Ever wondered why, when you pull the mixture to shutdown your aircraft engine, that, as the engine slows down a rather loud clattering noise is heard being emitted from the engine compartment until the propeller finally stops?
Well, that noise emanates from a device called an impulse coupling which is attached to one or both magnetos and is used to retard the ignition timing to close to top dead center (TDC) to assist with starting the engine… but more about that later.
Pictured above, is an example of a Slick Magneto. The purpose of the magneto is to generate an electrical spark of sufficient intensity to ignite the fuel/air mixture in the cylinder(s) at the correct time, so as to allow the engine to produce maximum engine performance and all this is done without any reliance on the aircraft's electrical system at all.
The spark delivered by a magneto is quite powerful, the voltage is in the region of 10kV initially and this drops to 1kV once the arc is established for normal firing. The coil in the magneto can produce up to a 20kV spark if necessary for very rich or lean conditions or for high-pressure mixtures produced by high compression ratios or turbocharging. Inside the cylinder, the aircraft sparkplug gap is small when compared to an automotive application, usually 0.016” to 0.021”. The greater the sparkplug gap, or the greater the pressure in the cylinder; the greater the voltage required to jump the gap, which translates to better ignition of the fuel/air mixture, but as usual, everything comes at a price.
The larger the sparkplug gap, the higher the stress on the high voltage components in the ignition system. Most experts recommend gapping your plugs often and on the smaller end of the range to reduce stress on the magneto and wear of the sparkplug electrodes.
So how does a magneto work?
A magnet which is attached or cast into the rotor of the magneto is driven by the engine and rotates such that alternating lines of magnetic flux pass through an electric coil known as the primary winding. These alternating lines of magnetic force produce a wave-like flow of electrical current in the primary winding. The faster the magnetic field moves through the coil, the higher the crest of the wave. The voltage produced in the primary winding is used to induce electrical flux into a second coil with about 100 times the number of windings of the primary coil. As a result the secondary coil is capable of producing much higher voltages than the primary coil; in fact about 100 times higher, or about 20,000 volts. (20kV)
In order to produce this kind of voltage efficiently, the energy in the primary coil must be transferred to the secondary coil at just the right time. The transfer is accomplished by interrupting (collapsing) the circuit in the primary coil just as the wave of electrical current reaches its peak. This interruption of electrical current in the primary coil forces the energy into the secondary coil via the inductive coupling. The interruption of current is accomplished by opening a set of contact-breaker points that are driven by a cam on the shaft of the rotor.
To prevent an arc jumping across the 'points' as they are opened, a capacitor is connected to the points in parallel; this capacitor is a storage device that stores the energy from the coil just long enough to allow the points gap to reach an opening that is too great for the primary coil (low voltage) to spark across. Without this capacitor, the magneto would not work properly because the sparking would be taking place at the points rather than at the sparkplug.
To maximize the voltage potential in the secondary coil, the rapidly changing (building) magnetic flux in the primary coil needs to be interrupted at the point in time when the flux is at its peak, or maximum rate of change, so the points need to open at just the right position during the rotation of the rotor to induce the collapse and maximize output. This is known as the “E-gap.” (Electrical gap or Efficiency gap)
The newer Slick 4300 and 6300 magnetos use a T-150 tool to set E gap. The tool locks the rotor in the exact position that the points should open.
There are two internal adjustments that must be set correctly for a magneto to operate properly: point gap and "E-gap". The point gap should be set first. To do this, the drive shaft of the magneto is rotated to the position at which the cam has opened the breaker points to the maximum extent. Then the point gap is measured with an ordinary wire-type feeler gauge. The points are then adjusted for the specified gap; refer to manufacturer's recommendations. Once the point gap is correct, the E-gap can be set. First, rotate the rotor slowly until you can feel a "magnetic detent." This is known as the "neutral position" of the rotor. Now, with a magneto timing box attached across the breaker points, rotate the magneto until the points just start to open. The number of degrees of rotation from neutral to point opening is called the E-gap and needs to be set to a specified value (refer to manufacturer's recommendation) so that the points open exactly when magnetic field induced in the coil by the rotor is at its maximum.
The correct adjustment of the E-gap is crucial to producing an energetic spark. If the E-gap is not set properly a poor spark or no spark at all may result. Also as the points erode and the actuator arm wears the gap changes and needs to be readjusted to produce the best possible spark.
Now that the E-gap is properly set, we need to set the engine timing properly for best operation. Normal engine timing is set to ignite the fuel/air mixture at about 25 degrees before top dead center (BTDC, again refer to manufacturer's recommendations.) The fuel/air mixture is ignited before TDC so as to ensure that maximum pressure on the piston is applied after the piston passes top dead center on the compression stroke and begins the power stroke. The magneto needs to be timed such that the spark plug fires early enough to accomplish this. The magneto rotor is positioned using timing marks typically located on the gears in the magneto, so that the points are just beginning to open. Some magnetos allow a pin to be inserted through the housing and gears to lock the rotor in the proper position. The engine is then set to the proper position and the magneto is inserted into the accessory case on the engine and is engaged with the drive gear or tang on the rotor. At this point, the timing is pretty close to being perfect. It may be slightly off though due to play in the gears. This can be checked by using a timing light or timing buzzer, and a fine adjustment can be made by first removing the locking pin (if fitted) and rotating the magneto housing very slightly in one direction or the other to get the timing spot-on.
Over time the points and the points actuator cam/ramp wear resulting in a change in the E-gap as well as in the timing of the engine. Resetting the magneto timing will ensure a spark at the correct position BTDC, but not necessarily an efficient spark, as the changes in the E-gap reduces the energy in the spark. Slick allows up to a 5 degree correction in timing by “Bumping the mag” but recommends resetting the E-gap if more correction than this is required.
Back to our impulse coupler... The unit has pawls that contact a stud or dowel pin in the magneto body and stop the magneto from rotating as the engine is turned by hand or by the starter motor. This energy is stored in a coil spring and at about TDC the pawls slip and the stored energy in the coil spring rotates the magneto at a high speed; producing a large spark to ignite the fuel.
Due to the fact that the spark is delivered at or close to TDC, the engine cannot kickback and the ignited fuel/air mixture will drive the crankshaft in the correct direction of rotation. Once above 350 to 500 RPM, centrifugal force keeps the pawls disengaged and the magneto delivers the spark to all cylinders at the correct ignition advance point.
Remember, a magneto is a self-contained source of electrical energy and the only way to “kill” a magneto, is to short out (ground) the primary coil. If your grounding wire were to break, the magneto would be live even with the key/switch in the off position.
For this reason ALWAYS
consider a propeller to be “live” and treat with respect.
Keep it safe out there.
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