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
IT HAPPENS EVERY TWO MONTHS. AT ABOUT NOON on Friday people begin to arrive at the little airport that serves Ada, Oklahoma, population 16,000, lying roughly 80 miles southeast of Oklahoma City. Alternately griddle-flat prairie and gently rolling hills, Ada probably has a little picture of a longhorn cow and an oil well next to its name on those schoolbook maps. But the people flying in here today don't deal in cattle or petroleum.
They come from across the nation, many of them flying their own aircraft. Beech Bonanzas and Barons are the most numerous types out on the parking ramp. But here's a single-engine Cessna on stork-tall amphibious floats, its registration and little red maple leaf indicating that it flew here all the way from Canada. These visitors share one thing: They're pilots who fly aircraft powered by reciprocating engines. General aviation aircraft. The little guys.
Some of them are here because they've heard that they've been running their engines wrong all these years and they want to learn how to do it right. Some have thousands of hours, and others are barely starting out. Some are openly skeptical, and some may even harbor a private urge to unmask all that is shown and said here as fraud and sham so they can depart vindicated. Some of these people will have a hard time accepting what they will hear because it is hard to admit you've been doing something the wrong way for a very long time, especially if you are a pilot.
They have come to a class with the vaguely worded title "Advanced Pilot Seminars." The session opens on Friday evening and ends on Sunday afternoon, and the lessons are delivered in intense doses. But compared to how airmen and -women have been trained in the past, what goes on here is really closer to the founding of a religion. Call it the First Church of Combustion.
Its bishop is George Braly (pronounced BRAW-ly), an aeronautical engineer and attorney with a Wilford Brimley mustache and a booming voice cultivated in the courtroom. John Deakin and Walter Atkinson have signed on as disciples since being converted in the mid-1990s, when they were the first, aside from Braly himself, to test the tenets of the new gospel in their own airplanes. Deakin has the wise look of a wood owl. He retired in 2001 as a captain with Japan Air Lines, and the Boeing 747 time in his logbook adds up to more than four years. Atkinson's day job is dentistry, but he is also rated as an airline transport pilot, airframe-and-powerplant mechanic, and flight instructor. In the right light, he's a pretty good double for actor Fred Willard.
Braly and the two disciples promise the converted a life of airplane engine happiness, with cooler operating temperatures, fuel savings on the order of three gallons per hour for a typical six-cylinder engine in a Beech Bonanza, and reduced life-sapping carbon deposits on the valves and pistons. All they ask is that the believers ante up for precision engine monitoring systems.
The three pilots became friends while they were exploring the same subject they are about to preach in the classroom, which has filled with 36 students, each leafing silently through a fat three-ring binder. On the binders' covers is the name of the course: "Engine Management Made Easy." The $995 tuition covers all meals (except one on Saturday night), which are taken on site to cut down on travel time to restaurants. And the students will find that they need every minute of classroom time they can get. Here is some of what they'll learn:
All reciprocating engines that burn gasoline are ruled by the incontrovertible laws of chemistry and physics. They produce power by drawing air into a cylinder, mixing it with a combustible amount of gasoline, sealing the cylinder, compressing the mixture, and igniting it at just the right moment with an electric spark. Most modern engines use some method of fuel injection to mix the gas and air. What's different about aircraft engines is that they operate at widely varying altitudes: As the airplane climbs, the air becomes thinner. With less air to support combustion, the amount of gasoline to be mixed with the oxygen molecules must be reduced accordingly. Which is why airplanes have an engine control you'll never find in a car: the mixture control. Whether it's a knob or a lever, the mixture control adjusts the flow of fuel to all the engine's cylinders.
Student pilots who train in general aviation aircraft have traditionally been taught that at some altitude during the initial climb (typically 3,000 to 5,000 feet), they should move the mixture control from full rich, the setting for takeoff, to lean, then even farther to lean after they pull back the throttle to the cruise-power setting. While leaning at cruise, they learned to keep a sharp eye on an instrument that displays the temperature of the engine exhaust gas. The instrument, the exhaust gas temperature (EGT) gauge, sometimes uses a graphical bar display. (Older gauges used a needle on a dial.)
When the gasoline-air ratio is such that combustion has used up both fuel and oxygen, combustion occurs at the highest possible, or peak, temperature. (This mixture is described by chemists as "stoichiometric.") If the mixture is rich, containing an excess of fuel, or lean, containing an excess of air, the temperature of the combustion process drops. In managing mixture control, it is not a matter of what the absolute temperature of combustion is but where the mixture is relative to peak temperature, which serves only as an easily measurable reference point. Aircraft engines are not operated at peak EGT despite the apparent chemical perfection of combustion there. Cooler operating temperatures are desirable; on that all agree.
"Lean the engine until the EGT needle reaches its maximum temperature," instructors intoned, as student pilots gently pulled knobs or moved levers, "and then move it back until you are running 50 degrees on the rich side of peak temperature." The occasional inquisitive pup might ask why this is done. Instructors would warn of toasted valves, burned spark plug electrodes, holed pistons, and engine failure.
And all pilots learned through experimentation and experience that the Continental and Lycoming engines on their airplanes began to run rough around the point of peak EGT. They ran especially rough if one continued to lean the mixture past peak temperature-the dreaded lean side of peak. Roughness suggests engine failure; passengers get wide-eyed and pilots feel their palms getting moist. No one asked why these four- and six-cylinder air-cooled engines ran rough when leaned. Here be dragons, said the conventional wisdom; just don't go there. But George Braly, who bought a Beech Bonanza in 1991 and shortly thereafter installed an instrument to measure the EGT for each cylinder, noticed that when he pulled the knob that leans the mixture and reduces the fuel flow, the six cylinders of his Continental IO-520 engine reached their peak temperatures at widely scattered points across that range of motion. Why didn't they all peak together? he wondered.
On Compuserve's online aviation forum, pilots of all stripes-and those with none-could debate freely and anonymously the precepts of their training. In 1991 Braly began wondering about engine mixture management in messages to John Deakin. In an e-mail, Deakin recounts that time: "It took Compuserve's AVSIG [AViation Special Interest Group] to bring us all together and serve as a catalyst." Braly led the way, Deakin recalls, "with the rest of us asking questions he could not, at first, answer. Drove him nuts, so he began (in about 1994) the long, long trail that leads to today."
Braly says that the prevailing opinion of the time was that the peak EGT spread he saw on his engine was attributable to the design of Continental's induction system-that there was something wrong with the airflow (it's actually quite good). But mechanics adjusted the fuel injection systems on these engines on the theory that the airflow to each cylinder was equal and perfect. Using four or six containers (often cola bottles, resulting in the coinage "Coke bottle test") to catch the gas and determine the volume delivered, they would carefully tweak the system until it was metering precisely the same amount of gasoline through each injector to its respective cylinder.
Continental engines use continuous-flow fuel injection systems: The injector spritzes fuel in a flow as steady as a garden hose, even when the intake valve to the cylinder has closed. Braly began to suspect that some of the fuel that accumulated when the valve was closed was making its way down the induction system to the adjacent cylinders. If he was right, some cylinders were getting the wrong amounts of fuel, and the variation would prevent all six cylinders from arriving at peak EGT simultaneously. And if the fuel flows that brought the cylinders to peak EGT were different enough, the power outputs from all the cylinders would differ at leaner mixtures, where the power falls off quickly. No wonder the engines ran rough.
Maybe fuel distribution to each cylinder shouldn't be equal. Maybe it should be different.
By 1993, Braly had teamed up with an Ada-based parts manufacturer, Tim Roehl, to form General Aviation Modifications, Inc. He and Roehl began to experiment with injector nozzles calibrated to deliver fuel at a rate precisely matched to the needs of each cylinder. They had help from new microprocessor-based systems that displayed in monkey-simple graphics all the important engine data: exhaust gas temperature-not just for the engine, but for each cylinder-cylinder head temperatures, and, for turbocharged engines, turbine inlet temperature.
Braly started looking for an expert to help with the process of getting a Supplementary Type Certificate from the FAA for GAMI's new injector. Someone recommended a Texan named Carl Goulet. "I had no idea at that time that he was the former head of engineering at Teledyne Continental Motors," Braly recounts. "We had a very short and very remarkable conversation. I was considerably his junior, and he said, 'Now young man, tell me what you plan to do.' I told him, and these were his exact words: 'Hot damn! Somebody's finally gonna fix this problem!' "
GAMI applied for an STC in 1996, and the FAA approved it in the same year. At Goulet's suggestion, Braly submitted a proposal to Teledyne Continental Motors offering to supply fuel injectors and provide customer support. Hearing nothing, GAMI began to market the modification to pilots. Deakin and Atkinson were among the first to install the new injectors in their engines.
Atkinson remembers that after getting the injectors installed and heading home in his airplane, he leaned the mixture by ear, the way he'd been used to. Ignoring the EGT gauge, he liked to pull the leanerator until the engine ran rough, telling him he was just lean of peak, then adjust the mixture from there. "Except this time it wouldn't run rough," he recalls. "I kept pulling it back and it just kept running smoothly."
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.)
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."
Some people depart the weekend in Ada unmoved and cling to the old ways. But to date, Braly and his disciples have converted several hundred pilots, who return to their homes merrily pulling the mixture controls on their engines with abandon. Pull the mixture, all ye souls! Fly lean of peak and be free! Until you have done it yourself, some of them say, you have not tasted of the fruit.
Fred Scott, a farmer and Beech Baron owner from southern Virginia, recalls his first experience: "A friend and I had climbed to 11,000 feet on the way back home from the school. And we pulled it back through the peak [EGT]. 'Engine's gonna melt,' they used to tell us. We're thinking, Well, we gotta believe. And sure enough, the head temperatures went down and it was like a book opening, a complete revelation to see the science you learned in the school made sense. All of a sudden it was all true. We were like two little kids."
Students at the seminar are surprised when they don't hear the bad-mouthing of Continental or Lycoming engines that's common among pilots-"Lyconentals," they call them, a derisive term that encapsulates the notion that both engines are old technology and essentially interchangeable. All three teachers certainly have the chops to criticize, but they're very complimentary of the durability and efficiency of both manufacturers' engines.
As for Braly, he and the GAMI crew are now working on an electronic spark ignition system that monitors intra-cylinder pressure and adjusts the spark timing automatically to manage the pressure generated by combustion and eliminate detonation ("engine knock," in car talk), a condition in which combustion occurs too early, raising intra-cylinder pressure and ultimately destroying the engine. Each seminar class watches as the big TIO-540, running on 100-octane low-lead aviation gasoline, is switched to rotgut unleaded auto gas without missing a beat. Braly can't give a firm date by which FAA certification will be complete. Pray it's soon.