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.

 

[cleverlever]

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

 

[KevinKoss]

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.

 

[cleverlever]

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.

 

[KevinKoss]

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.

 

[cleverlever]

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/

 

[KevinKoss]

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.

 

[mattsooty]

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

 

[GregLocock]

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.

 

[turbocohen]

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.

 

[cleverlever]

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/

 

[mattsooty]

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

 

[GregLocock]

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.

 

[KevinKoss]

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.

 

[mattsooty]

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

 

[schmidtj86]

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?

 

[KevinKoss]

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.

 

[ivymike]

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

 

[KevinKoss]

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

 

[Rob45]

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.

 

[ivymike]

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