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Vacuum advance is an important mechanism in the ignition system of an internal combustion engine.
Function of Vacuum Advance:
- Timing Adjustment: Vacuum advance automatically adjusts the timing of the ignition spark based on the engine’s intake manifold vacuum. This adjustment ensures optimal ignition timing under varying engine loads and operating conditions.
- Compensation for Part-Throttle Conditions: During part-throttle operation, when the engine is not receiving as much air-fuel mixture as at full throttle, the combustion process may take longer to complete. Vacuum advance compensates for this by advancing the ignition timing, allowing for more efficient combustion and improved engine performance.
Components and Operation:
- Diaphragm Mechanism: The vacuum advance mechanism consists of a flexible, spring-loaded diaphragm connected to the distributor’s breaker plate. This diaphragm is sealed and connected to the intake manifold via a vacuum passage.
- Vacuum Signal: When the engine is running, intake manifold vacuum varies depending on factors such as throttle position and engine load. At part-throttle conditions, when vacuum is higher, the diaphragm is pulled in by the vacuum signal.
- Advance Timing: As the diaphragm moves in response to the vacuum signal, it causes the breaker plate to rotate slightly, advancing the timing of the ignition spark. This advanced timing optimizes combustion and improves engine efficiency during part-throttle operation.
- Disconnection during Timing Adjustment: In most cases, the vacuum advance is disconnected before checking the ignition timing and adjusting the point gap. This ensures that the timing is set accurately without interference from the vacuum advance mechanism.
Throttle Position Dependency:
- No Vacuum at Idle: When the engine is idling, the throttle valve is closed, resulting in minimal or no vacuum in the intake manifold. Therefore, there is no vacuum advance during idle conditions.
- Increasing Vacuum with Part-Throttle: As the throttle is opened partially, intake manifold vacuum increases, activating the vacuum advance mechanism to advance the timing.
- No Vacuum Advance at Wide-Open Throttle (WOT): At wide-open throttle, there is very little vacuum in the intake manifold. Consequently, the vacuum advance mechanism is not activated, and the ignition timing remains static or advances only through other means like mechanical advance.
Overall, vacuum advance plays a crucial role in optimizing engine performance by adjusting ignition timing based on engine operating conditions, particularly during part-throttle operation.
See
- Ported vacuum advance
- Speed control vacuum advance