Magneto Ignition Systems
The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto of the good old days or a modern fully electronic microprocessor controlled FADEC system we see more and more each passing year.
For obvious safety reasons the ignition system may not rely on the aircraft electrical system and must be dual and each system operates one of the two spark plugs in each cylinder.
These ignition systems apply to AVgas (spark ignited) engines and not to diesels, which are compression ignited engines.
Before we head on to sophisticated electronic ignitions we will discuss the good old magneto ignition from a bygone era. Mainly as you will still find these in the original and certified aircraft engines.
Magnetos are basically small generators with a transformer and a breaker switch and include a distributor to guide the high voltage to the spark plugs, and this will be the subject of this page.
The sole purpose of the ignition system is to supply a high energy spark at the right moment thereby igniting the fuel air mixture so that the engine can start its power stroke. Such a system consists of a number of components:
- The source for the spark, either electronic or mechanical (magneto)
- Distributor to direct the spark to the correct cylinder/plug
- High tension leads to conduct the spark
- Two spark plugs per cylinder which ignite the mixture
These use a strong magnet rotating inside a coil. The magnetic field generates a voltage in the coil which is transformed to a higher voltage by a secondary coil with much more windings than the primary coil. A breaker contact in the primary coil circuit interrupts the flowing current and this interruption causes the magnetic field to collapse thereby generating a very high peak voltage in the secondary coil. This peak voltage is then conducted to the correct spark plugs by the distributor and high voltage leads.
Two magnetos are connected in such a way that one drives the top spark plugs and the other the bottom plugs on the engine (LEFT and RIGHT really means TOP and BOTTOM). The magnetos generate power independently of the aircraft electrical system, so that in the event of flat battery during flight the engine will keep running.
Timing is of the essence here, the breaker points (with parallel capacitor) are driven by a small cam and opening at the correct crankshaft angle. That's usually 25° BTDC (before top dead center). Also parallel connected to the breaker contact and capacitor is the ignition switch, either L or R. Which effectively stops the spark when closed.
The distributor is also part of the magneto. Its function is to guide the high energy voltage to the correct spark plug through one of the high tension leads. As each cylinder fires every two revolutions of the crankshaft, the rotor in the distributor must therefore rotate at half the crankshaft speed.
Harness and Spark plugs
The ignition harness shields the high voltage and conducts it to the spark plugs, often bound together. The wires are screened or in a metal braid or conduit to shield the radio against the high frequency ignition interference.
A spark plug has a central electrode and a metal body which are screwed into the cylinder. Ceramic insulation is used to insulate the central electrode from the engine. Built into the spark plug is a resistor giving a short duration spark and protection against corrosion of the electrodes; it also suppression radio frequency interference to some degree.
Two spark plugs and separate ignition circuits are used per cylinder for redundancy, safety and better ignition and combustion of the mixture.
During starting of the engine, its crankshaft rotates very slowly (around 120 RPM) and the magnetos at 60 RPM. Generated voltage is very low at that point. The ignition timing is normally fixed at 25° BTDC, and this is too early at this low RPM. Should a cylinder fire it would probably cause a violent kickback (rotates momentarily in the wrong direction) and that will damage the starter and maybe more.
A device called a impulse coupling is used to retard the ignition timing to almost at TDC and an acceleration of the magnet (with a coiled spring) in the magneto to spice up the voltage to help igniting the mixture at TDC. When the engine fires and its RPM rises the timing is set back to 25° for normal operation (between 500 and 2700 RPM). The moment the engine fires and runs idle the impulse coupling detaches and timing is reset to 25° BTDC
On some engines a vibrating system is used to create a shower of sparks with the left magneto when starting during these low RPM operations.