Airframe and Powerplant

Power to the Processor

November 1, 1998

Electronic engine controls emerge

Solutions to large, vexing problems don't always come in convenient packages. Often, the best approach is to tackle a thorny issue — such as keeping the current crop of opposed, air-cooled aircraft engines viable — from multiple angles. The two major engine manufacturers, Continental and Lycoming, are more concerned now than ever that, without significant development and infusions of technology, the sun will set on our familiar powerplants (see " Enginuity," October Pilot). The impetus is double-pronged. For one, pilots — particularly new pilots — are ever more astounded that in an age of turn-key, minimal-maintenance automobiles, aircraft engines hang onto ancient technologies such as magneto ignition and manual mixture and propeller control.

And then there's the fuel issue. Leaded aviation fuels are not likely to take us more than a decade or two into the future, so one of two scenarios is expected. The new breed of alternative-fuels (read: diesel) engines that are on the drawing board from a variety of manufacturers will supplant today's avgas-burners. This will probably be an expensive solution to the fuels problem and one that will not apply to all aircraft — it'll be a hard sell to get owners of older airplanes to pony up nearly the value of the whole airplane for a retrofit.

But there's another tack, one that embraces computer control of existing powerplants with an eye toward making them run on low-octane or some interim-octane unleaded avgas. To prepare for the day that lead is legislated out of avgas, petroleum engineers have been working hard to find a replacement for 100LL. They may, or may not, come up with a solution that's a one-for-one replacement.

Hedging on the outcome of the petro-engineers' work are the engine manufacturers. Continental and Lycoming have programs in place to trade pilot-operated, mechanical engine controls for computerized versions. Among the critical advantages is the potential to run the engines on a lower octane than they are currently certified to consume. (There is also the side issue of whether every engine certified on 100-octane fuel actually needs that much antiknock quotient; some of them, however, might truly need more.) Gains in engine longevity and efficiency are also in the cards, should the engineers' hands play out right.

Given the rapid maturity of computer technology, it's not surprising to see electronic controls emerge. Aurora Flight Sciences, under a NASA grant, developed an electronic ignition and fuel-injection system on a Cessna Skymaster (see " Airframe and Powerplant: Under New Power Management," March Pilot). Continental has been working on electronic engine management for nearly a year with new Teledyne subsidiary Aerosance. Lycoming and Unison Industries (maker of Slick mags) announced at Oshkosh a program called EpiC — for electronic propulsion integrated control. Along with electronic management of fuel and spark, EPiC and Continental's systems provide single-lever power controls. Both systems let the pilot retain physical control of the throttle plate assembly and force the electronics to match fuel flow and ignition timing to the conditions; in this sense, they are not truly fly-by-wire systems. Both systems will have computer-controlled prop speed, however, and these speeds will likely be set to obtain the best compromise of prop efficiency, noise signature, and vibration levels.

EPiC proportions

Unison's EPiC is a comparatively conservative approach to piston-engine FADEC — a term taken from the turbine world that stands for full-authority digital engine control. In a sense, the Unison/Lycoming setup aims to merely control by microprocessor many of the familiar systems — fuel delivery, turbo wastegate, propeller governor — not replace them with entirely new components.

Central to EpiC's certifiability is a built-in backup system. Each subsystem has a simplified, mechanical backup — the prop goes to high rpm; the ignition reverts to fixed timing; and the wastegate to some intermediate, fixed position. Such a philosophy trades what may turn out to be technological benefits of newer delivery systems for a simple, purely mechanical backup mode. Unison intends for the EPiC system to revert to a fully flyable mode in the worst-case, loss-of-power situation.

For fuel delivery, EPiC employs the injector lines and nozzles from the Bendix arrangement already installed in most IO-series Lycomings. An electronic valve meters fuel flow, which is verified by an electronic flow transducer. In the backup mode, EPiC will revert to a mechanically metered arrangement very similar to Continental's standard fuel injection. A set of metering spool valves connected to the throttle arm and cockpit mixture knob — which will normally be caged for electronic flight — will provide the backup to the electronic metering valve. Unison says the transition from electronic to mechanical fuel control will be seamless. The engineers will have their work cut out here; it's assumed that the mechanical backup will be parked in full-rich mode, which will provide a lot more fuel than the engine needs for high-altitude normal cruise. Speaking of mixture control, Unison says that EPiC will employ a pilot-selectable mixture control, for either best power or best economy. Expect the rich mixture to be out near the best-power setting of 100 degrees Fahrenheit rich of peak exhaust-gas temperature, with best economy near peak EGT. Unison says it will test lean-of-peak operation during development.

In addition, EPiC will use an electronic prop governor — that reverts to maximum rpm if the power goes off or the computer sends the wrong signals — and electronic wastegate control for turbo applications. Sparks emerge from a version of Unison's LASAR electronic ignition system, its guts housed in conventional magneto bodies. As with the current LASAR models, if the electronics pack it in, the mags revert to the standard self-powered, fixed-timing mode.

There are two ways to manage a FADEC system, both of which have been explored in cars and motorcycles for decades. An open-loop system uses sensors to tell the computer about engine speed, throttle position, various temperatures, and other operating parameters. The computer takes this information and compares it to a lookup map stored in memory. For every set of variables, the computer can calculate the correct ignition timing and the duration of the fuel-injector pulse width, which determines fuel flow. It does not, however, have a means of checking its work. Closed-loop systems, on the other hand, employ some means of feedback, either through an oxygen sensor — not currently slated for either the Lycoming or Continental systems because of problems of lead fouling — or exhaust-gas temperature. Closing the loop allows the microprocessor to determine whether the fuel delivery and timing are obtaining the desired results.

Unison expects the EPiC system to work in the open-loop mode. There are provisions for such a feedback sensor within the EPiC system, but don't expect to see the O2 sensor used until we transition to unleaded fuels.

Unison believes that replacing expensive mechanical systems — priced a governor or Bendix fuel servo lately? — with simpler electronic equivalents will result in a system no heavier or more costly than the one it replaces. Unison expects to see immediate fuel economy gains and a long-term boost in engine reliability.

Cessna is expected to be Unison's launch customer for the EPiC system; testing will take place on a Lycoming IO-540. Although Unison and Lycoming are predicting EPiC to appear first as a new-airplane installation, they also are talking in terms of the retrofit market.

Continental's solution

Continental's version of piston-engine FADEC is more sophisticated and employs different engineering philosophies. Most obvious is that Continental, instead of relying upon mechanical backups, has built in significant electronic redundancy. A single ignition/ fuel-delivery computer runs a pair of cylinders only — a six-cylinder engine would have three separate boxes — and employs two processors each. Should one processor go off line, the other can pick up the slack. As part of the package, Continental expects to certify a pad-mounted standby alternator to provide electrical redundancy.

Whereas the Unison/Lycoming system meters mass fuel flow to all injectors, the Continental FADEC uses tiny solenoids between the fuel-delivery spider and injector nozzles — the standard engine-driven pump is retained — which more resembles an automotive system than standard aircraft fare. These solenoids precisely meter fuel flow to each cylinder; they are also manipulated individually, giving Continental the opportunity to tailor the fuel/air ratios at each cylinder regardless of intake-tract quirks or environmental conditions. The remainder of the induction system can be either Continental's tuned, top-down system or the more common runner-updraft setup.

Continental intends to run its FADEC engines on the lean side of peak EGT — by about 50 degrees Fahrenheit — for best-economy operations up to 65-percent power. For takeoff, climb, and maximum-power cruise, the FADEC will command best-power mixture settings. Individual EGT and cylinder-head temperature probes will monitor the results, and the computer can call for additional fuel or reset ignition timing for any cylinder individually should either parameter exceed preset limits. Optional on the TCM setup is automated control of the boost pump.

Because the fuel-delivery computers will have access to flow information and various temperatures, expect to see cockpit instrumentation a step beyond what we have now. In addition to the multi-cylinder EGT and CHT readouts, expect to see calculated percent of power and integral, sophisticated fuel computers. Continental will offer an optional cockpit display that will include a health monitor, essentially an annunciator panel that will point out exceeded conditions to the pilot; this will relieve the pilot of having to include every engine instrument in his scan.

Continental is carrying out testing of its FADEC on an IO-240-powered Diamond Katana; the prototype was shown at Oshkosh this year. Diamond expects to put the system on a version of the D1 Katana as soon as it's ready. Diamond President Michael Slingluff says, "We are excited about the possibilities of the system and definitely want to see it into production." Continental also is eyeing the retrofit market.

Finally, it's worthwhile to note that Continental's and Lycoming/Unison's systems employ integral data logging. Should your shiny new, electronically managed engine go poof halfway to TBO, the manufacturer or maintenance facility will be able to view the operating conditions of the engine to see whether it's been abused. A better deal for owners is this: Presumably, with the system working properly, there will be no way for the pilot to abuse the engine — all key parameters will be controlled by the computer — so the engine makers will have a harder time hiding warranty coverage behind the old barricade of pilot incompetence. Big brother may be watching the way you fly, but it's possible he'll be on your side.


E-mail the author at marc.cook@aopa.org.