Manifold vacuum

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere. Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere. Manifold vacuum is an effect of a piston's movement on the induction stroke and the choked flow through a throttle in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine. In some engines, the manifold vacuum is also used as an auxiliary power source to drive engine accessories and for the crankcase ventilation system. Manifold vacuums should not be confused with Venturi vacuums, which are an effect exploited in carburetors to establish a pressure difference roughly proportional to mass airflow and to maintain a somewhat constant air/fuel ratio. It is also used in light airplanes to provide airflow for pneumatic gyroscopic instruments. 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 the product of the rotation rate of the engine, 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 (engine speed in a vehicle 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 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, output power is limited by the ambient barometric pressure. Superchargers and turbochargers boost manifold pressure above atmospheric pressure. Modern engines use a manifold absolute pressure (abbreviated as MAP) sensor to measure air pressure in the intake manifold. Manifold absolute pressure is one of a multitude of parameters used by the engine control unit (ECU) to optimize engine operation. It is important to differentiate between absolute and gauge pressure when dealing with certain applications, particularly those that experience changes in elevation during normal operation. Motivated by government regulations mandating reduction of fuel consumption (in the USA) or reduction of carbon dioxide emissions (in Europe), passenger cars and light trucks have been fitted with a variety of technologies (downsized engines; lockup, multi-ratio and overdrive transmissions; variable valve timing, forced induction, diesel engines, et al.) which render manifold vacuum inadequate or unavailable. Electric vacuum pumps are now commonly used for powering pneumatic accessories. Manifold vacuum is caused by a different phenomenon than venturi vacuum, which is present inside carburetors. Venturi vacuum is caused by the venturi effect which, for fixed ambient conditions (air density and temperature), depends on the total mass flow through the carburetor. In engines that use carburetors, the venturi vacuum is approximately proportional to the total mass flow through the engine (and hence the total power output). As ambient pressure (altitude, weather) or temperature change, the carburetor may need to be adjusted to maintain this relationship.