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Old 03-06-2009, 12:53 AM
Joe Perez Joe Perez is offline
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Default Gasoline, Methanol, AFR, Lambda, and Stoichiometry.

Been doing a lot of searching around and sticky-reading of late. Finding that a lot of links (particularly those to NACA papers) aren't working. But I haven't really come across a solid answer (or even a general consensus) to a question that I've been pondering for some time. 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 and ask for opinions here.

The question has arisen in my mind as to the effect that injecting methanol, or a mixture of methanol and water, has on perceived AFR, from the standpoint of a gasoline-calibrated wideband EGO2 sensor.

Now, a wideband O2 sensor really measures lambda, not AFR. Through the use of a scaling factor or a lookup table, the controller then converts this to AFR format, as 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 shows 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. Conversely, if we had a sensor calibrated for gasoline and we put it into an engine running on methanol, a stoichiometric methanol mixture (6.5:1) would produce a display of 14.7.


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. Let's not get sidetracked arguing about whether or not that's correct, just accept for this discussion that we're tuning for 12.5:1 on gasoline. By my math, 12.5:1 for gasoline is 0.85λ (12.5 / 14.7 = 0.85). So we do some dyno tuning, 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. Our objective in doing this is to use the methanol (or methanol / water mixture) as an anti-detonant, to allow for increased ignition advance, with the ultimate goal of increasing torque.

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 methanol being introduced is significant enough to affect the engine's AFR in a measurable way.

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.

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 big question: Assuming that 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 methanol to gasoline going into the engine, the overall ratio of the combined mixture of fuels to air is "ideal"?
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Old 03-06-2009, 08:50 AM
Richard L Richard L is offline
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I am not sure if the design principle of WB is any different to the normal Lambda sensor. I believe they both detect excess oxygen and then calculate lambda value for displaying purposes.

So before going further, we need to establish the above before venturing further based on assiumption of WB actually detecting AFR and not excess oxygen.
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Old 03-06-2009, 02:27 PM
raddy raddy is offline
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both wideband and narrow band lambda probe detect excess oxygen and provide voltage output on this base. then they use lookup table for voltage-lambda value for lambda calculation. and then, AFR is calculated by adjusted parameter for specific fuel from lambda value...only different is in voltage to oxygen ratio curve...
as narrow band provide only very rapid change of voltage, it can be used only for stoich ratio (there is sudden voltage change appear) measurement, while wideband provide much more wide voltage change, so can be used to measurement of wide ratio of oxygen...
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Old 03-06-2009, 02:36 PM
Joe Perez Joe Perez is offline
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Quote:
Originally Posted by Richard L
I am not sure if the design principle of WB is any different to the normal Lambda sensor. I believe they both detect excess oxygen and then calculate lambda value for displaying purposes.

So before going further, we need to establish the above before venturing further based on assiumption of WB actually detecting AFR and not excess oxygen.
I believe that this is the case. I base that upon the design & documentation of both the Innovate LC-1 and the AEM UEGO.

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:



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.


So 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. 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. 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 sensor calibrated for gasoline, which we presume to indicate 0.85λ regardless of the specific fuel involved, is optimal for a blend of two fuels (gasoline and methanol) at a ratio that is not precisely known.
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Old 03-06-2009, 03:35 PM
Joe Perez Joe Perez is offline
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I expect I should also clarify what I'm really driving at here.

If my supposition above is correct (that a wideband system calibrated for gasoline AFR will indicate 14.7 on a hypothetical engine in which 100% methanol is being burned at 6.5:1, because both mixtures correspond to exactly 1λ for their respective fuels) then the core issue becomes:

Let's say that I were hypothetically to set my wideband gauge (an LC-1 w/ XD-16) 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 it correct to assume that MBT for a mixture of gasoline and methanol (and water) is also 0.85λ, and that while tuning a turbocharged engine running said mixture of fuels, one should be striving to achieve 0.85λ?
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Old 03-06-2009, 05:23 PM
raddy raddy is offline
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I think that you are right, if you tuning by lambda, you can forget to AFR and fuel types, thats known thing... anyway, tuning by lambda is only REALLY correct....
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Old 04-06-2009, 01:33 PM
Joe Perez Joe Perez is offline
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Quote:
Originally Posted by raddy
I think that you are right, if you tuning by lambda, you can forget to AFR and fuel types, thats known thing... anyway, tuning by lambda is only REALLY correct....
That was really my area of uncertainty. Whether an indication of 0.85λ really does mean 0.85λ for the combined blend of fuels overall, regardless of what percentage of the fuel entering the engine is gasoline vs. methanol.

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 0.85λ, 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?
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Old 04-06-2009, 11:54 PM
keithmac keithmac is offline
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I`ve tried a fair few mixtures and quantities of water/methanol on my GTO Twin Turbo, from 500cc/min to 2000cc/min of pure methanol currently.

I have always tuned for 11.5:1 petrol AFR, regardless of the actual ratio of water/ methanol/ petrol and never had a problem with this method (not to say it`s right though). All wideband controllers derive AFR from the Lambda reading, so I`ve always tuned for the same lambda value..
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