Vacuum functions as a fundamental dynamic airflow of the internal combustion engine. Without the proper vacuum, a car would be deprived of the air-fuel mixture necessary to produce combustion. Vacuum is the pressure difference, usually measured in inches of mercury, between the inside of the intake manifold and the outside air pressure.
When the intake valve is opened, the air-fuel mixture is drawn into the combustion chamber by the downward movement of the piston, creating a suction or vacuum. When the piston reaches the top of its compression stroke, the air-fuel mixture is ignited by the spark plug, which sends the piston down on its power stroke. It follows the exhaust stroke, where the spent combustion gases are pushed out of the exhaust valve and into the exhaust system. When the piston makes another downward stroke, suction or vacuum is created again. Regardless of the speed at which the engine runs, the pistons draw incoming air and fuel into the combustion chamber.
Throttle and vacuum position
An engine that starts normally will produce about 3 to 5 inches of mercury or Hg. Vacuum increases and is highest at idle when the throttle position is closed or slightly open. This results from the restriction of the air flow that cannot move in large volume from the air inlet to the manifold. As the throttle is opened, more air enters the intake, causing a decrease in vacuum.
Vacuum efficiency and motor performance
The efficiency of the vacuum can be measured with a vacuum gauge. Engine vacuum will decrease as the engine is subjected to a heavy load with the throttle wide open, such as when climbing a hill or during rapid acceleration from a stop. The highest vacuum will occur when the engine decelerates from high speed or during coasting, and this happens because the throttle is closed but the engine rpm is high. Manifold vacuum specifications for different conditions are outlined in the vehicle manufacturer’s service manual.
Auxiliary vacuum components
Many vehicle components use multiple vacuum through ports and hoses. Brake boosters, which have vacuum assist, use vacuum to actuate a diaphragm that increases pressure when the brake pedal is applied. Some windshield wipers and door locks use vacuum-assisted servos to operate valves and connections. Older distributor vehicles use vacuum as a means of advancing the spark timing in the ignition system. PVC and EGR valves use vacuum to function as part of the emission control system. These auxiliary systems use the engine rpm or additional switching valves to regulate the proper amount of vacuum needed to operate.
Engine vacuum and barometric pressure: what is engine vacuum
At sea level, atmospheric pressure is about 14.7 pounds per square inch and maintains a column of mercury at 29.92 inches high, or Hg. As atmospheric pressure and temperature increase or decrease during climatic changes, there are slight changes in engine vacuum. Less dense or warmer air produces less vacuum due to loosely packed air molecules. The higher the altitude (less dense air), the lower the Hg or inches of mercury in the vacuum reading. For example, an engine idling at 22 Hg at sea level will show approximately 17 Hg at 5,000 feet altitude. At 10,000 feet altitude, the mercury will read about 12 Hg.
Proper vacuum conditions: how a vacuum is generated in an engine
Normal vacuum for an idling engine works at 14 to 18 inches of mercury when measured with a pressure gauge. Abnormally high idle rpm can indicate a vacuum leak somewhere between the throttle and the manifold, usually pointing to cracked hoses, leaking base plates, manifold leaks, faulty carburetor, or ported vacuum-operated switch valves. defective. This also results in an air mixture that is too lean. Low vacuum can result from low compression and blown valves.