The bottom line is this, the two well-known brands of electronic ignition systems for experimental aircraft are inherently flawed in their design and are both prone to generating kickbacks due to low voltage input.  Be aware that the Sky-Tec Factory Warranty does not cover damage done to a starter by a kickback.

The Problems

Voltage Drop:  When cranking (starting) an engine with an electronic ignition system installed, if a marginal voltage supply situation exists (low battery, weak or loose cabling, poor grounds, etc.), the buss voltage can drop dramatically as the engine approaches the top of a compression stroke.  This voltage drop is the result of the starter input current increasing as the workload (approaching compression) increases.  While this effect is true of all cranking motors, permanent magnet motor technologies can magnify the situation as their design (although powerful and extremely lightweight) can be 20%-30% more current hungry at very low rotational speeds such as initial inrush and/or stalling conditions.  As the buss voltage drops, the amount of voltage available to the electronic ignition my dip below the  minimum level required to properly calculate timing and generate appropriately-timed sparks.

Timing:  If an aircraft's electrical system is weak enough, the engine might actually rotate in a reverse direction after failing to reach the top of a compression stroke.  Neither of the two well-known brands of electronic ignition systems understand this "bounce-back" problem and neither is capable of understanding that the propeller's direction of rotation has actually reversed as both systems simply count time from an index point using Hall-type sensors.  Because of this limited/flawed design it is possible for both systems' sensors to misinterpret engine timing because again, neither are capable of determining the actual LOCATION of the crank/pistons - only the AMOUNT OF TIME that passes between index indicators.   Either system is capable of initiating spark in a cylinder with an inappropriately located piston (before TDC) possibly resulting in a kickback.

Fixes

Voltage Drop:  There are two commonly used solutions for dealing with the voltage drop issue.  Those are represented by the diagrams below.

In either case, you must choose a diode that will handle the current required by the ignition system (5A?).  The diode prevents a downward spike on the buss from being reflected on the input of the ignition system.  The battery shown in #1 can be an old computer (PC) UPS battery.  They work well in this application.  In #2, the capacitor may need to be very large, perhaps 50,000 mfd @ 20 volts or more.  

Timing:  No fix is known for either EI solution at this time.  And any possible fix would need to be instituted by the manufacturer(s).  

Punt:  If an impulse-coupled magneto is available on one side of the engine, simply pull the circuit breaker to the electronic ignition and put it in when the engine has started.

A Better Mousetrap?  We at Sky-Tec are intrigued by an all new EI design introduced to the experimental aircraft industry at Oshkosh 2004.  The company is called E-MAG Ignitions (short for Electronic Magneto) and their product is just that:  a self-contained (and even self-powered) electronic ignition solution that is as simple to install as removing an old magneto and replacing it with an eMag.  The only additional hook-up is a 12v power source.  Most important to its design (in our opinion) is the use of an optical crank position sensor (not a Hall-type, time-based sensor), a backup capacitor to minimize momentary voltage drops, fail-safe start-up cranking-specific ignition logic that locks-out all spark generation before TDC during start-up, and a low-voltage resolution that also locks-out sparks if all the other safeguards above should fail.

Sky-Tec has no relationship with E-MAG Ignitions other than the fact that they went out of their way during initial R&D to contact Sky-Tec to ascertain our ideas and opinions of how a new EI solution could/should function with regard to engine cranking.  We were flattered and grateful when we saw most of our ideas incorporated in their innovative design.  We hope to visit their operation periodically and monitor their eventual success in this market.  And, because of the multitude of starter-friendly cranking functions included in the E-MAG product, Sky-Tec will extend its standard warranty to include damage from kickbacks within the first two years of operation should the starter be properly installed with a properly-installed and maintained E-MAG ignition system (dual or single installed w/a properly functioning impulse coupler magneto).  We are that confident in how the E-MAG has been designed.

In short, while any ignition system is prone to failures and mishaps, the two most popular systems available seem especially prone to generating kickbacks. Possible remedies include disengaging the electronic ignition during engine starting, installing a secondary supply of voltage to the electronic ignition during engine starting, or utilizing an electronic ignition solution that understands the cranking process and is better designed to eliminate such errors.

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