From Wikipedia, the free encyclopedia
A
mass air flow (MAF) sensor responds to the amount of a fluid (usually a
gas) flowing through a chamber containing the
sensor. It is intended to be insensitive to the
density of the fluid.
[edit] Automotive mass airflow sensors
A mass air flow sensor is used to find out the
mass 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 use of a
turbocharger and this is an ideal application for a mass sensor. (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 an additional sensor or two, the engine's air mass flow rate can be accurately determined.
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 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.
[edit] Vane meter sensor
A vane, or paddle, projects into the engine’s intake air stream on a spring-loaded arm. 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.
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, velocity and the shape of the vane, see
drag equation.
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.)
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
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
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 Mitsubishi Lancers/EVOs, all
DSMs (Mitsubishi Eclipse, Eagle Talon, Plymouth Laser) and some Toyotas and Lexuses.
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.
Auto repair technician observation
The MAF sensor senses the incoming air into the engine. This sensor does not regulate the incoming air, this is done by the engine throttle plates. The MAF merely senses incoming air and relates a signal to the ECM. Air flow sensors come in three types. The Vane Air Flow sensor, Hot Wire MAF sensor and Hot Film MAF sensor. They all perform the same function but their operation is quite different.
The VAF sensor measures the air flow into the engine with a
spring-loaded air flap/door attached to a variable resistor (
potentiometer). The drag force of the incoming air pushes against the spring force of the air flap on the VAF sensor, which also moves the variable resistor’s sensing arm (wiper arm). As air flows into the engine the mechanical air flap rotates further, changing the voltage signal output.
The VAF sensor has 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. In addition to the regular air flow measuring function, some VAF sensors also employ an air temperature sensor (IAT sensor) and a fuel pump switch.
The IAT sensor is found inside the VAF casing and has the same electrical characteristics as a regular air temperature sensor. The VAF sensor flap also closes a set of contacts that activate the fuel pump relay coil (circuit opening relay). The contacts are actually closed as soon as the smallest amount of air pushes on the air flow flap. Once this happens the fuel pump starts running and the engine starts.
One of the main drawbacks of the VAF sensor is that it measures volume of air and not weight. As air temperature changes so does its weight. There are more air molecules present when the air is colder than when it is hotter. As air temperature decreases, more air is absorbed by the engine, so there are drastic changes needed in the air fuel ratio (depending on the temperature of the air). The air temperature sensor inside the VAF somewhat compensates by signaling the ECM of any changes in air temperature.
The hot wire MAF sensor is a fully electronic unit. It senses the amount of air flow into the engine by measuring the amount of current needed to maintain a constant temperature through a very thin (70 micrometers) platinum hot wire. Hence the name hot wire MAF sensor. It also measures air by weight, since it takes into consideration the air temperature as well.
This sensor works as follows. As the air enters the intake manifold through the hot wire MAF sensor it cools down the platinum wire, which is heated at a very precise temperature. When the MAF circuitry senses the platinum wire cooling down it increases the amount of current flow through the hot wire trying to maintain a specific temperature. This varying current flow is then converted to a voltage output signal by the MAF electronic circuitry and is used as an air flow indicator by the ECM. Hot wire MAF sensors have a signal that is directly proportional to air flow. So as air flow increases so does its voltage signal output.
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.
Conditions that affect operation
VAF sensors are mechanical in nature. Their measuring element (wiper contact, pivot bushings and sensor resistors) get worn out over time. A binding air flap door is also a major problem with these sensors. The air flap mechanism is extremely precise and does not tolerate any misalignments. Always make sure that the air flap can travel freely all the way to its full open position. A broken air duct pipe will also render the VAF useless, since most of the air will be bypassed and enter though the broken duct hole. A thorough air duct check is always a good idea. The resistors also tend to wear out over time, sending the wrong voltage signal to the ECM. This will certainly throw off the air-fuel ratio.
The air temperature sensor and the fuel pump switch are the other reasons for VAF failures. This fuel pump switch activates the fuel pump relay and its contacts also wear down over time, causing a no start-no no-fuel pressure condition. A simple continuity test will quickly reveal a bad fuel pump switch. The air temperature sensor also follows the same electrical characteristics of a normal IAT sensor and the same ohms to temperature tables could be used for diagnostics.
Hot Wire MAF sensors are very prone to sensing wire element contamination. A condition referred to by many technicians as “growing hairs” happens when debris, dirt from cheap air filters and outside air stick to the sensing wire element, shielding it from the incoming air. This shielding effect prevents the MAF sensor from correctly measuring the air flow and mass causing severe air-fuel ratio control problems. An ECM not in control while at pre-load is a strong indication of a dirty MAF.
In any fully electronic device, the electrical connections and circuitry fails after a certain lifespan of operation. An output signal voltage test will surely reveal a bad MAF sensor.
Hot Film MAF sensors tend to get electrical damage more often that the other type of sensors. The tap test ,as mentioned before, is a useful and simple procedure that usually reveals a bad hot film MAF sensor. Contamination or a broken air duct is also a problem for this type of sensor.
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 usual constructed form a large number of parallel pipes to achieve the required flow rating.
[edit] See also
References
External links
