My understanding of a wideband o2 sensor is that it DOES measure fuel in the air, not remaining air. To my knowledge it samples exhaust gas, reads as either lean or rich, and applies a voltage to it one direction or the other to either use up the free fuel, or consume oxygen. It compares that to reference voltage to get its output, which then, depending on the fuel, the air/fuel ratio is calculated.

That is the kicker too. A/F ratio IS dependent on the fuel being used. Methanol has a much lower a/f ratio for stoich than gas. If you were to run a methanol car on a dynojet, without somehow letting the a/f display or gauge know it was methanol, it thinks its gas, and you know that with methanol, the a/f needs to be 5.8:1 for best power, you'll keep adding fuel and adding fuel. Similarly, if I were to put my Wb sensor in a diesel, it would read incorrectly for diesel fuel, as it has a different stoich point. So, when you add nitrous to the equation of a gasoline a/f, you are effectively changing the stoich point. Now, if I remember correctly, stoich is when all fuel is burned completely. Great for emissions, but not power. That's why even N/A cars run what we'll call RICH at 13.0:1. My car ran as high as 16:1 at 2000 rpms, and only hitting 13:1 at redline, with no misses. It might have made more power running richer, since stoich has NOTHING to do with power production, only complete burn.

Now, I don't know stoich for N20, but Fred says 9.6:1. (Fred, a little off topic, but since nitrous isn't flammable, how can it have its OWN stoich point?) and NOS says 6:1, which makes since, you'd want it RICHER for best power production.

Another thing I think that got lost slightly is octane rating. Doesn't that have a big affect on how much a/f you need or how lean you can "get-away" running?

I guess I like Fred's answer since it is scientific and I tend to over-analyze things. It makes sense that you would be able to quantify how much fuel you would need for a particular motor that way. Maybe that's why so many guys burn holes in pistons on nitrous thinking its "rich enough". How far rich or lean have you gone "playing" with tuning on your motor Rich? Just curious to see how much more power there would be if it were richer or leaner. I've seen alot of turbo Hondas pick up 40 - 50 hp just going from 12.5:1 to 11.2:1.

 

Nope, the O2 sensor does not measure fuel.

Rich Krause

 

Rich:

My thinking is that an O2 sensor measures "lambda", and "lambda" = stoichiometry. It does not measure "absolute" mass of oxygen in the exhaust. It relies on the "partial pressure" of oxygen in the exhaust. Partial pressure is proportional to molar concentration and vapor pressure at the gas temperature. If you burn a fuel or an oxidizer that has a stoichiometry different than the that of "air + gasoline" you will end up with a different molar percentage of each component.... free oxygen, nitrogen, water, CO2, etc., in the exhaust. That will alter the molar composition, which will cause the O2 sensor to measure vs. the stoichiometry of the actual fuel and oxidizer being used. For a wideband, you typically need to tell the meter what fuel you are using, so it can convert "lambda" to A/F ratio.

Maybe I'm extrapolating a little to far here, since we are altering the chemical composition of the oxidizer, and not the fuel, but the fact that you are increasing O2 content relative to N2 content should significantly alter the chemical composition of the exhaust, and the reading of the O2 sensor.

Maybe I need to talk to the guys at Second Street and see how they account for this concept.

 

I conceptualize it the same way you do. The calibration would be off (potentially) if you used a different fuel. Exactly what effect using nitrous as the oxidizer has is something I have wondered about. I wasn't speaking from any specific knowledge when I posted that the source of the O2 doesn't matter, I wasn't thinking of the effect of the nitrogen. More than once, I have heard it said that "you cannot rely on the accuracy of AF ratio from a wide-band O2 sensor when using nitrous". However, I do know from experience that you do get repeatable results from a wide-band when using nitrous. And they will vary in the same direction and magnitude with fueling changes as in non-nitrous operation. So, I still think a wide-band may be used as a tuning tool with nitrous. And from experience, ~11.5:1 works safely and makes power that correlates well with the size of the nitrous shot.

So, even though the true AF ratio may be very different from 11.5:1, using the wide-band to acheive the 11.5:1 reading does work. At least for the 50-250hp nitrous shots I am familar with on a moderate to large displacement V-8. A very different wide-band reading might be appropriate for larger nitrous shots, smaller engines, etc.

Rich Krause

 

How do you calibrate an O2 sensor??? with FREE AIR!! not free fuel...

The o2 sensor breathes through the wires and measures the AMBIENT O2 and "compares" it with the O2 in the exhaust gases... that way it creates a voltage for the PCM to read it.. that is...

Try this... run the WB sensor all the way in the exhaust pipe with a lot of wire in to see what happens... it will simply read off the charts LEAN!...

 

I understand a narrow band O2 sensor works that way, but does a wideband work that way as well?

 

YES it does... I know that when I purchased my wideband setup, FIRST thing I had to do before running the unit was to hook up the o2 sensor to the wideband with the o2 sensor OFF of the car and then it had to be calibrated with FRESH AIR via the wideband software....

 

A few further thoughts on AF ratios and nitrous. Stoich for nitrous is ~9.6:1 (as Fred pointed out). Most nitrous setups are run 15-25% richer than stoich to help supress detonation and make max hp. So, the "actual" AF ratio of the nitrous/gas mixture will be richer than 8:1. If the overall AF ratio, as measured by a WB sensor, simply reflects the proportions of O2 and fuel supplied by the nitrous system and the "non-boosted:" system, it should be possible to calculate the "real" AF ratio, right?

I guess Fred implied/said this. The final AF ratio will simply reflect the combination of the AF ratios from the two sources. I have the data to calculate the AF ratio from the nitrous. And I have WB AF numbers from both nitrous and non-nitrous operation. So, I should be able to calculate the final AF ratio and compare that to the readings, thus establishing the accuracy of the WB readings under nitrous operation. The reason why I am posting is that I not too good at math (hence my short career as an engineering student and my switch to the biological sciences) and am having trouble setting up the equation.

Anyone want to help me out? Fred? Other engineers? I'd like to know how accurate the WB readings are.

Sorry if this post is confusing, do you guys understand what I mean?

Rich Krause

 

Basically the logic you are trying to figure out is this

9.6(n20)+13(non n2o)/2=11.xxx from the AF WB Reading??

you just wnat to know exactly if you know the non n2o AFR what is the stoich ratio of nitrous?

 

You can't just take an average (divide by 2) unless you happen to be getting 1/2 your HP from the N/A process. And you can't just use the 9.6 stoichiometric ratio for nitrous... you need to use your actual fueling rate, which as Rich points out is typically 15-25% richer than stoichiometric (or more). If you follow the NOS recommendations, you might be fueling at even richer rates, on the nitrous portion of the combustion.

The APPROXIMATE formula is pretty much explained in my earlier post, but to summarize:

[ (N2O A/F ratio X N2O HP) + (N/A A/F ratio X N/A HP) ] / [ N2O HP + N/A HP ] = actual A/F rate

There's a more detailed analysis of fuel requirements in the "Advanced Nitrous Tuning" section of this book.

I'm going to go back and review the BSFC data I have from the engine dyno's of my engine, running both N/A and with a 250-shot of nitrous. Just to see if any of this makes any sense. Also want to talk to the guys at Second Street and see how they handle the issue of nitrous stoichiometry and the wide band sensor.

 

It was just an idea! not specific and yes you are correct, you would need the % which are summed up there in your formula...

 

In my tuning sessions, I always shot for about 1 point lower A/F on the squeeze than NA. NA my car ran from 12.9 - 13.2 on avg, and with the nitrous around 11.7 - 11.9.

Made decent power for never having had the valve covers off.

 

That's a common rule of thumb. What we are wondering about is "what AF ratio does this actually represent?" I am going to do some calculations on this.

Rich Krause

 

A wideband measures O2 content in the exhaust.

Just because there is a higher concentration of O2 in the N2O, we are adding fuel to burn with it. If the O2 reading goes up or down, that's a direct indication of how well the jetting of the N2O is setup.

Most people go rich to quell any possible detonation or hot spots. Better safe than sorry.

 

You need to read the rest of the thread. The issue is whether the additional nitrogen in the exhaust when using nirtrous oxide interferes with the accuracy of an O2 sensor.

Rich Krause