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Manifold vacuum, or engine vacuum in a petrol 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 airflow through a throttle in the intervening carburetor or throttle body leading to the intake manifold.
Inlet manifold vacuum is measured using a pressure sensor located in, or connected to the intake manifold, in order to calculate the duration of fuel injection pulses. Originally, this system was called Jetronic, but the name D-Jetronic was later created as a retronym to distinguish it from subsequent Jetronic iterations.
Intake manifold vacuum is applied to the crankcase via the PCV valve. The airflow through the crankcase and engine interior sweeps away combustion byproduct gases. This mixture of air and crankcase gases then exits, often via another simple baffle, screen, or mesh to exclude oil mist, through the PCV valve and into the intake manifold. On some ...
The vacuum is only an assist and the brakes can still function, requiring greater force, if the booster vacuum is used up. Many older vehicles used vacuum-powered windshield wipers. Loss of manifold vacuum when the engine was working hard, or at wide open throttle, necessitated using a vacuum booster pump which was usually part of the fuel pump.
The way they do this is during a deceleration of the vehicle when there is low absolute pressure in the intake manifold (i.e., a high vacuum present in the intake manifold relative to the outside air) the powertrain control module (PCM) will open the EGR valve and then monitor the MAP sensor's values. If the EGR is functioning properly, the ...
The restriction causes a strong manifold vacuum which the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect. [1] This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivetrain to engage with the ...
A 'power valve', which is a spring-loaded valve in the carburetor that is held shut by engine vacuum, is often used to do so. As the airflow through the carburetor increases the reduced manifold vacuum pulls the power valve open, allowing more fuel into the main metering circuit.
That frustrates the purpose of the Stage II nozzle, which was designed to vacuum away any fuel vapors that come up that fill pipe during the refueling process. If the car's own vapor recovery system is working properly, then the Stage II nozzle will only be vacuuming normal fresh air and depositing that into the gas station's underground fuel ...