February 1, 1995
MARC E. COOK
Electronics have taken over the world. Nary a household appliance has been spared the influence of the microchip. And yet, high-tech navigators and communicators aside, light aircraft remain almost totally mechanical. Indeed, some aspects of general aviation technology lay mired in the past.
Certainly, the magneto fits this description. Maintenance intensive and inflexible, the mag is often cited as a prime limiting factor of engine performance. Indeed, with more sophisticated ignition and fuel delivery systems, an otherwise unchanged aircraft engine could realize significant gains in power, smoothness, and reliability.
Why do we want so badly to relegate the magneto to the likes of a 10-cent cup of coffee? One of the greatest shortcomings of the standard magneto — beyond the maintenance issues — is its fixed timing. Once set, the mag fires the plugs at a certain time before the piston reaches top dead center (BTDC), usually between 25 and 30 degrees BTDC.
But the engine's requirements far outstrip the mag's ability to fulfill them. For starting, the advanced timing doesn't work, so one of two types of ancillary systems must be employed to retard the spark for easier starting — either an impulse coupling or a retard-breaker arrangement. (Moreover, the mag's electrical output at low engine speeds is poor, so some way of boosting the current must be used as well.)
Optimum ignition timing for ideal flame propagation and detonation suppression varies significantly with engine speed, load, and combustion pressures. A magneto cannot accommodate those needs, since it is set for one flight regime: takeoff. In this sense, the mag is timed with sufficient advance to produce rated maximum power but not so much as to incite detonation. This compromises both starting, as mentioned, and cruise-power efficiencies.
Electronic ignition harbors no such compromise, and the advantages are manifold. Take modern automobiles: They employ computers to manage ignition and fuel-injection systems. In fact, the more savvy models also have the automatic transmission and variable valve timing actuators working through the same processor. A few cars use throttle-by-wire schemes wherein the foot pedal isn't even physically connected to the engine. One result: Today's cars have the highest specific output, greatest overall efficiency, and lowest emissions in history.
Production airplanes will soon have some of the technology that makes this possible in cars. Unison Systems, maker of the Slick brand of magnetos, has taken the wraps off of its LASAR system, the acronym standing for Limited Authority Spark Advance Regulator. A key part of that name is "limited." The Unison system doesn't totally do away with the magneto. Instead, the cranky old spark maker remains as a backup; with a pair of them you could say you have quadrupled ignition redundancy.
Of course, electronic ignition has already been used in production airplanes. The Porsche-powered Mooney PFM had it, and the Diamond Katana, with its 80-horsepower Rotax 912, makes do without a magneto in the traditional sense. In the Mooney's case, the ignition system resembled that of a car's, circa 1980. Battery power was required at all times for the system to operate, which was in part why the PFM's complex and heavy electrical system seemed more fitting for a King Air than a 201. The Rotax's system is self-powered; in essence there are the components of four small magnetos at the back of the engine, although each of the four coils is triggered by a small electronics package. Each coil powers two spark plugs on different cylinders, using what's called the lost-spark system. The coil fires once each crank revolution; in one cylinder, this occurs in time to ignite the fuel/air mixture, and in the other it happens during the exhaust stroke, where the spark is hardly noticed among all the other commotion. This is the system successfully used for decades by Rotax and others on off-road motorcycle ignition systems.
Unison's is entirely different. Attached to the conventional engine is a pair of conventional-looking mags. Each contains the guts of a normal mag — rotor, coil, distributor cap, and harness connections. It's the solid-state control box wired to those mags that makes the difference. It monitors engine rpm and manifold pressure, and based upon software calculations, powers and commands the conventional mag coil to supply a spark at a given time for a given duration.
In this mode, the only parts of the magneto really being used are the coil and distributor assembly; the windings and a set of normal breaker points continue to spin but do not influence the spark. There are no impulse couplings on the LASAR units, which will likely be a real boon for durability.
How does the computer know when to strike the spark? After a slew of dynamometer tests, Unison engineers will determine the optimum spark timing and duration for various engine conditions — low speed and high, low manifold pressure and high, as well as all the combinations in between. The ideal settings are then mapped out on a computer and the data transferred to the on-board box. In flight, the Unison system will compare parameters to this map and select timing and duration as appropriate.
In early testing — on an O-320 Lycoming and an IO-240 Continental — Unison found that the electronic system posted several significant changes in engine performance. Overall, exhaust-gas temperatures were lower, as much as 150 degrees Fahrenheit. Makes sense — with earlier and more complete combustion, there's less unburned fuel and air mixture going out the exhaust pipe. Cylinder-head temps rose slightly, also indicating more complete combustion and slightly greater power output. (It's too early to tell if elevated CHTs will be troublesome in the long run.) Unison also discovered that fuel consumption could be cut by as much as 16 percent and that power had increased marginally for a number of manifold-pressure/rpm combinations tried.
What will this mean for you? In flight, the Unison system should allow more aggressive leaning without transferring as much heat to the exhaust valves, seats, and plumbing. This is where some of the economy gains will be felt. (Remember, though, that fuel-injected and/or turbocharged powerplants will likely see more of an advantage this way than simple carbureted engines, thanks to better mixture distribution.) Moreover, starting and idling should be much quicker and smoother, and there'll be lower likelihood of plug fouling during ground operation.
And should the electronics die in flight, the two otherwise conventional mags will take over without a pause. In fact, Unison says the FAA personnel with whom it is conducting the certification tests aren't going to require any annunciation of electronic failures. Unison says that you'll notice the changeover, however, since the engine will run rougher and lose some power when it reverts to fixed-timing ignition. Also, after you shut down, it's unlikely you'll be able to get the engine started again on the fixed-timing mags sans impulse couplings. This will be Unison's way of telling you something's gone awry.
At press time, Unison had begun flight tests in a Cessna 172. Over the winter and into early 1995, the company expects to perfect the system on the Skyhawk and begin work on finalizing the production specifications. By April, in time for the annual Sun 'n Fun airshow and convention in Lakeland, Florida, a system intended for experimental airplanes will be available. Unison expects the system to be approved for certified airplanes a few months after that.
Unison believes that the approval method will require only a supplemental type certificate (STC) for the engine family, rather than having to do airframe-specific STCs. This will dramatically speed the process and will likely help Unison keep the lid on prices. Though the company will not commit to hard costs, it does say the LASAR arrangement will run about double that of a conventional mag, or about $2,000 for a complete retrofit.
What's most impressive about the Unison system is that there's room to grow. Right now, you will find an RS-232 data port on the firewall-mounted computer, which in the future could offer data linking. Also, the company is looking into developing a knock sensor to work in conjunction with the computerized timing map. When the time comes for general aviation to transition to an unleaded fuel, these features will be important and hugely useful. Precise spark timing and detonation detection may well be keys to keeping some of the high-output engines happy — perhaps even out of the junk heap — on the octane-challenged fuels of the future.
While we're not yet ready to fire up the hearse for the magneto's funeral, it's clear a better system lies just over the horizon. And, perhaps, not a moment too soon.
Unison Industries, 7575 Baymeadows Way, Jacksonville, Florida 32256; telephone 904/739-4000, fax 904/739-4006.
It's no secret that production airplanes are limited by certification standards. More often than not, the paperwork needed to add new electronics is more effort than the potential gains themselves. In part, this mindset has helped keep our airplanes in a technological time warp.
Experimental-category airplanes have no such restrictions, of course, and over the years homebuilders have reaped the rewards of newer technology such as electronic ignition. In fact, many trend-setting builders have been using electronic ignition for more than a decade with excellent results. In many cases, these systems coexist with conventional magnetos on production-based engines. For the hardy souls marshaling auto- engine conversions, the standard, car-based system usually remains; it worked in the four-wheeler so it should perform in the airplane, the thinking goes.
These days, however, enterprising homebuilders can take advantage of new ignition systems designed from the outset for aviation use. One such system is from Klaus Savier's Light Speed Engineering (805/933-3299). Savier, incidentally, was for many years the scourge of the CAFE races with his highly efficient and speedy Long-EZ. Light Speed's system has been sold since 1987, and it's intended to replace one of the two standard mags. Weighing just 3.5 pounds and starting at $795, the LSE system uses engine-speed and manifold-pressure sensors to calculate ideal ignition timing. LSE claims improved fuel consumption of between 5 and 7 percent at low altitude and up to 20 percent up high. In addition, LSE says the advanced and more accurate timing allows for lower exhaust-gas temperatures at the usual mixture settings. Except for needing a backup power source, there are few drawbacks to using two LSE setups and no magnetos.
Even when working with (or should we say against?) a standard magneto, the LSE arrangement does indeed provide easier starting and resistance to spark-plug fouling during ground operations. We have flown with the LSE system in the Berkut kit airplane, which carries a modified Lycoming IO-360. In flight, turning off the electronic system results in a definite change in engine tone and a reduction in airspeed. As if to suggest the LSE system could handle the duties all by itself, when you switch off the mag there's virtually no change in sound or speed. That's amazing, considering that the yawning combustion chambers in aviation engines really need the double flame front provided by two plugs to work effectively.
Electronic ignition may well be just around the corner for production airplanes, but the experimental set has been enjoying the benefits for some time.
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