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MAF Fuelling Strategy

MAF-based fuelling is an engine management strategy that calculates fuel delivery using a mass air flow (MAF) sensor's measurement of the air entering the engine. A valid MAF reading can account for air-density changes and some airflow changes directly, but it does not guarantee correct fuelling after every intake or engine modification. The sensor installation, transfer function, and trim calibration still need to suit the engine.

MAF fuel setup and tuning​

To tune a Capoworks ECU using MAF, you need a working MAF sensor and a suitable transfer function that converts its raw signal into kilograms per hour (kg/h). Details on setting up the sensor and its transfer function are on the MAF sensor page. For model selection and migration guidance, see Airmass Models and Migration.

Start with the MAF Trim table at 100 percent. The current firmware provides an 8x8 trim table with engine speed on the X axis and pre-trim standardized cylinder-filling percentage on the Y axis. Adjust the trim where a correctly operating, steady-state combination of RPM and airflow does not meet its target; use the AFR or lambda target table when you want a different target air-fuel ratio.

MAF airmass calculation​

The MAF sensor produces a raw signal, which the MAF transfer function converts into airflow in kg/h. The MAF airmass model then converts that measured flow and engine speed into airmass per cylinder:

airflow_g_per_s = maf_kg_per_h / 3.6
air_per_revolution = airflow_g_per_s / (rpm / 60)
cylinder_airmass = air_per_revolution / (cylinder_count / 2)

The divisor cylinder_count / 2 reflects the half of the cylinders that take in air during one revolution of a four-stroke cycle. See the implemented conversion in the pinned firmware source.

Standardized cylinder filling and MAF Trim​

To produce a load value for tables and corrections, the model compares the measured cylinder airmass with a standard charge for one cylinder:

standard_charge = ideal_gas_law(displacement / cylinder_count, 101.325 kPa, 293.15 K)
air_charge_load_percent = 100 * cylinder_airmass / standard_charge

The standard pressure is 101.325 kPa and the standard temperature is 293.15 K (20 degrees C). The ideal-gas calculation uses the gas constant and molar mass of air rather than a fixed air-density constant. See the standard-charge calculation in the firmware source.

air_charge_load_percent is the standardized amount of air in a cylinder before trim is applied. It is not a percentage of the MAF sensor's output. The MAF Trim table uses this pre-trim filling percentage on its Y axis and engine speed on its X axis:

trim_percent = MAF_Trim_table(RPM, air_charge_load_percent)
corrected_cylinder_airmass = cylinder_airmass * trim_percent / 100

A trim of 100 percent leaves the calculated airmass unchanged. Values above 100 percent increase the airmass used for fuelling, and values below 100 percent reduce it. The trim table is separate from the VE table. The configuration template defines it as an 8x8 table, and the model applies it after calculating air_charge_load_percent; see the table definition and the trim calculation.

Two MAF sensors​

When a secondary MAF sensor is configured:

  • If both sensors provide valid readings, their airflows are added.
  • If only one sensor is valid, the model uses twice that sensor's reading. This is a fallback and the airmass evaluation is marked degraded.
  • If neither sensor is valid, the model does not produce an airmass result.

The active behavior is visible in the pinned dual-MAF handling.

At this level, the MAF calculation is a conversion from measured mass airflow and engine speed to corrected cylinder airmass. Fuel delivery then uses that corrected airmass through the normal fuelling calculation.

For those interested, this link details the MAF sensor equations in more detail.