Sparking Respect: Unique facts about the aviation spark plug

Sparking Respect Unique facts about the aviation spark plug By Harry Fenton and Dick Podiak September 1999 Imagine this engineering project: Design an electromechanical device for the aircraft engine that will spark between 90 to 150...


Finally, the metal to metal connection between the ignition harness shielding, the ignition harness connector, and the spark plug provides a reliable electrical ground to suppress electromagnetic interference (EMI) generated by the ignition spark at the firing tip of the spark plug. Suppressing EMI is important. If unchecked, it can cause interference or "noise" in the aircraft's communication and navigation radios.

As a rule of thumb, the 5/8-24 connector is used on most normally aspirated engines that operate at less than 10,000 feet of altitude. The 3/4-20 harness, commonly referred to as a high altitude or all-weather harness due to its superior sealing characteristics, is used on virtually all turbocharged engines or normally aspirated engines that routinely operate at altitudes of 10,000 feet or higher. The 5/8-24 harness nut is installed using a 3/4-in. wrench, and the 3/4-20 harness nut is installed with a 7/8-in. wrench.

Another signature characteristic of aviation spark plugs is the electrode configuration. For the most part, automotive spark plugs use a single electrode design, while most aviation spark plugs favor a more complex, multi-electrode configuration.

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Contemporary electrode design features either a two-prong massive electrode configuration or a single, fine wire iridium electrode design. The fine wire, iridium plugs use a single electrode for a number of reasons. First, iridium is an expensive material, which dictates conservative use of the material. Second, the durability and resistance to lead corrosion of the iridium is excellent, which offsets the need for the second electrode to guard against fouling and to promote long plug life.

For the most part, aviation spark plugs feature an 18mm mounting thread diameter, although some vintage and low volume production engines can use a 14mm plug. The base thread size is a result of the need to have a large surface area at the electrode end to achieve scavenging, provide adequate surface area for heat transfer to cool the plug, and to accommodate the multi-electrode design.

The length of the mounting threads that engage the plug into the cylinder head define the reach of the plug. The reach of the plug is carefully engineered to optimally place the electrodes in the combustion chamber, to provide adequate cooling and scavenging. Dimensionally, an 18mm aviation spark plug with 13/16-in. of mounting thread is defined as a long reach plug, and an 18mm plug with 3/4-in. of mounting thread is considered a short reach.

However, some plugs suffer an identity crisis. For instance, a plug common to Lycoming four-cylinder engines features an extended nose insulator with long electrodes designed to resist fouling and improve the overall lead scavenging qualities of the plug while having the 3/4-in. or short reach threads. This design places the discharge end of the plug closer to the flame front to improve the anti-fouling and lead scavenging qualities of the plug. This extended insulator plug is sometimes incorrectly referred to as long reach, when in fact, the mounting thread is short reach.

The spark plug electrode end faces the harshest operating conditions in the engine. Each spark event displaces a minute amount of electrode material, while the combustion process generates a massive release of heat in excess of 3,000 F, and releases a cloud of harsh chemical byproducts such as carbon and lead salt deposits. Once the combustion process has been completed, the tip of the plug must cool, so that the entire process can continue. The discharge, chemical exposure, and cooling cycle all occur within an instant — 20 to 30 times each second — and each event has a degrading effect on the spark plug electrode. Accordingly, this hostile environment drives the design of the electrode and insulator.

The center electrode material typically is a composite of copper and nickel alloy materials, which provides excellent electrical conductivity, as well as good heat transfer qualities and resistance to wear. The side, or ground, electrodes are usually a high percentage nickel l alloy material.

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