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In a jet engine, the term "auto/continuous ignition" refers to both automatic and pilot selectable activation of the ignition system in circumstances other than during the start cycle.
The function of the ignition system is to provide an electrical spark to ignite the fuel/air mixture in the cylinders. The ignition system of the engine is completely separate from the airplane's electrical system. The magneto type ignition system is used on most reciprocating aircraft engines. Magnetos are engine driven self-contained units supplying electrical current without using an external source of current.
However, before they can produce current the magnetos must be actuated as the engine crankshaft is rotated by some other means. To accomplish this, the aircraft battery furnishes electrical power to operate a starter, which through a series of gears, rotates the engine crankshaft. This in turn actuates the armature of the magneto to produce the sparks for ignition of the fuel in each cylinder. After the engine starts, the starter system is disengaged, and the battery no longer contributes to the actual operation of the engine.
Under normal conditions, the ignition system of a jet engine is only energized during the acceleration portion of the start cycle. During the start, the ignition system is activated at about 20% N1 (as displayed on the N1 Indicator) and deactivated at about 50% N1, with actual values variable from engine to engine. Once the engine is started, combustion is self sustaining so long as fuel is supplied and the airflow is not interrupted.
A jet engine is sensitive to the flow characteristics of the air that enters the engine intake. If the airflow remains essentially normal, the engine will continue to run smoothly. However, should the airflow be significantly disrupted, a compressor stall or engine flameout could result. This level of airflow disruption is usually associated with abrupt pitch changes such as might be encountered in severe turbulence or during an aerodynamic stall.
Other conditions that could lead to disruption of sustained combustion include ice shedding, water ingestion from heavy precipitation or contaminated runways and bird strike.
Modern aircraft engines are required by to have a dual ignition system - that is, two separate magnetos to supply the electric current to the two spark plugs contained in each cylinder. One magneto system supplies the current to one set of plugs; the second magneto system supplies the current to the other set of plugs. For that reason the ignition switch has four positions:
OFF, L, R, and both with the switch in the "L" or "R" position, only one magneto is supplying current and only one set of spark plugs in each cylinder is firing. With the switch in the BOTH position, both magnetos are supplying current and both spark plugs are firing.
The main advantages of the dual ignition system are:
1. Increased safety. In case one system fails, the engine may be operated on the other until a landing is safely made. Consequently, it is extremely important for each magneto to be checked for proper operation before takeoff. This should be done in accordance with the manufacturer's recommendations. Unless the manufacturer specifies otherwise, it is advisable to turn the ignition switch from the "BOTH" position to the farthest "ON" position first, then back to "BOTH;" then to the nearest "ON" position and back to "BOTH." This sequence best assures that the magneto switch will be returned to the "BOTH" position for normal operation. Also, the performance of each ignition system will have been compared with a "BOTH" performance.
2. More complete and even combustion of the mixture, and consequently improved engine performance; i.e., the fuel mixture will be ignited on each side of the combustion chamber and burn toward the center.
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