Correcting your AFR

Hello,

I just attended the "Correcting your AFR" module in the EFI fundamental Course.

Concerning the compensation factor : does it aplies for all the rpm and loads , even for WOT ?

Fine tweeking the VE tables this way is based on an EXTERNAL WB O2 sensor, right ? not the ECU O2 sensor ?

Concretely : i plan on purchasing a EMU Black ECU : it's speed Density based, with VE Tables. For my Elise 2ZR engine, it comes with a wide band Bosch LSU 4.2 O2 sensor. What's the benefit vs a narrow band O2 sensor then ?? (other than auto tune)

I am a bit disapointed to not have heard about open loop / closed loop / LTFT / etc .. and the link with Fuel Tables and when it comes open or closed loop. That is something i Don't catch.

Thanks a lot for your answer.

BR

Mat

Hi Mat,

The AFR correction factor will work at any RPM or load. It is giving you the math equation needed to work out what Lambda you're at vs what lambda you're targeting.

Yes we also use a Wideband 02 sensor. As the name suggests it gives us a wider range of measurement where as a narrow band will only operate within a narrow range near stoichiometric. The wideband can either be an external device or integrated into the ecu.

We only really see narrow band sensors in the OEM world which are used in closed loop conditions. You can also use a narrow band in an aftermarket ecu, although once again you would only use for closed loop fueling after the engine has been fully tuned. If you can, using a wideband for closed loop will produce a far more accurate control.

Thanks Chris for all those elements,

In order i really know if i catched it or not, could you confirm (or not) these allégations :

- Most OEM ECU have narrow band O2 sensors and thus, can only go into Closed loop at idle and cruise (because here a lambda 1 is targeted) BUT can't operate in closed loop at WOT because another value than 1 (lower, ie. richer) is targeted at WOT or full load. Right ?

- Most Aftermarket ECUs benefit from a wide band O2 sensor : does this means that all operating conditions (idle cruise and WOT) are in closed loop in this case ?

- (Volountary provocative question :) )If my aftermaket ECU is based on a Wide band O2 sensor and is operating Always in closed loop, why should i bother with tuning properly VE table, for example ?? (i would say because the smaller corrections you have to make, the smoother is the engine operation ?)

Thanks again ,

Best regards,

Mathieu

ps: maybe those questions are covered in the "understanding AFR " course ?

Bonjour Mathieu!

I can't answer all your questions, but I assume you are correct regarding the Narrowband sensor. As they are typically 0-1V sensors, they can only tell if you're lean, rich, or stoich, but they can't say how lean or rich. I remember during some technical training with GM or Ford (mid 90s) mock up board that the narrowband sensor would cycle fast from rich to lean and that was consider Stoich. With a wideband sensor (0-5V) you can have an actual value of what your lambda or AFR is.

Also, at WOT, I don't think the sensor / ECU would react quick enough to make change to the AFR in closed loop. Also, let's say your wideband sensor goes bad and start giving inaccurate readings, you could damage your engine by relying only on closed loop for WOT operation. That's why there are set points to switch to open loop from TPS or MAP values. Some aftermarket ECUs have a "learn table" that can operate at all loads including WOT. They build up over time but you need need to manually accept the changes so it updates the fueling table.

Also, I think OEMs now use a wideband sensors somewhere on the exhaust piping for close loop. I guess it probably help achieve the strict emission regulation nowadays.

Anyway, I hope my thoughts on this are correct!

Have a good day,

Frank

Frank has pretty much covered your questions, just adding onto it.

Yes you're correct the narrow band can not be used in closed loop when at WOT because of richer values, the sensor only works around stoich.

You can use a Wideband during all conditions if you wanted too. Although as Frank said, if there is a sensor failure or fault and the ecu starts removing fuel etc it could damage the engine. I will only use closed loop for idle and cruise with a clamp of 5-10%. A lot of manufacturers now use wideband sensors and some of these are still enabled during WOT conditions.

Not tuning the VE table correctly and relying on the wideband only will give very poor drivablilty. You need to remember that an 02 sensor is a measurement after the event has happened. So if you fuel mixture is out by 40% it won't be corrected by the ecu until the wideband has measured this event and the ecu has corrected it. This means you have already run the engine at 40% lean/rich, giving really poor drivability and dangerous conditions to the motor. Most manufactures allow control up to around 25% (some will go to 50% in STFT and LTFT). I will tune a fuel table within 1% (2% tops) and allow the ecu up to 5-10% of closed loop correction if needed.

Some aftermarket ecus will allow you to autotune, you can set the whole fuel table to a value, enable autotune and drive the car and it will populate the fuel table based on correction from the wideband.

Check out the video below on auto tuning

[url=https://www.youtube.com/watch?v=HxSt1oOQaD8]https://www.youtube.com/watch?v=HxSt1oOQaD8

Thanks Chris and Franck !

It's more clear now.