Gasoline, Methanol, AFR, Lambda, and Stoichiometry.
 

Regarding the tuning of a gasoline engine with the use of a wideband O2 sensor, in the presence of methanol. I'd like to expose my thinking to third party review.

The question has arisen in my mind as to the effect that injecting methanol, or a mixture of methanol and water, has on AFR.

Now, a wideband O2 sensor really measures lambda. Through the use of a scaling factor or a lookup table, the controller then converts this to AFR format, and that's what most folks are accustomed to looking at on their gauge.

With gasoline, we know that 1λ = 14.7:1 AFR. So if the engine is running at precisely stioch (1λ actual), the display gauge reads 14.7.

Now, for methanol, the ratio for stoichiometric combustion is somewhere in the neighborhood of 6.5:1. So if we were running an engine on pure methanol, we'd have calibrated our wideband controller such that 1λ produces a display of 6.5. Hypothetically, if we were to take that same engine and run it on gasoline, then a stoichiometric mixture of gasoline and air (again, 1λ), despite being 14.7:1 in reality, would still produce a display of 6.5.


Agreed so far?


Getting back to practical matters, let's say that we're tuning a turbocharged engine. Ignoring detonation, conventional wisdom tells us that we want to shoot for an AFR of about 12.5:1 for best torque, assuming optimum ignition timing. By my math, 12.5:1 for gasoline is 0.85λ (12.5 / 14.7 = 0.85). So we do that, and now we have a fuel table that achieves 12.5:1 on gasoline.

Now, let's say we're going to introduce some methanol into the engine. First off, I'm going to assume that the presence of water has no effect upon AFR, so the exact concentration of water to methanol is unimportant. And second, I'm going to assume that the quantity of water/methanol being introduced is significant enough to affect the engine's AFR in a measurable way- we'll say a ratio of 25% meth/water mixture to fuel (by volume).

So we start injecting the mixture and, assuming we do not reduce our fuel trim, the mixture starts going richer than 12.5:1. So we obviously start decreasing fuel to bring the mixture back towards our target. And this is where the big question arises.

The only data I've been able to find suggests that peak torque on meth is achieved at about 5.5:1. By my calculations, this comes out to 0.85λ, which is exactly the same number we came up with for the peak-torque lambda for gasoline.

So the question becomes: Assuming our WBO2 system is calibrated for gasoline, is it safe to assume that when the display on it reads 12.5 (equating to 0.85λ) that regardless of the ratio of gasoline to methanol going into the engine, the overall ratio of the combined mixture of fuels to air is ideal?

joe that is the million dollar question. i would assume the amalgum would be at ideal however maybe a pyrometer and some 3 way catalyst collectors would enable us to analyze it and deduce our fuel waste %. I know just from experience and general tom foolery my caloric effeciency on such and amalgum is vastly superior to that of a normal intercooled gas fueled miata. So i tend to agree with you and state my bsfc's as proof.

Attachment 205444
My first setup 5 days after i first turned the key stock ecu fuel system etc, with a 12-1 obx fpr, megan racing manis, and small 16g at 8 psi with a helper spring opening the WG so i didnt overboost while i made sure it wouldnt blow up. Stock cat and aftermarket muffler. AFR's taken from the tailpipe outlet.
Attachment 205445

Joe, most WBO2s actually measure in a voltage range of 0-5v. Stoich (Lambda) and AFR are the conversions. Stoich and lambda are basically synonyms.

Let's say an engine is running with the WBO2 putting out 3.5v. In the look up tables for the controller, 3.5v equals 16:1 AFR. Then that engine that produces 3.5v from the WBO2 has an AFR of 16:1 regardless if it's burning alcohol, gasoline, propane or any other fuel. The stoich ratio (lambda) will change depending on the fuel used because each fuel has it's own "sweet" spot.

Did I misread what you were trying to state?

Yup, you're absolutely correct. The WBO2 measures lambda. And you tune for that. Some say the Methanol makes max power at a bit richer than 0.85 lambda though. That should mostly matter if you're running the engine on just methanol. For intake injection, just tune for 0.85 lambda on the gauge regardless whether you're spraying methanol or not :)

P.S. Btw the water is supposedly changing the O2 sensor readings, but so minor that you can ignore it

He was trying to state that one should ignore the type of fuel he is running - just gasoline, or gasoline + methanol injection, but just tune for the same reading on the O2 sensor (== lambda) and ignore AFR and all that crap. And he is right.

I think that what Joe is saying is that the wideband does not actually read an AFR, instead it reads the exhaust gasses distance from lambda. The wideband then converts that number to an air/fuel ratio that applies to gasoline. When you put a different fuel in the motor the AFR (that was calculated for gasoline) is now incorrect.

If the gauge was reading a true AFR guys running a 4:1 meth injection should be targeting something around a 11.1 air fuel ratio for peak torque (4 parts gas @ 12.5, 1 part meth @ 5.5). Since the gauge is actually reading the exhaust's distance from lambda peak torque will be seen at 12.5:1 regardless of the fuel that the motor is running.

(Hopefully I got that correct, please correct me if I'm wrong)

The zirconium dioxide oxygen sensor changes the flow of electricity through a circuit based on the concentration of oxygen in the exhaust gasses. Since all it determines is oxygen concentration one must determine if the presence of methanol increases or reduces the concentration of oxygen in the exhaust gasses. The stoich value of burning methanol is irrelevant to the sensor, because the exhaust created by stoich burning methanol may have a higher or lower concentration of oxygen than that of the exhaust of stoich burning gasoline (I'd look it up right now, but I'm driving on the interstate). This difference (and there likely will be one) must be factored into the calculations regarding the computation of fuel ratios based solely on exhaust gas oxygen concentrations.

which is why you should just learn lambda and know that it is a direct ratio of air to fuel based on complete combustion as "lambda=1".

so if you're cruising with some random fuel, you know you're good when your gauge says "1ish" or when you're under power, you know you want to be <1. and when you lift, you want >1.

I believe that you are mis-interpreting how a wideband system works.

And I'll explain why I believe that.


In the configuration software for the LC-1, there is a screen where you tell the system what fuel you are using. A screenshot is attached:

http://img28.picoodle.com/img/img28/...pm_a864c98.gif

Note that it says "... to calculate AFR from Lambda", suggesting that the sensor itself responds to lambda.

In the documentation for that section, of the software, it states the following:

On this page you can see the software version of the LC-1 and you can change the multiplier to
calculate AFR from Lambda. A number of different multipliers are already pre-selectable but you
can change it to a custom one for the particular fuel you are using.

From this, I infer that the LC-1's sensor element responds to lambda (which would be a constant for the stoichiometric ratio of any fuel) and then relies upon this setting to determine the correct multiplier it will use to convert that lambda value to an AFR value for display purposes.



The AEM gauge is much less configurable, and the documentation is not quite as clear. However it does state that "UEGO sensors use a “current pump” to determine the actual oxygen concentration within the sensing element" and in the "specifications" section in the back, it indicates "Measuring Range: 0.751 to 1.143 Lambda". Additionally, it notes that of the five switch-selectable calibrations, four are intended for use with gasoline, and implies that if an alternate fuel is used, the fifth calibration, lambda, should be selected. It then lists a table of the 1λ values for various fuels, along with a table showing what AFRs corresponds to the range of 0-5v output for all those fuels. In every case, the 1λ point for each fuel occurs at 2.35v, despite the fact that there is no user-selectable calibration for discriminating between, say, ethanol, methanol, propane or CNG. One is simply presumed to have selected the lambda display calibration for any non-gasoline fuel.


So I believe it to be a safe assumption that, like their nernst-cell cousins, wideband O2 systems natively respond to lambda. IOW, for the ideal stoichiometric ratio of any fuel to air, the sensor itself will detect 1λ, and the controller, assuming a linear 0-5v calibration, will output 2.35 volts. This would be true for gasoline at 14.7:1, methanol at 6.5:1, ethanol at 9:1, propane at 15.7:1, etc. Any of those combinations is 1λ, representing neither a surplus nor a deficit of free oxygen in the exhaust. It then falls upon the controller to perform an arithmetic operation to convert that into an appropriate AFR, which is done solely for the purpose of driving the user display. Were it not for the display, AFR could be ignored entirely and everything done in straight lambda.


Perhaps it would be easier to think about this in terms of a narrowband application, where the actual sensor itself directly produces an analog voltage, with no intermediate conversion steps. If you have a narrowband sensor in an engine running gasoline, and the mixture is exactly 14.7:1, then the voltage output of the sensor will be midrange, approximately 0.5v. If you then convert that engine to operate on 100% methanol, and you set it up so that you're running at exactly 6.5:1 AFR, then the output of the sensor will still be 0.5v. This is because both conditions represent perfect stoichiometric combustion, at exactly 1λ.The narrowband sensor has no concept of AFR as such, only lambda.

From all of the data that I can find, it appears that a wideband sensor (again, the physical sensor itself) operates in essentially the same way, responding to lambda, with the subsequent translation of lambda to AFR done by the microprocessor inside the controller, based upon a pre-determined conversion formula which is specific to the fuel being burned.



What I don't know is whether this validates my belief that, assuming an "ideal" (meaning best power) AFR is 12.5:1 for gasoline, and 5.5:1 for methanol, both equating to 0.85λ, does it follow that tuning for a displayed value of 12.5:1 on a wideband system calibrated for gasoline, which we presume to indicate 0.85λ regardless of the specific fuel involved, is optimal for any blend of two fuels (gasoline and methanol) at a ratio that is not precisely known.




Well, yeah. I think that's the basis of what I'm postulating. Not so much that AFR should be disregarded (as it's still how my brain wants to work when thinking of mixtures) but rather that, based upon the system's tendency to translate any mixture which achieves 1λ (regardless of the actual AFR that 1λ represents) into a displayed value of 14.7, that using that system of scaling is reliable for any mixture of fuels where the lambda value for best power is the same for each fuel in the mixture (0.85λ) regardless of what the ratio of fuel A to fuel B is.

Well, in a nutshell, I'm essentially trying to confirm that what you've said, which is also what I assume, is in fact true.

Let's say that I do have my gauge set to display lambda rather than AFR. If we assume that MBT for gasoline is 0.85λ, and MBT for methanol is also 0.85λ, is MBT for a mixture of gasoline and methanol (and water) at an unknown ratio also 0.85λ, and that while tuning a turbocharged engine running said mixture of fuels, one should be striving to achieve 0.85λ?

that's what wikipedia says...

Wikipedia says a lot of thing. :rolleyes:

Joe, you've done it again. My mind is officially blown. I see what you're saying and I am confident to go leaner with my mixture. Before, i was thinking that since stoich was 6.5:1 for methanol, I was going to be shooting for somewhere in between MBT AFR for methanol and MBT AFR for gas. That is not the case.

indeed. but it corroborates what you thought and what I thought before looking at wikipedia.

You're the one who started me down this path with your 7 GPH nozzle. You've been here long enough to know that you can't do something unconventional without somebody turning it into a dissertation. :D

Haha, very true. Although, I would say around these parts, you're the king of dissertations.

You are right, I will just blame it on trying to answer a post like this at 1:00am!:confused:

I think the only reason that the settable value is there for the LC-1 is for the purpose of the scaling factors in the analog outputs. You can set up the scale on the output using AFR or lambda; If you select AFR, then set it to read 0V at X.X AFR and 5V at Y.Y AFR, the value input in the drop down menu scales it into the equivalent lambda value. I think if you just set the outputs in terms of lambda, then it is insignificant. For example I set my output that goes to the dash gauge with lambda values. IIRC, 0.5λ = 0V, 1.5λ = 5V. I think in that case it ignores the value in the AFR multiplier.

Also the dash gauge is really just a volt meter, scaled to read for gasoline AFRs. You could just replace the scale on the gauge to read in λ instead of AFR. Or, like someone said above, know that a gauge reading of 14.7:1 AFR is λ=1 regardless of what fuel you are running. It is after all, like someone said, just measuring residual oxygen levels.

Yours is just a voltmeter I guess but the good ones are not. My TechEdge gauge communicates with the controller (serial RS-232) and displays the lambda the controller internally is reading (which excludes any errors caused by analog conversion, ground diffs, etc.). And you can configure the gauge to convert that lambda in whatever else units you want.
The good Innovate gauge is the same way - serial connection to their wideband.
I use the gauge to make sure the ECU is properly/precisely calibrated for the wideband. I.e. my AEM EMS needs a 0.96 scale (4%correction) when connected to a TechEdge WBO2A unit.

The LC-1 connection to my Adaptronic is via RS232. I like that because it removes a source of error.

Ah, that's cool. Does the adaptronic have a separate serial port for the O2 communication separate from the one for PC/Notebook connection?

yeah it do.

So, the question is:

When burning a more-or-less homogeneous mixture of gasoline and methanol, at a combined 0.85 lambda, will the gasoline behave in the same way as it would were it running all by itself at 0.85 lambda?

In other words, gasoline at 1λ (or even leaner) would be expected to produce high EGTs, detonate, etc., when run through an engine that's operating at 180 kPa. By contrast, gasoline running at 0.8λ, under the same conditions, tends to behave itself provided the ignition timing is not too aggressive. Will it continue to do so in an overall 0.85λ environment, even if the amount of gasoline present is less than would constitute a 0.85λ mixture were it not for the presence of some other fuel as well?

I think the answer to this could be close enough to the same behavior as one would see running ethanol blended fuel. So, if one fills up with say E10 or E20, uses WI (all water), and compensates for the slightly leaner condition my adjusting back to the target lambda (maybe the ECU does this on its own in some cases), is the car more knock prone, have higher EGTs, etc, or does it all more-or-less balance out?

Seems like this is a common enough scenario that an experienced tuner would know the answer. Perhaps the answer is out there in Internetland via the Google magic bus.

You'd think so.

I am finding a hell of a lot of good information on water-injection in general, much of it from NASA based on studies performed during WWII on aircraft engines. Turns out that this is, in fact, rocket science. At some point, as part of my upcoming modifications I'm planning to write a sort of thesis-style FAQ concatenating a lot of what I'm learning as well as capturing the data that I hope to generate while doing some experiments of my own. That's going to require a hell of a lot more motivation than I have just at the moment, however.

Despite all this, I have yet to find any research that specifically answers the question of how gasoline behaves in the presence of other fuels. I had not, however, considered the E10 / E85 angle, so perhaps bending my queries towards modern flex-fuel vehicles may produce some answers. The downside here is that most of the old stuff is freely available, whereas access to much of the more contemporary research is subscription-based or requires SAE membership.

That's one thing I really miss about college- having easy access to good libraries.

so I was browsing around in Heywood and there was no simple answer for mixed fuels.

but there are two slightly different equations for the combustion process because of the alcohol's oxygen content.

so I suppose you could write a long chemical equation using both fuels and their mole fractions.

or you could assume that the lambda is simply weighted by the fuel ratios. which I think is acceptable because you're breaking both down into their raw constituents which recombine with air and presumably aren't interacting some other way to give you your combustion products.

stoich for methanol is 6.5:1
stoich for gasoline is 14.7:1

so if you burn 90% gasoline and 10% methanol, you'd have .9(14.7) plus .1(6.5) is 13.88 as your new stoich value. and your wideband would read lambda=1 at that AFR.

?!

I thought we all agreed that because 14.7 for gasoline is 1λ, and 6.5 for methanol is also 1λ, and because the sensor itself actually reads lambda and then the controller converts it to AFR based on a lookup table, that stoich for any homogeneous mixture of gasoline and methanol will show as 14.7 on a display gauge that has been calibrated for gasoline.

ok so the answer to your original post's question is "yes"....

sorry I've been sick and a little spaced out.

i just was hung up on finding out the specifics of the chemistry involved.

I feel kind of bad for resurrecting this thread being that it is almost 5 months old. Having said that, I just have to tell you guys that reading through this conversation has not only been a pleasure, but it has also answered every single one of the questions I had sat down to research on tuning with Meth Injection. I am a Honda guy, but if you guys regularly have such intelligent and readable technical discussions (unlike the majority of my Honda-Enthusiast brethren), I will most certainly be returning to this forum.

Thanks

We're glad you enjoyed the thread. Let's hope you have thick skin also and do not run away once the members start giving you grief for resurrecting this thread. :giggle:

If you're looking for good info about anything mechanical (Miata related), you've come to the right place.

Go to the "Meet and Greet" section and introduce yourself properly.

Enjoy your stay.

Don't feel bad. Weighted Companion Cube loves you.

I'd like to know who told you 12.5 was best for torque also as A Graham Bell (F1 and NASCAR fame) tries to hit 15.2 without shagging the fuck out of the engine. So why?

OK this does not make sense to me. Lambda is relative to stoich for the specific fuel isn't it? 1lambda=stoich right?

How then would the Lambda sensor automagically adjust itself for whatever is stoich for your mixture?

I made a little table showing what stoich would be for various ratios of meth to water:

Attachment 241650

So for lambda to be properly configured for your mixture you would need to set lambda to equal the rightmost value.

Back to Joe's original question,

"Assuming our WBO2 system is calibrated for gasoline, is it safe to assume that when the display on it reads 12.5 (equating to 0.85λ) that regardless of the ratio of gasoline to methanol going into the engine, the overall ratio of the combined mixture of fuels to air is ideal?"

My answer is no. Stoich isn't necessarily doesn't provide us with best torque or safety for gas or meth.

Assuming 5.5 gives us the most power and safety for meth and 12.5 does the same for fuel, we get this table:

Attachment 241651

At 25% meth, 75% fuel we would be at 10.75 AFR.

pff tell you what this thread might be mathmatically correct in some respects but i wont change how i do shit becouse to me the data is all inconclusive. I know the shit i have seen and done works and is real that is enough for me until i see something better.

There's no adjustment needed. Free O2 is free O2, and surplus HC is surplus HC, regardless of whatever the mass ratio would be that produces neither.

I don't really care (for the purposes of this thread) what the ideal AFR for MBT is for any given fuel. It could be 10:1, it could be 15:1. That's far too application-specific.

My quandary is this: If we assume that X represents the ideal lambda ratio for gasoline, and Y represents the ideal lambda ratio for methanol, does it follow that (X * %gas) + (Y * %meth) = the ideal lambda ratio for any given mixture of the two? Or put another way, does the presence of one fuel affect the behavior of the other insofar as ideal λ for MBT is concerned?