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Air Flow and Mass Sensing Devices
Air Flow and Mass Sensing Devices:
A Mass Air Flow/Air Flow Meter/Speed Density Primer
When one reads about mass air flow sensor contamination, it is important to recognize that a modern engine can use one of five distinct categories of air measuring devices, each of which works on very different physics principles. In order to assist your understanding of the various strategies used on fuel injection-equipped engines to measure and calculate air flow or mass, we've assembled the following primer.
In order to precipitate combustion, internal-combustion engines need air, fuel and spark. Before the advent of fuel injection, carburetors were used to distribute an appropriate amount of fuel for a given amount of air. Carburetors function on Bernoulli's principle: Air pressure drops as airflow increases, and the resulting vacuum is used, in conjunction with mechanical bits, to meter a proper fuel mixture.
In the 1980s, fuel injection became the standard in automotive engines as a more precise method of meeting fuel needs for a given airflow, paying dividends in efficiency and thus performance, fuel economy, and emissions. Modern electronic fuel injection systems feed pressurized fuel to computer-controlled, solenoid-type injectors which deliver accurate quantities of fuel into the engine's intake system or combustion chamber.
Engine sensors and inputs, in conjunction with an engine computer (ECU or Engine Control Unit), measure airflow and mass in order to calculate the proper amount of fuel so as to as closely approach the stoichiometric, or chemically correct, ratio of air mass to fuel mass for fuel to be burned completely: 14.64:1. These inputs can include an exhaust gas oxygen sensor (O2 sensor), wideband 02 sensor or "lambda" sensor, air flow or air mass sensor, intake air temperature sensor, manifold air temperature (MAT) sensor, manifold absolute pressure (MAP) sensor, throttle body position (TPI) sensor and engine revolutions per minute (RPM).
Vane Air Flow Meter
The Vane Air Flow Meter (VAFM), was one of the first airflow sensors in use and has been largely phased out of new car production. In a vane, or as it's commonly called, "flapper-style" air flow meter, intake air flow exerts force against the surface area of a measuring plate, which is deflected in proportion to the volume of air flow meeting the plate. Movement of the measuring plate is transferred through a shaft to a slider on a potentiometer resistor, which produces a variable voltage signal proportional to the air intake volume. Once air volume, expressed as force exerted over a surface area, is measured, air mass is calculated in conjunction with an air temperature sensor.
Another mechanical-type AFM is the plunger type, which uses a sliding sprung cone-shaped plunger, rather than flap, to measure airflow, deduced from its position on a calibrated scale. Due to the mechanical nature of measurement, vane and plunger type meters are largely impervious to the effects of contaminants, but less accurate than other AFM/MAF types.
Karman Vortex Air Flow Meter
An older style of AFM still in limited use, the Karman Vortex air flow meter uses an air stream-centered conical vortex generator to create vortices, essentially precisely-shaped air disturbances, in the measurement chamber. First, however, the air stream passes through a honeycomb grid to straighten airflow and reduce turbulence.
Located on either side of the chamber of one type of vortex meter are a transmitter and receiver that send and receive a signal, either infrared light or ultrasonic waves. The amount of distortion in signal caused by the vortices, whose frequency increases with airflow, is measured and compared against a known set data points, from which an inference about air flow is made.
Another type of Karman Vortex Meter features a pressured directing hole in which sits a metal foil mirror which oscillates in proportion to the vortex frequency. The movement of the mirror deflects light from an LED on and off of a photo transistor or diode, opening and closing a 5-volt loop to generate a signal. Air flow, combined with a temperature reading from an internal sensor, is used to generate a measurement of air mass.
Speed Density systems do away completely with a mass air or flow device, instead marrying inputs like RPM, throttle position (TPS), manifold absolute pressure (MAP), which compensates for altitude, and incoming air temperature to calculate air mass. Various aftermarket products exist to convert MAF-equipped cars to speed density, which are traditionally used in forced induction applications with high airflow numbers.
Unlike air flow meters, hot wire or hot film mass air flow sensors directly measure air mass, as the convective qualities of air are affected by factors like temperature, humidity and density. Hot wire sensors create an analog signal and hot film sensors a digital frequency signal.
Most common hot-wire MAF sensors use a platinum wire or filament heated to a prescribed, maintained temperature above ambient, located centrally in the incoming air stream. These sensors function on the electrical principle that resistance increases with temperature. As intake air moves past the wire or film, the cooling effect causes a measurable drop in resistance, and thus lower voltage is required to maintain the prescribed temperature. The hot-wire MAF control unit is sent a reference voltage of 5V, and returns around .4V to.5V at idle and from 4.5V to 5V at full throttle. Based on a fixed data set, an accurate assumption of air mass is made.
Readings can stray from the target values as a result of contamination of the wire and for the sake of accuracy, an additional input from an integrated intake temperature sensor is sometimes used. Many hot wire MAFs incorporate a burn off cycle when the ignition is switched off, heating the element to over 1800-degrees F to clear it of contamination. Hot wire sensors are the most physically delicate and easily contaminated of all the air flow and mass sensors.
Hot-film MAF sensors function much like a hot wire sensor, and used a centrally-heated film or metallic grid-type element. One side of the film encounters cooling airflow, while the shielded backside maintains a consistent temperature, and the current differential between the two is measured and relayed as a square-wave digital frequency output, between around 30Hz at idle and 150Hz at wide-open throttle. Hot film sensors tend to be more robust and less susceptible to contamination than hot-wire types.
Mass Air Flow Sensor Contamination:
Contamination of hot-wire type sensors does occur. Usual suspects include substances like silicone potting compound, dirt, oil and spider webs.
Potting compound, used in the manufacture of the sensors to environmentally seal them can migrate onto the wire.
Oil most often enters a MAF in the form of vapor via an engine's PCV system. In an effort to lower emissions, positive crankcase ventilation systems use a PCV valve to draw fuel and oil vapors from the crankcase, this can allow blow-by gases to make it past the piston rings, and reintroduces them into the intake system upstream of air flow metering devices. When a throttle body or airflow meter needs cleaning, the oily film that is removed is largely a product of this system.