impulse coupler

[baobab72]

hi folks
i am not too familiar with piston engines, more familiar with jet ones.
what is an impulse coupler?
how does the inversion of polarity of the magnet affect the emf induced in the primary winding?
how does the distributor work?
how doe the breaker points work?
and finally when it is mentioned that a circuit is grounded, what does that mean in plain english?

many thanks

baobab72

[Oktas8]

An impulse coupling is an addition to a standard magneto. It is designed to produce a high-energy voltage at very low rpm, such as may occur during start.

When a circuit is grounded, it means that both sides of the circuit are connected to earth. In the case of an ignition system, it means that the electrical pulse that would normally generate a spark is actually sent to "ground" or "earth" - the circuit is shorted out so that no spark occurs. This is what happens when a magneto is turned "off".

When a magnetic field is passed across an electrical conductor, a voltage is generated. The polarity of the voltage depends on the pole of the magnet that is used and the direction of rotation. So if the magnetic polarity is reversed, the voltage and current will be inverted. The voltage generated depends on the speed of movement, the size of the magnetic field and the number of wires that the field passes over. At engine start, the speed of movement is too low to generate a high enough voltage, so the impulse coupling holds back the magneto shaft until a spring is fully under tension. A cam then releases the shaft and the tensioned spring causes it to spin very rapidly to catch up with the engine's crankshaft that drives the shaft. This rapid albeit brief rotation generates a sufficiently strong voltage to produce a good spark.

The voltage and current generated within the magneto is not sufficient to power a spark plug. At set points within each cycle, breaker points open like a switch turning off. This causes the current in the magneto to collapse abruptly. When the current in a coil of wire changes rapidly, a very large voltage is created to oppose the change in current. The faster the current changes, collapsing to zero in this case, the higher the voltage is created. This new higher voltage is sufficient to drive the spark plug. (Actually a secondary coil is used to step the voltage up even more but I've left that step out to simplify.)

The distributor simply connects the high tension voltage output, to each set of spark plugs in turn. Think of a clock with a high voltage applied to the second hand, and spark plugs connected at 3 o'clock, 6 o'clock, 9 o'clock and 12 o'clock. You can see that the voltage would be connected four times a minute as the second hand rotates. The distributor works to a similar principle.

This explanation is entirely off the cuff. I hope it answers at least some of your questions. There are plenty of resources on the internet, including wikipedia, describing how these items work in more detail - and quite likely more accuracy!!!

Cheers,
O8