Mass flow sensor From Wikipedia, the free encyclopedia A mass airflow sensor A mass (air) flow sensor (MAF) is used to find out the mass flowrate of air entering a fuel-injected internal combustion engine. The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each cylinder. (See stoichiometry and ideal gas law.) There are two common types of mass airflow sensors in use on automotive engines. These are the vane meter and the hot wire. Neither design employs technology that measures air mass directly. However, with additional sensors and inputs, an engines ECU can determine the mass flowrate of intake air. Both approaches are used almost exclusively on electronic fuel injection (EFI) engines. Both sensor designs output a 0.0–5.0 volt or a pulse-width modulation (PWM) signal that is proportional to the air mass flow rate, and both sensors have an intake air temperature (IAT) sensor incorporated into their housings for most post OBDII vehicles. Vehicles prior to 1996 could have MAF without an IAT. An example is 1994 Infiniti Q45. When a MAF sensor is used in conjunction with an oxygen sensor, the engines air/fuel ratio can be controlled very accurately. The MAF sensor provides the open-loop controller predicted air flow information (the measured air flow) to the ECU, and the oxygen sensor provides closed-loop feedback in order to make minor corrections to the predicted air mass. Also see MAP sensor. Hot wire sensor (MAF) A hot wire mass airflow sensor determines the mass of air flowing into the engine’s air intake system. The theory of operation of the hot wire mass airflow sensor is similar to that of the hot wire anemometer (which determines air velocity). This is achieved by heating a wire suspended in the engine’s air stream, like a toaster wire, with either a constant voltage over the wire or a constant current through the wire. The wires electrical resistance increases as the wire’s temperature increases, which varies the electrical current flowing through, or the voltage over the circuit, according to Ohms law. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit or causing a smaller voltage drop over the wire. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The current or voltage drop is proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement into a calibrated signal which is sent to the ECU. If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass airflow. Unlike the vane meters paddle sensing element, the hot wire responds directly to air density. This sensors capabilities are well suited to support the gasoline combustion process which fundamentally responds to air mass, not air volume. (See stoichiometry.) This sensor sometimes employs a mixture screw, but this screw is fully electronic and uses a variable resistor (potentiometer) instead of an air bypass screw. The screw needs more turns to achieve the desired results. A hot wire burn-off cleaning circuit is employed on some of these sensors. A burn-off relay applies a high current through the platinum hot wire after the vehicle is turned off for a second or so, thereby burning or vaporizing any contaminants that have stuck to the platinum hot wire element. The hot film MAF sensor works somewhat similar to the hot wire MAF sensor, but instead it usually outputs a frequency signal. This sensor uses a hot film-grid instead of a hot wire. It is commonly found in late 80’s early 90’s fuel-injected vehicles. The output frequency is directly proportional to the air mass entering the engine. So as mass flow increases so does frequency. These sensors tend to cause intermittent problems due to internal electrical failures. The use of an oscilloscope is strongly recommended to check the output frequency of these sensors. Frequency distortion is also common when the sensor starts to fail. Many technicians in the field use a tap test with very conclusive results. Not all HFM systems output a frequency. In some cases, this sensor works by outputting a regular varying voltage signal. 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) There are some drawbacks: dirt and oil can contaminate the hot-wire deteriorating its accuracy installation requires a laminar flow across the hot-wire the sensor contains a thin platinum wire, which can break if handled incorrectly
Posted on: Thu, 15 Jan 2015 09:19:59 +0000