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.

 

[GregLocock]

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

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

 

[turbocohen]

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.

 

[ivymike]

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?

 

[mattsooty]

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

 

[cleverlever]

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

 

[ivymike]

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.

 

[mattsooty]

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

 

[ivymike]

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?

 

[cleverlever]

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.

 

[KevinKoss]

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.

 

[cleverlever]

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.

 

[turbocohen]

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

 

[optimum]

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.

 

[macmet]

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.

 

[NormPeterson]

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

 

[macmet]

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.

 

[mattsooty]

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