An impulse coupling is a mechanism primarily associated with engine ignition systems, particularly in older engines or those using magnetos for ignition. Based on the provided reference, a key function of the impulse coupling is to delay the spark when the engine is not starting but running at normal operating speeds.
Understanding the Function
The primary purpose of an impulse coupling is to aid in starting an engine, especially one with a magneto ignition. Magnetos typically produce a strong spark only at higher rotation speeds. During starting, when the engine is being cranked at very low speeds, the magneto might not generate sufficient voltage for reliable ignition.
The impulse coupling addresses this by temporarily spinning the magneto armature faster than the engine crankshaft is turning during the cranking process. This rapid rotation creates a strong spark at the correct timing for starting.
Spark Timing and Impulse Coupling
The provided reference specifically highlights the impact of the impulse coupling on spark timing during different engine states:
- During Starting: The impulse coupling engages to provide an advanced spark (earlier relative to the piston's position) and a strong jolt to the magneto, ensuring ignition even at low cranking speeds.
- During Normal Running Speeds: According to the reference, the "impulse coupling delays the spark. When the engine is not starting but running at normal operating speeds the spark occurs just prior to the piston reaching the top dead [center]." This indicates that the coupling disengages or alters its function once the engine reaches a certain speed, allowing the spark timing to return to the normal running position, which is slightly later (closer to Top Dead Center) compared to the starting spark timing.
Essentially, the impulse coupling ensures:
- A sufficiently strong spark during low-speed cranking.
- An appropriate spark timing (often advanced) for easy starting.
- A transition to the normal running spark timing once the engine is running.
This mechanism is crucial for reliable starting, particularly in applications like older aircraft engines, motorcycles, or some industrial engines that utilize magneto ignition systems.
