Displays a 3D table used for setting target air/fuel ratios throughout the operating range of the engine.  In speed/density mode, this table will appear as “Target Air/Fuel Ratio vs. RPM & MAP” and will appear as “Target Air/Fuel Ratio vs. RPM & TPS” in Alpha-N mode.

There are two primary factors that will determine what role this table will play in the tuning process.  One is whether speed/density mode or Alpha-N mode is selected, and the other is whether the ECU is running in open loop mode or closed loop mode.

In speed/density mode, the ECU first determines, based on several sensor measurements, the volume of air being consumed by the engine.  Once it has determined the volume of air, it then looks to this table to determine what air/fuel ratio is currently being targeted.  When a ratio is selected, this information is used to determine the proper amount of fuel to inject in order to produce this ratio.  For this reason, the target air/fuel ratio table is always a critical part of the calibration in speed/density mode.  In contrast, Alpha-N mode does not use this table as part of its base fuel calculation.  In Alpha-N mode, an injector pulsewidth is simply looked up from the base fuel table.

Regardless of whether a speed/density calibration or an Alpha-N calibration is being used, the system can be run in closed loop mode.  When operating in closed loop mode, the ECU will compare the measured air/fuel ratio to the air/fuel ratio targeted within this table.  If there is a difference, the fuel delivery will be automatically adjusted in an attempt to make the measured and targeted air/fuel ratio from this table as close as possible.  Running in closed loop mode

In open loop mode, the differences in measured and targeted air/fuel ratios won't be made up as they are in closed loop mode.  The oxygen sensor will still be actively measuring air/fuel ratios, but no action based on this information will be taken within the ECU.

To break it down, there are four different ways this system can be run:

• Speed/density mode, open loop.  The target air/fuel ratio table is used as part of the base fuel calculation only.
• Speed/density mode, closed loop.  The target air/fuel ratio table is used as part of the base fuel calculation, and the ECU will attempt to correct any differences between measured air/fuel ratios and the values in the target air/fuel ratio table.
• Alpha-N mode, open loop.  The target air/fuel ratio table is not used at all in this configuration.
• Alpha-N mode, closed loop.  The target air/fuel ratio table will not be used for the base fuel calculation, but the ECU will attempt to correct any differences between measured air/fuel ratios and the values in the target air/fuel ratio table.

Air/fuel ratio selection

There is a particular ratio of air to fuel that results in all of the oxygen molecules combining chemically with all of the fuel molecules. This ratio is 14.7:1, meaning 14.7 parts of air for every 1 part of fuel.  Running an engine at this particular air/fuel ratio helps to minimize emissions and maximize fuel economy, but this air/fuel ratio is generally not safe to run at high RPM and/or engine load conditions.  Since most oxygen sensors produced are only meant to accurately measure a 14.7:1 air/fuel ratio, they shouldn't be used in applications where much richer or leaner target air/fuel ratios are selected.  The wide-band oxygen sensor used with this system is capable of accurately measuring a much wider range of air/fuel ratios.  This system can measure any ratio from as rich as 9:1 to as lean as 16:1.