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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 intake manifold of an engine. It is a measure of the amount of restriction of ...
Vacuum-Boost sensors can display negative readings, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged or turbocharged), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. The suction is caused ...
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.
A locomotive engine which gives a leak-down of 10% on a leak-down tester is virtually perfectly sealed while the same tester giving a 10% reading on a model airplane engine indicates a catastrophic leak. With a non-turbulent .040" orifice, and with a cylinder leakage effective orifice size of .040", leakage would be 50% at any pressure.
OBD-II PIDs (On-board diagnostics Parameter IDs) are codes used to request data from a vehicle, used as a diagnostic tool.. SAE standard J1979 defines many OBD-II PIDs. All on-road vehicles and trucks sold in North America are required to support a subset of these codes, primarily for state mandated emissions inspections.
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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.