Why Aircraft Feature Two Spark Plugs Per Cylinder?

A majority of aircraft rely on the combustion of fuel-air mixtures to create the propulsion necessary for flight, and the process of ignition is very controlled and regulated to ensure proper performance and operations. Many aircraft ignition systems take advantage of spark plugs for combustion, often coming in pairs for each cylinder unlike the standard one spark plug configuration found in automobiles. The presence of dual spark plugs is extremely important, however, ensuring reliability, power, and even combustion.

In the instance that an aircraft engine featured only one spark plug per cylinder, it would be imperative that that plug and the magneto maintained operability throughout the entire operation. If the magneto were to fail, the ignition system would halt, resulting in a loss of engine power. This would quickly spell danger as the aircraft would either slowly descend to the ground or outright drop. Beyond such dangers, the failure of a single spark plug configuration in a 4-cylinder engine would mean that the aircraft would lose 25% of its total power. While a somewhat low loss, such a decrease in power could mean losing the ability to maintain a set altitude. With two spark plugs for burning a fuel-air mixture, redundancy is maintained for the means of safety.

The presence of two spark plugs also can have a major effect on power, creating a twin flame front within the cylinder as they fire simultaneously. Rather than having a single side ignited, two spark plugs will ignite the mixture on opposite sides of the cylinder. This has two major effects on the properties of combustion, one being that the mixture would burn more quickly. Additionally, mixtures would also burn more completely with a dual set-up. When such operational characteristics are upheld, more power for the aircraft may be achieved which is always beneficial.

Another benefit of having two spark plugs per cylinder is for creating a more even combustion process. With a single spark plug, fuel-air mixtures would burn top to bottom from the center, slowly moving outward. With two spark plugs placed on opposite sides of the cylinder, fuel can be burned from the edges moving inward. This can ensure that hot-spots are well avoided, and pressure will be more even across the cylinder. Maintaining even pressure distribution is highly important for high-compression, air-cooled engines that are widely used across general aviation aircraft.

The last major benefit of a dual spark plug configuration is related to spark plug fouling. In general, the Avgas that is popularly used by many aircraft contains lead, TEL being used specifically. TEL is important to prevent engine knocking which is when pockets of fuel-air mixtures are uncontrollably detonated within the cylinder. Knocking is quite detrimental, raising the temperature and pressure of the detonation area. While knocking can be deterred with TEL, the resulting build up of lead deposits on spark plugs from incorrect vaporization can lead to misfiring. If one spark plug begins to foul, having a redundant plug will ensure continued operations.

With the various benefits that two spark plugs bring per cylinder, it is important that they are well maintained and replaced as necessary. Parts Needed Yesterday is an online distributor of aircraft parts, offering top quality spark plug and ignition system components that have been sourced from leading global manufacturers that we trust. Take the time to fully explore our offerings, and our team of industry experts is always on standby to provide you personalized quotes for your comparisons. If you have any questions regarding our services or are ready to initiate the purchasing process, give our team members a call or email at your earliest convenience, and they would be more than happy to assist you however necessary.


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