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March 1, 1998
MARC E. COOK
Two schools of thought prevail regarding development of the next generation of piston aircraft engines. One says that we need to forsake these cranky old designs and get on with implementing new, cutting-edge powerplants that take advantage of everything we've learned about internal-combustion engines in the last five decades. Usually, this faction points to automotive-based engines as the seed of future flowers.
Then there is the camp that says our basic engines — for the most part flat, air-cooled, direct-drive — are ideal for the task and that we can make the most gains in longevity and efficiency by tossing out antiquated fuel- and spark-delivery devices. Teledyne Continental's purchase of a controlling interest in Aerotronics Controls, Incorporated, may presage a long-anticipated shift in aircraft-engine technology. ACI, started in 1996, has been working on electronic engine control for piston engines, and it intends, under TCM's stewardship, to bring such a system to market.
Several companies have tried, and many more have tested, electronic engine controls. Mooney's Porsche-powered PFM used a single-lever power control and electronic ignition. Its fuel-injection system, a derivative of the Bosch L-Jetronic, was mostly mechanical in nature — just a step or so more advanced than our existing injection systems. And the PFM's single-lever power control was provided by the expedient of tying the throttle to the prop governor, resulting in the engine's running at wide-open throttle during normal cruise, regardless of altitude. (This tactic is, from an efficiency standpoint, ideal, but it incurs some operational shortcomings.)
What many forward-thinkers envisage for the next great step in engine management is substantial computer control — with authority over almost all firewallforward systems. The processor will determine the ideal settings of manifold pressure, rpm, mixture, and cooling flow for the commanded amount of power and the prevailing conditions of outside-air temperature and airspeed. The pilot will push a single lever and watch a digital display of power (in percent or absolute) until the desired amount shows up in big numbers on the panel. Everything else will be automated.
Sound a bit like science fiction? It's not. Turbine aircraft have for years used full-authority digital engine controls — called FADEC. In the piston world, Aurora Flight Sciences, the Manassas, Virginia-based firm that develops ultra-high-altitude unmanned vehicles, has conceived and tested a computer-controlled, single-lever power control for piston engines under a NASA grant. It was tested on the front engine of a Cessna O-2 (the military version of the centerline-thrust-twin 337.)
Aurora's main business is developing unmanned research aircraft that can climb to very high altitudes (in excess of 60,000 feet) and gather scientific data. To have usable power from a piston engine at those rarified altitudes, multistage turbocharging is required. Aurora's ULEV, Perseus B — which spawned the electronic-control system used on the O-2 project — uses a three-stage turbo system to feed an 80-hp, liquid-cooled engine at 60,000 feet. Because of the difficulty of managing the turbochargers in such thin air, electronic fuel injection and computerized engine management were developed.
A derivative of the Perseus's system was fitted to the Cessna. At the core is a 32-bit computer that manages three new systems: electronic fuel injection that's overlaid upon the standard mechanical, continuous-flow setup standard on the Continental 210-horsepower IO-360; electronic ignition in place of one magneto; and modified electric servos (from an S-Tec autopilot) that control throttle-plate and prop-governor position. By combining control of these systems, the computer has tremendous power and flexibility.
Aurora's electronic fuel injection is primarily a mapped system, using lookup tables stored in the computer to associate a certain fuel flow with particular combinations of manifold pressure, engine rpm, ambient temperature, and requested percent of power. It then modulates the six Siemens fuel injector nozzles atop a modified Continental tuned-induction system to deliver the right amount of fuel. Like automotive applications — in fact, the injectors are the same as those used in the Dodge Viper sports car, which has a 400-hp, 8-liter V-10 — the amount of fuel provided by the injectors is proportional to how long the fuel-control solenoid is left open. Increasing this pulse width increases fuel flow. As initially tested, the Aurora system gangs all injectors together electronically, so they open and close simultaneously and provide the same amount of fuel. Future applications could see these injectors separated and the computer dictate different fuel flows for each cylinder according to its needs.
In addition to checking the fuel injection's progress with various temperature sensors, the Aurora configuration employs an oxygen sensor in the exhaust stream. Used in cars for almost two decades, this sensor "smells" the exhaust system for the presence of oxygen released as the fuel/air ratio reaches the chemically correct relationship, called stoichiometric. This mixture setting closely, though not precisely, corresponds to peak EGT. In the R&D airplane, the Aurora computer commands operation at or near stoichiometric for most cruise power settings and near best-power mixtures for takeoff and climb; some configurations run the engine on the lean side of peak, but only at reduced power settings. Because the computer is so flexible, there's no reason that a production version couldn't specify any range of fuel/air ratios for any flight regime consistent with moderate temperatures and smooth running.
Part of the beauty of the Aurora system is that the original mechanical setup remains. An electronic bypass valve prevents the standard mechanical fuel pump from delivering its goods to the throttle body and injection spider, thereby allowing the electronic system to be the sole provider of fuel. But if the electronics drop off line, the valve opens (it's held closed by electricity) and, after a second's hesitation, the TCM system pressurizes and begins feeding the engine. From the cockpit, the sensation is a bit like running a tank dry, with a brief, complete loss of power.
Electronic ignition takes the place of one conventional magneto in the Aurora project. This was done primarily because there were so many other unknown systems on the airplane that it made good sense to keep at least one known source of ignition. Ignition timing varies by engine rpm and approximated power output; the computer can read manifold pressure as well as throttle-plate position, making the selection of the most desirable spark timing both straightforward and precise. Aurora's setup naturally knows to retard spark for starting and, combined with the no-fuss nature of the fuel injection, makes for an easy-starting engine. Production examples of this technology would probably use dual independent electronic ignition systems.
Because the desire here is to manage all systems automatically, some means of manipulating the throttle plate and prop governor were needed. Aurora called on S-Tec to supply modified autopilot servos. Admittedly, the power of these servos is overkill for moving the throttle and prop controls, but their integral declutching mechanism is an important part of the system's fail-safe design. As commanded by the computer, the servos move the throttle and prop controls — with movement visible in the cockpit because the standard levers remain in place — according to the prescribed logic for the phase of flight.
Getting this Cessna 337 aloft is a bit more complicated than doing so in an unmodified version. First, you turn on the electrics and let the computer perform a self-test routine. Later, just after engine start, you must run tests on the electronic fuel injection and single-lever power control separately, in addition to the usual pretakeoff checks. At least starting the engine is straightforward: Turn on the pumps and mags, punch the starter, and the Continental jumps to life with less complaint than normal. (The test airplane suffered a bit from poor idle after a cold start, explained by test pilot Ken Zugel as a lack of development of the engine mapping at or near idle. Most of the work had been done in the critical high-power ranges.)
It's when you swing onto the active that the Aurora-equipped airplane really seems odd. For the front engine — fitted with the electronic controls — you can jam the power-control lever to the hilt in a way that would savage the stock engine. The computer manages the rate at which any of the controls is moved, so even the most ham-fisted pilots will seem like kindly caretakers to the engine.
For takeoff, the throttle and prop controls go full forward and the injection system delivers the same fuel flow as the stock TCM setup would for maximum power. In this sense, all is familiar. At the first power reduction, pull back on the power control to the desired numbers; in a production version you would see a percent-of-power display to guide you. Aurora has programmed the system for maximum efficiency, so the throttle stays wide open as the prop control comes smoothly back. For our flight, the climb detent dropped the prop back from the takeoff setting of 2,800 rpm to 2,500 rpm. Pull the power control back further and the prop will continue to draw down engine speed. During testing, Aurora programmed the system to bring engine speed down to 2,300 rpm before reducing throttle. (Through the entire demo, the hardest job for pilot Zugel was keeping the unmodified rear engine in sync with the automated front.)
During cruise, there's just not much to do. Set the power you want and watch the gauges to make sure that all's well under the cowling. The fuel injection system picks the best mixture setting, the ignition goes into its most-efficient curve, and the throttle and prop controls take care of themselves. In the O-2, the cowl flaps were still manually controlled, but a production version of this undoubtedly would integrate existing cowl flaps into the control loop for true set-it-and-forget-it cruising. One interesting note: Because the injection seeks a mixture setting close to peak EGT, the cylinder temps on the O-2 were high, running in excess of 400 degrees Fahrenheit on a cool day. Future applications could easily run richer or leaner to mitigate temperatures — in fact, the Aurora project switched the IO-360's log-runner induction for Continental's tuned system in hopes that it would run smoothly at desirably lean fuel/air ratios.
For descent and landing, the computer commands a reduction in throttle position until the airplane is at what the engineers have determined is final-approach power. Then, the prop control returns to the high-rpm setting to be ready for a sudden go-around. The logic for this part of the system is still under consideration. On one hand, with the prop coming back up during the low-power part of the approach, you get the benefits of extra drag and improved response to power-control movement. Conversely, this scheme presents a noisier approach and may in fact be unnecessary because during a commanded max-power go-around, the computer could just as quickly run the throttle and prop controls to the stops simultaneously.
It's all a bit odd, watching the Cessna's engine controls moving on their own, marching to the unseen commands of the computer behind the pilot's seat — but fascinating, too. There's so much potential for the system: integrating prop synchrophasing in twins; allowing the computer to help you determine which engine has quit cold and beginning the prop-feathering process for you; and working the fuel and ignition to maintain even, consistent temperatures regardless of the airspeed or atmospheric conditions. Aurora also sees economy in two senses. First, the company's engineers predict that fuel savings of as much as 15 percent are possible. Second, with the pilot out of the loop, so to speak, it would be impossible to damage the engine from improper management.
Aurora isn't guessing at the costs of installing one of these systems on an existing airframe. There are too many variables from airplane to airplane, and it's too early to tell how much custom-built servos and computers would cost — to say nothing of the certification costs. Moreover, the company's bright thinkers admit that all attempts must be made to simplify and reduce the weight of the installation. It's up to companies like Aurora and Continental's new acquisition to bring this technology off the drawing board and into our airplanes. Certainly, it behooves TCM to make its bread-and-butter product survive; this technology is the key.
E-mail the author at [email protected].
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