12P Flex Fuel Discussion

[Jayme]

dont think of it in terms of wideband. your wideband is a bad example... a wideband reads in lambda, then calculates the AFR for the fuel type you are using. if you left your wideband in lambda, then tuned for petrol, had it all perfect and had petrol suitable AFR's in your commanded AFR table, then swapped to E85, your wideband would read lean because you need more fuel.

if you changed the AFR commanded table from 14.7 values to the equivalent E85 values (14.7 petrol = 9.76ish E85) then your wideband in lambda would start looking normal again. if your wideband was in petrol AFR mode, it would read 14.7, if your wideband was in E85 AFR mode, it would read 9.7. so end result is that while there are many ways to tune for different fuels, the method to tune for different fuels, if you want the commanded AFR to look right (for your fuel type) is to only alter the commanded AFR, but you have to tell your wideband what fuel you are using if you want your wideband data to match your commanded AFR table.

below is a simple conversion for differnet fuels. this is the actual AFR value.
Lambda -1.00
Gasoline -14.70 :1
E85 Alcohol -9.77 :1
E100 Alcohol -9.01 :1
Alcohol (Methanol)-6.40 :1
Propane -15.50 :1
Diesel -14.50 :1
Nitro -1.70 :1

[Dylan]

Google Volumetric Efficiency or search on here. VE is how much air is filling the cylinder. That won't change depending on fuel type. You can increase the VE instead of target AFR to go richer but that's the wrong way to do things and creates trouble in other area's.

[TdracerTd]

Yes, I understand how the different fuels relate back to lambda. My question is how is changing the commanded afr going to change how much fuel you are putting in? Yes, in closed loop and yes if you have wideband VE learn turned on. I run e85 and my wife band displays petrol afrs. It makes no difference except the reading on the dial.

I have to admit, I don't use wideband be learn and all my closed loop and blm are turned off (because racecar ). But I can change my AFR tables all day and it doesn't make a difference to the reading on my wideband. It's nice to have them correct for logging though.

[TdracerTd]

Google Volumetric Efficiency or search on here. VE is how much air is filling the cylinder. That won't change depending on fuel type. You can increase the VE instead of target AFR to go richer but that's the wrong way to do things and creates trouble in other area's.

Yes, but what do you change in your tune when you need more fuel at a given load point to match your commanded?

I understand ve. But thanks for the suggestion to google it.

Remember. The ecu is looking at a voltage from the wide band. It will not change if you have enough fuel for the amount of air for a given fuel. To me bu changing between petrol and e85 afrs will confuse things more than simply adding more fuel to achieve a certain lambda. But that's just how my brain works.

[Dylan]

6.4 AFR (for this example) compared to 14.7 is over double that's how the ECU adds more fuel. You can have your wideband display petrol AFR's and it'll be reading accurate as it's just calculated from lambda. It's a pain for you trying to convert it all the time when tuning though.

[Dylan]

Set your BPC to match injector and engine capacity. Set target AFR's to match fuel type the tune VE to achieve actual AFR's.

Then say you change to a different fuel type but still same engine capacity and same injectors.
Just change target AFR tables and the ECU will increase or decrease the pulse width depends which way your target AFR went. Because the VE was accurate to suit the air flow of your engine that will work fine.

[Jayme]

the bit I think you are missing TDracer, is that the commanded AFR changes the injecotr pulse width, as the final fuelling equation included commanded AFR. if you change the commanded AFR from 14:! for example to 7:1, your inejctor pulsewidth will literally double. thats why it works.

in the case of wideband, you are tlaking VE learn, which will probably not work in a multi fuel scenario, unless you rescaled the 0v and 5v values inside the 808 to suit the different fuels. then perhaps you could get it to work.
this is the exact reason a lot of later ecus moved to EQ ratio or lambda.... to stop the confusion around AFR readings and different fuel

[VL400]

Thinking in terms of the code for the ethanol correction I will most likely be doing this on the final injector PW. This sorts cranking and run conditions and does not screw with the AFR which will get very very confusing and difficult to manage.

The main reason is exactly what (I think) TracerTD is trying to say and others are missing by thinking in actual conditions rather than what the ECU logging would display. We all agree the wideband outputs a voltage proportional to lambda, and we just add some calcs to make it in to a AFR. So if the ECU modifies the target AFR - lets say its an impossible instant change from petrol to E85 and we are in closed loop targeting stoich. The only thing that is modified is the target AFR, so you would see the target change from 14.7:1 to 9.7:1, the PW would change accordingly but the wideband would still read 14.7:1. Everything would actually be happy and running at stoich. Confused? The wideband reads lambda and we didnt change the 0V and 5V AFR values for the calc.

That is why I think I will be using the final PW, if we had an ECU that worked in lambda it would be less confusing. Its not totally impossible to change it to lambda though.

[VL400]

Oh and one more thing is the VE does change with fuel. The engine will be more efficient on better fuels such as E85, and you will get more spark advance in fuel knock limited areas. You dont do the ethanol % correction on the VE but it will change slightly so it is something to keep in mind.

[Dylan]

That sounds workable. In the case of Td, I took his question as a general how dies the target AFR table effect the pulse width. If he was talking about as VL400 has said, I apologise for adding any extra confusion.

Carry on troops!