MAP sensor

A manifold absolute pressure sensor (MAP) 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 manifold pressure information to the engine's electronic control unit (ECU). The data are used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion (see stoichiometry). A fuel-injected engine may alternately use a MAF (mass air flow) sensor to detect the intake airflow. A typical configuration employs one or the other, but not both.

MAP sensor data can be converted to air mass data using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.

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. 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)

or equivalently

(Engine Mass Airflow Rate) ≈ RPM × MAP / (absolute temperature)

Example

The following example assumes the same engine speed and air temperature.

• 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 manifold absolute pressure 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.