Speed-Density

A speed-density strategy monitors the intake air pressure and temperature to calculate the theoretical mass (density) of air that enters the chamber each cycle. However, the speed of the engine will affect the actual mass of air which enters the chamber due to restrictions and resonance effects of the intake and exhaust systems. This can be handled with a lookup table of Volumetric Efficiency (Ve) against engine speed. This allows you to calculate the amount of fuel to inject based on the chemically ideal ratio of 14.7:1.

The advantage of speed density is that when making a modification to the intake or exhaust systems, only the Ve table must be modified to account for the changes in volumetric efficiency.
 

Alpha-N

An alpha-n engine control method is simpler because it looks up the mass of air for each throttle angle (alpha) and engine speed (N) operating point, which results in a large table of several hundred points. When mechanical modifications are made to these engines, many or all of the operating points must be recalibrated.