GM > Engine > Airflow > Dynamic Airflow

Airflow and Cylinder Airmass Calculations
The PCM has a number of airflow calculations it performs based primarily on input from the Mass Airflow sensor (MAF), Manifold Absolute Pressure (MAP) sensor, Inlet Air Temperature (IAT) sensor and other inputs. The PCM relies very heavily on its ability to accurately estimate the mass of air entering each cylinder during the intake stroke. It uses this information to accurately meter the correct mass of fuel into the cylinder to achieve the desired (commanded) air/fuel ratio (AFR).

The dynamic airmass calculations include various filters and strategies to predict the cylinder airmass from MAF, MAP, TPS, RPM, IAT and VE table inputs. The system is divided into various "zones" of operation (very similar to fuel trim cells). The zones are shown in the diagram below:

As engine operating conditions change the current zone changes and this selects the appropriate filter equation to be used in the dynamic airmass calculation.

Dynamic Airflow

Prediction Coefficients

Steady State
A critical part of the dynamic airmass calculation is determination of when the engine is operating at a steady state condition or unsteady (transient) state. During steady state the PCM uses a filtered MAF signal as the basis for airmass calculations, the PCM also calculates a "VE Correction Factor" during steady state. The VE correction factor is simply the ratio of the MAF airmass to the VE airmass and is used to "offset" the VE calculated airmass when a transient is encountered.

During unsteady state, the PCM uses the VE table to calculate airmass and it is offset (multiplied) by the last calculated VE Correction factor. As long as the VE Correction factor is within limits it will accurately offset any differences between MAF and VE table airmasses and the transition between steady and unsteady (transient) prediction will be smooth.

VE Correction Factor
The VE Correction Factor is used to maintain a relative offset between the VE calculated airmass and the MAF measured airmass to ensure smooth transition between steady state and unsteady state behavior. To protect against unstable behavior the VE correction factor has limits in place and a reset condition.