=US3714932

Anybody interested in a patent can just download the whole patent as a PDF, so they get the text and the pictures.
(There is a weblink to save the complete document and a little text checking procedure to go through just to make sure you are not some soft-robot trying to leech the whole database.)

=

I can see sense in the idea that

Internal EGR is superior to external EGR in reducing hydrocarbon emissions because the hydrocarbon emissions are more concentrated in the last part of the exhaust stroke.

Unburnt hydrocarbons can arise from crevices where the hydrocarbons can hide from the flame front, eg between the cylinder and piston above the piston rings. They are compressed into the crevices during compression, and the volume of heat sinking metal around them makes them harder to ignite.

 

[icrman]

I guess soon the blower on a AA fuel dragster will be replaced by an EGR system.

 

[KevinKoss]

I am trying to put EGR on a small air-cooled engine as a possible cost-saving alternative to a catalyst in controlling HC and NOx emissions.

Does anybody know some makers of "dumb" (as in no closed-loop control) EGR valves for engines of 500cc and less?

NOx is obvious, but I think HC will go down as well because a small percentage of the unburned HC will have a chance to fire again, and it will also displace some of the intake charge. These small air-cooled engines consistantly run rich to keep them cooler under load.

Plus the EPA and CARB requirements have an additive standard for these engines, so one can lower either NOx or HC and still be green.

Thanks for your help

 

[turbocohen]

May I ask what the additive standard for the engine is?

 

[patprimmer]

Close the exhaust valve later and open the inlet later, and suck exhaust gas in on the first part of the induction stroke.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[KevinKoss]

The 2006 std is 16.1 g/kWh (HC + NOx) and drops to 10.0 g/kWh for 2007.

I am investigating a open loop catalyst as well, no sensors.
But, I believe EGR would be cheaper and might get us by until more stringent regulations come in 2012...

In regards to the cam timing - we really can afford to loose advertised HP to make this happen, which I believe would happen.

Thank you for your input.