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EGR and how it adds to fuel economy???

EGR and how it adds to fuel economy???

EGR and how it adds to fuel economy???

(OP)
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.

RE: EGR and how it adds to fuel economy???

#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.

CODE

      REM Written in BBC BASIC - will run on free demo version from here:-
      REM http://www.cix.co.uk/~rrussell/products/bbcwin/bbcwdemo.exe
      
      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.

CODE

      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.

CODE

      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.

CODE

      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

CODE

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.)

RE: EGR and how it adds to fuel economy???

(OP)
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 sad  , it did work out in the end smile.

  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

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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

RE: EGR and how it adds to fuel economy???

"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...

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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

RE: EGR and how it adds to fuel economy???

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?

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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..

RE: EGR and how it adds to fuel economy???


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

Quote:

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.

RE: EGR and how it adds to fuel economy???

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

RE: EGR and how it adds to fuel economy???

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

RE: EGR and how it adds to fuel economy???

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

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

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.

RE: EGR and how it adds to fuel economy???

Internal EGR is implemented via variable valve timing and internal EGR is cooler than external EGR which reduces the potential detonation limited power loss.

Internal EGR is superior to external EGR in all respects.  This was all known 30 years ago.  Makes you wonder why they just didn't do internal EGR from the get go rather than waste all the interim cost of retooling to accomedate external EGR

RE: EGR and how it adds to fuel economy???

The small engine market is right now on the verge of being where the automotive world was in the mid-70s.  Primitive cats, inexpensive EGR, etc.

Variable Valve Timing to incorporate internal EGR is not something the small engine market can afford at this time (although the preferred method).  These are carbed and air-cooled, some still have a splash-feed lubrication.  

Replacing with a similar but different camshaft in is one thing, putting a closed loop electronics system with two OVERHEAD camshafts would triple the cost of the engine.  Remember these engines just recently adopted overhead valves (from side valves).  

Hence, I refer to my original question:
Does anybody know of manufacturers of "dumb" EGR valves for smaller engines?

Regards.

RE: EGR and how it adds to fuel economy???

I would ask you to rethink that statement that the small engine industry can't afford variable valve timing.

What if the single cylinder engines were intake valve throttled?  Thats how the small engine industry was birthed.

You can pick up internal EGR by altering valve event size or valve timing.  By going the Intake Valve Throttling route you pick up the internal EGR and far more importantly you get the cleanest ,leanest cold start known to mankind since the air fuel mixture is subjected to sonic atomization.

The cost to implement in a single cylinder wouldn't be any more than for a conventional compression release mechanism.

RE: EGR and how it adds to fuel economy???

I am trying to digest how this would be possible on a single cam, as cheaply as possible.

There is compression release, but it is simply a spring mass system based on RPM.

Please describe in detail how you imagine being possible.

RE: EGR and how it adds to fuel economy???

I have 13 patents related to variable valve timing.  To answer this question you would need to tell be if you are talking flathead or OHV application.

There are over 700 patents pending related to variable valve timing that you can easily view at the USPTO site.

As I understand it the largest single challenge facing the small engine industry is a cheap way to reduce cold start emissions.

Would be glad to help you but need more info on your application.

You might be interested in these sites and links at these sites. http://cleverlever99.blogspot.com  http://modifiedatkinsoncycleengine.blogspot.com/

RE: EGR and how it adds to fuel economy???

cleverlever,
OHV application.  I will take a scan of what's out there on the US patent website.

I am having a difficult time picturing the mechanism in my head, I need more details.

The cold start emissions aren't really the concern for emissions.  Emissions that are reported to EPA/CARB are taken from a engine that is warmed up.  Cold start is more of a concern for the consumer (which is important, just not for my situation) - do you really want rotator cuff surgery after trying to pull start your lawn mower?

If we can't pass the standard, we cannot produce and sell the engine.  I would like to find the cheapest method to do this.  Catalyst will work, I know this... but I don't want to be that narrow minded... I think EGR could be cheaper.  

I appreciate your help thus far.

RE: EGR and how it adds to fuel economy???

Kevin,

1/ I very much doubt whether EGR will improve your HC emissions, infact it will probably increase both HC & CO.

2/ Open loop 2 (or 3) way catalyst without sensors would not help you. A certain amount of lambda dither is required to allow the catalyst to both oxydize & reduce the exh gases, without a sensor this will be neigh on impossible.

3/ As for VVT, I dont think that is really what your product needs. I cannot see how the additional on cost could be explained away!
 
With the EGR you may be in a position to trade off NOx reduction with HC production and actually meet your targets, however it will probably be quite a steep trade off curve!

What is the emission cycle that EPA/CARB require you pass? I am not familiar with their test cycles in this class. The trick may be in 'beating the system'...

MS

RE: EGR and how it adds to fuel economy???

Disagree. In the early days of emissions control (air pumps and such stupidity) EGR /was/ used to improve HC, CO and NOx

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: EGR and how it adds to fuel economy???

As for small engines, I am still amazed to see a lack of simple feedback controls.  A few japanese imports use electronic carbs and injection systems with electronic controls that are O2 sensor ready.  If all engine mfg's have to meet regs then it will level the playing field.  OK, another $20 for a cleaner running closed loop engine.  What a bargain.  Where can I buy one..  seriously.

One of the best features of my motorcycle of choice is the low smog closed loop engine and catalyst.  Touring in a group of bmw's is much easier on the lungs than most japanese and damn near all US brand bikes.  I would brag about it if my lawn mower was closed loop with a catalyst/muffler.  I bet that no american mfg's have upper management with the forsight or the balls to beat Honda to market with clean small engines.

RE: EGR and how it adds to fuel economy???

The biggest emission challenge facing the internal combustion engine is reducing cold start emissions.  All the electronic gizmo's in the world can't surpass the clean start characteristics of Intake valve throttling due to the sonic atomization benefit not being dependent on engine temperature.

Variable valve events are simple on a single cylinder engine because you don't have to deal with cylinder to cylinder synchronization.

You also get the advantage of increased low speed torque which is a huge factor in single cylinder engines.

Another advantage of the IVT approach is the potential to implement Atkinson Cycle Technology.  http://modifiedatkinsoncycleengine.blogspot.com/

RE: EGR and how it adds to fuel economy???

Greg,

Disagree all you want...Just please explain to me how reducing the combustion efficency by way of adding EGR is actually going to improve the 2 pollutants that are formed because of combustion inefficency.

I think what you are wrongly classing as EGR (EGR with airpumps???) is in fact SAI, a different thing entirely. This is only used during catalyst light off and will indeed cause a reduction in HC & CO from the tail pipe.

MS

RE: EGR and how it adds to fuel economy???

EGR was used to improve emissions before (proper) cats were used. I can only give the hand waving explanation that any HC in the exhaust was given a  second chance to burn, likewise the CO. I don't know about the supposed effect on NOx

For what it is worth EGR was still included as an option in some passenger car engine development exercises as recently as 10 years ago, but I don't know when it last made it into production.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

RE: EGR and how it adds to fuel economy???

Thank you for all the interesting debate.

I know open loop catalysts work, some manufacturers are using them today.  The EPA also just did a study on exactly this and found it to work.

The production costs on these engines are like $150-300, adding a $12 cat-muffler is hard to swallow, plus there are heat concerns.  I think EGR would be a cheaper method to reduce the HC+NOx emissions (and cooler).  So that is why I was asking for the manufacturer of small engine EGR valves(maybe mopeds/motorcycles in European/Asian markets), which I still don't know.

Yes engines today are trying to light-off as soon as possible because almost all emissions are made within the first 2 minutes of warm-up... but that is an automotive challenges - small engines are decades behind.

RE: EGR and how it adds to fuel economy???

Kevin,

A 2 way oxidation catalyst will work to reduce the HC & CO, that much is true. However as previously said it will prove challenging to keep the AFR such that there is no break through from the catalyst becoming 'saturated' with pollutants.

The EGR will not reduce the HC, though it will reduce the NOx at part load. At WOT you can forget about using EGR unless you design in some sort of venturi set up, even then you may struggle to flow sufficent quantity.

My apporach to your problem would be to try and run the engine as lean as possible to reduce my HC then add EGR piecemeal to try and bring the NOx down.

In general though all the tosh about HC & CO being given another chance to burn is absolute tosh, the whole raison d'etre of EGR is that it is INERT.

As for EGR still being in production, it is, infact the last engine I calibrated with EGR has fairly recently gone into production in the new 4.4l Range Rover.

MS

RE: EGR and how it adds to fuel economy???

EGR's obvious main purpose is NOx reduction (inert gas temperature cushion). Any HC benefit is secondary (heating intake charge thus improving vaporization (it's hot EGR) - the second go-around is kind of non-sensical in a steady state mode). CO benefits, uhh, maybe from better vaporization, but seems like a bit more of a long shot.

EGR has one advantage, though - it's hardware emissions improvement, such that even if the EGO fails, or open loop in general, EGR can have benefits. Throttling losses are probably another slight perk.

If you look at it from a constant torque production standpoint - you'll see that with EGR
a) More throttle opening is required (less throttling loss)
b) Less NOx is produced (reduced peak temperature)
c) More timing is required for MBT (drawback, in my book)
d) More cylinder pre-combustion pressure is achieved (HC benefit perhaps??)

I still can't picture how a single throttle has more loss than individual throttles per cylinder, when looked at from a constant torque output (which is how I measure economy). Anyone care to clarify?

RE: EGR and how it adds to fuel economy???

I know the engine runs rich, all air-cooled do.  But the catalysts plan for that and are able to last the 1000hrs of life required.

I am going to lean things out, to see if I can lower HC without raising NOx too much.  When those benefits are no longer measurable, then I will have to add some sort of exhaust treatment (whether EGR or catalysts).  I just thought the EGR would reduce NOx efficiently and cheaper than a cat.

Also, in my particular application, temperatures are also a concern.... so running a little leaner might raise the temps too much, (also adding a cat might do the same) so EGR seemed like the best option.  Lower temps, lower emissions, less cost vs. catalyst.

I understand that exhaust is supposed to be inert, but when you are running about 0.85 lambda (common for these engines), there are some "leftovers".  It may not be a measureable change in HC, more of wishful thinking on my part.

Thanks for everyone's input.

RE: EGR and how it adds to fuel economy???

sounds like you'll need an air pump to go with that catalyst...?

RE: EGR and how it adds to fuel economy???

In the EPA's study, they looked at check-valves that would let a little air in the exhaust due to the pressure pulses from the valves open/closing... but they found they were not needed.

There is enough oxygen still remaining that the cat is beneficial

RE: EGR and how it adds to fuel economy???

Greg:
You need more than hand-waving to make EGR reduce HC and CO:  you need some magic too,  since it just didn't (in the '70s) woprk that way,  and it doesn't now.
The amounts of EGR used at that time were far too small for the "second chance combustion" to amount to anything.

EGR reduces NOx by reducing peak cylinder pressures/temperatures.
More air reduces HC and CO.

RE: EGR and how it adds to fuel economy???

... so does EGR allow you to run a bit leaner w/o violating temp constraints, thus reducing HC&CO out?

RE: EGR and how it adds to fuel economy???

Rob45, I agree, I never entirely believed the HC and CO bit to be honest - after all if you are using 10% EGR that will only reduce your HCs by 10%, not enough to matter, but I didn't work in engines at the time, so I accepted what I was told.

So it must have been NOx then?





Cheers

Greg Locock

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RE: EGR and how it adds to fuel economy???

Egr allows running a little closer to Stoich by allowing less pumping loss via dilution with co2..  I wouldnt say leaner.  In the lab I compared raw egr with externally supplied CO2 from the exhaust, the difference was that we could throttle after initial startup using only sequestered exhaust sourced CO2.  The direct injected Engine running on CNG demonstrated the useful improvements were negligible nox and improved part load fuel economy using lean burn strategies and reduced nox at stoich for world point loads aka 38psi bmep.  After weeks of experimentation improvements to cold CO were realized by reducing cold enrichment, not increased egr use.

Low pumping loss and oxygen dilution allow spark ignition direct injection (SIDI) and port injected engines to approach diesel efficiency sans higher compression ratios however engines with inadequate sources of ignition cannot tolerate much egr.  

Supplying 30-90% intercooled aftercat sourced exhaust gas as a means of throttling is tricky, costly and high risk while misfire and reentrant partial burns make for tricky idle speed control.. especially when cold.  

Last time I looked Honda and other state of the art low emissions engines still use 5-15% EGR.

RE: EGR and how it adds to fuel economy???

  I wouldnt say leaner
... not "leaner," just "less rich," eh?  That's a bit like saying "60deg is not colder than 70deg, it's just less warm," isn't it?

And what's the pumping loss got to do with the AFR you're able to run?

RE: EGR and how it adds to fuel economy???

Turbocohen, your post has a few interesting points in it; what do you mean: -

"Egr allows running a little closer to Stoich by allowing less pumping loss via dilution with co2..  I wouldnt say leaner"

Why would the engine not be running at Stoich in the first place (assuming it is a gasoline) so how/why would EGR allow you to run closer to Stoich if you werent there in the first place???

Personally I have never heard of EGR being used on cold start, it is usually not even used until the coolant temp is at least 60degC. So why would one ever assume that it is going to reduce cold CO in the first place??? Surely the standard means of reducing cold emissions is all about reducing, where possible, cold enrichment and trying to get stable closed loop control of AFR at Stoich?

Also, as Ivymike asked, what on earth does AFR have to do with pumping work?

Where is this lab of yours and who is it you work for? It sounds to me like you may be doing some interesting stuff but also reinventing the wheel on the way!

MS

RE: EGR and how it adds to fuel economy???

I think reinventing the wheel is an accurate summary of this thread.  External EGR is like from the stoneages.

Just look at the auto industry.  The world is moving to internal EGR for reasons previously mentioned.

Have any of you thought about the complications of fire from refueling and mulching when you talk about adding devices that create additional hot surfaces on a lawn mower engine?

Every one assumes lean burn makes the engine run hotter but with Intake valve throttling you can get lean to the point that exhaust temps drop below cat light off requirements.

It would seem to me the the goals being discussed in this thread could be more productively achieved by focusing on reducing the cost of mechanism necessary to implement IVT

RE: EGR and how it adds to fuel economy???

Why would the engine not be running at Stoich in the first place (assuming it is a gasoline) so how/why would EGR allow you to run closer to Stoich if you werent there in the first place???

Running on the rich side of stoich was (is) a common way to keep in-cylinder temps down on engines without catalytic converters.  It is still done in many applications w/cat for brief periods, especially during full load acceleration.  Using EGR in formerly fuel-cooled applications may allow leaner (closer to stoich) operation.  

RE: EGR and how it adds to fuel economy???

Come on we are going from the ridiculous to the absurd - talking about reducing incylinder temps with EGR instead of using enrichment! If one were that worried about emissions the enrichment in the first place would ensure a pretty full bag result but with 0 NOx.

Also, a point to note is that often the actual incylinder peak temperature will increase with external EGR. The reduction in NOx is more to do with the rate of heat release and the thermochemistry behind the conversion of HC into CO2 without production NO & NO2 species first. ie combining the Oxygen with Hydrogen & Carbon before it get chance to combine with the Nitrogen...

As for Intake Valve Throttling, sure it looks good on paper but it is simply not mature enough as a technology in this (& most) applications, it would add a huge oncost in the auto world and for very little benefit.

Also in the stringent world of on highway (for instance CARB) emissions legislation there can be no running away from stoich apart from component protection, there is no lean burn PFI engines but there certainly is external EGR. I refute that the technology belongs in the stoneage, yes internal EGR is a good thing but even with VVT on both intake & exhaust there is still a lack of residual gas in the chamber to bring NOx down sufficently. Plus there is a natuarl limit, the small factor of a Valve/Piston interaction....

MS

RE: EGR and how it adds to fuel economy???

That story is consistent with this 1991 article about Honda's lean burn engines:

http://www.findarticles.com/p/articles/mi_m3165/is_n9_v27/ai_11321093

Instead, the 1992 California Civic VX will feature a version of the VVT engine with conventional 3-way catalyst control... The Civic with the fuel-efficient learn-system engine will arrive in 49-state Honda dealers in October. The Honda lean-system engine operates at air/fuel ratios believed to be a high as 25:1 during "normal operation" and switches to 3-way catalyst control during the "powermode" with NOx controlled by exhaust-gas recirculation during "transition," says a Honda spokesman.

------------

...but how does that fit with the fact that Honda still sells "lean burn" engines in a variety of on-road applications?

RE: EGR and how it adds to fuel economy???

IVT in a single cylinder engine is comlicated and expensive?

Have you ever seen a single cylinder IVT Cadillac from the early 1900's?

As a side bar - wouldn't all this be much easier to achieve by just setting small engines up as E85 fueled?

I built IVT prototype engines for Ford GM and Chrysler in the early 1980's. I don't believe IVT is practical in multi cylinder engines but its pretty simple in a single cylinder engine.

RE: EGR and how it adds to fuel economy???

You can't just go "Yellow" as GM totes on it's ads without other concerns.  First off, E85 is not available everywhere.  Secondly, small engines cannot afford a 40% reduction in power and economy and just expect Joe Consumer to buy into that (small engines use a much higher percentage of their rated power than the automotive market)... and most importantly (for me) most of the engines I am working on are LPG and CNG, gasoline as well.

Tell me how IVT can be implemented on a single cylinder or V-twin and I will investigate.

I am looking for the easiest and cheapest method to implement this in 6 months time... apparently nobody makes EGR valves for small engines because that is all I was asking for in the first place.  

RE: EGR and how it adds to fuel economy???

I would not advocate IVT in a multi cylinder engine.

I would suggest a search at the USPTO and SAE engineering papers on the subject to learn more about IVT.

E85 will always be inferior to gasoline in MPG but on power E85 is great IF you use high compression.

E85 may be unavailable in some areas now but its going to see more use because its politically correct.

RE: EGR and how it adds to fuel economy???

Have you considered ported egr?  a simple venturi upstream of the throttle plate and a crankcase referenced backpressure valve supplying exh gasses might do.

RE: EGR and how it adds to fuel economy???

If you open the intake valve during the exhuast stroke you will contaminate the mixture enough to lower NOX. I dont know how this would work on throttled engines, but it seems to work very well in CI engines.

RE: EGR and how it adds to fuel economy???

Well, I don't have an answer to whether or not EGR helps economy, but I have a question from the original post.  I must be missing something b/c no one else has mentioned it, so you say

"...too late and it(MBT) will occur at a larger cylinder
volume and will result in lower combustion efficiency."

Is TDC not the largest possible volume?  How can occuring after reaching TDC increase the volume?  And how does this lower combustion efficiency, I would think you would still burn the same, if not more, fuel.

This is not my field, I'm just curious as this thread was interesting to read.

RE: EGR and how it adds to fuel economy???

I would think that TDC would be minimum volume, given that the piston is occupying as much of the cylinder volume as it possibly can . . . then just picture what happens to the "empty space" above the piston as it descends again.

Norm

RE: EGR and how it adds to fuel economy???

Ya, evidentally I was thinking of it backwards, it would be minimum volume.

So is it simply because if the MBT occurs before TDC the combustion goes against the piston causing knock and the larger volume is irrelevant.  

And if it doesn't ignite until after TDC the volume is larger because the volume for combustion expands as the piston moves away from TDC?

I guess that makes sense and would explain why nobody else mentioned this.

RE: EGR and how it adds to fuel economy???

The addition of EGR & optimised spark advance will improve fuel economy because of the reduction in pumping losses for a given airmass per stroke (the measure of load in a throttled SI engine).

Spark advance in an SI engine is pretty much always set Before TDC (and hence therefore so is MBT) - this will not cause detonation in itself. An explaination of the cause of detonation is something altogether different to this thread.

This thread seems to be going round in circles!

MS

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