Answers - Powerplant Ignition and Starting Systems


1. b
Almost all magnets, regardless of their retentivity, lose some of their magnetic strength when their lines of flux pass through the air. Therefore, during the overhaul of a magneto, when the rotating magnet is removed, it should be placed in a soft iron keeper to prevent loss of magnetism. A "keeper" is a piece of soft iron that is used to link the magnetic poles and provide a highly permeable path for the flux.

2. a
There are two common methods for checking the strength of a magneto magnet. If the magnet is removed from the magneto, it is checked with a magnetometer. However, if the magneto is assembled, the magnet's strength is checked by holding the points open and then checking the output of the primary with an AC ammeter as the magneto is rotated at a specified speed.

3. b
The number of degrees between the neutral position of the rotating magnet and the position where the contact points open is called the E-gap angle. When the rotor is at the E-gap angle the cam opens the breaker points and current stops flowing through the primary coil. This sudden stoppage of current flow causes the magnetic field in the coil core to collapse and induce a voltage into the secondary winding.

4. a
The greatest flux density within a rotating magnet-type magneto occurs when the poles of the magnet are fully aligned with the field shoe faces. This position is known as the fill1 register position.

5. b
The internal timing of a magneto is most easily set when the magneto is being assembled prior to installation.

6. c
The ignition harness on reciprocating engines serve several purposes. It supports the wires going from the magneto to each spark plug and protects them from engine heat, vibration, and weather. Ignition harnesses also serve as a conductor for the stray magnetic fields that surround the wires as they momentarily carry high-voltage current. Therefore, by routing the ignition leads through metallic conduits, the stray magnetic fields are easily conducted to ground, thereby reducing electrical interference with aircraft radio equipment.

7. b
Any time the gap between the breaker points in a magneto is increased, the points will open early. When the points open early, the spark within the cylinder occurs in advance of when it is supposed to. Furthermore, early opening points also cause a decrease in spark intensity.

8. a
Some magnetos contain a safety gap that protects the secondary coil. The safety gap is connected in series with the secondary circuit by two electrodes. One electrode is attached to the high tension brush holder and the other electrode is grounded to the ground plate. With this configuration, if excessive voltage builds in the secondary circuit due to an open that does not allow the spark to jump the spark plug electrodes, the excessive voltage jumps the safety gap to the ground connection. This helps ensure that the voltage cannot rise high enough to damage the insulation within the secondary coil.

9. c
When the timing marks within a magneto are aligned, the magnets are in the E-gap position and the breaker points are just beginning to open.

10. c
When internally timing a magneto, the breaker points should begin to open when the rotating magnet is a few degrees past the neutral position. The neutral position is defined as the point where the rotating magnet is 45 degrees past full alignment with the pole shoes.


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