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Today there are a couple of electronic ignition alternatives for certified airplanes and several other choices for experimental airplanes. Electronic ignitions can have higher energy output than a magneto and can also implement variable ignition timing. The magneto has fixed ignition timing, typically 20 to 25 degrees before Top Dead Center (TDC) of the cylinder. The magneto will retard timing to 0 deg only for starting. The ignition spark occurs in advance of the cylinder TDC so that the flame front will produce maximum cylinder pressure at the appropriate time in the cylinder cycle. A magneto set for 20 degree fixed timing will fire 5.6 ms (0.0056 sec) before TDC when the engine is idling at 600 rpm. Increase the engine speed to 2400 rpm and the 20 degree advanced spark occurs 1.4 ms before TDC. Electronic ignition accounts for this difference in time interval by further advancing spark timing based on engine rpm and manifold pressure. This keeps the peak cylinder pressure in better alignment with the cycle over various engine speeds.
When considering a particular brand of ignition system, here are some features to consider when comparing:
Continued Airworthiness – Slick magnetos require a 500 hr inspection. My last inspection cost $420. There is at least one electronic ignition manufacturer that requires a 100 hr inspection. Most others don’t. You don’t want to trade one required inspection for another.
Individual coil packs – All in one electronic ignitions that are the same size as a magneto may only have one coil. This coil has to fire every cylinder and will have no more power than a magneto. You can tell by the spark plug used with the magneto if it is higher power or not. This is explained in the next paragraph.
Spark Plugs – The low output from a magneto can only jump a spark plug gap of 0.016 to 0.019 inches. Higher output electronic ignitions can jump a 0.035 inch gap. Because standard massive electrode spark plugs can not be gapped to 0.035 inch, an automotive sparkplug or a Tempest high energy spark plug has to be used with the high output electronic ignitions.
Electronics mounted on the engine – Electronics mounted on an engine are subject to heat and vibration which may reduce reliability. Some electronic systems mount their controller on the cool side of the firewall. All in one magneto replacement systems have it on the engine. I’m not saying this is a showstopper but something to think about when comparing systems.
Timing Source – The electronic ignition must know crank shaft position to know when to fire the spark. It seems to be that about half of the systems on the market derive this timing from the engine magneto gear just as the magneto does. The other half of the systems derive timing from magnets mounted to the flywheel and a sensor mounted to the engine. This requires removing the propellor to drill the flywheel to mount the magnets for the crank position sensor.
Electronic Advance – There is at least one electronic ignition system that is a direct replacement for a magneto that has no ignition timing advance. I would pass on a system like that as there are all in one systems that have that feature.
That was a quick tour of what to look for in an electronic ignition system. I’ll write a future article telling you what I picked for my experimental RV-10 and how the installation went.