What is Manifold Pressure Sensor in Aircraft

1. Although it’s technically termed as aircraft manifold pressure monitoring, the word “pressure” in this case, is a bit of a misnomer. This is because the aircraft manifold pressure data is not about pressure but suction. The cylinders in any aircraft engine are like a large suction pump because the cylinders are constantly sucking air into itself. The MAP sensor therefore, if anything, is reading suction not ram air pressure. So at idle power your MAP gauge might display between 10 and 12 inches when actual pressure outside is 30 inches. This means your engines are actually starving for air creating a vacuum pressure within the intake manifold. As the piston descends with the inlet valve open, a partial vacuum is created. It is this vacuum sucks in fuel into the intake. Therefore, greater the vacuum, greater the air-fuel mixture and hence greater the power output from the aircraft engine. So, the more air-fuel mixture pumped into the cylinders, the more power the engine develops and enables us to fly higher or faster. Now, if we measure air pressure in the induction system, just before it enters the aircraft cylinders, we will have a good idea of how much power we can develop. In any normally aspirated aircraft engine (non turbo-charged), the manifold pressure gauge usually has a range of 10 to 40 in. hg. In a turbocharged engine, the limits as per manufacturer settings. Effectively, when the engine is shut down, the MAP should read roughly the same as current atmospheric pressure setting.

2. Generally, MAP Sensors are used in fuel injected aircraft engines. These days, the manifold absolute pressure sensor provides instant manifold pressure data (as described above) to the engine's electronic control unit (ECU). The data presented by the MAP is used to figure out the density of the air and calculate the engine's air mass flow rate. These figures in turn help the pilot determine the required fuel mix for the most optimum (or economical) combustion. This helps work out the ignition timing. Alternatively, fuel-injected engine can also use the mass airflow sensor (MAF sensor) to detect the intake airflow. In most naturally aspirated engines, there is either of the above two. In forced induction engine however, both tend to be deployed. Usually the MAF sensor is parked on the intake tract pre-turbo while the Manifold Pressure Sensor is installed on the charge pipe leading to the throttle body. A lesser known fact is that the aircraft’s MAP sensor data can easily be converted to air mass data using the speed-density method. For this, the air temperature and engine speed in RPM Sensor are used. Typically, modern EDM’s use MAP sensor data for on-board diagnostics (EBD), to test the functionality of the exhaust gas recirculation (EGR) valve. JPI Sales: 1-800-345-4574 714-557-3805 FAX: 714-557-9840 sales@jpinstruments.com JPI Technical Support: 1-800-345-4574 714-557-3805 support@jpinstruments.com J.P. Instruments Inc. 3185-B Airway Ave Costa Mesa, CA 9262