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what is the difference between afr target and fuel table

Understanding AFR

Discussion and questions related to the course Understand AFR


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hi i have a beginner doubt, what is the propourse of the afr target map if we have the fuel map

The afr target map is used for closed-loop operation. In closed-loop operation feedback from the O2 (Lambda) sensor is used to adjust the value from the fuel map to reach the AFR target.

When not in closed loop mode, the amount of fuel delivered comes from the fuel map (plus any trims/compensations).

Some systems use the target AFR map to allow you to quickly tune the fuel map by the necessary amount to achieve the target -- this may be a learning mode, or quick-setting or similar name.

So when is it necessary to use closed loop vs not use it ?I think this is what in most confused about cause the way I see it ,why wouldn't you always have every table in closed loop hitting your target consistently?

Can you tell us what ECU and engine you are working with? Basically newer standalones and many newer stock ECUs can be closed loop all the time except the first few seconds of cold start.

The AFR target map is a table that you create to represent the air to fuel ratio you would like your engine to run.

The Fuel Map is the table you then tune to calibrate the EFI system to deliver the amount of fuel required to reach the targets you've entered in the AFR map.

Once that process is complete, you *should* be able to then change the values in the AFR target map, and as your fuel map is calibrated (this is the tuning process), the fuel being delivered to the engine will increase or decrease to match the new target. This is the tricky point of EFI calibration, as there are many, many other aspects that affect the AFR the engine will run (primarily injector model data), so all these other things need to be correct to get the system to function as intended. Many times they aren't, and a certain amount of 'fudge factor' is burned into the fuel table during the initial tuning process to get the engine to run at the AFR's in the AFR target table. This fudge means that when you then change the AFR target, the actual AFR the engine runs wont match it 100%.

This isn't closed loop behaviour, it occurs because the background fueling equation that actually determines the amount of time to open the injector for includes the value you enter in the AFR target map.

When you enable closed loop operation, the AFR target map then helps out in another way, the ECU looks at the current AFR, compares it to the target AFR, and uses additional fuel trimming parameters to adjust the fueling to ensure the two match.

Also, many ECUs use Target AFR as a reference point -- if you go far above/below you can trigger fuel/timing cuts. Also, your fueling numbers would not produce the same AFR if you drove up a big mountain (or down one, if you started up top) ... the ECU can compensate for some of these changes. As for smart learning ECUs -- they generally cannot keep up for WOT runs, so you're more likely to blow your motor if you depend on them -- they're great for optimizing cruising, though.