The function of a spark plug in an ignition system is to conduct a short impulse of high-voltage current through the combustion chamber wall. Within the combustion chamber, the spark plug provides an air gap across which the electrical impulse produces a spark to ignite the air-fuel mixture.
Although aircraft spark plugs are relatively simple in construction and operation, they can be a source of engine malfunctions. However, when properly maintained and operated under recommended engine operating procedures, spark plugs can provide long periods of reliable, trouble-free service.
Operating Environment
Spark plugs operate under extreme temperatures, high voltages, and very high cylinder pressures. In a four-stroke engine operating at 2,100 rpm, each spark plug must produce approximately 17.5 high-voltage sparks per second to ignite the air-fuel mixture.
At this firing rate, the individual sparks appear almost continuous, while the spark plug is exposed to combustion temperatures exceeding 3,000 °F. At the same time, the spark plug is subjected to gas pressures as high as 2,000 pounds per square inch (psi) and ignition voltages of up to 20,000 volts. Given these severe operating conditions and the potential for power loss if ignition does not occur correctly, proper spark plug operation is essential for reliable engine performance.
Construction and Components
The three main components of a spark plug are the electrode, insulator, and outer shell. [Figure 1]
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| Figure 1. Spark plug cutaway |
The outer shell is threaded to fit into the cylinder head and is usually made of finely machined steel, often with a protective plating to resist corrosion and help prevent thread seizure. Close-tolerance threads and a copper gasket prevent combustion gases from escaping around the spark plug. Internal seals between the outer metal shell and the insulator, and between the insulator and the center electrode assembly, prevent gases from leaking through the plug.
The outer end of the spark plug is threaded to receive the ignition lead from the magneto. All-weather spark plugs form a waterproof seal between the ignition lead and the plug to prevent moisture from entering the connection.
Spark Plug Insulator
The insulator provides a protective core around the electrode. In addition to providing electrical insulation, the ceramic insulator core transfers heat from the ceramic tip, or nose, to the cylinder head.
The insulator is made from an aluminum oxide ceramic that has excellent dielectric strength, high mechanical strength, and good thermal conductivity. Spark plugs used in different engines vary in heat range, reach, electrode design, and other characteristics required for the specific installation.
Massive vs. Fine-Wire Electrodes
Spark plug electrodes are available in several designs, ranging from massive electrodes made of nickel-based alloys to fine-wire electrodes made of materials such as iridium or platinum. [Figures 1 and 2]
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| Figure 2. Fine wire electrodes |
Compared with fine-wire electrodes made of iridium or platinum, conventional massive-electrode materials generally have a lower melting point and may be more susceptible to erosion and corrosion. The main differences between massive-electrode and fine-wire spark plugs include cost, service life, and performance.
Iridium and platinum are precious metals with very high melting points, making them well suited for fine-wire spark plug electrodes. Although fine-wire spark plugs generally cost more, they can provide longer service life and improved performance.
Fine-wire spark plugs can provide more effective ignition because their smaller electrodes expose the spark more directly to the fuel-air mixture. In comparison, the larger electrodes of massive-electrode plugs can partially shield the spark from the mixture, which may reduce ignition efficiency.
Iridium electrodes can also support a larger spark gap, producing a more intense spark that can improve ignition performance. The rate of electrode erosion and spark gap wear is influenced by operating conditions and the properties, including the melting point, of the electrode material.
Heat Range
The heat range of a spark plug is a measure of its ability to transfer heat from the combustion chamber to the cylinder head. The plug must operate at a temperature high enough to burn off carbon deposits that can cause fouling, yet remain cool enough to prevent pre-ignition. A fouled spark plug may fail to produce a spark across its electrodes.
Pre-ignition can occur when an overheated spark plug electrode or other hot spot becomes hot enough to ignite the air-fuel mixture before the normal ignition event. The length of the insulator nose is the principal factor in determining the spark plug’s heat range. [Figure 3]
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| Figure 3. Hot and cold spark plugs |
Hot plugs have a long insulator nose that creates a longer heat-transfer path, while cold plugs have a shorter insulator nose that transfers heat more rapidly to the cylinder head. [Figure 3]
If an engine were operated at only one speed, spark plug design would be greatly simplified. Because flight demands impose different loads on the engine, spark plugs must maintain a temperature high enough to resist fouling at low speeds and light loads while remaining cool enough to avoid overheating at cruise and takeoff power.
The choice of spark plugs for a specific aircraft engine is determined by the engine manufacturer after extensive testing. When an engine is certificated to use hot or cold spark plugs, the appropriate heat range is determined by factors such as the compression ratio, the degree of supercharging, and the engine's operating requirements. High-compression engines tend to use colder heat-range plugs, while low-compression engines tend to use hotter heat-range plugs.
Spark Plug Reach
A spark plug with the proper reach ensures that the electrode end inside the cylinder is correctly positioned for effective ignition. Spark plug reach is the length of the threaded portion that is inserted into the spark plug opening or bushing in the cylinder head. [Figure 4]
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| Figure 4. Spark plug reach |
Using a spark plug with the wrong reach can cause improper combustion, difficult removal, pre-ignition, or engine damage. In extreme cases, a spark plug with excessive reach may contact a piston or valve.
If the threaded portion extends too far into the combustion chamber, carbon deposits can accumulate on the exposed threads, making the spark plug difficult to remove. These deposits may also become hot enough to act as an ignition source and prematurely ignite the air-fuel mixture. Therefore, only spark plugs approved for the specific engine installation should be used.