Mass flow sensor

A mass airflow sensor.

A mass air flow sensor 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 piston stroke. (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 electronic control unit 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.

When a MAF 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.

Vane meter sensor (VAF sensor)

The VAF sensor measures the air flow into the engine with a spring-loaded air flap/door attached to a variable resistor (potentiometer). The vane moves in proportion to the airflow, and a voltage is generated in proportion to the distance the vane moves, or the movement of the vane directly regulates 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 airs density, 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.

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). The General Motors division (GM) was the first car company to use the hot wire sensor.^{[citation needed]} This is achieved by heating a wire with an electric current that is suspended in the engine’s air stream, like a toaster wire. The wire's electrical resistance increases as the wire’s temperature increases, which limits electrical current flowing through the circuit. When air flows past the wire, the wire cools, decreasing its resistance, which in turn allows more current to flow through the circuit. As more current flows, the wire’s temperature increases until the resistance reaches equilibrium again. The amount of current required to maintain the wire’s temperature is directly proportional to the mass of air flowing past the wire. The integrated electronic circuit converts the measurement of current into a voltage 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 amount of air entering the engine. So as air 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

"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) where the inductance of a tiny sensor changes with the air mass flow over that sensor. The sensor is part of an oscillator circuit whose oscillation frequency changes with sensor inductance; hence the frequency is related to the amount of air (cubic feet per minute) passing over the unit. This oscillating electrical signal is then fed to the car's ECU. These MAF units (such as the one pictured) have 3 pins, denoted +, - and F. F carries the square-wave frequency between - and F. They are powered by +5 V_DC from the ECU's regulated power supply.

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 or Electronics Cleaner should be used, not carburetor or brake cleaner. These are alcohol or CFC-based solvents, rather than the harsh petroleum distillates used in the other cleaners... The sensors should be gently sprayed from a careful distance to avoid physically damaging them. 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

Animation of the phenomenon. Courtesy, Cesareo de La Rosa Siqueira.

A Kármán vortex sensor works by setting up a laminar air stream. The air stream is disrupted by a vertical bow in the sensor. This causes a wake in the air stream and subsequently the wake will collapse repeatedly and cause Kármán vortexes. The frequency of the resulting air pressure oscillation is proportional to the air velocity.

These vortexes 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 vortexes. 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.

More information can be found here:^[1]

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 mass 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

1. ^ "Air Flow Sensors". http://www.autoshop101.com/forms/h34.pdf. Retrieved 15 September 2009. 

External links

• A Hot Film sensor with theory of operation