MAP sensor
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The examples and descriptions in this article apply strictly to four-stroke cycle gasoline engines. Other engine types such as diesel, or two-stroke cycle can differ in the exact implementation, but the general theme still applies.
A MAP sensor (manifold absolute pressure) is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous pressure information to the engine's electronic control unit (ECU). This is necessary to calculate air density and determine the engine's air mass flow rate, which in turn is used to calculate the appropriate fuel flow. (See stoichiometry.)
An engine control system that uses manifold absolute pressure to calculate air mass, is using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. Not all fuel injected engines use a MAP sensor to infer mass air flow, some use a MAF sensor (mass air flow).
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[edit] How the MAP value is used
The manifold absolute pressure measurement is used to meter fuel. The amount of fuel required is directly related to the mass of air entering the engine. (See stoichiometric.) The mass of air is proportional to the air density, which is proportional to the absolute pressure and inversely proportional to the absolute temperature. (See ideal gas law.) Engine speed determines the frequency, or rate, at which air mass is leaving the intake manifold and entering the cylinders.
- (Engine Mass Airflow Rate) ≈ RPM × (Air Density)
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- or equivalently
- (Engine Mass Airflow Rate) ≈ RPM × MAP / (absolute temperature)
[edit] Example
This example assumes the same engine speed and air temperature.
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- Condition 1:
- An engine operating at WOT (wide open throttle) on top of a very high mountain has a MAP of about 15" Hg or 50 kPa (essentially equal to the barometer).
- Condition 2:
- The same engine at sea level will achieve 15" Hg of MAP at less than WOT due to the higher barometric pressure.
The engine requires the same mass of fuel in both conditions because the mass of air entering the cylinders is the same.
If the throttle is opened all the way in condition 2, the MAP will increase from 15" Hg to nearly 30" Hg (~100 kPa), about equal to the local barometer, which in condition 2 is sea level. The higher absolute pressure in the intake manifold increases the air's density, and in turn more fuel can be burned resulting in higher output.
Anyone who has driven up a high mountain is familiar with the reduction in engine output as altitude increases.
[edit] Vacuum comparison
Vacuum is the difference between the absolute pressures of the intake manifold and atmosphere. Vacuum is a "gauge" pressure, since gauges by nature measure a pressure difference, not an absolute pressure. The engine fundamentally responds to air mass, not vacuum, and absolute pressure is necessary to calculate mass. The mass of air entering the engine is directly proportional to the air density, which is proportional to the absolute pressure, and inversely proportional to the absolute temperature.
Note: Carburetors are largely dependent on air volume flow and vacuum, and neither directly infers mass. Consequently, carburetors are precise, but not accurate fuel metering devices. Carburetors were replaced by more accurate fuel metering methods, such as fuel injection.
[edit] Barometer and vacuum calculations based on MAP
The MAP sensor can be used to directly measure the BAP (barometric absolute pressure).
- BAP = MAP (When either of the following conditions are true.)
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- When the engine is not turning.
- When operating at WOT (nearly equal to the barometric pressure)
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Once the BAP is known, the MAP sensor can be used to calculate intake manifold vacuum.
- BAP - MAP = Manifold Vacuum
- or
- BAP = MAP + Manifold Vacuum
- or
- MAP = BAP - Manifold Vacuum
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- When the engine is running, the difference between the BAP and the MAP is known as intake manifold vacuum. The ECU learns the BAP just before cranking the engine, i.e., when MAP equals BAP.
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As atmospheric pressure decreases with increasing altitude, vacuum must also decrease to maintain the same MAP in order to maintain the same torque output. This is accomplished by opening the engine's throttle more as altitude increases. However, the BAP learned at the beginning of the trip becomes obsolete as altitude changes.
Sometimes an engine control system will use both a BAP sensor and a MAP sensor to continuously maintain an accurate barometer and manifold vacuum. However, neither vacuum nor barometer are necessary for fuel determination, although they are helpful for other engine functions. The critical information is the air's density in the intake manifold, and the speed of the engine, i.e., the speed-density method.
The BAP sensor is often located within the ECU, and the MAP sensor is usually located near the intake manifold.
(See Earth's atmosphere.)
