Mass flow sensor

A mass air flow sensor or short LMM (including mass air flow meter (MAF ) mass air flow sensor or LMS ) is a inserted in the control and measurement technology flow sensor that determines the mass per unit time flowing through air ( mass flow ).

The measured mass flow of the air is proportional to the molar amount of oxygen contained and, therefore, can be used for control of combustion processes, in particular in internal combustion engines can be used.

Equivalent to the mass flow rate is calculated on the standard state of the air or gas flow.

Obtaining the measure of combustion processes

The LMM is a particularly common in the automotive, gas turbine and condensing technology mass flow sensor, the output signal in kg / h of air or ln / min ( liters of air per minute at STP ) is usually calibrated. It thus differs from a flow sensor or an anemometer, since it provides, in contrast to this the related to the standard state of the air flow.

The measure mass air flow sensor is used, inter alia, for diesel and gasoline engines and gas turbines to control the injected fuel quantity. Only with the knowledge of the exatkte air mass, the critical for the combustion process ratio between air and fuel mass are accurately set (lambda = 1).

Also in diesel engines, it provides a manipulated variable for the exhaust gas recirculation.

Many, used inter alia in motor vehicles sensors are now working on the principle of hot wire anemometer with characteristic correction:

The power consumption of heated sensor elements serves as a benchmark. Two different energized heating elements are kept at a fixed temperature and the necessary electrical power is measured. With increasing air mass flow therefore increases the electrical power consumption of the sensor. The integrated electronics converts the values ​​on criteria such as the temperature of the air around a map in a mass flow.

Another method determines the temperature, the pressure and the flow rate of the air / gas and calculates the adiabatic equation of state on the mass flow. This, as a Karman vortex air mass meter designated LMM determines the flow velocity by ultrasonic Doppler velocimetry transverse to the swirling flow of air for this purpose. For this purpose, any portion of the sensor must protrude into the intake manifold, this method is very reliable.

Are also promising air mass sensors micro -mechanical based. However, these are more sensitive to dirt and therefore have shorter lifetimes.

Term discrimination and historical development

LMM is often referred to as " air flow meter ". Measuring the amount of air in the form of the volume would not take into account the temperature, pressure and humidity dependency of the passing air and thus would be insufficient to determine the molar amount of control of the combustion process. The measured values ​​were dependent on the pressure ( suction vacuum and atmospheric pressure and height above sea level) and temperature of the air flowing past and therefore for precise engine management and emission control too imprecise a purely volumetric acquisition.

Earlier mechanical based sensors were used to measure the aspirated air volume. In this air flow meter, referred to as donors, for example, squeezing the intake air, a sensor flap, which is connected to a potentiometer. Depending on the flow rate, the electrical resistance of the potentiometer changes. The resistance value is the motor control device for determining the intake air amount. Said air flow meter are used, for example, in the KE-Jetronic.

This measurement method is, however, due to the mechanical tolerances and the above reasons less precise than that of a mass air flow sensor ( MAF ).

Turbo diesel engines were fitted to control the maximum injection quantity with a direct mechanical means acting on the injection pump pressure cell. As a control variable used was a line that thumbed the speed-dependent negative pressure between the air filter and charging turbine.

Construction in cars

The LMM has great significance for the engine management, but it is only a relatively small component.

In the picture above is the LMM of a passenger car diesel engine ( VAG, built in 1995-2003). The sensor itself is located inside the intake pipe. He is attached to a 50 mm long arm which holds it in the middle in the air flow of the intake pipe. This arm is used in assembly by a fit to the intake manifold and bolted to this. A seal prevents air bleed. The electrical contacts are made via a built-in plug (right).

Another design used in the car is the already mentioned Karman vortex air mass meter, which is particularly used by Japanese automakers since the early 1980s. He has initially mostly a honeycomb lattice, which soothes the air for the following measuring section. In a defined section of the directed air flow to an ultrasonic sensing element is one or more barriers, swirl the air sucked in a specific pattern. The ultrasonic sensors detect the sound samples, so that the accurate air flow can be detected. Complemented by a temperature sensor and a pressure sensor currently enforced air mass is determined.

Action of thermal LMM

The LMM transmitter is when the vehicle internal combustion engines, usually positioned downstream of the air filter in the intake pipe in order to determine the per unit of time available for the combustion air of the molar mass.

Conventional thermal sensors operate on the principle of hot wire anemometer; are within the sensor, two platinum wires heated by electric current or layer resistors. One is directly cooled by the passing air, the other is shielded.

By the electric current flow, the two heating resistance elements, but the passing intake air to cool the heating element is not shielded more than shielded from the intake air. This therefore heats up more, and is thus highly resistive.

From the resistance values ​​of the two heating elements and their difference can be made using a characteristic field taking into account other engine parameters derived the following values:

• Intake air temperature
• Humidity of the intake air
• Mass flow of the intake air ( molar mass per time )

After passing the LMM the intake air is supplied depending on the type of the motor from either the forced induction ( turbo or supercharger) or directly through the intake pipes to the engine.

Importance for the motor control

The engine management system of modern diesel and petrol engines in motor vehicles requires the determination of the per cylinder filling the available air or oxygen molar mass to determine the maximum injected or admixed amount of fuel. The mass flow is not only dependent on the engine speed, but also on the engine temperature, air temperature and the ambient air pressure.

Furthermore, the autonomous working turbocharger has a strong influence on the amount of air sucked because he is driven by exhaust gas and thus needs time to run up and Abtouren. Its flow rate is not only dependent on the engine speed, but also on the load on the motor.

A closed-loop emission (catalyst) requires an oxygen sensor (lambda probe) and / or LMM, so that nowadays in almost all motor vehicles with pollutant reduction - whether petrol or diesel - a LMM is used.

In motor vehicles, older construction date mechanical air flow meter yet been used. In turbo-diesel engines (among Golf II ) was the eg by a (lower ) pressure measurement between the air filter and turbocharger.

Failure / defect

In case of failure of the LMM engine control system must detect this defect. This is done based on implausible measured values, which then activate a so-called fail-safe program. Purpose,

• To protect the motor against overload
• To preserve the environment from unnecessary exposure
• The driver / the driver by OBD Check Engine Light ( On Board Diagnostic, "Check Engine" ) and to motivate visit to the workshop due to the greatly reduced engine performance.

Failures of the LMM can be promoted among others by the following events:

• Penetration of spray water through the air filter when driving fast in heavy rain
• Backflow of oil vapors from the crankcase ventilation shortly after the engine has
• Leaks on the clean air side in front of the LMM, whereby the LMM is damaged by sand and other particles ( air filter)
• Usual wear and tear after a long running time

These events lead to long-term functional impairments, as to settle foreign substances on the measuring element. In practice, the LMM then measures a too low intake air mass. Because it is a main control variable for the calculation of the injection quantity, less fuel is injected and the engine power drops.

At times slightly incorrect MAF readings lead to various symptoms, which are not always easy to allocate the driver and workshop. Since marginal LMM defects are logged by the engine electronics is not necessarily the fault memory and thus do not have the diagnostic connector are available for the workshop, LMM defects could by some car companies at the beginning of the generation of engines with exhaust gas control after OBD2/Euro2/Euro3- standard are localized only after extensive error analysis.

Indicators of impaired function are reduced benefits in the mid- speed range, especially in diesel engines. These so-called " pull-through holes" show up at full acceleration especially in the area of maximum torque, which is often described as palpable uneven acceleration in third gear, the maximum power at rated engine speed (and thus the terminal velocity ) is rarely affected. In addition, a so-called " moods" of the engine ( " running in wet conditions much worse " ) occur.

Some passenger car gasoline engines start at not plausible signal at times to " cut ", ie the speed increases at idle, in seconds clock and drops out again. But misfiring in conjunction with poor throttle response and " beaters " into the intake manifold can be an indication of a faulty MAF. In some vehicles, it is possible to identify, by separating the electrical connector of the MAS. The engine management system detects the interruption and on the basis of stored maps can approximately calculate the mass of air drawn from the engine speed, throttle position, intake air temperature, air pressure and boost pressure. This means that the vehicle runs better than with infected defective LMM. This only works on vehicles whose engine management system does not respond to this error is detected with a fail-safe program.

Self-cleaning

LMM, which operate on the principle of hot wire anemometer, (not after each ) burned free in car after turning the ignition off. To the hot wire is heated for a short time to about 1000 ° C, whereby the dirt can be removed by evaporation.