Mass flow sensor - wikipedia Mass flow sensor This article has multiple issues. Please help improve it or discuss these issues on the talk page. ( Learn how and when to remove these template messages ) This article may require cleanup to meet Wikipedia 's quality standards. No cleanup reason has been specified. Please help improve this article if you can. ( November 2009 ) This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( November 2009 ) A mass airflow sensor A mass ( air ) flow sensor ( MAF ) is used to find out the mass flow rate 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 engine 's 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 engine 's 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. Contents 1 Moving vane meter 2 Hot wire sensor ( MAF ) 3 Coldwire sensor 4 Kármán vortex sensor 5 Membrane sensor 6 Laminar flow elements 7 See also 8 References 9 External links Moving vane meter The VAF ( volume air flow ) sensor measures the air flow into the engine with a spring - loaded air vane ( flap / door ) attached to a variable resistor ( potentiometer ). The vane moves in proportion to the airflow. A voltage is applied to the potentiometer and a proportional voltage appears on the output terminal of the potentiometer in proportion to the angle the vane rotates, or the movement of the vane may directly regulate the amount of fuel injected, as in the K - Jetronic system. 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. The drag force depends on air density ( air density in turn depends on air temperature ), air velocity and the shape of the vane, see drag equation. Some VAF sensors include an additional intake air temperature sensor ( IAT sensor ) to allow the engines ECU to calculate the density of the air, and the fuel delivery accordingly. The vane meter approach has some drawbacks : it restricts airflow which limits engine output its moving electrical or mechanical contacts can wear finding a suitable mounting location within a confined engine compartment is problematic the vane has to be oriented with respect to gravity. in some manufacturers fuel pump control was also part on the VAF internal wiring. 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 wire 's electrical resistance increases as the wire 's temperature increases, which varies the electrical current flowing through the circuit, according to Ohm 's law. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit, since the supply voltage is a constant. As more current flows, the wire 's temperature increases until the resistance reaches equilibrium again. The current increase or decrease is proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the proportional 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 meter 's paddle sensing element, the hot wire responds directly to air density. This sensor 's 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 Coldwire sensor A Holden Commodore 's MAF sensor The GM LS engine series ( as well as others ) use a coldwire MAF system ( produced by AC Delco ) that works similarly to the hot - wire MAF system ; however, it uses an additional `` cold '' resistor to measure the ambient air and provide a reference for the `` hot '' resistor element used to measure the air flow. The mesh on the MAF is used to smooth out airflow to ensure the sensors have the best chance of a steady reading. It is not used for measuring the air flow per se. In situations where owners use oiled - gauze air filters, it is possible for excess oil to coat the MAF sensor and skew its readings. Indeed, General Motors has issued a Technical Service Bulletin, indicating problems from rough idle all the way to possible transmission damage resulting from the contaminated sensors. To clean the delicate MAF sensor components, a specific MAF sensor cleaner or electronics cleaner should be used, not carburetor or brake cleaners, which can be too aggressive chemically. Instead, the liquid phase of MAF sensor cleaners and electronics cleaners is typically based on hexanes or heptanes with little to no alcohol content and use either carbon dioxide or HFC - 152a as aerosol propellants. The sensors should be gently sprayed from a careful distance to avoid physically damaging them and then allowed to thoroughly dry before reinstalling. Manufacturers claim that a simple but extremely reliable test to ensure correct functionality is to tap the unit with the back of a screwdriver while the car is running, and if this causes any changes in the output frequency then the unit should be discarded and an OEM replacement installed. Kármán vortex sensor A von Kármán vortex street A Kármán vortex sensor works by disrupting the air stream with a perpendicular bow. Providing that the incoming flow is laminar, the wake consists of an oscillatory pattern of Kármán vortices. The frequency of the resulting pattern is proportional to the air velocity. These vortices can either be read directly as a pressure pulse against a sensor, or they can be made to collide with a mirror which will then interrupt or transmit a reflected light beam to generate the pulses in response to the vortices. The first type can only be used in pull - thru air ( prior to a turbo - or supercharger ), while the second type could theoretically be used push - or pull - thru air ( before or after a forced induction application like the previously mentioned super - or turbocharger ). Instead of outputting a constant voltage modified by a resistance factor, this type of MAF outputs a frequency which must then be interpreted by the ECU. This type of MAF can be found on all DSMs ( Mitsubishi Eclipse, Eagle Talon, Plymouth Laser ), many Mitsubishis, some Toyotas and Lexus, and some BMWs, among others. Membrane sensor An emerging technology utilizes a very thin electronic membrane placed in the air stream. The membrane has a thin film temperature sensor printed on the upstream side, and one on the downstream side. A heater is integrated in the center of the membrane which maintains a constant temperature similar to the hot - wire approach. Without any airflow, the temperature profile across the membrane is uniform. When air flows across the membrane, the upstream side cools differently from the downstream side. The difference between the upstream and downstream temperature indicates the mass airflow. The thermal membrane sensor is also capable of measuring flow in both directions, which sometimes occur in pulsating situations. Technological progress allows this kind of sensor to be manufactured on the microscopic scale as microsensors using microelectromechanical systems technology. Such a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. See also MEMS sensor generations. Laminar flow elements Laminar flow elements measure the volumetric flow of gases directly. They operate on the principle that, given laminar flow, the pressure difference across a pipe is linear to the flow rate. Laminar flow conditions are present in a gas when the Reynolds number of the gas is below the critical figure. The viscosity of the fluid must be compensated for in the result. Laminar flow elements are usually constructed from a large number of parallel pipes to achieve the required flow rating. See also List of sensors Manifold absolute pressure ( MAP ) References Jump up ^ Gilles, Tim ( 2011 ). Automotive Service : Inspection, Maintenance, Repair ( 4th ed. ). Cengage Learning. ISBN 978 - 1 - 1111 - 2861 - 6. Jump up ^ `` Air Flow Sensors '' ( PDF ). Retrieved 15 September 2009. External links A Hot Film sensor with theory of operation A video example of cleaning a MAF sensor An example of how to clean a MAF sensor, 3 wire How To Test a MAF Automotive engine Part of the Automobile series Basic terminology Bore Compression ratio Crank Cylinder Dead centre Diesel engine Dry sump Engine balance Engine configuration Engine displacement Engine knocking Firing order Hydrolock Petrol engine Power band Redline Spark - ignition engine Stroke Stroke ratio Wet sump Main components Connecting rod Crankcase Crankpin Crankshaft Crossplane Cylinder bank Cylinder block Cylinder head ( crossflow, reverse - flow ) Flywheel Head gasket Hypereutectic piston Main bearing Piston Piston ring Starter ring gear Sump Valvetrain Cam Cam follower Camshaft Desmodromic valve Hydraulic tappet Multi-valve Overhead camshaft Overhead valve Pneumatic valve springs Poppet valve Pushrod Rocker arm Sleeve valve Tappet Timing belt Timing mark Valve float Variable valve timing Aspiration Air filter Blowoff valve Boost controller Butterfly valve Centrifugal - type supercharger Cold air intake Dump valve Electronic throttle control Forced induction Inlet manifold Intake Intercooler Manifold vacuum Naturally aspirated engine Ram - air intake Scroll - type supercharger Short ram air intake Supercharger Throttle Throttle body Turbocharger Twin - turbo Variable - geometry turbocharger Variable - length intake manifold Warm air intake Fuel system Carburetor Common rail Direct injection Fuel filter Fuel injection Fuel pump Fuel tank Gasoline direct injection Indirect injection Injection pump Lean - burn Stratified charge engine Turbo fuel stratified injection Unit injector Ignition Contact breaker Magneto Distributor Electrical ballast High tension leads Ignition coil Spark plug Wasted spark Electrics and engine management Air -- fuel ratio meter Alternator Automatic Performance Control Car battery ( lead -- acid battery ) Crankshaft position sensor Dynamo Drive by wire Electronic control unit Engine control unit Engine coolant temperature sensor Glow plug Idle air control actuator MAP sensor Mass flow sensor Oxygen sensor Starter motor Throttle position sensor Exhaust system Automobile emissions control Catalytic converter Diesel particulate filter Exhaust manifold Glasspack Muffler Engine cooling Air cooling Antifreeze ( ethylene glycol ) Core plug Electric fan Fan belt Radiator Thermostat Water cooling Viscous fan ( fan clutch ) Other components Balance shaft Block heater Combustion chamber Cylinder head porting Gasket Motor oil Oil filter Oil pump Oil sludge PCV valve Seal Synthetic oil Underdrive pulleys Portal Sensors Acoustic, sound, vibration Geophone Hydrophone Microphone Seismometer Automotive, transportation Air -- fuel ratio meter Blind spot monitor Crankshaft position sensor Curb feeler Defect detector Engine coolant temperature sensor Hall effect sensor MAP sensor Mass flow sensor Omniview technology Oxygen sensor Parking sensors Radar gun Speed sensor Speedometer Throttle position sensor Tire - pressure monitoring system Torque sensor Transmission fluid temperature sensor Turbine speed sensor Variable reluctance sensor Vehicle speed sensor Water sensor Wheel speed sensor Chemical Breathalyzer Carbon dioxide sensor Carbon monoxide detector Catalytic bead sensor Chemical field - effect transistor Electrochemical gas sensor Electrolyte -- insulator -- semiconductor sensor Electronic nose Fluorescent chloride sensors Holographic sensor Hydrocarbon dew point analyzer Hydrogen sensor Hydrogen sulfide sensor Infrared point sensor Ion selective electrode Microwave chemistry sensor Nitrogen oxide sensor Nondispersive infrared sensor Olfactometer Optode Oxygen sensor Pellistor pH glass electrode Potentiometric sensor Redox electrode Smoke detector Zinc oxide nanorod sensor Electric, magnetic, radio Current sensor Electroscope Galvanometer Hall effect sensor Hall probe Magnetic anomaly detector Magnetometer MEMS magnetic field sensor Metal detector Planar Hall sensor Radio direction finder Test light Environment, weather, moisture Actinometer Bedwetting alarm Ceilometer Dew warning Electrochemical gas sensor Fish counter Frequency domain sensor Gas detector Hook gauge evaporimeter Humistor Hygrometer Leaf sensor Psychrometer Pyranometer Pyrgeometer Rain gauge Rain sensor SNOTEL Snow gauge Soil moisture sensor Stream gauge Tide gauge Weather radar Flow, fluid velocity Air flow meter Anemometer Flow sensor Gas meter Mass flow sensor Water metering Ionising radiation, subatomic particles Bubble chamber Cloud chamber Geiger -- Müller tube Geiger counter Ionization chamber Neutron detection Particle detector Proportional counter Scintillation counter Semiconductor detector Scintillator Thermoluminescent dosimeter Wire chamber Navigation instruments Airspeed indicator Machmeter Altimeter Attitude indicator Depth gauge Fluxgate compass Gyroscope Inertial navigation system Inertial reference unit Magnetic compass MHD sensor Ring laser gyroscope Turn coordinator Variometer Vibrating structure gyroscope Yaw - rate sensor Position, angle, displacement Accelerometer Angular rate sensor Auxanometer Capacitive displacement sensor Capacitive sensing Gravimeter Inclinometer Integrated circuit piezoelectric sensor Laser rangefinder Laser surface velocimeter Lidar Linear encoder Linear variable differential transformer Liquid capacitive inclinometers Odometer Photoelectric sensor Piezoelectric accelerometer Position sensor Rotary encoder Rotary variable differential transformer Selsyn Sudden Motion Sensor Tachometer Tilt sensor Ultrasonic thickness gauge Variable reluctance sensor Velocity receiver Optical, light, imaging Charge - coupled device Contact image sensor Electro - optical sensor Flame detector Infrared Kinetic inductance detector LED as light sensor Light - addressable potentiometric sensor Nichols radiometer Optical fiber Photodetector Photodiode Photoelectric sensor Photoionization detector Photomultiplier Photoresistor Photoswitch Phototransistor Phototube Position sensitive device Scintillometer Shack -- Hartmann wavefront sensor Single - photon avalanche diode Superconducting nanowire single - photon detector Transition edge sensor Tristimulus colorimeter Visible - light photon counter Wavefront sensor Pressure Barograph Barometer Boost gauge Bourdon gauge Hot - filament ionization gauge Ionization gauge McLeod gauge Oscillating U-tube Permanent Downhole Gauge Piezometer Pirani gauge Pressure gauge Pressure sensor Tactile sensor Time pressure gauge Force, density, level Bhangmeter Force gauge Hydrometer Level sensor Load cell Magnetic level gauge Nuclear density gauge Piezoelectric sensor Strain gauge Torque sensor Viscometer Thermal, heat, temperature Bimetallic strip Bolometer Calorimeter Exhaust gas temperature gauge Flame detection Gardon gauge Golay cell Heat flux sensor Infrared thermometer Microbolometer Microwave radiometer Net radiometer Quartz thermometer Resistance thermometer Silicon bandgap temperature sensor Special sensor microwave / imager Thermistor Thermocouple Thermometer Proximity, presence Alarm sensor Doppler radar Motion detector Occupancy sensor Passive infrared sensor Proximity sensor Reed switch Stud finder Touch switch Triangulation sensor Wired glove Sensor technology Active pixel sensor Back - illuminated sensor Biochip Biosensor Capacitance probe Carbon paste electrode Catadioptric sensor Digital sensors Displacement receiver Electromechanical film Electro - optical sensor Fabry -- Pérot interferometer Fisheries acoustics Image sensor Image sensor format Inductive sensor Intelligent sensor Lab - on - a-chip Leaf sensor Machine vision Microelectromechanical systems Photoelasticity Quantum sensor Radar Ground - penetrating radar Synthetic aperture radar Radar tracker Sensor array Sensor fusion Sensor grid Sensor node Soft sensor Sonar Staring array Transducer Ultrasonic sensor Video sensor technology Visual sensor network Wheatstone bridge Wireless sensor network Related List of sensors Retrieved from `` https://en.wikipedia.org/w/index.php?title=Mass_flow_sensor&oldid=794702914 '' Categories : Engine sensors Flow meters Gas technologies Mass Hidden categories : Articles needing cleanup from November 2009 All pages needing cleanup Cleanup tagged articles without a reason field from November 2009 Wikipedia pages needing cleanup from November 2009 Articles needing additional references from November 2009 All articles needing additional references Talk Contents About Wikipedia Deutsch Ελληνικά Français Bahasa Indonesia Italiano Latviešu Bahasa Melayu Nederlands Русский Svenska Edit links This page was last edited on 9 August 2017, at 15 : 38. 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Question: what is the purpose of mass air flow sensor
Answer: to find out the mass flow rate of air entering a fuel-injected internal combustion engine