What is the purpose of air flow meter in car?

The air flow meter in a car tells the engine management computer how much air the cars engine is using. This is important because the fuel to air ratio must be maintained for proper combustion of the fuel. If there is too little fuel for the amount of air, the engine will tend to stutter misfire and accelerate poorly. If there is too much fuel for the amount of air, the engine will smoke, have poor efficiency , high emissions, and also have poor performance. When the computer in in "closed loop" operation, it calculates how much fuel to deliver based on the measurement received from the air flow meter. The computer is smart and many can calculate what the air flow is supposed to be without the air flow meter being hooked up. The engine rpm, manifold pressure and temperature can be used to get a very close estimate of the air flow at the lower engine rpm range. Sometimes the computer operates in "open loop" and does such calculation. Open loop takes place in some cars when the car just starts and is warming up, or if there is a problem with some part of the system, so you can still operate the vehicle. The oxygen sensor on a car is also used while in the closed loop mode to assist in determining how much fuel to inject. The injectors control the fuel by opening and closing very fast, the longer the injector is open, the more fuel. Fuel pressure upstream of the injector is held constant, in that way the fuel delivered is directly proportion to the amount of time that the injector is open.