6.2.4 Ignition Systems
Principles of Magneto Ignition
In any ignition system the basic necessity is to deliver a powerful electrical current to the spark plugs, so to detonate the fuel/air mixture within each cylinder. Since both the inlet and exhaust valves are closed during ignition, the pistons are forced down the cylinder by rapidly expanding gasses. Work is produced and in turn the crankshaft is rotated and the propeller turns.
Construction and Function
A magneto is simply a permanent magnet that rotates within a conductor and coil to create an alternating electrical current. This current is entirely independent of the aircraft’s electrical system and does not rely on battery power, it is instead a self-exciting entity. The magneto, which can be mechanically or engine driven, generates sufficient electrical charge to the spark plugs in each cylinder after passing through the distributor – igniting at just the right time and in a specific sequence. All of this takes place once 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 intermitted 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.
A dual ignition system with two individual magnetos is what is commonly used in small aircraft these days. Separate sets of wires and spark plugs improve the redundancy and reliability in the ignition system. Each magneto operates independently to fire its own spark plug within their allotted cylinders. Ignition of the fuel-air mixture is therefore improved with dual spark ignition and results in a marginally higher power output. In the event of one magneto failing, the other will be unaffected, improving pilot safety considerably. This redundancy allows the engine to continue somewhat normal operation, although engine RPM can be expected to be slightly reduced. Understandably 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
A spark is produced in the plugs because the magneto spins a magnet inside an iron coil core. This generates an alternating current within the coil and produces up to 20 000 volts which are 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 and running efficiency. Incorrect timing can lead to a premature power stroke known as kick-back. This results from normal magneto timing set for a higher R.P.M settings. Ultimately, this can lead to the crankshaft being forced in the wrong direction. The ignition therefore needs to be delayed on startup (during low R.P.M operations).
This start-up problem is solved through the use of Impulse coupling. An impulse coupling device works in two ways. Spring weights and a spring-loaded coupling are used initially to prevent the magneto from turning. Once the spring is fully wound it releases the magnet which then spins at a greater velocity. The benefits of this are two-fold.
First, it accelerates the rotation of the magnet producing a higher voltage (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 cause interference during normal operations.
The spark plugs are a useful indicator in determining the engine condition. During each Mandatory Periodic Inspection (M.P.I) the plugs are removed for inspection and testing. Normal engine function is indicated by a light grey coating of the end of the plugs. If detonation is suspected, excessive wear will be a good indication. Improper mixture control will leave Black sooty-like deposits in cases where mixture has not been sufficiently leaned, whereas engines operated with too lean a mixture will leave behind white powdery traces. Black oily deposits suggest excessive oil consumption and if hard brittle deposits are found lodged in the spark plug gap, it means lead in the fuel is not being removed during combustion. If disregarded, these deposits can build up enough to cause the plugs to ground without a spark. This often results in a “mag drop” which can be recognized by a rough running engine and an excessive loss in R.P.M. When this occurs on the ground during magneto checks, aborted the flight immediately.
The ignition system can be identified as defective or damaged 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 individually. 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 damaged wires between the magneto and the spark plugs, fouled spark plugs or incorrect ignition timing. “No drop” in R.P.M is also abnormal and should be considered 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, by simply turning the propeller by hand. Even with the battery and master switches OFF, the magneto is self-exciting and the engine can fire and turn over if the ignition switch is left ON and the propeller is moved. If this occurs, the only way to stop the engine is to starve the engine of fuel by move the mixture lever to the Idle Cut-Off (I.C.O) position. The system should immediately be checked by a qualified AMO. To avoid a premature start, ensure the engine is stopped by moving the mixture to the I.C.O position thereby draining the fuel lines, the magneto switch is turned to the OFF position after each flight and be extremely cautious when in the vicinity of the propeller.