An air flow meter is used in some automobiles to measure the quantity of air going into the internal combustion engine. All modern electronically controlled diesel engines use air flow meter, as it is the only possible means of determining the air intake for them. In the case of a petrol engine, the electronic control unit (ECU) then calculates how much fuel is needed to inject into the cylinder ports. In the diesel engine, the ECU meters the fuel through the injectors into the engines cylinders during the compression stroke.
The vane (flap) type air flow meters (Bosch L-Jetronic and early Motronic EFI systems or Hitachi) actually measure air volume, whereas the later "hot wire" and "hot film" air mass meters measure speed of air flow.
The flap type meter includes a spring which returns the internal flap to the initial position. Sometimes if the spring is tensioned too tightly, it can restrict the incoming air and it would cause the intake air speed to increase when not opened fully.
Differential pressure is also used for air flow measurement purposes.