[Joe Perez]

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"?