6.2.4 Ignition Systems
Principles of Magneto Ignition
The basic requirement of any ignition system is to deliver an intense electrical current to the spark plugs. This ignites the fuel/air mixture in each cylinder. Since the valves are closed during ignition, the pistons are forced down because of the now rapidly expanding gasses, producing work and in turn rotating the crankshaft and thus the propeller.
Construction and Function
A magneto is simply a permanent magnet that rotates within a conductor and coil to generate an alternating electrical current. This current is completely independent of the aircraft’s electrical system. The mechanically or, engine driven magneto, generates the required electrical energy to the spark plugs in each cylinder – Igniting at just the right time. This takes place when the starter is engaged and the crankshaft begins to turn and it continues to operate whenever the crankshaft is rotating.
The distributor, which consists of a rotor that spins inside the non-conductive distributor block and makes contact with terminals embedded within the block. Each terminal is connected to a spark plug. The rotor (carrying a high voltage charge from the magneto circuit) comes in contact with each terminal, and the current is conducted to the applicable plug in the correct sequence.Most modern aircraft make use of a dual ignition system with two individual magnetos. Separate sets of wires and spark plugs improve the redundancy and reliability of the ignition system. Both magnetos operate independently to fire their own spark plug within their allotted cylinders. Combustion of the fuel-air mixture is therefore improved with dual spark ignition and results in a slightly higher power output. In the event of one magneto failing, the other will be unaffected. This redundancy allows the engine to continue somewhat normal operation, although engine RPM can be expected to be slightly reduced, resulting in a lower power output. Operation of the magnetos are controlled in the cockpit through various ignition switch positions:
BOTH / START
Purpose and Principle of Impulse Coupling
In order to produce a spark in the plugs, the magneto spins a magnet within an iron coil core. This generates an alternating current within the coil and produces up to 20 000 volts which is used to fire the spark plugs. In order for sparks to be effective, the magnet needs to be rotating at speeds of at least 500 R.P.M. Anything below this results in weaker sparks and reduces engine start-up efficiency. A premature power stroke known as kick-back results from normal magneto timing set for a higher R.P.M setting. Ultimately, this can lead to the crankshaft being forced in the wrong direction. On startup (during low R.P.M operation) the ignition therefore needs to be delayed.
This start-up problem is solved through the use of Impulse coupling. This device works in two ways. The magneto uses spring weights and a spring-loaded coupling which prevents the magneto from turning at first. Once the spring is fully wound it releases the magnet at an accelerated rotational velocity. The benefits of this are two-fold. First, it accelerates the rotation of the magnet producing a higher voltage or, better spark, and second the ignition spark is delayed during start-up. Once the engine is running, the centrifugal force of the flyweights ensures the impulse coupling is disconnected and does not interfere during normal operations.
The spark plug is a useful indicator of engine condition. At each Mandatory Periodic Inspection (M.P.I) the plugs are removed for inspection and testing. Normal engine operation is indicated by a light grey coating of the end of the plugs. Excessive wear may indicate detonation. Black sooty-like deposits will appear in cases where mixture has not been sufficiently leaned. Whereas engines operated with too lean a mixture will leave white powdery traces behind. Black oily deposits suggest excessive oil consumption. If hard brittle deposits are found lodged in the spark plug gap, it means fuel lead is not being removed during combustion. If disregarded, these deposits can build up sufficiently causing the plugs to ground without a spark. This often results in a “mag drop” which can be identified by a rough running engine and an excessive loss in R.P.M. When detected on the ground during magneto checks, the flight should be aborted immediately.
The ignition system can be identified faulty during the pre-takeoff run-up checks. This is done by observing the reduction in R.P.M that occurs between when the LEFT and RIGHT ignition are selected independently. The maximum allowable reduction and ‘total drop’ limits are listed in the POH. If the engine stops running when switched to one magneto or if the rpm drop exceeds the allowable limit, the aircraft should not be flown until the system is serviced and the problem is corrected.
Possible causes of an unacceptable mag drop could be the result of fouled spark plugs, damaged wires between the magneto and the spark plugs, or incorrect timing. “No drop” in R.P.M is abnormal and considered another cause for concern. In this case, the aircraft should not be flown and sent in for immediate inspection.
“No drop” is an indication that one of the magnetos is not grounding and can result in a premature start, simply by turning the propeller. Even with the battery and master switches OFF, the engine can fire and turn over if the ignition switch is left ON and the propeller is moved. This happens because the magneto is self-exciting. If this occurs, the only way to stop the engine is to move the mixture lever to the Idle Cut-Off (I.C.O) position, then have the system checked by a qualified AMO. Ensure the magneto switch is turned to the OFF position after flight and be extremely cautious when in the vicinity of the propeller.