Nira i7x - Fuel Delivery

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2.3.3 Mass Airflow Sensors (MAF)

Most modern engines have MAF sensors. These essentially weigh the

incoming air before it enters the engine. The engine control system receives

information about how many grams of air/millisecond have passed through

the MAF sensor.

Using a MAF has a major advantage in that it reports only the amount of air

that actually enters the engine. Engine wear during a long life is factored in

automatically using this approach. Additionally, MAF sensors simplify the fuel

calibration significantly. This was especially important many years ago when

the computational power of the microprocessor in the engine management

system was limited.

Today, MAF is the standard because of strict emission standards that apply

during the entire life of the engine. There are, however, several

disadvantages:

 MAF sensors are slow to react, expensive and sensitive to damage.

There are often problems using MAF sensors on aggressively tuned

engines because the sensor can break due to high airflow

 The sensor itself is mounted in a housing, which represents a

restriction to the intake air

 There are also problems with oil entering the intake tract from the

turbo due to the restriction

2.3.4 Alfa-N and Speed-Density

During the mid-1990’s many cars used speed-density systems. Speed-density

systems have no restrictions in the intake tract (as compared to MAF sensor

systems) and the precision was good enough over the lifetime of the engine

to meet the emission rules of the time.

On low cost vehicles, Alfa-N systems were common. Alfa-N systems are very

inexpensive to manufacture because they only require a high-precision TPS.

Unfortunately, as the engine wears, the TPS value no longer is a correct

measure of how much air the engine takes in and this leads to emissions

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