curisu

iTrader: (3)

After searching this forum, I found this thread:
https://www.evolutionm.net/forums/sh...d+accuracy+cat However, it did not really address my questions, hence the new thread.

A little context for my situation and in tuning in general:
I own a IX where the stock secondary o2 sensor is on the cat. In weighing my options, I decided to go with a new IP that has an o2 bung after the cat. My wideband o2 sensor has been installed there. A bit of trivia that Pete at WORKS told me is that most tuners use a wideband that is placed in the exhaust; almost invaribly they will pick up the AFRs post-cat. My assumption is that the tail-pipe widebands are per-tune calibrated to read stoich at cruise loads.

My question(s) are:
1. in your experience, does have the wideband o2 sensor post-cat seriously change the readings? My personal experience in logging data for the last 5 days is that while cruising, even post-cat, my AFRs will settle into stoich or 0.1 leaner (14.7 - 14.8)
2. related to the above, if a car has leveled its fuel trims so that idle and cruise (LTFT low and LTFT mid) are within +/-3 of 0, will the exhaust gases still coat the tip with soot?
3. if the wideband o2 cannot be recalibrated for after-cat placement, what is the rule-of-thumb for readings? Do I enrichen the reading a bit, say interpret 14.7 as 14.6-14.5?

Any and all input is welcome, thanks!

jfitzpat

A wideband sensor works by oxidizing. A cat works by using the composition of the exhaust to tear down hydrocarbons. So post cat readings will always be leaner. As far as I know, there is no good rule of thumb, since the change will be assymetric, particularly on the rich side (where a turbo engine is normally run).

-jjf

curisu

iTrader: (3)

Thanks for the response. I'm still getting up-to-speed on EFI tuning - apologies if I'm asking questions that should be obvious to anyone formally or properly trained.

If the AFRs post-cat are within such a small variance, does that mean we're venting 1 part fuel for every 14.7-14.8 parts air? Does this in fact mean that the catalytic converter is doing very little at cruise, let alone at WOT?

For those of you who have tuned fuel trims and maps, and have achieved the nirvanic state of little-to-no soot on the exhaust tips, what are your AFRs at cruise?!?

jfitzpat

I'm sorry, I didn't mean to sound terse or condescending. I was just trying to say, without giving a long lecture on the chemistry involved, that, from the wideband sensor's point of view, post CAT gas is leaner, and the exact amount is hard to quantify in a rule of thumb. The cat is not venting fuel, but it is changing the composition of the gas - so from the sensor's point of view, which runs a pump cell to try to achieve equalibrium, the gas appears leaner.

From an emissions standpoint, stoich is desirable because 2 of the 3 things we are trying to clean out of the exaust are low, and the third, nitrogen oxides, are still not at their peak. Also, the temperature of the gas is high, meaning lots of energy available for conversion. That is why stoich is so often a closed loop target.

When we go WOT, the ecu generally enrichens the mixture considerably. ROP, nitrogen oxides drop, but carbon monoxide and hydrocarbon output goes up. This is because there is not enough o2 to feed the secondary part of combustion that breaks most the hydrocarbons down. So, when you are rich, there is generally more soot - because there are more hydrocarbons, but less heat and fewer nitrogen oxides - so the cat is less able to convert the hydrocarbons into carbon dioxide and water.

But WOT, you generally need to be rich because ROP means both more power and less heat.

I'm sorry if that doesn't answer your questions. It is actually a surprisingly complex subject.

-jjf

curisu

iTrader: (3)

I appreciate your input - I didn't take your response as terse or condescending, and I really appreciate the detail you put into your post. Thanks.

I have a layman's idea of what catalytic converters do and I'm trying to find mneumonic associations to get a better feel for EFI tuning in general. I was under the impression that on the exhaust-side of the combustion cycle the gases that are passed to the down pipe and on to the cat contain the particulate matter from the combusted fuel as well as the unburnt fuel. I was also under the impression that the unburnt fuel is then processesed by the cat to go through a similar cycle to reduce the actual hydrocarbon output. I figured that California reduced maximum octane ratings to encourage hotter burning exhuast to cause better catalytic conversion levels. This being said, I was under the impression that pre-cat gases contained fuel that the wideband o2 sensor measures.

Would I better correct in deducing from your explanation that the o2 sensor measure the "oxygen" part of the ratio, not the "fuel"? In effect, even past the cat, could the o2 sensor still be fairly accurate in measuring the oxygen levels in ratio to the processed exhaust gases?

In hindsight, I guess the tools would have been called "Wideband Fuel Sensor" if it measure actual fuel levels. Silly me.

jfitzpat

Simply speaking, the sensor really measures lambda, or a relative, proportion of stoich. lambda 1.0 is stoich, <1.0 is rop, >1.0 is lean of peak. AFR is actually just calculated from this measurement, and is only an estimate. Consider that gasoline is 14.7:1, but ethanol is 9.<something>:1, so when you put ethanol in as an addtive the stoichiometric ratio of the fuel may not be exactly 14.7:1. But the lambda probe measures 1.0 at stoich, and this is multiplied by 14.7 for display.

A good explanation of how the sensor works can be found here. Put really simply, we are looking for residual O2 LOP, and hydrocarbons ROP. We don't typically run LOP, we run peak or richer (actually, we really dither around peak - which requires the cat to temporarily store oxygen - but let's not go there, at least yet!). So we are concerned with the ROP side. One of the things a 3 way cat does is convert hydrocarbons, so... the sensor thinks that the exhaust gas is leaner.

The efficiency of most 3 way cats at converting hydrocarbons is a sharp, funny curve, which is why a rule of thumb is hard to formulate. Generally speaking, the 'lean error' will get smaller the farther you get away from stoich, but not in a very linear way. Remember, the big reason we run at stoich isn't efficiency (best economy is somewhere around 1.05 lambda) or power (best power is typically around .85-.86 lambda). We run at stoich principally for cat efficiency.

-jjf

P.S. Please note, just because 'Best Power' is somewhere around .85 lambda (say an AFR of 12.5:1) doesn't mean that we can run there. We usually have to run quite a bit richer at high power settings to avoid overheating and detonation.

curisu

iTrader: (3)

Very succinct explanation - thank you. I'm learning a ton from your post and that link you cited.

If I were to put together a 10-sec precis, would the following be correct?
The main difference between narrowband and wideband o2 sensors is that there is a smooth curve in the voltage readings at or around stoichmetric (14.7:1 AFR) which allows for a more precise reading of the reconstructed/calculated AFR that the engine is consuming. In contrast, the narrow band's voltage output abruptly drops at 14.7 resulting in the ECU bouncing (dithering as you put it) between ROP and LOP to maintain an average stoich of 14.7:1. Since there was very little information given about narrowband ranges, I assume that the two narrowband o2 sensors in the exhaust piping each measure LOP or ROP, and the ECU takes those readings to guess how to enrich or enlean the mixture; this also assumes that each sensor only cares about a very "narrow" band of mixture, +/- 14.7:1.

The predicted pre-combustion AFR is reconstructed by measuring the o2 level in the exhaust gases. The process involves known calculations and some given constants (geez, it's been 13 years since my last o-chem class at university!). Given the above, post-cat wideband accuracy depends largely on the processing efficiency of the cat at given engine loads. I'm guessing this is where that "sharp, funny" curve occurs.

Given my assumptions and the knowledge gained from you and the link at megamanual.com, is there a way of inductively determining the cat's efficiency and thus the impact on the wideband readings?

OT:
If a wideband sensor is literally a hundred times more accurate with the single sensor, why do car manufacturers only use narrowband sensors in the fuel control systems? Wouldn't the single sensor be cheaper to source, install and replace on failure? Or is it a durability question - the narrowbands are rated to last 5+ years and the wideband needs to be replaced annually (on a daily-driver)?

jfitzpat

It is actually a wider and more predictable current curve around stoichiometric, but fundementally correct.

Typically no, the two sensors are for reduancy or different banks. The sensor, like a wideband, reads a singularity (stoich), and you can tell which side you are on (just not how much).

Most of the time, a typical passenger auto is looking for stoich (maximum cat efficiency). In cases where it is not, typically a different mechanism of estimating AFR (ex. MAF and injectory duty cycle) is used.

Well, one good way would be to mount a wideband on both sides and compare... Seriously, short of measuring what is going in and what is coming out, it is pretty difficult. Remember, the Cat is trying to elliminate three things, all of which have their own efficiency curves.

First of all, consider what they want to measure. Stoich itself is quite accurate on both wideband and narrow band sensors. Measuring values other than stoich with a wideband sensor requires additional electronics, also, the sensor typically drifts over time (this opens a seperate topic of wideband measurement principles, which, while interesting, is probably one I can't comment on because of forum rules).

So, for a typical 'car' application, narrowband gives the ECU the exact info it needs (stoich), with good accuracy, longer sensor life, and simpler electronics.

Widebands are starting to appear in stock cars (we've done some licensing), and I expect the trend to continue. But, while stoich remains a very good spot for cat efficiency, I expect narrow bands to remain in use for a lot of installations.

-jjf

cij911

iTrader: (14)

jjf / others -- Where is the best place to put the wideband sensor on the EVO? Is there a ideal location / distance from the turbo (I have heard 16" + )?

I have searched the boards and saw a nice picture of the sensor mounted on the DP before the catalytic converter, but would like to know Innovative's mounting recommendation. Thanks

curisu

iTrader: (3)

Since the wideband sensor reads o2 mass in the exhaust gases, I believe it would not make any difference in the location of the sensor as long as it was placed ahead of the catalytic converter and sufficiently downstream from the headers to allow the gasses to cool (to protect the pickup).

Given a choice, I would have liked to have placed mine a few inches forward of the flange connecting to the cat.

curisu

iTrader: (3)

Hi jfitzpat,

I can't really express how grateful I am for your input - it's invaluable to my education!

Now that I'm a little better educated, I went back and re-read this thread, and I wanted to you to clarify this statement:
Intuitively, since exhaust flow will increase in volume when in open-loop (high-load/high TPS or WOT) runs, I would guess that wideband readings post-cat become more accurate. Would that be correct?

The reason I came to this conclusion is that while logging some pulls last night, I noticed that the EGR temps decreased as the AFR became ROP. I don't have an EGT probe in place so I can't tell if the gas temps going out the headers proper are higher or lower, but I inferred correlation here. The relatively lower EGR temps lead me to think that the cat's effieciency would likewise decrease, resulting in a more accurate reading at my wideband sensor. Would this be a correct read of the situation?

jfitzpat

Yes. As you move away from stoich (nec. for wide open throttle), the cat efficience drops off dramatically. At the cat is less efficient, the exhaust gasses being monitored by the sensor are less altered. Still, it is hard to get precise readings because the efficiency of the cat changes under different conditions.

Regarding Sensor placement:

There are three basic considerations:

* Gas integrity
* Gas Temp
* Bung Temp

In the case of a single sensor on a single turbo setup, Gas Integrity is easy - you want to be pre-cat, since the cat alters the composition of the exhaust.

If you wanted to get cyl by cyl measurements, gas integrity becomes a larger undertaking. A common solution is extra plumbing, where small lines are pulled off close to each cyl run to a sensor, then back to the main exhaust (tyipcally post turbo). We see stuff like this quite a bit in pro racing setups, particularly when a carb is involved (since leveling fuel/air distribution is a constant battle).

Max gas temp is a limit imposed by the sensor. Typically it is on the order of 800-900 degrees C. Honestly, we don't see this limit violated very often, but it is there.

Bung temp is also a limit imposed by the sensor. It is typically about 300-350C cooler than max gas temp. This is the one that 'gets' a lot of installs, particularly since it doesn't show up as an error on most controllers, just scewed readings and short sensor life.

There are several ways to deal with this, for example we have several app notes on fabing your own heat sink. We also sell some more sophisticated heat sinks (intended to deal both with gas and bung heat).

However, on an Evo, the DP, anywhere from 16-20" after the turbo to just before the cat should be a reliable, heat sink free spot on many evos. At least up to 300-325HP. After that, heat sinking may or may not be nec. (If your wideband does not report bung temp errors, you can check yourself with a temp probe).

-jjf

cij911

iTrader: (14)

JJF -- Thanks....I had the bung welded on the downpipe right before the rear flange (before the cat).

Bassicfun

iTrader: (10)

Old thread of course, but incredibly accurate and detailed information, well worth bringing back up if that is acceptable by the moderators. If not, my apologizes.