Air Mass Sensors
An air mass sensor measures the amount of air which is entering the engine. Since air mass varies depending on the temperature of the air, the engine control unit measures it to determine the correct amount of fuel, which needs to be mixed with the air. Having the proper mixture of fuel and air allows the engine to run more efficiently. Mass airflow sensors are used in conjunction with an oxygen sensor, which allows the engine's air and fuel ratio, to be controlled very accurately.
Air density changes depending on the altitude, temperature, and the amount of forced induction. All of these variables change as a vehicle accelerates or encounters weather changes. There are two types of mass airflow sensors found on most vehicles. These two sensors are exclusively used on fuel injected engines. The first of the mass air flow sensors is the vane meter sensor. This sensor measures the airflow with a flap, which is secured with a spring. This spring loaded door opens and closes depending on the amount of airflow. The movement of the door is also the component,which regulates how much fuel needs to be mixed with the air. Many vane meter sensors use an air-fuel adjustment screw, which can be opened or closed, for a small air passage. This screw also controls the air-fuel mixture by allowing a specific amount of air hit the spring loaded door. Turning this screw either enriches the mixture, or makes it leaner.
The other mass airflow sensor which is commonly used in vehicles, is the hot wire sensor. This sensor determines the amount of air allowed to flow into a vehicles air intake system. This method works by heating a wire with an electric current, which is suspended in the engine's air stream. The electric resistance in the wire increases, as the temperature rises. As air flows past the wire it cools, which decreases resistance, and allows for more current to flow through the circuit. Amounts of current required to maintain the wire’s temperature is proportional to the mass of air flowing, past the wire. The measurement of current is converted into a voltage signal, which is sent to the engine control unit.
These sensors' capabilities are suited to support gasoline engines, because they respond well to air mass, not air volume. When the air is denser, more heat is removed from the wire, which indicates a higher mass airflow. Hot wire sensors occasionally use a mixture screw, which is electronic and uses a variable resistor, instead of an air bypass screw. This sensor typically has a burn off cleaning circuit. This is used to burn off or vaporize, any material that may have been stuck to the platinum wire.
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