Top Dead Center, or TDC, is the furthest position of piston travel within the combustion chamber to what is considered the “top” of the combustion chamber. The magneto alignment to the engine synchronizes the spark generated by the magneto to be delivered to the combustion chamber at the correct moment of the combustion cycle of a particular cylinder. When installed on an engine, the magneto is physically connected, or “timed” to the rotation of the engine crankshaft. Internal timing is the alignment of the various moving internal components of the magneto to ensure that the spark generated by the magneto is of the maximum intensity and discharged at the correct magneto to engine timing position to ignite the fuel mixture in the combustion chamber. The magneto must be “timed” to deliver a spark so that the process of combustion, which occurs over “time”, can occur as the piston travels within the cylinder. To do this, the “Internal” and “External” timing of the magneto must be exactly correct. These combustion pressures push against the piston to drive the crankshaft in a forward direction to turn the propeller.įor the combustion process to begin the magneto must produce a high energy spark at the exact right moment of time relative to piston travel. At the peak point of piston travel, the fuel is completely burned up, and thermal energy is released resulting in high combustion pressures in the cylinder. The combustion process begins before the piston is at the point of maximum travel within the cylinder and continues to burn until the piston reaches its peak point of travel. While seemingly an instant explosion, the combustion occurs over a period of time relative to the travel of the piston within the cylinder. Piston travel is accomplished via crankshaft rotation of all of the connected engine parts to push the piston. Magneto timing to the engine is directly based on the “time” of when and where the piston is located during its travel within the cylinder relative to the combustion chamber. A basic understanding of the basics is critical to the maintenance and ongoing safe operation of the aircraft ignition system. However, Kelly Aero Product Support sometimes finds that not all Kelly Aero customers completely understand the specific terms or the interaction of these terms with one another. These terms are basic knowledge to aircraft technicians and educated owners. The sum total of all of these terms becomes what is broadly referred to as Magneto Timing. However, the word “timing” as applied to magnetos are a set of terms that means one thing, and many things, depending upon the context of the terms: Internal Timing, External Timing, and Advance Timing are all examples of individual timing terms. “Timing” is equally a simple and extremely complicated term relative to the installation and operation of magnetos. By Harry Fenton, Director of Business Development and Product Support
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