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b. Mass Air Flow Sensor (MAF) – Located in the air intake duct between the filter element and the throttle body, input from this sensor regulates the amount of fuel to be metered to each cylinder in an attempt to achieve the stoichiometric ratio. A derivative of the hot-wire anemometer (a heated wire cooled by the air passing over it), using a Wheatstone bridge and a variable-resistance heated element the MAF can produce a near-linear signal from which the mass flowrate of air is easily determined by the engine control module. The greater the mass flowrate of air over the sensor, the greater the voltage required to heat the wire element. The actual airflow measurement is likely the most important variable in determining the amount of fuel to be metered to the engine. Although this device is very accurate, it tends to be somewhat delicate and expensive.
c. Manifold Absolute Pressure Sensor (MAP) – Used on all “speed-density” systems (those not measuring airflow directly), this device measures the absolute pressure of air in the intake manifold using a silicon diaphragm strain sensor and the piezoresistivity phenomenon. The output of the MAP sensor is used in conjunction with intake manifold air temperature, engine displacement, RPM, exhaust gas recirculation amount and various constants to determine the amount of incoming combustion air and consequently the amount of fuel to be metered. Closed throttle plate would represent close to a vacuum in the intake manifold, whereas wide-open throttle should be near atmospheric pressure, the maximum intake manifold pressure for a normally aspirated engine.
· Speed-density method used in MAP systems is described as follows:
Recall that the mass flowrate of air is represented by the product of the air density and its volumetric flowrate. The instantaneous density is calculated by multiplying the density of air at standard conditions by the ratios of MAP and MAT data with respect to standard atmospheric conditions for temperature and pressure. The volume flow rate of air into the engine is simply the product of engine RPM and half of the displacement (it only draws one half the total engine displacement in one revolution under ideal conditions). Figure in corrections for exhaust gas recirculation and other small factors, and the mass flowrate of air into the intake manifold at any given instant is easily determined by the onboard engine computer.
· Note: Some vehicles are equipped with both MAF and MAP systems, relying on MAF data unless a malfunction is encountered, whereupon the engine control module will default to the speed-density method.
The dual systems also use MAP as an instantaneous BP sensor during the start up operation of an engine. Without the MAP sensor Barometric pressure is inferred into the ECM since there is nothing to actually measure it.
Another use of the dual system is for transient fueling operations. MAF systems without a MAP sensor have no way of knowing what the actual pressure of the manifold is. For this reason manifold pressure again has to be inferred through a series of very sophisticated software. Even with the best of software, inference of manifold pressure is not exact. A MAP sensor in the system allows for actual measurements of manifold pressure to be used along with the actual air flow measurement of the MAF sensor.
c. Manifold Absolute Pressure Sensor (MAP) – Used on all “speed-density” systems (those not measuring airflow directly), this device measures the absolute pressure of air in the intake manifold using a silicon diaphragm strain sensor and the piezoresistivity phenomenon. The output of the MAP sensor is used in conjunction with intake manifold air temperature, engine displacement, RPM, exhaust gas recirculation amount and various constants to determine the amount of incoming combustion air and consequently the amount of fuel to be metered. Closed throttle plate would represent close to a vacuum in the intake manifold, whereas wide-open throttle should be near atmospheric pressure, the maximum intake manifold pressure for a normally aspirated engine.
· Speed-density method used in MAP systems is described as follows:
Recall that the mass flowrate of air is represented by the product of the air density and its volumetric flowrate. The instantaneous density is calculated by multiplying the density of air at standard conditions by the ratios of MAP and MAT data with respect to standard atmospheric conditions for temperature and pressure. The volume flow rate of air into the engine is simply the product of engine RPM and half of the displacement (it only draws one half the total engine displacement in one revolution under ideal conditions). Figure in corrections for exhaust gas recirculation and other small factors, and the mass flowrate of air into the intake manifold at any given instant is easily determined by the onboard engine computer.
· Note: Some vehicles are equipped with both MAF and MAP systems, relying on MAF data unless a malfunction is encountered, whereupon the engine control module will default to the speed-density method.
The dual systems also use MAP as an instantaneous BP sensor during the start up operation of an engine. Without the MAP sensor Barometric pressure is inferred into the ECM since there is nothing to actually measure it.
Another use of the dual system is for transient fueling operations. MAF systems without a MAP sensor have no way of knowing what the actual pressure of the manifold is. For this reason manifold pressure again has to be inferred through a series of very sophisticated software. Even with the best of software, inference of manifold pressure is not exact. A MAP sensor in the system allows for actual measurements of manifold pressure to be used along with the actual air flow measurement of the MAF sensor.
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