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 2 of 5)
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.