Electronic ignitions have been in aviation for more than 20 years, yet they remain something of a mystery even to Experimental-aircraft builders. That’s no surprise, as the gritty details of internal combustion normally aren’t of practical concern to pilots. Yet e-ignition is an area a builder can explore, or ignore, with meaningful consequences.
To illuminate electronic ignition possibilities, we’ll review ignition basics and take a quick pass through the nuts and resistors of what’s available. This is a large subject, so it will be broken into a two-part series starting this month with a discussion of what an ignition system needs to do and that standard airport discussion topic: computerized car engines. Next time we’ll get deeper into what’s available for your horizontally opposed powerplant.
Starting a Fire
At its most basic, an ignition system needs to ignite a fire in the engine’s combustion chamber. That the fire must be started is pretty obvious. The only thing I’ll add is that it has to be truly lit; smolder and smoke don’t count. However, there’s plenty to say about lighting the fire at the correct time. Ignition timing is the fulcrum over which most ignition arguments are bent.
What’s to Be Burned
Then there’s the question of what you are trying to light on fire. It’s more complex than saying it’s the air-fuel mixture. That mixture is not a constant thing but is surprisingly dynamic because the ratio and density change often.
For starters, the pilot has direct control over the air-fuel ratio via the mixture-control knob. This is a good thing on the macro level—we can correct the mixture for decreased oxygen at altitude—but on the micro level, it means the air-fuel mixture is close to ideal all the time, but almost never ideal. The red knob is a coarse adjustment, as it controls all cylinders at the same time and it’s at the mercy of a human who may be busy talking on the radio or tending to other piloting tasks.
Seen via a 1-second exposure, a storm of sparks appears to stream to both massive-ground electrodes. A single spark from the center to one or the other of the ground electrodes is what lights the fire in a magneto-equipped engine. The dual ground electrodes are a hedge against electrode wear, as the spark always jumps from and to the sharpest edge it can find.
Also, while the mixture knob allows corrections of the air-fuel ratio, however coarsely, it does nothing about the change in density of the ratio. In other words, at sea level, the molecules in a 12:1 air-fuel mixture are packed together like New Yorkers, while at 12,000 feet, those molecules are spread out like Texans. The ratio is the same, but the density is different, and a less dense mixture is more difficult to ignite and keep lit.
Another factor in air-fuel density is compression ratio and forced induction.
Reliability—real or perceived—is a good thing when flying over the mountains at night, and it’s one reason magnetos are still aviation’s standard ignition. An inherent downside to mags is that they are at the end of a gear train and employ shafts and bearings of their own. This introduces the timing inaccuracy called spark scatter. Electronic ignition more directly measures crank position using magnetic or Hall-effect sensors for more stable, accurate timing.
Playing with Matches
We’ve all seen a B-movie thriller where someone lays out a trail of gasoline a respectful distance from the intended explosion and then lights a match. When the match hits the fuel, the fire zips along the trail before the inevitable fireball at the end. And the director always shows the fire running down the trail.
The same thing takes place in a combustion chamber. The spark plug fires and lights the air-fuel next to it, and then the flame kernel zips out in all directions, igniting more and more of the air-fuel mixture. What’s zipping along is called the flame front, and the take-home point is that it requires a fair amount of time for the combustion event to move across the combustion chamber.
How fast the flame front moves varies. A rich, dense air-fuel mixture, high cylinder pressure (sea level, high compression ratio or turbo boost) and high cylinder temperatures speed the flame front. The closer the air-fuel ratio is to chemical perfection—stoichiometric—the faster the flame front moves. Go the other direction with these parameters, including either rich or lean of stoichiometric, and the flame front slows down. Not a factor is rpm; flame-front speed is independent of engine speed.
A poster child for what the knowledgeable experimenter can build, Vito Wypraechtiger’s Formula 1 racer sports two completely redundant electronic-ignition systems. Looking over the instrument panel toward the prop flange, you see the dual batteries in the lower right and dual Light Speed spark boxes at lower left.
A Big Oven
Then there is the combustion chamber, the room where the match meets the gas. Consider that it’s when the piston is pretty much at the top of the stroke that we’re interested in, because that’s when the spark plug fires. It doesn’t matter how long the piston stroke is, but bore diameter is a primary consideration. Bigger bores mean a longer time for the flame front to get from one end to the other. (Thinking point: Why does the rpm drop during the mag test at runup, anyway?)
Wypraechtiger is a Swiss F1 Reno air racer with roots in car racing, electronics and a day job as a Red Bull Air Racing technician. His Scarlet Screamer Cassutt derivative takes electronic ignition to extremes, as shown by its extensive cockpit instrumentation from UAV navigation and dual Light Speed Engineering ignition systems boasting cockpit adjustability. A busy man, Wypraechtiger changes both the mixture and ignition timing twice per 3-kilometer lap, depending on whether the airplane is turning or on the straights. This is necessary as the load on the engine changes when the induced drag rises in the turns. Sounds like the perfect job for computerized engine management.
Also important are the spark plugs and how fat an arc they strike. Mechanically, what matters for ignition efficiency is how much of the spark is exposed to the air-fuel mixture.
The spark-plug gap, the distance between the electrodes, is a physical barrier to exposing the spark to the air-fuel molecules. Tight gaps light up small numbers of molecules compared to wide electrode gaps.
Theoretically, massive-electrode plugs mask the spark from the mixture more than fine-wire plugs do. On top of that, massive-electrode plugs are something of a heat sink, and they can absorb or quench some of the useful heat in the immediate vicinity of the spark and air-fuel interface.
So a massive-electrode spark plug with tight gaps may have less chance of striking a viable flame kernel than a fine-wire plug with a large gap. The catch is that it takes a high-voltage ignition system to jump the wide-plug gap.
Voltage is also important in overcoming high cylinder pressures from high compression or supercharging. But once the gap is bridged, excess voltage is of no benefit.
Of course, the mass of the spark is also significant. As I once heard it explained by an ignition engineer, it’s tough to light a Yule log with a match. But if you have 10,000 matches, you can get the log lit. The temperature of each match remains the same, but one match rapidly after another adds up to more heat mass. You could also light all 10,000 matches at once for a greater heat mass.
The corresponding ignition system parameters would be the amperage (not the voltage) of the spark jumping the plug gap, and how many times the spark plug fires per ignition event. An ignition system that throws a higher amperage fireball between the electrodes, or does it multiple times, is better at igniting a larger, hotter flame kernel. And it’s the well-established flame kernel that ignites the flame front.
Ken Christley, the in-house tuner at Kenne Bell, an aftermarket automotive supercharging manufacturer, can tell you how involved e-tuning can get. He’s adjusting the engine management in a 700+-hp Ford Mustang, a process that takes 40 to 80 hours on a fully instrumented and data-logged chassis dyno. Problem-child applications can take months, and he has the benefit of a stock factory-tuned car to start with. His task is simply to accommodate the management software to the supercharger.
All About Timing
At this point, it’s easy to understand that when the spark plug fires is a primary consideration of efficient combustion, power and fuel economy. Mechanically, it’s necessary to closely control ignition timing, and thereby cylinder pressure, to maximize the thrust imparted through the leverage of the piston and connecting rod to the crankshaft. Because airplane engines experience a wide range of air-fuel mixtures and densities, and therefore variations in flame-front speed, they respond favorably to variable ignition timing.
One area not so important to the aviation engine is ignition advance required by increasing rpm. Our airplane engines run at near constant rpm, so, purely from an engine-speed standpoint, fixed timing is workable—except for starting, which is why there is a spark retard in the impulse coupling.
Most aviation electronic ignition systems use one ignition coil for every two cylinders—standard practice on cars until the advent of coil-on-plug. This results in a “waste spark,” where one coil firing results in two sparks, one in a cylinder on its power stroke while the other is sent to die harmlessly in a cylinder on its overlap stroke.
Another concept regarding ignition timing is MBT, or Maximum Brake Torque spark. This is the minimum amount of ignition advance (how soon the ignition is fired before top dead center) required to reach the engine’s maximum torque output. The only reason I mention it is to point out that a minimum ignition advance is required for each engine to achieve max (or rated) power, and once MBT is achieved, there is no gain to further timing advance, assuming a fixed air-fuel mixture.
At the other end of the timing scale is Knock Limit Spark, the most advanced ignition timing before detonation occurs. Knock Limit Spark varies primarily with densities and temperatures, though fuel octane and other factors are definitely in play. Under high loads, ignition timing advance is often limited by spark knock and not MBT; in fact, that’s the norm with high-compression or supercharged engines.
Clearly the engine’s designer must define the minimum and maximum ignition advance, even if there is no ignition advance, as with aviation magnetos. In that case, the ignition timing can be set no more advanced than the KLS under maximum load—low density altitude, wide-open throttle. Every other condition is thus compromised if the timing is fixed.
Many aviation electronic ignitions use automotive spark plugs. They are inexpensive enough that you can throw them away every 200 hours instead of cleaning them. They are also designed for wide gaps and are available in affordable platinum, iridium or standard form such as these Denso plugs specified by Light Speed. The brass adapters fit the auto-style plugs to the aviation cylinder heads.
Putting what we know together, the typical dual-magneto aviation ignition fires two massive-electrode spark plugs 25° before TDC in a very-large-bore cylinder. (That timing spec can be different for every engine, though 25° is the most common.) Each spark plug is fired once per ignition event. Timing is fixed: There is no ignition advance or retard, except retard during starting as provided by one of the magneto’s impulse coupling. Two spark plugs are placed at each side of the bore to provide two flame fronts and thus consume all of the air-fuel mixture in the time provided, along with providing redundancy for safety. The typical 5-inch-or-larger-bore aviation cylinder is easily large enough to require dual spark plugs to burn all of the mixture in time. There is a functional benefit beyond redundancy.
The magnetos provide a reasonably compact, self-energizing ignition system that requires no battery, alternator or backup system. This makes them rather reliable: Once turning, the magnetos will keep sparking with no outside input. The battery can fall out of the airplane, the wiring behind the dash can burn, the EFIS can go dead and cosmic rays can play havoc with the GPS, but the magnetos will keep snapping away. The magneto spark is medium voltage and modest amperage, but long duration (it lasts for a relatively long number of crankshaft degrees). Because they are simple mechanical devices, magnetos are easy to troubleshoot, and pilots are assured the magneto won’t go freelancing the ignition timing.
Mixture control, as we’ve seen, is all over the map. Mixture density varies with altitude, but the fuel burned is a glorious 100-octane blend with excellent anti-knock qualities. That’s good, as airplane engines have a tough duty cycle—they spend most of their life churning out a large percentage of their maximum capability.
Here’s a hint at the work Detroit goes through to develop an engine. The bit of blue is the left valve cover on a 2013 662-hp Ford Shelby GT500 V-8 on a Roush Industries development dyno. The tangle of yellow wires descending at right supports pressure sensors embedded in the block and cylinder heads, four sensors per cylinder. Incredibly expensive, such testing is necessary when fully developing an engine for the real world, and you can bet the results figure in the engine-management software, as cylinder pressures are carefully limited to ensure reliability.
Some auto ignitions still use a fast-acting, high-amperage capacitive discharge spark box with wide-gap spark plugs, but “coil-on-plug” ignitions—what an old A&P would call a “low-tension” ignition—are the new norm. With coil-on-plug there is no separate CD box; all amplification is done in the coils, which sit individually atop each spark plug. Most systems multi-strike (fire the plug more than once per ignition event) at idle and low rpm, but at around 3000 rpm, there is not enough time and the systems revert to single strike.
Automotive ignition timing is computerized, controlled cylinder by cylinder many times per second. Timing varies greatly as dictated by a bewildering number of parameters, including sophisticated knock detection made possible by knock sensors.
The knock sensors are significant because they are an important defense against detonation. They are difficult to employ on aircraft because the loose-tolerance, air-cooled engines are noisy as donkeys in a tin barn, making the knock tough to separate from the clanking mechanicals.
It’s important to understand that auto ignitions are just part of an integrated engine-control system. The same computer calling the ignition timing 10 times per second is controlling the fuel delivery, the electronic throttle, variable camshaft timing and about 100 other things. In aviation we’d call this Full Authority Digital Engine Control (FADEC). Lately, at Lycoming, it goes by the name IE2.
Also important is that because auto-engine management is complex, it takes meaningful engineering resources to calibrate. Four engineers working a year and a half to map a new engine’s software is considered remarkably fast work.
Considering that most airplanes already have a battery, the typical electronic ignition will save weight. Further, not hosting at least one magneto means there’s an open drive for a backup alternator, or that cheapest of alternatives, a blanking plate such as the black NFS-logoed unit on the Cassutt.
Aftermarket Aviation Ignitions
If they aren’t complete engine management, what are the aftermarket electronic aviation ignitions offered today? By and large, the most popular are just ignition and have nothing to do with fuel or anything else. A handful of systems are standalone engine-management designs; they are aimed at advanced hobbyists or racers with a complete understanding of an engine’s spark and fuel needs.
Furthermore, among the simple e-ignitions is an amazingly diverse group of offerings, some dead basic, others feature rich, but mainly with a capacitive discharge spark box for a hotter spark along with rudimentary adjustable timing in reference to manifold pressure. We’ll explore their differences next time.
What Can Electronic Ignition Do for You?
So what should you expect when going electronic? The main benefit to expect is better fuel economy from variable ignition timing, especially at altitude. The fixed timing of magnetos is easy pickings, even for an airplane engine that spends most of its life at 2200 to 2500 rpm. Some method of inferring cylinder pressure and varying the ignition timing accordingly helps because the flame-front speed varies with engine load. As it is, two mags firing at 25° before TDC are pretty optimal for maximum power at sea level.
With more timing accuracy from its solid-state, no-moving-parts design, electronic ignition can more accurately deliver greater amperage than common-sized magnetos for faster starts and smoother idling. The hotter spark should reduce misfiring, which might be felt as a smoother-running engine, especially when running lean. Few moving parts means reduced maintenance and no magneto overhauls.
Maybe it’s not mandatory, but easy monitoring and data logging of an electronic ignition would be helpful so that users will know what the spark timing is, which would have both educational and troubleshooting advantages.
One thing you ought not expect is increased maximum power. Climb and top speed shouldn’t increase, because magnetos are already good in the simple, wide-open-throttle regime.
In Part 2, we’ll review the electronic-ignition hardware.