Airframe and Powerplant

A forward-thinking ignition system could allow existing engines to run on future fuels

Powerplant engineering involves negotiating a constantly changing stream of compromises. Perfect-world designers want every product to be its very best technically, to light the market’s fire and win the admiration of fellow engineers. Marketing steps in to decide if there’s an audience and whether it will pay what it takes to have blue-sky technology. Production managers weigh in on issues of producibility and maintainability. Finally the money managers decide which of the engineer’s great ideas gets the green light and which are consigned to the waste can—or the "deleted items" folder on his computer.

Over time, savvy engineers decide what they can "get away" with and immediately pass over what could be a groundbreaking technology because they know it won’t get past the bean-counters and, if this idea is for production aircraft, on the grounds that it could be prohibitively expensive to certify. So these engineers automatically filter the crazy and absurd and get right down to work on more practical matters.

The minds at General Aviation Modifications Inc., known in the fuel-injector world as GAMI, aren’t that bright. Instead, under the direction and curiosity of Chief Engineer George Braly, the company has come down the road that any reliable aviation engineer would discard as the wrong direction. Braly, together with GAMI President Tim Roehl, has been looking at powerplant data in excruciating detail since the company began playing with balanced fuel delivery. In that time, Braly and Roehl noticed that while it’s possible to make an engine run efficiently and smoothly at very lean mixtures, there were some significant limitations posed by fixed-timing ignition—limitations that in Braly’s mind could and should be fixed. So in true hobbyist/breadboard fashion, work began on a fairly revolutionary aircraft ignition system. Its working name is Prism, for pressure-reactive intelligent spark management.

Let’s step back for a moment and consider what happens in a normal aircraft engine with fixed timing. Typically, air-cooled aero engines have their running ignition timing set about 25 degrees before top-dead center (BTDC); the spark occurs before the piston reaches the top of the cylinder on the compression stroke. The timing range is actually 20 to 28 degrees, depending upon engine type (turbocharged or not), compression ratio, and approved fuel types. Sometimes powerplant designers will alter an engine’s static ignition timing to set total power output precisely or to define the detonation margins under worst-case operation. Except for starting, this ignition timing is held constant, regardless of engine speed, manifold pressure, load, and mixture strength. For decades, what most pilots and mechanics have come to appreciate is that this fixed-timing scheme works fairly well—the engines make decent power and exhibit acceptable economy while detonation is a comparatively rare event outside of a magneto malfunction or ingestion of improper-grade fuel.

But Braly and Roehl noticed something different—something, in fact, that most experienced aero-engine engineers will acknowledge over a friendly bottle of beer—that the engine’s needs are not well-served by fixed timing. The leading computer-control schemes on the boards for gas engines from Continental and Unison take this into account; they will use timing maps that will allow an otherwise standard engine to return better power and fuel specifics (see "Airframe and Powerplant: Bag of Chips," January Pilot). These maps will be based on in-flight and dynamometer testing and will be fixed according to a combination of engine speed and manifold pressure. (To a lesser extent, the electronic-control systems will also adjust timing for secondary reasons such as high cylinder temperatures.)

The GAMI brain trust believes that such a system, while a massive step forward, misses a fundamental issue—the engine’s ignition-timing needs also vary by fuel/air ratio. This is something that Braly and Roehl found while testing their GAMIjector fuel injectors on a variety of engines. Early in the development of the GAMIjectors, Braly figured that direct measurement of the pressure pulse created by combustion would be instructive. In the days when lots of new aircraft engines were being developed, the engineers also looked at cylinder pressures to detect the presence of detonation or destructively high cylinder pressures. But they did so with comparatively primitive equipment, staring squinty-eyed into oscilloscopes and using a combination of hard-won knowledge and intuition to decipher the thin green line. Today, there are a variety of solid-state, fiberoptic-based sensors that are far more reliable and accurate than anything that was available in the 1960s. Moreover, these new sensors produce voltages that are easily turned into digital signals to feed a data-logging computer. In one stroke, you have more repeatable data that are vastly more conveniently analyzed than with the manual method.

What the GAMI crew found—first in ground tests of a turbonormalized Continental IO-550 and then in a ground-based, two-cylinder industrial engine—surprised them. First, the air/fuel ratio burned in the cylinder dramatically influenced the shape and absolute peak of the pressure pulse. (This isn’t really news, as the internal-combustion textbooks all point this out, and the non-uniformity of the exhaust-gas temperature plots at lean mixtures suggested this phenomenon.) Rich mixtures—at or near best-power—produce sharp, steep pressure curves. Lean mixtures produce shallower curves with lower absolute peak pressures. (By increasing manifold pressure, this lean mixture can still produce the same amount of torque as a rich mixture, thanks to the greater area under the pressure curve.) With fixed timing, the lean mixtures were completing their combustion cycle late enough to create high exhaust-gas temperatures and, in some cases, late enough to waste power.

Wouldn’t it be nice, Braly considered, to move the spark around so that it would not only be in the right place with regard to engine speed and manifold pressure, but also for mixture strength? But how do you do this? It took something of a leap and considerable research in the Society of Automotive Engineers archives to come up with the answer.

It’s a conclusion that’s almost frightening in its simplicity: Move the spark timing to put the peak pressure pulse where you want it for every cycle. Within that scheme are several important side benefits. A feedback loop of pressure-pulse position to ignition timing self-compensates for all manner of mixture strengths and by its very nature doesn’t even need to know the engine speed or manifold pressure in order to calculate the right time at which to fire the plugs. All it really needs to know is when any given cylinder has reached top-dead center, so that it can calculate the position of the pressure pulse. This reduces the number of sensors and greatly simplifies the installation.

Prism could also create more horsepower in existing engines on existing fuels. Why? With fixed timing, the peak of the pressure pulse may occur anywhere between 8 degrees after top-dead center (ATDC) and 25 degrees ATDC, depending upon engine speed and mixture strength. But there is an optimum position for the peak, based on connecting-rod angularity, cylinder breathing, and valve timing. Generally, the sweet spot is between 18 and 20 degrees ATDC. GAMI has designed the development version of Prism to calculate and hold the peak of the pressure pulse at any point by changing the programming, but will determine by model the ideal setting for any given engine type. GAMI couldn’t boost maximum or takeoff horsepower because of engine and airframe certification rules, but Prism could allow a normally aspirated engine to maintain maximum-cruise power to higher altitudes, thereby marginally boosting cruise speed.

Setting ignition timing to ideally position the pressure pulse seems like such a simple solution, so how come no one has thought of it before? Turns out, someone has. In fact, the Nissan Motor Corp. briefly used such a feedback system on its Infiniti flagship. According to an SAE paper outlining the system, it had limited authority to skew ignition timing and was used mainly to keep a highly stressed engine away from detonation. It is not being used in the current automobiles.

There are other cascading benefits of a Prism-like technology. For example, if the computer is continuously plotting the combustion-pressure pulse, it also can be programmed to distinguish detonation. (Detonation has a wonderfully recognizable pressure signature.) Armed with a digital snapshot of detonation, Prism can look for the enemy and take corrective steps should detonation arrive. Rather than fight, Prism’s mandate is to run away: In simple terms, when the detonation routine is active, Prism will retard ignition timing until the detonation stops for a given amount of time. Then, on the assumption that it was a transient experience, Prism will go back to normal timing. If detonation occurs with a certain frequency or intensity, Prism can then go into a new routine that favors keeping the cylinder heads on the block over bulk power production.

GAMI is hopeful that the long-term benefits of Prism will lie in the murky future of leaded fuels. The company’s small ground engine—lovingly dubbed "the little engine that could" because of its stoic duty in the line of thousands of detonation events mild and severe—has been run on regular 100LL avgas, 89-octane auto fuel, and 75-octane naphtha. Even under maximum load, the test engine will operate happily all day long on 100LL but enter into light detonation with the auto gas. It knocks and clatters like a big rig on the naphtha under fixed timing. With Prism operating, the spark timing retards, power and peak combustion pressures drop, and the little engine lives to see another day.

Sure, a stationary engine and a real-life aircraft powerplant are two different things—how do you know that the technology translates? That’s just the question GAMI considered. To answer it, Braly and Roehl built a sophisticated engine test stand at their Ada, Oklahoma, headquarters. (It’s named the Carl Goulet Memorial Engine Test Stand, a nod toward the ex-Continental engineer who is credited with putting Braly on the right track.) On that stand is a Continental IO-520-F, running at the end of 1999 with a full six-cylinder (12-channel) Prism system. Outside in the fuel farm are a tank of 100LL, a tank of 89-octane auto fuel, and a barrel of nonleaded avgas; it’s 100LL base stock without the lead and has an octane rating of about 92.

In the first 50 hours of test-stand operation, the Prism technology has proven itself. The IO-520 has been tested at both normal and elevated cylinder-head temps. It has run on all three fuels, and in all cases Prism has done whatever it takes to place the pressure pulse at the ideal 18 to 20 degrees ATDC and ward off detonation. In this testing, Braly and company have discovered that engines like the IO-520 operate in stages of incipient to light detonation more often than is commonly believed.

With the basic concept proven, GAMI is working hard packaging the hardware, tweaking and making certifiable the software, and further plumbing the depth of the IO-520’s capacity for abuse. It has hired on FAA-designated engineering representatives and laid groundwork with the FAA for certification. Many of the key pieces of hardware are designed and tested. For example, the pressure measurement will take place with small pressure transducers connected to the controller through fiberoptic cables. (This step removes a large obstacle: testing for electromagnetic interference.) They are built into CNC-machined bosses that mate to modified spark plugs, so there is no need to alter the cylinder head and the spark plug remains easily replaceable; it’s anticipated that production versions of Prism will have two independent cylinder-pressure transducers per engine. A small box mounted to one of the vacated magneto drives will provide TDC references times two, again via fiber optics. The second magneto pad could take a small backup alternator to supply the system’s dedicated electrical bus. It’s also a possibility that a small battery could be used, but GAMI’s yet-to-be-certified Supplenator, basically a self-exciting alternator, would give the pilot as many landing options as there is gas in the airplane.

By all reasonable measures, we’re at least a year away from an off-the-shelf Prism unit for general aviation. GAMI has to prove the mechanical components, design a cockpit interface, and get the whole package certified. When it arrives, it will do so full of promise—multifuel capabilities, permanent detonation avoidance, potentially better engine life through moderation of peak cylinder pressures, and greater power recovery operating at lean mixture settings. And if Braly and Roehl’s dream should come true, you can bet there’ll be powerplant engineers slapping palm to desk, saying to no one in particular, "Now, that’s the way to do it."

Links to additional information on electronic ignition fuel injectors may be found on AOPA Online ( www.aopa.org/pilot/links/links0002.shtml). E-mail the author at [email protected].