Electronic engine management systems are here ... nearly
A shiny white Diamond Katana touches down on the Fairhope, Alabama, runway; rolls along for a few hundred feet; and then accelerates down the tarmac. Its little four-cylinder engine churns away, and the airplane lifts off and climbs smartly for another trip around the patch. Just another ordinary touch-and-go with student and instructor trading the secrets of flight? Not this time.
Indeed, this Katana might be on one of the more important training flights you’ll find, and it has almost nothing to do with the pilots. It’s aloft for the purpose of putting the first fully electronic piston-engine control system to the test. As a test mule of Teledyne Continental Motors’ FADEC, this innocuous little Canadian airplane is spearheading what TCM (and many others) believe is the future of the aircraft piston engine. (FADEC is the industry moniker for full-authority digital engine control.)
It’s true that Continental has the jump on the market by being the first to fly its FADEC setup, but competition remains stiff. Textron Lycoming and Unison Industries have formed a partnership to develop EPiC, a form of electronic engine management. At press time, both Lycoming and Unison had run engines in test cells with EPiC and were expecting to be flying them by the first part of the year.
It’s useful to consider why Continental, Lycoming, and Unison would develop a computerized control system. After all, the existing fuel-delivery and ignition systems work just fine, don’t they? Most of the time, yes, that’s true. They are tried, proven, and for the most part reliable. Pilots have become accustomed to operating two, three, or more levers in the cockpit to manage engine power, engines last a long time, and the Earth continues to orbit the sun.
But there are two compelling issues at hand here—first, a general lack of technical development in powerplants compared to those found in other forms of transportation, and, second, the understanding by engine makers that leaded aviation fuel will not be around forever. It’s perhaps the fuel issue that is most imposing to the engine manufacturers. Although it is believed that many of the naturally aspirated engines and even some of the turbo models will survive on something with less knock resistance than 100LL, the engineers won’t know for sure until each engine model is tested.
With the fuel issue still firmly in the future, today’s impetus for developing these systems comes down to airframe manufacturers’ desires for something to differentiate new airplanes from old ones (Lycoming and Unison’s angle) and fitting the vast fleet of existing airplanes with better engine management (Continental’s primary tactic).
In what has been called a gutsy and aggressive move, TCM recently purchased Mattituck Aviation Services, a well-known shop specializing in Lycoming-engine overhauls. Continental is not interested in selling boxes to be fitted in the field; it wants to control the installation and the quality of the underlying engine. At EAA AirVenture this year, for example, TCM showed a Cessna 172 fitted with its FADEC system; the installation and initial development were carried out by Mattituck. It’s also TCM’s intention to go after the abundant Cessna Skyhawk models and the Piper Cherokee 180/Archer family for the full FADEC treatment.
Lycoming and Unison’s tack is considerably different—relying not so much upon the retrofit market but forging alliances with original-equipment airframe manufacturers. Their intent is to develop EPiC for new aircraft as a certified package and then, only after that pipeline is well filled, look toward the aftermarket. The team is working with Cessna as its principal customer, although seven other airframe manufacturers have signaled their desire to be on board.
The basic EPiC system is designed with certifiability in mind, using an electronic management system overlaid upon a basic mechanical scheme; in normal operation, though, the mechanical components do not limit the flexibility of EPiC. Twin dual-mode magnetos will be mounted on the engine—based on Unison’s Lasar system, which mates electronic ignition with many of the traditional magneto parts to achieve full system redundancy. The Bendix fuel injection servo found on injected Lycomings will be replaced by a simple mechanical system for backup. A modified propeller governor will have electronic control with a preset default speed in case the computer looks the other way. The pilot maintains control of the throttle and has access to a mechanical mixture control for use when the automatic system has been shut down.
In concept, EPiC is a lot like an open-loop automobile engine management system. A computer with a permanent memory map of engine parameters calls the shots. A series of sensors—manifold pressure, inlet-air pressure and temperature, cylinder-head temperature, fuel flow, and outside air temperature—take into account the operating conditions of the engine and meter fuel and spark accordingly. Open loop means that there is no direct feedback to the computer (save for the fuel flow) about what the engine is doing—automobiles today use oxygen sensors as one feedback channel. EPiC uses fuel flow as a check on the rest of the parameters and to help tailor the system for the inevitable engine-to-engine differences.
The system has considerable flexibility. The ignition system, for example, will allow spark advance to move as far as 40 degrees before top-dead center (BTDC), while a fixed mag is normally timed around 25 degrees BTDC. In addition, the timing can be set at TDC for easier starting. In the initial testing of EPiC, Unison engineers decided to implement dual sensors across the board; this gives the system input redundancy. If one sensor goes wacky, EPiC will continue to perform normally on the backup. The pilot is advised by a panel instrument that there has been a fault found.
Precise fuel and ignition settings will be found on the dynamometer and chosen for optimum power output and/or efficiency. (The pilot will be able to select performance or economy modes. Performance is essentially best-power mixtures, while economy will run the engine to or slightly rich of peak EGT.) One other item Unison created as the result of testing is a pulsed-fuel mode for starting. The engineers say that it starts the engine quickly hot or cold—forever relieving the pilot of difficult starting rituals.
So what’s in it for the pilot? Much easier engine management. Set the throttle for takeoff, and EPiC will optimally manage fuel flow, prop rpm, and ignition timing. No more fiddling with EGT values or mixture management in the climb, cruise, or descent. In the event of a system fault—say a ship-wide power failure or a computer glitch—the system re-verts to pure mechanical. Spark timing is taken over by the mags at a fixed interval, fuel delivery reverts to the manual system supplied by a simple mechanical fuel pump, and the prop resets to the default value. The pilot will see a fault indication, note a slight loss in power—then he’ll have to reach over and manually lean the mixture.
For its part, the TCM FADEC system has progressed significantly since we last looked in on it, although the root technologies remain intact. Unlike EPiC, Continental’s FADEC is pretty daring technologically. There are no true mechanical backup systems involved; instead, the electronic systems are designed to be fully redundant. Every two cylinders in TCM’s FADEC are served by a computer module that itself has two independent computers. They both "watch" the engine sensors and perform the firing of the fuel injectors and ignition system. (Unlike EPiC, TCM’s system will be able to control the flow of each fuel injector to tailor fuel/air ratios on a cylinder-by-cylinder basis. EPiC gangs all injectors together and simply controls mass flow.) Should the main computer stumble, the auxiliary unit will step in and take over control of the fuel and spark delivery. On a six-cylinder engine, there are three independent ECU modules.
Power for the system will come from the airplane’s main power grid, but a small backup battery will be installed as well. In testing, this battery—diode-isolated from the rest of the airplane except to take a trickle charge—was able to power the four-cylinder system for three hours. A preflight check of the system will disclose to the pilot the condition of each computer system as well as the state of backup-battery charge.
Continental has imbued its FADEC with much more sophisticated control logic. As a baseline, the FADEC uses a stored map that tells the computers when to hit the spark and when and for how long to hold the injector solenoids open. (On a pulsed system like FADEC, fuel flow is proportional to both inlet pressure and the duration of the injector pulsewidth.) Continental has made sure that the maximum fuel requirements of the engine are well below the flow rates of the injectors. This allows the system to time the burst of fuel to the opening of the intake valve. This helps to keep fuel from puddling in the intake tract and promotes better atomization.
In addition, TCM has refined the operating modes of the FADEC. A new throttle-position sensor tells the computers when maximum power has been demanded by the pilot and sets fuel flow and ignition timing accordingly. For the Katana test mule, flight-test engineers discovered that the old system—based purely on engine rpm—would let the computers dither the logic from maximum-power to cruise and back again because of prop-speed fluctuations. For all high-power operations, the FADEC uses best-power fuel/air ratios—normally, these engines are set up quite rich to account for atmospheric variability—with a routine to watch cylinder-head temperatures. Additional fuel can be used for cooling if CHTs get out of hand. In addition, spark timing can be retarded to help keep the heads cool.
In cruise, the pilot can select one of two modes. A maximum-performance mode will set the fuel injection to best-power mixture. It does this by first going to the lookup table and setting a baseline fuel flow and spark timing. Then, after a brief stabilization period, it cycles the mixture to find peak EGT on each cylinder. (It does this for all cylinders at once to prevent unnecessary vibration.) When it finds peak, it then enriches by 100 to 125 degrees for best power. If the pilot has chosen the best-economy mode, the FADEC will perform the lean cycle and settle on a fuel/air ration on the lean side of peak EGT.
So much technology is great, but when will you actually fly with this stuff? Continental has been aggressively developing its FADEC. It’s flying right now on the aforementioned Katana and a Cessna 172. Tests were due to start in late November on a Beech Baron and a Cessna Centurion. A Beech Bonanza was waiting in the wings, expected to be flying by late December. Although the system was envisioned to work best with the top-down, tuned induction systems, Continental feels that the system will work well with the runner/updraft induction systems found in the Baron’s IO-550-C engines and the 210’s IO-550-F.
Because Continental’s FADEC requires an internal engine modification—addition of small holes in the cam-drive gear to provide top-dead-center (TDC) and ignition-timing references for redundant hall-effect sensors—the company plans to focus on selling full kits, including rebuilt or new engines. It’s expected that FADEC will add around $5,000 to the cost of the engine change. As a payback, the company expects to see a 15-percent fuel savings over traditional fuel-injected engines and a 20-percent increase on the carbureted Lycomings that are part of the plan.
Lycoming and Unison are taking an intentionally conservative approach. Airplanes will be flying as you read this, with a predicted certification of the engine system on an IO-540 in April. At about that time, the system will go into an accelerated testing program at Unison on a Cessna 182, while Cessna will carry out a parallel program that will also incorporate changes necessary to the type certificates (or development of supplemental type certificates) for installation on production-line airplanes. (The airframe has not been chosen, but it will likely be a 182 or a 206.) Then, by autumn, the system will be made available to the seven airframe manufacturers that have expressed an interest in making EPiC standard or optional equipment. Finally, work on retrofits will begin. Because this is a joint program between Unison and Lycoming, the emphasis has been to serve the OEM market. Don’t expect to see EPiC retrofit packages before 2002.
Development of both manufacturers’ systems has been plugging along for most of 1998 and 1999. And determined flight testing from both concerns will take place early this year. So don’t be surprised if that Katana represents not just a bit of flotsam of the sea of aero-engine technology but truly the tip of the iceberg.
Links to additional information on electronic engine management may be found on AOPA Online ( www.aopa.org/pilot/links/links0001.shtml). E-mail the author at [email protected].
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