Magnetos Under Pressure
By Harry Fenton
One of the fundamental problems with turbocharged piston engines that operate at high altitudes is that the ignition system requires special design features to compensate for the lack of air density, due to lower ambient pressure, encountered at high altitudes. One special feature on turbocharged engines to allow them to perform at altitude is pressurized magnetos. Let's take a look at how they work.
An electrical arc across the spark plug initiates each combustion event in the engine cylinder. The source of the high voltage required to start the electrical discharge across the spark plug gap is the ignition coil. The ignition coil must not only supply enough breakdown voltage to form an arc at the spark plug, but must also provide enough breakdown voltage across all of the other "gaps" in the ignition system. For an aircraft magneto, there are typically two "air gaps" that the ignition coil must arc across. One is the well-known gap between the spark plug electrodes and the other is the air gap inside the magneto between the distributor gear electrode and distributor block electrodes.
A typical normally aspirated aircraft engine operating at sea level and takeoff power may require 10 kV (10,000 volts) to breakdown the plug gap and 1 kV (1,000 volts) to breakdown the gap between the distributor gear and distributor block electrode. The total voltage needed from the ignition coil to make a spark at the spark plug is therefore 11kV. Once an arc is formed across the spark plug gap, the spark continues as long as current from the ignition coil continues to flow through the all of the air gaps in the system. Immediately after the plug gap breaks down and current flow begins, the plug gap voltage drops from the initial 10 kV to about 1 kV. There is a similar decrease in the voltage required to sustain the arc between the distributor gear and distributor block electrodes. So, once the air gaps break down and the spark plug begins to fire, the coil only needs to supply about 1 kV total to sustain the arc across the spark plug gap.
Concerning how long each of these events takes place, the ignition coil operates at the relatively high voltage (11 kV) for only a small amount of time (0.5 percent of the duration of the arc) and operates at the lower voltage for 99.5 percent of the time. Unfortunately, even though the magneto operates almost all of the time at the lower voltages, it must work reliably at the higher voltages needed to initiate the spark. The physical magneto design must therefore accommodate this highest voltage, and this subsequently drives the magneto's outside dimensions and weight.
As the air density and ambient atmospheric pressure decreases with altitude, the voltage required to breakdown the distributor gap and any other high voltage conductor spacing in the magneto, drops. For any given throttle setting in a normally aspirated engine, the cylinder pressure at the time of the ignition spark event also drops due to the reduced manifold pressure caused by lower atmospheric pressure at higher altitudes. The result is less voltage is required to fire the spark plug. When an engine is turbocharged, however, the spark plug voltage demand remains relatively high (near sea level conditions) because the air pressure in the engine cylinder does not decrease with altitude. This requires that the magnetos produce the same high voltages at high altitude as at sea level.
The techniques and opinions on how to start aircraft engines are as varied as the airplanes, engines, and operators that encompass this topic.
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