IGNITION ADVANCE MECHANISMS
IGNITION ADVANCE MECHANISMS
There are two general methods used in modern engines to advance and retard the ignition timing automatically in relation to engine speed and operating conditions after the initial timing is set manually, most manufacturers call these methods as automatic advance mechanisms.
1. Centrifugal advance mechanism.
2. Vacuum advance mechanism.
1. Centrifugal advance mechanism. It consists of two fly weights (advance weights), cam, spring and base plate. Fig. 7.13. The flyweights are carried by the distributor drive shaft through the base plate which is fixed to the drive shaft. The fly weights are pivoted on the base plate, and attached to the cam through springs. The cam is connected with the distributor drive shaft through the springs, fly wheel and base plate.
When the engine speed increases, the weights move out due to centrifugal force rotating the plate and cam in the anticlockwise direction. This movement affects the desired advance. This timing of the spark varies from no advance at low speed to full advance at hive speed.
2. Vacuum advance mechanism. Fig 7.14 shows the vacuum advance mechanism. It consists of a diaphragm which automatically advances and retards the ignition timing according to the engine speed and operating condition. The diaphragm forms to chambers, one of which is connected to the induction manifold and the other is open to the atmosphere. A linkage connects the diaphragm to the distributor. When the diaphragm is in its normal position, the contact beaker is held fully retarded. When the engine speed increase, the induction manifold depression is high and the diaphragm is pressed up by atmospheric pressure. The movement of the diaphragm moves the contact breaker in the opposite direction to rotate and thereby advancing the ignition. A decrease in vacuum allow the diaphragm to return back to its original setting retarding the ignition.
that the centrifugal advance mechanism takes much care of speed only and not the load conditions, whereas the vacuum advance mechanism mechanisms is applied on the distributor. In this design, part of the ignition advance is due to the centrifugal force and part of the vacuum produced in the intake manifold. The combination of the two mechanisms gives the practically perfect spark timing for all driving conditions.
References
- Automobile Engineering (Vol. 1 & 2) - K.M.Guptha
- Automotive Mechanics - Joseph Heitner
- Automobile Engineering - Harbans Singh Reyd
- Automotive Mechanics - William H. Course
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