This is going to be rather brief because Alpha-N is rarely used as the primary strategy for engine management today.  You should at least know what it is because it is often used as a “sanity check” for primary control strategies.

Alpha-N is also sometimes called “TPS maps” because the only sensor that is used for determination of fueling is the Throttle Position Sensor.  (And measured RPM, or how fast the motor is spinning)  Fuel and timing requirements for the engine are expressed as a function of RPM and TPS.

Alpha-N is used most of the time in tricky situations:

1. When the MAP sensor or MAF sensor has failed and the primary control strategy is deemed to be invalid.  Something-is-better-than-nothing is the idea.  (“Load with Failed MAF” is an example from Ford-land)
2. In conjunction with ITBs (Individual Throttle Bodies) due to the extremely low vacuum created by them (making Speed-Density tricky) and the desire to avoid needing to fit a potentially restrictive Mass Air Flow sensor (making MAF impossible).  Again, something-is-better-than-nothing is the idea.
3. In conjunction with ITBs and MAP as a load multiplier. (PowerFC D-Jetro for GTR Skyline, most notable example)  ITBs + Boost – Alpha-N output is multiplied by a MAP sensor to come up with a composite load index.
4. In conjuction with Speed-Density and some kind of blending algorithm.  This approach is often used with very large camshafts that pull little vacuum at idle.  Basically, TPS and MAP are allowed to contribute varying amounts to the overall load calculation.   Net result: more stable and meaningful load index close to idle when MAP sensor readings are unstable.  Found on the Electromotive TEC3 among others.

Alpha-N is very poor at dealing with hills (think about engine load going up and down hills at a constant throttle position), temperature variations and just about anything else that you’d care about except close to wide open throttle where it does fine.