This article is about Vacuum Spark Advance Mechanism
Along with the centrifugal spark advance, another method is used to advance ignition spark under load. This is commonly called the vacuum operated spark advance unit.
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The vacuum advance unit supplements the mechanical advance; however, it operates differently as it varies spark timing in relation to engine load.
Engine load can best be determined by manifold vacuum. Under light engine load conditions, there is a high vacuum in the intake manifold caused by the restriction of the partially closed throttle valve; consequently, there is a smaller amount of air/fuel mixture delivered to the combustion chambers.
Because the mixture will not burn as rapidly, ignition must take place earlier in the cycle. So consequently the vacuum advance unit mounted on the distributor is connected directly to engine intake manifold vacuum.
The carburetor is used on some applications to control the vacuum operated advance unit located oft the distributor. On these units, a calibrated spark advance port is located in the air horn (carburetor bore) Just above the carburetor throttle valve.
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It is connected by a vacuum line directly to the distributor vacuum advance unit.
This unit contains a spring-loaded diaphragm which rotates the breaker plate assembly to advance the spark timing, The diaphragm is connected by a vacuum passageway to the spark advance port which is located above the throttle valve (on the atmospheric side)when the engine is idling.
In this position, no vacuum is applied to the diaphragm and the breaker plate is retarded by spring pressure.
In the curb idle position, the throttle valve is below the spark advance port so consequently no vacuum is applied to the advance unit and the spark advance remains in the retarded position. As the throttle valve is opened, it swings past the vacuum port which applies vacuum to the diaphragm, moving it against spring pressure and advancing the breaker plate.
When the engine is accelerated, intake manifold vacuum drops according to the degree of throttle opening which retards the position of the breaker plate assembly accordingly.
Thus, we can see that two things control spark advance. One is engine speed; the other is the load placed upon the engine.
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The load is determined by how much the throttle is open and how fast the engine is running which determines the amount of air flow through the carburetor.
The air flow (determined by air speed) and the throttle opening determine the fuel/air ratio entering the cylinder.
Because the fuel/air ratio determines the burning rate, the vacuum advance can be thought of as primarily regulated by throttle opening. And the engine speed regulates the centrifugal advance which is necessary simply because in-
creased engine speed leaves less time for the fuel/air mixture to burn.
Some autos eliminate the centrifugal advance by using in addition to the vacuum spark advance mechanism discussed above, one which also has a vacuum opening at the carburetor venture.
This simply provides vacuum advance in direct relation to the engine speed (the same as a centrifugal advance mechanism).
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Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.