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The system usually consists of a tube, a one-way valve and a vacuum source (such as the inlet manifold). The unwanted gases, called "blow-by", are gases from the combustion chamber which have leaked past the piston rings. Early engines released these gases to the atmosphere simply by leaking them through the crankcase seals.
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 intake manifold of an engine. It is a measure of the amount of restriction of ...
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.
In a naturally aspirated engine, air for combustion (Diesel cycle in a diesel engine or specific types of Otto cycle in petrol engines, namely petrol direct injection) or an air/fuel mixture (traditional Otto cycle petrol engines), is drawn into the engine's cylinders by atmospheric pressure acting against a partial vacuum that occurs as the piston travels downwards toward bottom dead centre ...
Some early gas engines worked on the "vacuum" or "atmospheric" principle in a similar way to the Newcomen steam engine.A mixture of gas and air was eaten by the cylinder and ignited; the mixture expanded and part of it escaped through the exhaust valve; the valve then closed, the mixture cooled and contracted, and atmospheric pressure pushed the piston in.
Vacuum Modulator is an engine load sensing device that converts engine vacuum into a transmission valve body input.. Most vacuum modulators operate with manifold vacuum (below throttle blades) that offer more vacuum at idle, and proportionately changes (rises and falls) with engine load as opposed to operating on engine speed.
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Alternatively, a manifold vacuum sensor can be used. The manifold vacuum sensor signal, the throttle position, and the crankshaft speed can then be used by the engine control unit to calculate the correct amount of fuel. In modern engines, a combination of all these systems is used. [5]