CA SMOG, HIGH NOx

[Ecomike]

Originally Posted by 5-90

Theoretically, you want to have zero O2 remaining - it shows that the fuel being delivered is sufficient.

You also want to have nearly no HC or CO - because either of those represents unburned or partially-burned fuel, respectively.

NOx isn't a product of combustion, it's a product of combustion temperature, which makes it a "flyer" on emissions testing results (HC and CO are from fuel not burning correct, CO2 is from fuel being burned properly, O2 is what is needed to burn the fuel. CO2 is going to be fairly low because it's almost naturally low enough to be considered a "trace gas," and the atmosphere only runs about 20-22% O2 anyhow (it's 70-odd percent nitrogen tho...)

Y'know, so much of the emissions headache could be avoided (and tailpipe emissions reduced to nearly nothing) if they would just allow us to use water/MeOH fogging in place of the wretched EGR system (and you could tune an engine to run much better on the fog than on EGR!)

I disagree on two points (possibly typos?), one is the theoretical zero O2. I suspect it takes a good bit of excess O2 to reach the 14.7 stoich number, since that is not an excessively high rich burn number. I have also seen test data showing multiple percent excess O2, but that may have been tailpipe 6X dilution numbers on the O2, so the data is not always on equal footing. I could be wrong on that?

The other I think is a typo: " CO2 is going to be fairly low because it's almost naturally low enough to be considered a "trace gas".

The O2 is replaced by CO2, so if O2 goes from 21% to zero, the CO2 will go way up as the O2 is replaced by CO2. I have seen CO2 numbers average around 14% in many reports, which would suggest 6% remaining O2, but there is a huge increase in H20 gas that dilutes them all that is not measured or reported, and I have seen O2 numbers reported well below 1% on many reports (which is why the O2 numbers running from 6% to under 1% have always puzzled me so far. Only thing I can figure is the O2 reported is sometimes a diluted O2 due to tester differences, and the huge increase in CO2 and H20, from fuel combustion, dilutes the N2 and remaining O2....)

[O-Gauge Steamer]

Early HO engines operate at 39psig nominal fuel pressure. I am not sure when the brake happened, but I suspect 95 down to be the lower rail system. If the rail has a return line then there you go. 93 HOs definately have the lower pressure. Leastways on YJs they do and I can not imagine two fuel systems in the OBDI years...

If it is a 95- HO, the pressure should be 39lbs. The regulator is connected to the manifold and varies the correction of the fuel pressure based on the manifold vacuum level. Under low vacuum (accelerating) the regulator allows more fuel to stay in the rail.

Sounds like it is going lean due to fuel starvation. Cleaning the injector goes a long way but sometimes, another set needs to be installed. Personally, I'd take a stab at the fuel pressure to get it back to nominal.

[Ecomike]

If the HO-OBD-I years (91-95) are indeed the same as Renix on fuel pressure, then his numbers he posted are perfect on fuel pressure. Mine runs from 29-41 max, and his post was a tighter range:

"its about 34psi at primed cold state/engine off and varies from 32-38psi revving the engine" which is just fine. So it sounds like we can eliminate the fuel system as an issue.

Since you have not checked the plugs, check them next to see if the color is off on any plugs, indicating that one or more cylinders are running too lean and too hot?

I wonder if premium gas would lower engine and Cat temps a little, and thus lower the NOx a little?????

I also wonder if adding some methanol or acetone might lower the combustion temps enough to pass it, but that is more of a desperate move, chat idea for now?

[jeepsrock]

First question is, can low fuel pressure cause the engine to run perfectly but result in a slightly lean condition ?

Second question is, where can i actually find out what the pressure is supposed to be under different condition for my year ? My Haynes book seems to be worthless for things like this, all my searching online results in info for very early Renix XJs or recent model ones ?

Thanks,
Pete

[Ecomike]

Originally Posted by jeepsrock

First question is, can low fuel pressure cause the engine to run perfectly but result in a slightly lean condition ?

Second question is, where can i actually find out what the pressure is supposed to be under different condition for my year ? My Haynes book seems to be worthless for things like this, all my searching online results in info for very early Renix XJs or recent model ones ?

Thanks,
Pete

If there is a vacuum operated valve on the front of the fuel rail, and a hose at the rear of the fuel rail, you have the Renix fuel pressure regulator set up, and your pressures are fine, so they are not the droids you are looking for, LOL.

http://www.autozone.com/autozone/rep...tem +Pressure


"31-39 PSI (Pounds per Square Inch)Position the ignition switch and the A/C system in theoff position. Install a fuel pressure gauge to thevehicles fuel rail or fuel pressure test port.Confirm that no leaks are present, and visuallyverify the PSI reading on the gauge with the ignitionin the on position."

Have you pulled the spark plugs, and inspected them???

[jeepsrock]

My fuel Rail has a vac regulator as well as two lines, towards the front of the Jeep.

I will pull a few plugs tonight and report back..

Pete

[Ecomike]

Originally Posted by jeepsrock

My fuel Rail has a vac regulator as well as two lines, towards the front of the Jeep.

I will pull a few plugs tonight and report back..

Pete

LOL, pull them all, else the one you don't pull will be the droid we were looking for!!!

I thinking just one cylinder might be running lean, and show the signs on the plug?

[5-90]

Originally Posted by Ecomike

I disagree on two points (possibly typos?), one is the theoretical zero O2. I suspect it takes a good bit of excess O2 to reach the 14.7 stoich number, since that is not an excessively high rich burn number. I have also seen test data showing multiple percent excess O2, but that may have been tailpipe 6X dilution numbers on the O2, so the data is not always on equal footing. I could be wrong on that?

The other I think is a typo: " CO2 is going to be fairly low because it's almost naturally low enough to be considered a "trace gas".

The O2 is replaced by CO2, so if O2 goes from 21% to zero, the CO2 will go way up as the O2 is replaced by CO2. I have seen CO2 numbers average around 14% in many reports, which would suggest 6% remaining O2, but there is a huge increase in H20 gas that dilutes them all that is not measured or reported, and I have seen O2 numbers reported well below 1% on many reports (which is why the O2 numbers running from 6% to under 1% have always puzzled me so far. Only thing I can figure is the O2 reported is sometimes a diluted O2 due to tester differences, and the huge increase in CO2 and H20, from fuel combustion, dilutes the N2 and remaining O2....)

Hm.

As I understand it, "stoichiometric combustion" means there's enough oxygen to combust all fuel, there's enough fuel to consume all oxygen, and there's none of either left. Most of the "ash" from combustion of hydrocarbons is actually water vapour (there's far more hydrogen than carbon in a hydrocarbon chain - although it has been some time since I took O chem...) and measuring humidity of exhaust gas RT ambient air would be illustrative. Theoretically, you should have zero oxygen remaining in an unaltered hydrocarbon combustion system, running properly with an effective feedback loop in place.

And, that addresses both the O2 and CO2 (IIRC, seeing 12-18% CO2 is a marked increase over general atmospheric content - but it's still going to be relatively low, else we'd have asphyxiated ourselves years ago as a result of combustion of hydrocarbons in general.)

Looking at the composition of the atmosphere (Wiki chart - http://en.wikipedia.org/wiki/Atmospheric_chemistry) it can be noted that CO2 is indeed a trace constituent of the biosphere - Hell, there's more argon in the atmosphere than there is CO2 (and argon is and remains chemically inactive, being a noble gas.) (For those who don't want to read through the link, or lack intermediate chemistry to understand the numbers, here's a link directly to a graphic representation of the issue - http://upload.wikimedia.org/wikipedi...on_Langley.svg. It also features a raw percentage breakdown - CO2 comprises not quite 0.04% of ambient air...)

So, reading CO2 of over ten percent in the exhaust stream very much is a marked increase in content, and makes sense.

However, if combustion in the engine is truly stoiciometric (~14.7:1AFR) and the feedback loop is effective, accurate, and the information followed, then it stands that remaining %m O2 should be zero, and %m HC should be likewise zero (stoichiometry simply refers to the combustion of hydrocarbons and the oxygen available in relation to that - CO is partial combustion, and therefore still "combustion" in se. Ergo, CO is not measured as a byproduct to indicate that stoichiometry is not followed, it is measured to ensure that combustion is total.)

The provision of excess O2 available to combustion means that combustion will not follow stoichiometry, by definition, that results in a "lean burn" condition (there's not enough fuel available to the quantity of oxygen provided,) and can result in elevated combustion temperatures. This relationship will continue until the LBL (Lean Burn Limit) is reached, and the chain reaction of combustion can no longer be supported (I don't recall the LBL for gasoline, but I think it's up around 20-22:1) Lean burning is typically characterised by an absence of HC and CO, the presence of exhaust gas O2, and an increase in NOx (due to elevated combustion temperatures)

Going the other way, we run into having excess fuel available, which is a "rich" burn condition. This has the effect of lowering combustion temperatures, and excessively rich combustion can cause "washdown" where the oil remaining after the oil control rings wipes the sides of the cylinder bore gets literally washed off of the metal. This accelerates ring wear (in addition to the increased HC and CO emissions.) This will continue until the RBL (Rich Burn Limit) is reached - again, I don't recall exactly, but I'm thinking it's about 7-8:1. Rich burn conditions are indicated by the absence of O2, increased HC and CO, and depressed NOx (excess heat is absorbed in finishing the job of vapouorising the fuel.)

It's interesting to note that, despite all of the fuss over reaching stoichiometric burning, it's actually not an "ideal state" for engine operation!

"Best cruise operation" is typically slightly lean, running at about 15.2:1.

"Best power operation" is typically slightly rich, usually about 12.8-13.0:1

Go figure.

[Ecomike]

LOL, as usual the EPA likes to redefine things, like stoich, LOL!!!

True chemist stoich is where all of A reacts with all of B to make nothing but compound AB. If the product is A-2B, then you need 1A + 2B => A-2B so the stoich ratio is 1:2

Everything I have read says 14.7:1 is a rich ratio which is the number EPA likes, so how can it be a true chemists stoich ratio if it is on the rich fuel side of a true stoich?

But there is more too it than that. In order to run the system rich, so that there is enough excess fuel to heat the Cat converter, the engine must be run rich (excess fuel), but in order to burn all the fuel, 100%, meaning all the H goes to H2O, and all the C goes to CO2, you need excess oxygen, and some of the oxygen turns into NOx, so you need even more excess O2.

Got a head ache on that one yet? LOL

Frankly I have no idea why they use the term stoich except to define that it IS a fuel to O2 (or O2 to fuel?) ratio, even if it is not a true chemists stoich?

I Googled the hell out of it one week, and never found anything solid anywhere on the internet in writing that nailed down the BS of what the real numbers are and what they really mean!!!!

I did run across some info that said that the PD process control of the ECU and O2 sensor, where it rapidly oscillates from slightly rich to slightly lean, is designed to pulse feed excess fuel to the cat converter to keep it hot enough to work. so there is a pulse oscillation process going on there!!!!

IIRC the Cat converter is a surface adsorpion, conversion-reaction, desorption process that generates its own heat!!!! Add in the pulse wave of the O2 sensor PD control of the ECU swings, and the pulse waves of the engine valves opening and closing and it gets real interesting!!!!!! Long story short, even at cruise there is a wave cycle superimposed on a wave cycle going on the PD control and 2 step combustion (step one the engine, step two the Cat converter).

To a chemist, stoichimetric ratio is a fixed number, it does not change, from a rich stoich to a lean stoich ratio, it is what is, a fixed mol ratio for complete reaction of a A and B to make AB.

[5-90]

Originally Posted by Ecomike

LOL, as usual the EPA likes to redefine things, like stoich, LOL!!!

True chemist stoich is where all of A reacts with all of B to make nothing but compound AB. If the product is A-2B, then you need 1A + 2B => A-2B so the stoich ratio is 1:2

Everything I have read says 14.7:1 is a rich ratio which is the number EPA likes, so how can it be a true chemists stoich ratio if it is on the rich fuel side of a true stoich?

But there is more too it than that. In order to run the system rich, so that there is enough excess fuel to heat the Cat converter, the engine must be run rich (excess fuel), but in order to burn all the fuel, 100%, meaning all the H goes to H2O, and all the C goes to CO2, you need excess oxygen, and some of the oxygen turns into NOx, so you need even more excess O2.

Got a head ache on that one yet? LOL

Frankly I have no idea why they use the term stoich except to define that it IS a fuel to O2 (or O2 to fuel?) ratio, even if it is not a true chemists stoich?

I Googled the hell out of it one week, and never found anything solid anywhere on the internet in writing that nailed down the BS of what the real numbers are and what they really mean!!!!

I did run across some info that said that the PD process control of the ECU and O2 sensor, where it rapidly oscillates from slightly rich to slightly lean, is designed to pulse feed excess fuel to the cat converter to keep it hot enough to work. so there is a pulse oscillation process going on there!!!!

IIRC the Cat converter is a surface adsorpion, conversion-reaction, desorption process that generates its own heat!!!! Add in the pulse wave of the O2 sensor PD control of the ECU swings, and the pulse waves of the engine valves opening and closing and it gets real interesting!!!!!! Long story short, even at cruise there is a wave cycle superimposed on a wave cycle going on the PD control and 2 step combustion (step one the engine, step two the Cat converter).

To a chemist, stoichimetric ratio is a fixed number, it does not change, from a rich stoich to a lean stoich ratio, it is what is, a fixed mol ratio for complete reaction of a A and B to make AB.

Correct as far as it goes.

However, stoich != AFR (or FAR,) and is is the AFR/FAR that varies, not stoich. The stoichiometric ratio of a fuel is immutable and unchanging - you can't alter it without either altering the oxygen content of the atmosphere (oxygen enrichment, another planet) or altering the fuel (at which point, you're not dealing with gasoline anymore, are you?)

It's still goofy because it's generally defined using octane (either n-octane or i-octane) and there ain't that much octane in gasoline, but it's workable. Besides, true stoich isn't even 14.7:1 - that's just a convenient number (I think it's about 14.68...:1)

And, the catalytic converter reaction heat is self-sustaining once it gets going, but the catalyst has to reach "light-off" temperature in order to kickstart the reaction. Once it's lit, it will stay "lit" as long as the engine is running (which has much to do with why startup AFR is closer to "best power" mix during open loop operation.)

As far as the EPA? We all know about them - what the EPA does bears little on actual reality, methinks. "Best Power" and "Best Cruise" were determined by experimentation - read Taylor and Ricardo for more information on that, as their works are definitive.

As far as oscillation of AFR, tht's more a product of the narrowband HEGO (NHEGO) sensors commonly used. The NHEGO has a very sharp response curve running from about 12.5:1 to about 16:1 - beyond that range, response is bloody flat (this applies to both titania and zirconia type NHEGO sensors.) I still don't understand why wideband HEGO sensors aren't in common use - they don't cost that much more than NHEGO units, give much finer response over a much wider range (it's predictable and responsive through the range between RBL and LBL,) and with a small alteration in programming, it can be used for much finer feedback control over fuel delivery and trim. I think they're finally coming into use on some higher-end vehicles, I'm informed that MBZ and BMW are starting to have OEM six-wire HEGO sensors, but I only believe those are WHEGO units (it's a bit odd, because I believe the typical WHEGO sensor is a four-wire unit. I'm not sure why offhand, I haven't checked. Probably to get the finer signal, a self-grounding sensor isn't useful anymore?)

Frankly, I don't think the EPA gets even close to using "stoichiometric" to define anything - as you've noted, any time they used it would be likely to be incorrect anyhow. And, how much of EPA regs really have any useful basis in actual science - apart from known toxic effects of a substance, or something similar? Very few EPA (or CalEPA) regs I've seen are based on practicality or science, and their use of language bears as much on "common English" as "mess hall food" bears in "fine dining." (I've had mess hall food, I've had fine dining. Not even close...)

But you are entirely correct in your basic presumption - however, combustion is rarely stoichiometric, for a number of reasons.

[kdailey4315]

Logic and facts are not the EPA's strong suit and don't even get me started about CARB.

[Ecomike]

5-90, if you ever find a citation that confirms what the units are for this mythical 14.68:1 ratio, I would like to see it.

I searched for a week, and never found any one that confirms the units, like lbs:lbs, gallons:cu. ft, or mols:mol......

If it once was Mols:Mols, 10% ethanol tossed that ratio out the window, as the C and H mol count is no where close to the C and H count in octane, LOL!

[5-90]

Originally Posted by Ecomike

5-90, if you ever find a citation that confirms what the units are for this mythical 14.68:1 ratio, I would like to see it.

I searched for a week, and never found any one that confirms the units, like lbs:lbs, gallons:cu. ft, or mols:mol......

If it once was Mols:Mols, 10% ethanol tossed that ratio out the window, as the C and H mol count is no where close to the C and H count in octane, LOL!

Correct - I'm quite sure that the actual stoichiometric ratio has changed - and probably continues to change, due to the uses of ethers and alcohols in fuels (the EPA keeps screwing things up for no very good reason. We were better off when it was "motor spirit" and still a mix of straight hydrocarbons...)

However, turn your research toward The Internal Combustion Engine in Theory and Practice (two volumes, Charles Fayette Taylor) and The High-Speed Internal Combustion Engine (Sir Harry Ricardo.) The detailed information is a bit dated - Taylor was written in the 1960's, and Ricardo even earlier. However, the principles have not changed.) More detailed information can probably be found in The Bosch Automotive Handbook - which is at least on its 6th edition, since that's what I have on the shelf (Bosch 6th is copyrighted 2004.)

Various texts on fuels should be available at your local library, and you can do more detailed research at any university that has a School of Engineering or an Organic Chemistry programme. (It may be covered under inorganic chemistry, but I'm sure it will be under O chem.) I would suggest the use of a textbook that deals with alternative fuels as a start, as it would have to cover more recent blends of gasoline as basic material.

However, Bosch 6th gives a theoretical air mass requirement of 14.7-14.8kg per kg of fuel (so my head was a bit off. Bosch, 6th Ed. p. 324.)

Taylor is available from MIT Press.
Ricardo takes some looking.
Bosch is available from Bentley Press.

[O-Gauge Steamer]

For the ARententive Ones among us:

http://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio#Air.E2.80.93fuel_ratio_.28A FR.29

[Ecomike]

5-90,

"However, Bosch 6th gives a theoretical air mass requirement of 14.7-14.8kg per kg of fuel (so my head was a bit off. Bosch, 6th Ed. p. 324.)"

Thanks, I always suspected it was a mass based number, which is not something Chemists would normally do, LOL. (Did you forget I am a bio-chemical engineer?) While I don't work in combustion engineering, I work in other areas like toxic industrial waste water treatment, and air emissions, industrial....and lab work...permitting, etc. So I work on the fringes of it.

I think the real key is ECU process control, the feed back control loop, and measured rate of oxygen depletion as the control. I wonder if an aftermarket header has any issues with O2 sensor locations? He might need to tweak his MAP sensor, or other sensors to force it a little richer to compensate if the aftermarket header O2 sensor and combustion scrubbing changes are causing his higher NOx? If the aftermarket header is the problem, I also wonder if boosting the oxygenated fuel concentration, or using a colder plug for the inspection would help? Or a colder T-stat for the inspection?


I wonder if our OP has pulled the spark plugs yet? Photos????