Manifold Vacuum - Overview

Overview

The rate of airflow through an internal combustion engine is an important factor determining the amount of power the engine generates. Most gasoline engines are controlled by limiting that flow with a throttle that restricts intake airflow, while a diesel engine is controlled by the amount of fuel supplied to the cylinder, and so has no "throttle" as such. Manifold vacuum is present in all naturally aspirated engines that use throttles (including carbureted and fuel injected gasoline engines using the otto cycle or the two-stroke cycle; diesel engines do not have throttle plates).

The mass flow through the engine is determined by the rotation rate of the engine, multiplied by the displacement of the engine, and the density of the intake stream in the intake manifold. In most applications the rotation rate is set by the application (road speed in a car or machinery speed in other applications). The displacement is dependent on the engine geometry, which is generally not adjustable while the engine is in use (although a handful of models do have this feature, see variable displacement). Restricting the input flow reduces the density (and hence pressure) in the intake manifold, reducing the amount of power that is produced. It is also a major source of engine drag (see engine braking), as the engine must pump material from the low-pressure intake manifold into the exhaust manifold (at ambient atmospheric pressure).

When the throttle is opened (in a car, the accelerator pedal is depressed), ambient air is free to fill the intake manifold, increasing the pressure (filling the vacuum). A carburetor or fuel injection system adds fuel to the airflow in the correct proportion, providing energy to the engine. When the throttle is opened all the way, the engine's air induction system is exposed to full atmospheric pressure, and maximum airflow through the engine is achieved. In a "naturally aspirated" engine, total engine output is thus determined by the ambient barometric pressure. Superchargers and turbochargers can "boost" manifold pressure to above atmospheric pressure.