First Church of Combustion

Never operate your airplane engine lean of peak exhaust gas temperature. These guys aren’t buyin’ it

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George Braly had been reading Internet postings by veteran airline pilots from the propeller days saying that they used to run their big radial piston engines on the lean side of peak EGT. Deakin had loads of Pratt & Whitney R-2800 time and could affirm to Braly that the names of instruments and methods may have differed, but leaning was leaning, and airline crews had been ordered by their companies to run their engines lean in order to reduce fuel consumption. But in the bargain they got cleaner spark plugs, valves, and cylinders, and perhaps the most important bonus of all-cooler operating temperatures-all as pure gravy. Braly couldn't understand why what worked in one piston engine wouldn't work in any piston engine. Born with enough tenacity for two people, he kept talking, asking questions, and reading.

That fall, a veteran pilot on the AVSIG forum told Braly that he had an old American Airlines book on how to operate the Wright R-3350. You might find it interesting, said the old vet. Perhaps the most complex powerplant ever to propel an airliner, the mighty -3350 squeezed every ounce of energy from the combustion process, in one version even using the exiting exhaust gas, already stripped of most of its energy by the turbo-supercharger, to turn a set of turbines that were geared back to the propeller shaft in order to capture the last twistlet of torque.

As soon as Braly got the old operating manual and read about the American pilots' lean-of-peak technique, he grabbed the factory graphs and charts for his Continental engine and calculated where his engine would be on the power curve if it were running under the same settings the -3350s were run at. The point corresponded to a power and fuel flow setting at exactly 50 degrees Fahrenheit lean of peak EGT. "It was the eureka moment," he recalls. By the summer of 1997, Braly, Deakin, and Atkinson were routinely flying with their engines running on the lean side of peak EGT and were ready to tell the world.

For a couple of years they hosted flying clubs at GAMI's hangar at Ada on Saturday mornings. In 2001, Atkinson put together a slide show to make the whole thing clearer.

Braly still wasn't satisfied with the data he was getting from the sensors and instruments. In 1998 Goulet had urged him to probe deeper, telling him that he needed to develop the means for measuring real-time cylinder pressure events. Goulet said he himself had spent a lot of time looking at combustion pressure data and that Braly would never really understand the engines until he understood the combustion events. "Within 60 days after that, I was flying with the first prototype combustion pressure sensors," Braly says. Now he could record the rise in pressure within the cylinder as the mixture began to ignite. On the first flight, Braly compared lean and rich combustion events at the same horsepower and found that the lean event produced significantly lower cylinder pressures and lower cylinder head temperatures. It was the second eureka moment.

Braly began to imagine a facility where he could study engines all day every day without having to go flying. He wanted to be able to change conditions like ignition timing, fuel octane, intra-cylinder pressure, and air inlet temperature-in short, to build a laboratory around an off-the-shelf, six-cylinder aircraft engine that he could poke and prod and see what happened. In 1999, GAMI built just such a lab around a six-cylinder Lycoming TIO-540. "We probably know more about the Lycoming TIO-540 than any other engine, and we probably know more about it than anyone else in the world," Braly says. Later that year, when Carl Goulet died, the lab was named for him.

With the lab up and running, Braly could gather data and translate it into operating knowledge any pilot could use. He expresses it in a graph showing the impact on an engine of the fuel mixture (opposite); this set of curves forms the new orthodoxy of engine operation. The horizontal axis can be thought of as the movement of the mixture control from full rich on the left to lean on the right. The topmost curve indicates that EGT peaks at a certain value and forms the reference point (dotted line) for managing the engine's operation. The second curve plots cylinder head temperature across the range of fuel flow, showing that a pilot can expect CHT to max out at a point slightly to the rich side of peak EGT. Almost perfectly parallel to CHT is the internal cylinder pressure curve. Just beneath it, the horsepower plot reveals that maximum power is reached in an even richer area of the fuel flow range. A curve of computed points, at the very bottom of the group, plots horsepower per pound of fuel burned per hour-a way of expressing fuel efficiency.

Braly's work showed-and the seminar teaches-that once the fuel injection systems of Lycoming and Continental engines have been adjusted to deliver the proper quantities of fuel to each cylinder, pilots can operate their engines over on the right side of that set of curves. (And way over on the left too, with rich mixtures at high power. It's the range in the middle students will learn to avoid.) Pilots in the classroom learn that cylinder head temperature-a critical measure of engine health-rises because of rising intra-cylinder pressure. Operate on the right side of the curve with a lean mixture and CHT drops off nicely.

But over on the right side of the curves, the horsepower falls off too. How did the airlines recover the power lost when they ordered their crews to lean the mixture? The old books and the veteran pilots revealed the simple answer: They moved the throttle back up from its reduced cruise-power setting until they got the horsepower back to no more than about 65 percent of rated power. And when they did that, they found themselves with an engine that was operating at the peak of the last curve-max fuel efficiency. In effect the First Church of Combustion is preaching the gospel of using both fuel and air, rather than just fuel, to manage engine power.

(For a more detailed explanation of the Wright Aeronautical Division-WAD-Leaning Procedure, read Braly's narrative at www.gami.com; click on "Future Series." Turbocharged engines offer a more complex picture.)

About George C. Larson

George C. Larson served as editor of Air & Space from 1985 to 2005. He is currently an inactive pilot, but holds a commercial pilot's license, with instrument and multi-engine ratings. He is between airplanes at this time, but has owned or operated a Grumman American AA-5B Tiger and a Mooney 201. He has been writing about aviation since 1972, when he joined the staff of Flying Magazine.

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