Reciprocating Engines and the Importance of Spark Plugs

Reciprocating aircraft engines are almost always fitted with independent, dual ignition systems, allowing for increased safety in terms of redundancy in the case of a system failure. With dual magneto configurations, the engine can even continue to run in the instance of the electrical system failing. As a result, new technology often continues this sense of redundancy, always ensuring that there is some form of backup to keep the engine running during a standard flight operation. In this blog, we will discuss the operations of a piston engine and the importance of spark plugs, allowing you to better understand how such systems function and are maintained.

For a piston engine, fuel-and-air mixtures are ignited and burned within a cylinder at a slow rate. During the power stroke of the piston, combustion gases expand and create a large amount of force. While heat drops, the mixture will continue to burn as it begins moving through the exhaust port upon the exhaust stroke. With the up and down movement of the piston during the combustion process, linear movement can be converted into rotating energy with the use of a crankshaft. As the crankshaft is attached to the propeller of the aircraft, the rotating energy allows for the propeller blades to spin and create the propulsion necessary for flight. While a seemingly complex procedure, the process of burning fuel-and-air mixtures for combustion is fairly simplistic as a whole.

In order to optimally ignite the fuel-and-air mixture within a cylinder, two flame fronts are established with the use of a dual spark plug configuration. Sparks are necessary for combustion, such electrical energy being used to ignite the mixture of fuel and air within the cylinder. As combustion is the driving force behind propeller operation, it is paramount that all spark plugs operate in peak condition. With standard use, combustion deposits of carbon and lead can build up over time, potentially spelling hazards if left unmitigated. To control the formation of such deposits, the operating temperature of the plug insulator core tip can be raised for deposit removal. As spark plug fouling is a common issue that can hinder the health of such components, one should avoid long periods of idling on the ground and in the air, and leaning should be carried out properly during flight.

The service life of a spark plug can be directly tied to the size of the electrode gap, and misfiring during idle operations is a clear sign of an issue. If the mixture is leaned and one is traveling at cruise power, misfiring may also occur. With increased instances of intermittent firing, plugs will begin to foul. As the fouling of spark plugs directly affects engine performance, it is important that such issues are avoided or remedied right away.

During scheduled inspections, such as annual and 100-hour inspections, a technician will pay special attention to the magnetos and spark plugs. Magnetos must be correctly timed and internally lubricated, and manufacturer recommendations will be set for magneto-to-engine timing. If the aircraft has been stored for a long period of time without use, there may be a need for magneto coil and spark plug replacement. For the overall health of the engine, electrical system, and other flight pertinent systems, battery components, bearings, exhaust system parts, bearings, and other related aerospace components should be regularly inspected as well.

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