Air flow meter
An air flow meter, is a device that measures how much air is flowing through a tube. It does not measure the volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 of the air passing through the tube, it measures the actual speed of the air flowing through the device in a defined time segment. Thus air flow meters are simply an application of mass flow meter
Mass flow meter
A mass flow meter, also known as an inertial flow meter is a device that measures mass flow rate of a fluid traveling through a tube. The mass flow rate is the mass of the fluid traveling past a fixed point per unit time....

s for a special medium. Typically, mass air flow measurements are expressed in the units of kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

s per sec
Seč is name of several locations in central Europe:* Seč, a city in Pardubice Region of the Czech Republic** Seč dam next to the Seč village* Seč, a village in the Trenčín Region of Slovakia...


In automobiles

An air flow meter is used in some automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s to measure the quantity of air going into the internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

. All modern electronically controlled diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

s use air flow meter, as it is the only possible means of determining the air intake for them. In the case of a petrol engine
Petrol engine
A petrol engine is an internal combustion engine with spark-ignition, designed to run on petrol and similar volatile fuels....

, the electronic control unit
Electronic control unit
In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....

 (ECU) then calculates how much fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 is needed to inject
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....

 into the cylinder ports. In the diesel engine, the ECU meters the fuel through the injectors into the engines cylinders during the compression stroke.

The vane (flap) type air flow meters (Bosch L-Jetronic
Jetronic is a trade name of an fuel 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 of the technology offering technological development...

 and early Motronic
Motronic is the trade name given to a range of digital engine-management systems developed by Robert Bosch GmbH, commonly known as Bosch.-Motronic ML1.x:Motronic ML1.x was one of the first digital engine-management systems developed by Bosch...

 EFI systems or Hitachi) actually measure air volume, whereas the later "hot wire" and "hot film" air mass meters measure speed of air flow.

The flap type meter includes a spring which returns the internal flap to the initial position. Sometimes if the spring is tensioned too tight, it can cause restrict the incoming air and it would cause the intake air speed to increase when not opened fully.

Differential pressure is also used for air flow measurement purposes.


Air flow meters may fail or wear out. When this happens, engine performance will often decrease significantly, engine emissions will be greatly increased, and usually the Malfunction Indicator Lamp (MIL) will illuminate. In most countries in Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, and in places in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 where emissions inspections are obligatory, a lit MIL is cause for a vehicle to fail the inspection. Some engines do not idle with an air flow meter failure.

Air flow meter for R&D of cars

In the development process of internal combustion engines with engine test stand
Engine test stand
An engine test stand is a facility used to develop, characterize and test engines. The facility, often offered as a product to automotive OEMs, allows engine operation in different operating regimes and offers measurement of several physical variables associated with the engine operation.A...

s, an air flow meter/air flow measuring unit is used for measuring the continuous gravimetric
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest...

 air consumption of combustion engines.

See also

  • List of sensors
  • Mass air flow sensor (MAF)
  • Anemometer
    An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...

  • :Category:Engines
  • :Category:Engine fuel system technology
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