All About Ignition Systems

Since I have now been around the block with ignition systems, I decided to write up what I think I have learned about them.

Magneto System

The magneto system is the time-honored way to provide spark to an aircraft engine. They date from the days when airplanes did not have an electrical system, and so the ignition system had to be self-powered. The same system can be found on your lawn mower. It was not long until two magnetos, with two spark plugs in each cylinder, were provide so that if one failed the other would power the airplane to a safe landing. The system remains to this day with only minor changes.

Reliable and well proven, the magneto suffers from two shortcomings: fixed timing, and a weak spark at lower engine speeds. Fixed timing means less power and/or economy at lower power settings. The weak spark at low engine speeds makes hot starts harder, especially with fuel injected engines, and also makes for a rough idle, and occasionally fouled spark plugs from a long idle period.

Electronic Ignition Systems

The main reasons to add the complexity of electronic ignition systems are first, to provide a hotter spark than is generated by a magnet swinging by a coil, and second, to be able to vary the ignition timing according to engine load and speed. To read more about the benefits of variable timing, see

http://www.cafefoundation.org/research.htm  and look at Ignition Dynamics I, II, and III

Unison LASAR System

LASAR stands for Limited Authority Spark Advance Regulator. It is a FAA certified system meant for new OEM and retrofit markets. It was recommended by my engine builder, Lycon, so I bought it with my engine.

The standard LASAR system consists of a pair of standard Slick magnetos, that have been modified with a movable plate on the points to allow the timing to be both retarded for starting, and advanced for cruise operation. The timing is controlled by a "brain box" typically mounted on the firewall, that senses RPM and manifold pressure, and modulates the timing according to a pre-programmed timing map. If the "brain" should fail, or if battery power is lost, the timing is returned to the baseline value that is set for the particular engine. If a magneto should fail in the normal sense, the other magneto will power the engine as usual, and the working magneto will revert to baseline timing.

Since the LASAR system is based on standard magnetos, it suffers from the same weak spark problem at lower RPM as they do. It does, however, vary the timing with engine load. In my opinion, the timing map, or curve, is much to aggressive. In my RV8, and in many other closely cowled kit planes, overheating occurs on both climb out and cruise. At first I just opened up the outlet on my cowl, and I got fairly close to acceptable numbers with that, although a long climb out in warm weather would still run my CHTs up quite high (440+). I tried pulling the breaker for the LASAR system, and with it in back-up mode (i.e. timing at baseline, in my case 25 degrees BTDC) the CHT would reduce about 40 degrees. I put in a toggle switch/breaker so I could control it, and found myself turning off the system anytime I was climbing, or even cruising on a warm day.

I suspect this came about from early testing on a Cessna 172, which has by far excessive cooling airflow, and didn't show the overheating that results from the curve. Also, Unison engineers were pushed to show a performance benefit for the system, which they were hard pressed to do anyway.

The LASAR system has a new major drawback that the old magneto system does not. That is, if the brain is dead, the magnetos have no way to retard the spark for starting. This means that if the system goes out on your way somewhere, you better land where there are mechanic facilities, because you are not leaving until it is fixed. Also, hand propping in the case of a dead or weak battery is impossible. Unison later addressed this problem with what the call the "Bush Kit", which includes a standard impulse coupling on one mag, at additional cost.

The LASAR system has two new minor drawbacks. One, a special timing box is required to set or check timing, to the tune of several hundred dollars. Second, to trouble shoot the system, a laptop computer with special interface cable is required.

When I first flew my airplane, at about the 4 hour mark, one magneto failed. When I called Unison for help, they were kind enough to send me the laptop interface cable for free, since it was a brand new system. It told me to replace the left mag, and Unison did that under warranty. At first they sent the wrong mag, then finally the correct one. Altogether it took about 10 days to get me flying again. I was glad that I had returned to my home base on one mag, it would have been a large inconvenience on the road somewhere.

When my hangar neighbor was ready to fly his new RV, also equipped with a Lycon engine and LASAR ignition, he was unable to get it started at all. I took my laptop and special cable over there, and it would not tell us what was wrong. By now, LASAR had decided customers should work through their dealers rather than through the factory, but after some arguing, they agreed to work with us. After 3 weeks of swapping parts in the blind (they would only send us one at a time), we finally got the engine running.

The big advantage of the LASAR system is that it is FAA certified, and that it does have a back-up mode in case of electrical failure.

Because of the above, I decided to remove the LASAR ignition system. I tried to get Unison to exchange it for a pair of standard magnetos and harnesses, but they refused.

I am not aware of any new certified aircraft using the LASAR system, and I suspect it is a major money looser for Unison.

August, 2004

Light Speed Engineering Electronic Ignition

After reading and talking to other experimental pilot/builders, I decided to try the Light Speed E.I.unit. I talked to founder Klaus Savier at Oshkosh and decided to go with a dual Plasma III system, with the crank sensor inputs. With this system, one drills 4 holes in the starter ring gear and installs 4 tiny magnets. A circuit board is mounted on the nose of the engine case. Sensors on the circuit board sense the passage of the magnets, and signal the brain box. Based on RPM and manifold pressure, the brain fires the coils that drive the spark plugs.

It is a CDI (Capacitive Discharge Ignition) system that delivers much more energy to the spark plug than a magneto does, and delivers the same energy at idle or cruise RPM. This improves the hard starting performance, since the stronger spark will fire better under less that optimal fuel/air mixtures. Also, along with a timing advance at idle, the idle is far smoother than a magneto fired engine.

A major advantage of the crank sensor system is that there are now no moving parts in the entire system. The holes in the accessory case that held the magnetos are simply capped. There is no distributor, but instead a separate coil for each spark plug. Each system of the dual Plasma III is an independent system, each using its own magnets, sensors,  coils, and spark plugs. In my installation, one of them runs off a separate 6 AH battery, so that if my main electrical system fails, I can still fly to a safe landing on one set of plugs.

The LSE system has a more conservative timing map, so that the engine does not overheat on climb out, and CHTs run about 370 degrees on a hot day in cruise. The system will function down to about 5 volts, so it will run for quite a while after an alternator failure. With the backup battery isolated, even if I leave my master switch on, I can hand prop it on the back up ignition system.

Since I have high compression (10:1) pistons, I installed my lead magnets at the 40 degree position, rather than the standard 45. When the engine is running, the timing advance is referenced from these magnets. When starting, the spark timing uses the zero degree magnets to generate a retarded spark. I also chose to install the optional display, that reads out the advance timing angle, just so I would know what it is doing. At a standard cruise, 8500 ft., 2500 RPM, 23" M.P., the advance angle is 27 degrees, or 2 degrees more than the baseline 25 for a parallel valve engine. I feel this will probably leave and adequate detonation margin for everyday flying, and judging by CHTs, more margin than the certified LASAR system. At 12,500 ft, and 19" M.P., it is showing about 30 degrees advance.

There is an option from LSE to use "hall effect modules" that run off the magneto gears. In these units, the magnets are fixed at 45 degrees apart, so that if a 40 degree position is chosen for the particular engine, starting happens at 5 degrees after TDC. Klaus maintains that this is not a problem. This has caused some confusion on some of the builder email lists. I did not choose this option because it involves moving parts, and costs more than the crank sensor option.

Because I removed two magnetos, and added the backup battery, the weight difference between the systems is negligible. The system uses standard automotive spark plugs at $1.35 each.

Ask me what I think in about 1000 hours :o)

May, 2007

I have only flown 200 hours since installing the Lightspeed system, but it has been flawless. I no longer clean spark plugs, just toss them and buy new ones every year.