Presentation, Air Flow Meter

Intro

The mass air flow sensor  measures the amount of intake air  , when the air goes through between an air cleaner and a throttle body. The sensor gives the ECU information about the output signal. Then the ECU controls the fuel injection and ignition timing from the MAF's data. It is critical that the sensor signal uses to determine injector's operation time. 

There are two common types of mass airflow sensors in use on automotive engines. These are the vane meter and the hot wire

Vane type air flow meters

There are seven terminals on a vane type air flow meter ,  it has a mechanical flap that measures the flow of air into the intake system with a  spring-loaded air flap/door attached to a variable resistor.The vane air flow meter works in a very similar way to the TPS sensor.When the amount of intake air changes with vane's movement, the resistance and voltage are also changed. When the vane closes, the resistance value is the highest with the highest signal.On the other hand, when the vane fully opens, the resistance value is the lowest with the lowest voltage. Therefore, the ECU knows the amount of intake air from the signal voltage changes.

Many VAF sensors have an air-fuel adjustment screw, which opens or closes a small air passage on the side of the VAF sensor. This screw controls the air-fuel mixture by letting a metered amount of air flow past the air flap, thereby, leaning or richening the mixture. By turning the screw clockwise the mixture is enriched and counterclockwise the mixture is leaned.
The vane moves because of the drag force of the air flow against it, it does not measure volume or mass directly.
Air flow meters may fail or wear out. When this happens, engine performance will often decrease significantly, engine emissions will be greatly increased

If the MAF sensor is faulty, the ECU would not control the fuel injection and the ignition timing properly due to the wrong signal given. As a result, the idle would be unstable and run rough but when under acceleration it  would run normal but have less power.

Bad points of a Vane:

it restricts airflow which limits engine output

its moving electrical or mechanical contacts can wear

finding a suitable mounting location within a confined engine , because vane is larger

the vane has to be oriented with respect to gravity

Some of the benefits of a hot-wire MAF compared to the older style vane meter are:

• responds very quickly to changes in air flow
• low airflow restriction
• smaller overall package
• less sensitive to mounting location and orientation
• no moving parts improve its durability
• less expensive
• separate temperature and pressure sensors are not required (to determine air mass)

Disadvanges of a hotwire

• dirt and oil can contaminate the hot-wire deteriorating its accuracy
• installation requires a laminar flow across the hot-wire

http://www.youtube.com/watch?v=Xkyfjo7spFc
http://www.youtube.com/watch?v=CXuO5OmKPyY By Shamsher & Jal