The fuel air mixture in the combustion chamber needs to be ignited at the correct moment to ensure efficient combustion and power generation by the engine. This is the job of the ignition system, be that the old fashion magneto of the good old days or a modern fully electronic microprocessor controlled FADEC fuel injected system we see more and more each passing year. For obvious safety reasons the ignition system may not rely on the aircraft electrical system and must be dual and each system operates one of the two spark plugs in each cylinder.
The plain magneto ignition suffers from a number of problems which have long been resolved with vacuum, rpm and mechanical ignition advancing combined with memory mapped microprocessor controlled FADEC systems or even with sophisticated electronic ignition systems.
It still is quite amazing that aircraft piston engines are using these old school magnetos where developments in car engine has discontinued the use of magnetos years and years ago in favor for a much more reliable full electronic motor management.
Its time for aviation engine manufacturers to develop an ignition system capable of igniting avgas and other fuels under all circumstances with a higher reliability than what we are used to now. For a reasonable price of course.
With a traditional (classic, from the good old days) dual magneto ignition system the timing is set to 25° BTDC. During engine start the right magneto is grounded and the timing of the left magneto is retarded by the impulse coupling to TDC (0°). When the engines reaches 200 RPM the impulse coupling disconnects and timing falls back to 25° BTDC. When the ignition switch is released the right magneto also begins to fire. Ignition spark timing will be fixed from that point on for any RPM or altitude or any change in operating condition.
During ascend the air molecules get further apart (lower air density) and this slows the burning of the fuel/air mixture and as a result there will be a loss in engine power. Ignition timing needs to be adjusted (advanced) as altitude (MAP), power setting (MAP), mixture leaning, RPM, temperature and atmospheric conditions have a remarkable influence on the time the mixture needs to combust completely.
The reason for ignition advance is that the air/fuel mixture does not start to burn immediately, it takes time from the moment the spark ignites to the moment when peak cylinder pressure is reached (ideally just after TDC) and during this time the crankshaft continues to rotate. RPM spark advancing is done to compensate for higher RPMs (thus higher angular speeds compared to idle) and vacuum advancing compensates for higher engine loads at partial open throttle settings.
This is all best done with an electronic ignition with MAP (altitude, manifold), temperature and RPM sensors to compensate these variables and restore engine power. It will contribute to engine complexity but the fuel savings are 10 to at least 15% depending on which system you choose.
An electronic ignition improves reliability over the old style magnetos. But this comes at a price in the form of more complexity and some added weight (think about the extra backup battery). Full Authority Digital Engine Controller (FADEC) systems are also able to control the mixture per cylinder on fuel injected engines, where as most electronic ignitions do not.
This system controls ignition timing through a mechanical, RPM and vacuum advance. You will need a backup power supply just in case the main battery fails during flight. Another feature is direct fire ignition: in this system the spark plugs are fed directly from the coils and there is no distributor and the plugs fires on the exhaust stroke too (wasted spark). Four stroke Rotax engines use a similar system.
Crankshaft RPM detection is through a high resolution crank trigger wheel which offers a spark accuracy of 1/4° crankshaft rotation. This means that during rapid engine RPM acceleration the system will not misfire. Ignition firing is during 20° crankshaft rotation, the longest spark duration in the industry.
E-Mag is a remove the old magneto, drop-in electronic ignition system look-a-like. You will need a backup power supply, just in case the aircraft system fails. There is compensation for spark advance though a MAP sensor and RPM detector.
The P-Mag is essentially an E-Mag with an internal alternator to supply power as a backup system (it sustains ignition at 800 RPM, and at 1500 RPM or higher the aircraft electrical system becomes the backup). There is thus no need for a separate backup power supply in the electrical system, in fact this one is preferable to an E-Mag, reason being the internal power supply. A waste spark system is used on these units too.
These ignitions have a data recording facility and a LED for setting the correct TDC timing. More detailed info in the E-Mag installation guide. There are plans to be FAA certified.
This is a FAA-PMA approved microprocessor based electronic ignition. They claim to improve starting even when cold and with worn out spark plugs, reduced fuel consumption (claims 12%), smoother idle running and more power from the engine, about 7%. LASAR stands for: Limited Authority Spark Advance Regulator.
These are high energy CDI (capacitor discharge ignition) type systems with RPM and vacuum (MAP) ignition advance timing (Plasma III), a dual system will need a backup battery. From the website: "The Plasma II series is a single spark system which has a fixed spark duration at all RPMs. The Plasma III has a dual output stage which provides a continuous spark for about 20 degrees of crankshaft rotation at all RPMs. Extensive flight testing with cockpit variable spark duration showed maximum performance gain with an uninterrupted 20 degree spark".
Most of these systems can be used in combination with one of the old magnetos still in place, should you wish to do so. Just keep in mind that modern electronic systems are much more reliable than the magneto ignition from a bygone era.