The function of the spark plug in an ignition system is to conduct a short impulse of high-voltage current through the wall of the combustion chamber. Inside the combustion chamber, it provides an air gap across which the impulse can produce an electric spark to ignite the fuel/air charge. While the aircraft spark plug is simple in construction and operation, it can be the cause of malfunctions in aircraft engines. Despite this fact, spark plugs provide a great deal of trouble-free operation when properly maintained and when correct engine operating procedures are practiced.

Spark plugs operate at extreme temperatures, electrical pressures, and very high cylinder pressures. A cylinder of an engine operating at 2,100 rpm must produce approximately 17 separate and distinct high-voltage sparks that bridge the air gap of a single spark plug each second. This would appear as a continuous spark across the spark plug electrodes at temperatures of over 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 electrical pressure as high as 20,000 volts. Given the extremes that spark plugs must operate under, and the fact that the engine loses power if one spark does not occur correctly, proper function of a spark plug in the operation of the engine is imperative.

The three main components of a spark plug are the electrode, insulator, and outer shell. [Figure 4-33] The outer shell, threaded to fit into the cylinder, is usually made of finely machined steel and is often plated to prevent corrosion from engine gases and possible thread seizure. Close-tolerance screw threads and a copper gasket prevent cylinder gas pressure from escaping around the plug. Pressure that might escape through the plug is retained by inner seals between the outer metal shell and the insulator, and between the insulator and the center electrode assembly. The other end is threaded to receive the ignition lead from the magneto. All-weather plugs form a seal between the lead and the plug that is water proof to prevent moisture from entering this connection.

Figure 4-33. Spark plug cutaway.

Figure 4-33. Spark plug cutaway.

The insulator provides a protective core around the electrode. In addition to affording electrical insulation, the ceramic insulator core also transfers heat from the ceramic tip, or nose, to the cylinder. The insulator is made from aluminum oxide ceramic having excellent dielectric strength, high mechanical strength, and thermal conductivity. The types of spark plugs used in different engines vary in respect to heat range, reach, massive electrode, fine wire electrode (Iridium/platinum), or other characteristics of the installation requirements for different engines.

Figure 4-34. Fine wire electrodes.

Figure 4-34. Fine wire electrodes.

The electrodes can be of several designs from massive electrodes or Nickel-base alloy to fine wire electrodes. [Figure 4-33 and 4-34] The massive electrode material has a lower melting point and is more susceptible to corrosion. The main differences include cost and length of service. Fine wire iridium and platinum electrodes have a very high melting point and are considered precious metals. Therefore, the cost of this type of spark plug is higher, but they have a longer service life with increased performance. Fine wire spark plugs are more effective than massive electrode plugs because the size shields its own spark from some of the fuel air mixture. Less than efficient combustion occurs due to uneven ignition. The iridium electrode allows for a larger spark gap, which creates a more intense spark that increases performance. The spark gap of any electrode is vulnerable to erosion and the melting point of the electrode material.

Figure 4-35. Hot and cold spark plugs.

Figure 4-35. Hot and cold spark plugs.

The heat range of a spark plug is a measure of its ability to transfer the heat of combustion to the cylinder head. The plug must operate hot enough to burn off carbon deposits, which can cause fouling, a condition where the plug no longer produces a spark across the electrodes, yet remain cool enough to prevent a preignition condition. Spark plug preignition is caused by plug electrodes glowing red hot as a glow plug, setting off the fuel-air mixture before the normal firing position. The length of the nose core is the principal factor in establishing the plug’s heat range. [Figure 4-35] Hot plugs have a long insulator nose that creates a long heat transfer path; cold plugs have a relatively short insulator to provide a rapid transfer of heat to the cylinder head. [Figure 4-35]

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 be designed to operate as hot as possible at slow speeds and light loads, and as cool as possible at cruise and takeoff power.

The choice of spark plugs to be used in a specific aircraft engine is determined by the engine manufacturer after extensive tests. When an engine is certificated to use hot or cold spark plugs, the plug used is determined by the compression ratio, the degree of supercharging, and how the engine is to be operated. High-compression engines tend to use colder range plugs while low-compression engines tend to use hot range plugs.

Figure 4-36. Spark plug reach.

Figure 4-36. Spark plug reach.

A spark plug with the proper reach ensures that the electrode end inside the cylinder is in the best position to achieve ignition. The spark plug reach is the length of the threaded portion that is inserted in the spark plug bushing of the cylinder. [Figure 4-36] Spark plug seizure and/or improper combustion within the cylinder can occur if a plug with the wrong reach is used. In extreme cases, if the reach is too long, the plug may contact a piston or valve and damage the engine. If the plug threads are too long, they extend into the combustion chamber and carbon adheres to the threads making it almost impossible to remove the plug. This can also be a source of preignition. Heat of combustion can make some of the carbon a source for ignition, which can ignite the fuel-air mixture prematurely. It is very important to select the approved spark plugs for the engine.

The FADEC System – Low-Tension Retard Breaker Vibrator

Engine Ignition and Electrical Systems

This system, which is in limited use, is designed for light aircraft reciprocating engines. A typical system consists of a retard breaker magneto, a single breaker magneto, a starting vibrator, transformer coils, and a starter and ignition switch. [Figure 4-32] To operate the system, place the starter switch S3 in the on position. This energizes […]

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The FADEC System – High-Tension Retard Breaker Vibrator

Engine Ignition and Electrical Systems

To provide for more spark power during the starting cycle, the shower of sparks system was developed, which provides several sparks at the spark plug electrodes during starting. The starting vibrator, or shower of sparks, consists essentially of an electrically operated vibrator, a condenser, and a relay. [Figure 4-31] These units are mounted on a […]

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The FADEC System – Impulse Coupling

Engine Ignition and Electrical Systems

Many opposed reciprocating engines are equipped with an impulse coupling as the auxiliary starting system. An impulse coupling gives one of the magnetos attached to the engine, generally the left, a brief acceleration, that produces an intense spark for starting. This device consists of a cam and flyweight assembly, spring, and a body assembly. [Figure […]

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The FADEC System – Booster Coil

Engine Ignition and Electrical Systems

The booster coil assembly, used mainly with older radial engine ignition systems, consists of two coils wound on a soft iron core, a set of contact points, and a condenser. [Figure 4-25] The booster coil is separate from the magneto and can generate a series of sparks on its own. During the start cycle, these […]

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The FADEC System – PowerLink Ignition System

Engine Ignition and Electrical Systems

The ignition system consists of the high-voltage coils atop the ECU, the high-voltage harness, and spark plugs. Since there are two spark plugs per cylinder on all engines, a six-cylinder engine has 12 leads and 12 spark plugs. One end of each lead on the high-voltage harness attaches to a spark plug, and the other […]

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The FADEC System – Electronic Control Unit (ECU)

Engine Ignition and Electrical Systems

An ECU is assigned to a pair of engine cylinders. [Figure 4-23] The ECUs control the fuel mixture and spark timing for their respective engine cylinders; ECU 1 controls opposing cylinders 1 and 2, ECU 2 controls cylinders 3 and 4, and ECU 3 controls cylinders 5 and 6. Each ECU is divided into upper […]

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The FADEC System – Low-Voltage Harness

Engine Ignition and Electrical Systems

The low-voltage harness connects all essential components of the FADEC System. [Figure 4-22] This harness acts as a signal transfer bus interconnecting the electronic control units (ECUs) with aircraft power sources, the ignition switch, speed sensor assembly (SSA), temperature and pressure sensors. The fuel injector coils and all sensors, except the SSA and fuel pressure […]

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FADEC System Description

Engine Ignition and Electrical Systems

A FADEC is a solid-state digital electronic ignition and electronic sequential port fuel injection system with only one moving part that consists of the opening and closing of the fuel injector. FADEC continuously monitors and controls ignition, timing, and fuel mixture/delivery/injection, and spark ignition as an integrated control system. FADEC monitors engine operating conditions (crankshaft […]

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