How OBD2 calculate the fuel flow rate and Mass Air Flow

I figured out that the formula stated in the paper for fuel flow below:

fuelFlow = (MAF * 3600) / (14.7f * 820) ; // l/h

is off by a single 0(needs an extra 0 added to the right of decimal).So we need to divide by 10, so the actual formula for fuelFlow now becomes:

fuelFlow = ((MAF * 3600) / (14.7f * 820)) / 10 ; // l/h

Apart from this the MAF calculated using the formula 1 below:

IMAP = RPM x MAP / IAT / 2 MAF = (IMAP/60) x (VolEff/100) x EngDisp x (MMAir/R)

probably results in a close estimation of MAF that is highly dependent on the accuracy of VE(Volumetric efficiency).I assumed it to be 80%.If there is some kind of way to calculate VE based of obd parameters not using mass air flow itself, then it would be great. Otherwise you just have to assume on VE percentage.

how does the ecu determine the load on the engine? [Archive] - The Subaru SVX World Network (subaru-svx.net)

The ECU needs to know the quantity of air it is getting, so it can work out the quantity of fuel to mix with it.

A MAF sensor measures it directly.

With a MAP based system you need to measure pressure and temperature and then calculate the quantity based on those readings. When you step on the gas, Mass Airflow changes almost instantly but Manifold Pressure lags behind. So you have to do TPS delta compensations to guess how the airflow is changing while you wait for the pressure to catch up.

MAF systems are faster to respond and more accurate. So they run smoother, especially at low rpm where precision is important. They suffer from less hesitation and provide better drivability. That's why most modern ECUs use MAF.

MAP tuning becomes a lot more useful when you increase the horsepower beyond what the manufacturer designed for. When your airflow exceeds what the MAF can measure, you either have to fit a bigger MAF (with reduced precision), or go to a MAP based system.

Some aftermarket systems like EcuTek RaceROM and Cobb Accessport can use MAF at low rpm and switch to MAP when the MAF gets maxed out. But obviously neither of those is available for the SVX.

Yes the Mass Air Flow sensor is far more accurate than the Manifold Absolute Pressure sensor, that was used before emissions demanded better fuel ratios. Though I think you have their actions, in reverse.:cool:

The MAP Sensor will signal the pressure change instantly, the MAF is slower as the air has mass, and pressure does not. When you floor the pedal, the pressure changes instantly, along with the fuel, but the air is not there to match it, so the ratio has to be compensated.

The MAF sensor reads the air mass that is entering the cylinder, so the air/fuel ratio can be set exactly to match the air.

Harvey.

Reference of MAP sensor

Fuel Injection Diagnosis: It's All About the Air | MOTOR