stoichiometric vs. AF sensors

[Paul Workman]

Wide band sensors: gasoline vs. alcohol = The question comes up when ethanol is introduced to the fuel, and what accomodation if any is necessary, far as AF reading goes...

Down to cases:

Ethanol is oxygenated, and therefore requires less air to burn the fuel, i.e., given the "stoich" for gasoline is 14.7 parts air to 1 part fuel (14.7:1) , and for ethanol it is 9.1:1.

The O2 sensors measure free oxygen percentage in the exhaust gas (as I understand it). The oxygen is detected by the wideband sensor and then electronically analyzed and interpreted by a processor before being displayed as the AF ratio.

The thing making me is that the wideband O2 sensor is an indirect reading; measuring O2 level in the exhaust, and not the actual air volume entering the engine. Adding or subtracting gasoline will result in a difference in the exhaust O2 %, which eventually results in an AF value readout - but based on gasoline fuel calibration, methinks.

But, suppose we were to switch the wideband sensor over to an exhaust pipe of a motor running on pure ethanol. Would the proper 9.1:1 (actual) AF mixture going into the engine be interpreted by the same exhaust O2% wideband equipment as a 14.7:1??

I think maybe so (based on what I think I know about the wideband sensors and their attendant processors/calibration). Put another way, the O2 percentage in the exhaust gas would have to result in a reading of 14.7:1 in order for the actual alcohol 9.1:1 stoich to be correct when interpreted by a wideband exhaust sensor calibrated for gasoline...NO?

One of you automotive engineering types prolly know, huh?

P.

[LGAFF]

Paul that is where lambda comes in...

[1990 quasar blue]

So you're saying for example if I strap my car to a dyno and hook up the wideband that it should be calibrated for the 10% ethanol that we are running here as opposed to 100% gas? If so, that makes sense to me. I believe Dom recently said that we can change stoich in the calibration from 14.7 to 14.3 which would compensate for the 10% ethanol. That alone leads me to believe you're right about a wideband needing a calibration change as well.

[Paul Workman]

Originally Posted by 1990 quasar blue

So you're saying for example if I strap my car to a dyno and hook up the wideband that it should be calibrated for the 10% ethanol that we are running here as opposed to 100% gas? If so, that makes sense to me. I believe Dom recently said that we can change stoich in the calibration from 14.7 to 14.3 which would compensate for the 10% ethanol. That alone leads me to believe you're right about a wideband needing a calibration change as well.

Well, I read that too (the 14.3:1) but I'm not at all certain that is correct...is my point.

As far as lamda tables go, that refers to injector dwell and consequently affecting the actual AF ratio according to alcohol %.

Let me rephase the question:

What would the wideband rig indicate for AF if the actuual fuel were alcohol with the INTAKE MIXTUR set at 9.1:1, keeping in mind the wideband jig being a constant??

P.

[1990 quasar blue]

http://m.*************.com/c6-scan-a...r-for-e85.html

According to these guys it looks like they're looking for mid 8s for their afr numbers.
(I hope I understood you better this time.)

[LGAFF]

Paul, LAmbda is current air fuel/stoich

Get a copy of Greg Banishes book......ISBN 978-1-932494-90-7



Lambda (λ) is the ratio of actual AFR to stoichiometry for a given mixture. Lambda of 1.0 is at stoichiometry, rich mixtures are less than 1.0, and lean mixtures are greater than 1.0.

There is a direct relationship between lambda and AFR. To calculate AFR from a given lambda, multiply the measured lambda by the stoichiometric AFR for that fuel. Alternatively, to recover lambda from an AFR, divide AFR by the stoichiometric AFR for that fuel. This last equation is often used as the definition of lambda:


Because the composition of common fuels varies seasonally, and because many modern vehicles can handle different fuels, when tuning, it makes more sense to talk about lambda values rather than AFR.

Most practical AFR devices actually measure the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas.

[Paul Workman]

Originally Posted by LGAFF

Paul, LAmbda is current air fuel/stoich

Get a copy of Greg Banishes book......ISBN 978-1-932494-90-7



Lambda (λ) is the ratio of actual AFR to stoichiometry for a given mixture. Lambda of 1.0 is at stoichiometry, rich mixtures are less than 1.0, and lean mixtures are greater than 1.0.

There is a direct relationship between lambda and AFR. To calculate AFR from a given lambda, multiply the measured lambda by the stoichiometric AFR for that fuel. Alternatively, to recover lambda from an AFR, divide AFR by the stoichiometric AFR for that fuel. This last equation is often used as the definition of lambda:


Because the composition of common fuels varies seasonally, and because many modern vehicles can handle different fuels, when tuning, it makes more sense to talk about lambda values rather than AFR.

Most practical AFR devices actually measure the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas.

Gotcha. And, I see how easy it is to confuse the actual AFR with that predicated indirectly via the O2% in the exhaust (i.e., the wideband rig).

There's two differnt things afoot:

1) The actual AFR at the injectors, and

2) the other is the O2% measured by the sensors which is used to calculate or predict what the actual AFR would be for a given fuel.

Your last statement receives props from Charles O Probst in his book, "Corvette Fuel Injection" ((pp 18, ch 2) ISBN 0-7386-0861-9). Both statements seem to support what I suspected: - In a closed loop operation, O2% in the exhaust remains the same as long as the appropriate AFR exists for the particular fuel being used.

So, if (as Probst and other turners indicate) if a 14.3 or 14.2:1 is good for gasoline, then I'm wanting to draw to the conclusion the AFR for the respective fuel would be adjusted to the proper ratio, as long as the O2% in the exhaust remains constant. In other words, one does not need to run out to buy a wideband rig with multiple lamda tables in it. Simply pretend the oxygenated fuel was gasoline. Then set the fueling tables according to established AFR wideband "readings" and the actual AFR for the oxygenated fuel will fall into place. (Of course software capable of handling flex fuel would monitor specific gravity and make lamda adustments on the fly. Not possible with our old programming or processors, I guess.)

Or so it seems to me at this point

P.