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Air Flow Sensor

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What is a mass air flow sensor and how does it work? Understanging answers on these essential questions will give you some good ideas about engine’s performance increase. A Mass Air flow Sensor (MAF sensor) is a car electronic fuel injection system component. It is installed between the air filter and the intake manifold of the engine.The air mass information is necessary for the engine control unit (ECU) to balance and deliver the correct fuel mass to the engine.

What does air flow sensor do?

Air flow senser

Now, when you know what air flow sensor is, there is one more question left: what does a MAF sensor do. 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.)

The performance mass air flow 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.

Types of Air Flow Sensors

There are two common types of mass airflow sensors in usage 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.

Types of Air Flow Sensors

Both 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 car MAF is used in conjunction with an oxygen sensor, the engine's air/fuel ratio can be controlled very accurately.

On electronic fuel injection (EFI) cars, the airflow sensor (air flow meter) could be an air flow restriction, depending on which type of air-flow sensor is used. The hot-wire airflow sensor, which is often called the performance mass air flow sensor, doesn't present much restriction to the air flow as it is usually about the same size as the bore of the air intake duct. If you need to increase the bore of a MAF sensor, you can swap out the mass airflow sensor for a larger one from the same car manufacturer, but you would need to reprogram the engine contrl unit (ECU) so that it can accurately measure the increased air flow rate. If your stock ECU is not reprogrammable, you may need to install a programmable aftermarket ECU. These air flow meters usually have a fine wire mesh at either end of the bore. Removing this wire mesh will lead to a 1% to 2% increase in power at maximum RPM.

The Karmen vortex-type airflow sensor

Air flow sensor solutions

The vane-type air-flow sensor, with its swinging flap is more reliable than the hot-wire mass air-flow sensor but it does present some air flow restriction. You can swap out these air flow sensors for larger units, but you would need to reprogram the ECU to ensure the correct air flow readings. Alternatively, you could reduce the spring tension on the swinging flap. However, this will lead to the flap swinging open further, and reaching full-open before peak air-flow is reached. The ECU calculates the air-flow rate on how far and how fast the flap swings open; therefore, the ECU will need to be reprogrammed. However, the flap should never reach full open before peak air flow is reached as the ECU would not be able to read increases in air flow once the flap is full open. No amount of reprogramming will get the ECU to increase fuel delivery if it is not able to sense an increase in the air flow rate.

The vane-type air-flow sensor

The Karmen vortex-type airflow sensor that is used my Mitsubishi is an oscillatory airflow sensor that measures the vibrations of the downstream vortexes caused by the barrier placed in the air stream. These car MAF sensors represent the greatest restriction to air flow, and they are the most difficult to adapt for better air flow. Some tuners use a bypass around the air-flow sensor, but this is terrible for modified street cars as the air flow will bypass the air-flow sensor at idle and during cruise conditions.

The best performance solution on these engines is rather expensive. It entails a second air intake controlled by a secondary throttle plate, a MAP controlled ECU and secondary fuel injectors. This secondary air intake branches into the air intake duct after the air-flow sensor. Under normal conditions, air flows through the stock air intake and through the air-flow sensor; but under heavy acceleration and at high RPM, the secondary throttle plate is opened by a pressure-sensitive solenoid switch to allow air through that secondary air intake system. A MAP sensor controlled ECU is then used to deliver the extra fuel through a secondary set of fuel injectors.