A mass (air) flow sensor (MAF) is a
sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
used to determine the
mass flow rate
In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot (''ṁ ...
of
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
entering a
fuel-injected
Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.
All com ...
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
.
The air mass information is necessary for the
engine control unit
An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by re ...
(ECU) to balance and deliver the correct fuel mass to the engine. Air changes its density with temperature and pressure. In automotive applications,
air density
The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature a ...
varies with the ambient
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
,
altitude
Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
and the use of
forced induction
In an internal combustion engine, forced induction is where turbocharging or supercharging is used to increase the density of the intake air. Engines without forced induction are classified as naturally aspirated.
Operating principle Overvi ...
, which means that mass flow sensors are more appropriate than
volumetric flow
In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes ). ...
sensors for determining the quantity of
intake
An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...
air in each cylinder.
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 flow rate
In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot (''ṁ ...
of intake air.
Both approaches are used almost exclusively on
electronic fuel injection
Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a mani ...
(EFI) engines. Both sensor designs output a 0.0–5.0 volt or a
pulse-width modulation
Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts. The average value of voltage (and current) fed ...
(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
on-board diagnostics (OBDII) vehicles. Vehicles prior to 1996 could have MAF without an IAT. An example is 1994
Infiniti Q45
The Infiniti Q45 is a full-size luxury sedan manufactured and marketed by Nissan's Infiniti brand for model years 1989-2006, over three generations.
The first generation Q45 (1989–1999) was based on the Nissan President; the second generati ...
.
When a MAF sensor is used in conjunction with an
oxygen sensor
An oxygen sensor (or lambda sensor, where lambda refers to air–fuel equivalence ratio, usually denoted by λ) or probe or sond, is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analysed.
It was ...
, the engine's air/fuel ratio can be controlled very accurately. The MAF sensor provides the
open-loop controller
In control theory, an open-loop controller, also called a non-feedback controller, is a control system in which the control action is independent of the "process output", which is the process variable that is being controlled."Feedback and cont ...
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 manifold absolute pressure sensor (
MAP sensor
The manifold absolute pressure sensor (MAP sensor) is one of the sensors used in an internal combustion engine's electronic control system.
Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provi ...
). Since around 2012, some MAF sensors include a
humidity sensor
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depen ...
.
Moving vane meter
![Průtokoměr s otočnou klapkou](https://upload.wikimedia.org/wikipedia/commons/2/2c/Pr%C5%AFtokom%C4%9Br_s_oto%C4%8Dnou_klapkou.jpg)
The VAF (volume air flow) sensor measures the air flow into the engine with a
spring
Spring(s) may refer to:
Common uses
* Spring (season)
Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...
-loaded air vane (flap/door) attached to a variable resistor (
potentiometer
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrume ...
). The vane moves in proportion to the airflow. A voltage is applied to the potentiometer and a voltage appears on the output terminal of the potentiometer proportional to the angle the vane rotates, or the movement of the vane may directly regulate the amount of
fuel injected
Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.
All comp ...
, as in the
K-Jetronic Jetronic is a trade name of a manifold injection technology for automotive petrol engines, developed and marketed by Robert Bosch GmbH from the 1960s onwards. Bosch licensed the concept to many automobile manufacturers. There are several variations ...
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
In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid. The equation is:
F_\, =\, \tfrac12\, \rho\, u^2\, c_\, A
where
*F_ is the drag force ...
. 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)
![MAF sensor bosch](https://upload.wikimedia.org/wikipedia/commons/a/a5/MAF_sensor_bosch.jpg)
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, by applying a
constant voltage over the wire. The wire's
electrical resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
increases as the wire’s temperature increases, which varies the
electrical current
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
flowing through the circuit, according to
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equat ...
. 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 proportional voltage 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
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
.)
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 1980s and early 1990s 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.
A micro-bridge uses the same principles but arranged on a silicon chip.
Coldwire sensor
The
GM LS engine
"LS engine" is the colloquial name given to the third and fourth generation small-block V8 gasoline engine used in General Motors' vehicles. The name evolved from the need to differentiate the Gen 3/Gen 4 small blocks from the original Gen 1 ...
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
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
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
hexane
Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14.
It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relatively ...
s or
heptane
Heptane or ''n''-heptane is the straight-chain alkane with the chemical formula H3C(CH2)5CH3 or C7H16. When used as a test fuel component in anti-knock test engines, a 100% heptane fuel is the zero point of the octane rating scale (the 100 poin ...
s with little to no
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
content and use either
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
or
HFC-152a as aerosol
propellants
A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...
. 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
Kármán vortex
Karman or Kármán is a Hungarian surname. Notable people with the surname include:
* Harvey Karman (20th century), inventor of the Karman cannula
* Janice Karman (born 1954), American film producer, record producer, singer, and voice artist
* J ...
sensor works by disrupting the air stream with a perpendicular bow. Providing that the incoming flow is
laminar
Laminar means "flat". Laminar may refer to:
Terms in science and engineering:
* Laminar electronics or organic electronics, a branch of material sciences dealing with electrically conductive polymers and small molecules
* Laminar armour or "band ...
, 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
In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement.
The current categorisation is that a supercharger is a form of forced induct ...
), 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
In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...
). 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
DSM
DSM or dsm may refer to:
Science and technology
* Deep space maneuver
* Design structure matrix or dependency structure matrix, a representation of a system or project
* Diagnostic and Statistical Manual of Mental Disorders
** DSM-5, the fifth ed ...
s (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
A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
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
The microscopic scale () is the scale of objects and events smaller than those that can easily be seen by the naked eye, requiring a lens (optics), lens or microscope to see them clearly. In physics, the microscopic scale is sometimes regarded a ...
scale as microsensors using
microelectromechanical systems
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
technology. Such a ''microsensor'' reaches a significantly higher speed and sensitivity compared with
macroscopic
The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic.
Overview
When applied to physical phenomena an ...
approaches. See also
MEMS sensor generations
MEMS sensor generations represent the progress made in micro sensor technology and can be categorized as follows:
;1st Generation :MEMS sensor element mostly based on a silicon structure, sometimes combined with analog amplification on a micro c ...
.
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 linearly proportional to the flow rate.
Laminar flow
In fluid dynamics, laminar flow is characterized by fluid particles following smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mi ...
conditions are present in a gas when the
Reynolds number
In fluid mechanics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be domi ...
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
This is a list of sensors sorted by sensor type.
Acoustic, sound, vibration
*Geophone
*Hydrophone
*Microphone
* Pickup
*Seismometer
*Sound locator
Automotive
* Air flow meter
*AFR sensor
*Air–fuel ratio meter
* Blind spot monitor
*Cranksh ...
*
Manifold absolute pressure
The manifold absolute pressure sensor (MAP sensor) is one of the sensors used in an internal combustion engine's electronic control system.
Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provid ...
(MAP)
References
{{DEFAULTSORT:Mass Flow Sensor
Engine sensors
Flow meters
Gas technologies
Mass