First Church of Combustion
Never operate your airplane engine lean of peak exhaust gas temperature. These guys aren't buyin' it.
- By George C. Larson
- Air & Space magazine, July 2004
(Page 4 of 5)
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.)
In June 2002 about 35 pilots from the Dallas chapter of the Experimental Aircraft Association flew to Ada to hear about the new way to operate their engines. They departed converted, and that visit led to the first formal seminar in Ada in September 2002 for paying customers.
It's hard to overcome the orthodoxy of the operating manuals and the notion that if engines were meant to be run this way, the aircraft and engine manufacturers would revise the manual. Four years ago, Textron Lycoming issued an advisory to its customers explaining the company's operating recommendations. "Operating an engine 'on the edge' is possible," the advisory states, "provided the pilot is extremely precise, has good instrumentation, and monitors the engine condition full time. For 98% of the pilots, it is an invitation to potential trouble."