During the starting process for a reciprocating engine, the overall output of the magneto may be low as a result of slow engine cranking speed. The cranking speed is often affected by the voltage induced to the magneto circuit, that of which can be bolstered with a stronger magnet or by having an increased rate of relative motion between the conductor and ignition magneto. As these two factors are unchangeable in a finalized magneto ignition system, the only way to raise induced voltage is to increase the speed at which the magnet itself rotates. In order to generate the speed necessary for a magnet to successfully create a jumping spark for ignition, the magneto is attached to an auxiliary ignition unit.
In their standard configuration, auxiliary ignition units are attached to the left engine magneto and receive energy from the onboard aircraft battery. For a reciprocating engine, the most common types of auxiliary ignition units include the booster coil, starting vibrator, impulse coupling, and electronic ignition system. As the engine begins its operations, its rotational speed will be slower than normal. To ensure that the piston does not cause the assembly to rotate in the opposite direction, the ignition system needs to be slowed to delay the ignition spark. Depending on the type of auxiliary ignition unit present, the method of slowing the spark may differ.
Booster coils are the oldest style of auxiliary units, often being used for older radial engine ignition systems. Such assemblies consist of a set of contact points, a condenser, and two ignition coils that are wound around a soft iron core. The booster coil produces its own sparks during the starting process, and these are routed to the right cylinder ignition lead through the distributor rotor. While a stationary contact allows for battery voltage to be applied to the system, the second winding features a grounded end and an end terminated at a high-tension terminal.
The impulse coupling is another auxiliary ignition unit that is most often found within opposed reciprocating engines. With such units, a single engine-connected magneto will have its acceleration temporarily boosted so that the starting spark is bolstered. Generally, these assemblies consist of a cam and flyweight assembly, body, and spring. To achieve the magneto speed necessary for a spark delay, the spring is attached to the magneto. The stud or stop pins present in the assembly are useful for catching the flyweight during rotation, allowing for the magneto spring to be wound. When the magneto flyweight contacts the impulse coupling body and detaches, the resulting twist creates a spark for jumping the gap.
In some instances, a shower of sparks is needed for starting, and this can be achieved with a high-tension retard breaker vibrator. Such assemblies feature an electrical vibrator, condenser, and relay, all of which is enclosed within a metal case. Unlike other forms of axillary ignition units, the starting vibrator utilizes battery power to create pulsating direct current that is supplied to the magneto primary coil. During operations, the vibrator points will continuously close, causing pulsating DC to flow. With a constant building and collapsing magnetic field, a shower of sparks is created at the desired spark plug air gap.
Alongside such examples, there are various other auxiliary ignition units that one may take advantage of for their reciprocating engine. As each type differs in its overall makeup and compatibility, it is important that you choose one based on your aircraft engine type and other requirements. Additionally, such assemblies should be well maintained and inspected on a regular basis as a result of their importance to standard aircraft operations. When it comes time to begin sourcing the various parts and components that you require, look no further than One Click Purchasing.
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