Prerequisite Knowledge:
A piston engine needs three things to function: gas, air, and a spark. This article will examine different systems in place that can give an engine its spark.
Spark Plugs
Spark plugs are essential to the operation of a piston engine, without them there would be nothing to ignite the fuel/air mixture in the combustion chamber of a cylinder. The engine in your car generally only utilizes one spark plug per cylinder, but aircraft piston engines generally use two. Not only does using two spark plugs per cylinder instead of one add redundancy in case of a spark plug failure, but it also increases the efficiency of the engine. By creating two sparks in different spots within a cylinder, less of the fuel/air mixture is left unburnt. This more complete combustion increases the power outputted by the engine.
Spark Plug Fouling
Over time, lead and carbon deposits can slowly build up in the engine's cylinders, as well as the spark plugs. These deposits "foul" the spark plugs, severely hindering their reliability and efficiency when the engine is running. In a cylinder with two spark plugs, the bottom spark plug will generally be the one to foul the fastest, as the deposits will generally settle on the bottom of the cylinder.
Spark plug fouling can be avoided by properly leaning the engine, and using the right type of fuel for the engine.
A piston engine needs three things to function: gas, air, and a spark. This article will examine different systems in place that can give an engine its spark.
Spark Plugs
Spark plugs are essential to the operation of a piston engine, without them there would be nothing to ignite the fuel/air mixture in the combustion chamber of a cylinder. The engine in your car generally only utilizes one spark plug per cylinder, but aircraft piston engines generally use two. Not only does using two spark plugs per cylinder instead of one add redundancy in case of a spark plug failure, but it also increases the efficiency of the engine. By creating two sparks in different spots within a cylinder, less of the fuel/air mixture is left unburnt. This more complete combustion increases the power outputted by the engine.
Spark Plug Fouling
Over time, lead and carbon deposits can slowly build up in the engine's cylinders, as well as the spark plugs. These deposits "foul" the spark plugs, severely hindering their reliability and efficiency when the engine is running. In a cylinder with two spark plugs, the bottom spark plug will generally be the one to foul the fastest, as the deposits will generally settle on the bottom of the cylinder.
Spark plug fouling can be avoided by properly leaning the engine, and using the right type of fuel for the engine.
Ignition System Types
There are several ways spark plugs can get the electricity needed to create a spark.
Magnetos
There are several ways spark plugs can get the electricity needed to create a spark.
Magnetos
Magnetos are devices that use the engine's rotation to generate the current needed for the spark plugs. Magnetos are spun by the engine, and they act like small generators, creating the high voltage electricity that is sent to the spark plugs, via the spark plug wires. Whenever the crankshaft is turning, the magnetos are being spun, and the magnetos are not grounded (see below) current is being sent to the spark plugs.
Generally, aircraft have two magnetos per engine, with each magneto powering half of the total spark plugs in the engine. In every cylinder, one spark plug is powered by one magneto, whereas the second spark plug is powered by the other magneto.
Generally, aircraft have two magnetos per engine, with each magneto powering half of the total spark plugs in the engine. In every cylinder, one spark plug is powered by one magneto, whereas the second spark plug is powered by the other magneto.
In light general aviation aircraft, the magnetos are controlled by a key or switch. Below is a picture of the magneto switch of the Cessna 172:
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- Both: Both magnetos are ungrounded, and they both power the engine. All spark plugs are powered. "Both" is generally used for all normal operations.
- Start: Same function as "Both", but the engine starter is also activated. This is generally spring-release, meaning the key flips back to "Both" when you let go (a lot like starting your car).
When operating on a single magneto, a small loss of power occurs. This is normal when operating on a single magneto, as only a single spark plug is firing in each cylinder (which results in incomplete combustion of the fuel/air mixture inside the combustion chamber).
Electrical Ignition
A second, relatively newer ignition system, is known as electrical ignition. This system has become more and more popular in recent years, due to it holding several advantages over magnetos.
The electrical ignition system does not create power directly from the motion of the engine, but rather it draws electricity from the aircraft's battery, and uses that to power the spark plugs. Even when electrical ignition systems became more common in cars, magnetos were still preferred due to their redundancy and closed system design. A magneto is completely independent of the aircraft's electrical system; it creates it's own power straight from the turning of the engine crankshaft. This means that if your aircraft battery dies in flight, the magnetos will still be able to power the spark plugs and keep the engine running. With an electrical ignition system, this would not be possible, as they draw their power from the aircraft battery. This meant that if your battery died in flight, your engine would also fail. However, electrical ignition systems have become significantly more reliable as of recently, and they have their own built in redundancies to eliminate that risk.
Electrical ignition systems are also more efficient than magneto ignition systems. With a magneto system, the spark plugs fire at the exact same point in the four stroke cycle, no matter what the power setting is. This is not ideal, as the spark plugs should fire earlier at higher power settings for added efficiency. With electrical ignition systems, the point at which the spark plugs fire can be varied, so they are always sparking at the most ideal point.
That's it for ignition! The next article will cover fuel systems.