Lower temps, lower power
Seventy-five-percent power can be generated with the engine fuel-air mixture set rich of peak — it also can be generated when the mixture is set lean of peak. Reproducible data garnered from extensive test-stand research presents a compelling argument for lean-of-peak fuel management techniques. The same data also supports rich-of-peak mixtures — mixtures that are a lot richer than engine manufacturers and pilot's operating handbooks (POHs) have been touting in print.
According to studies at the engine test stand at the General Aviation Modification Inc. (GAMI) headquarters in Ada, Oklahoma, whenever fuel-air mixtures are selected that result in exhaust gas temperatures (EGTs) between 125 degrees Fahrenheit on the rich side of peak and 50 degrees on the lean side of peak, combustion pressures within the cylinders are markedly higher than when the EGTs are outside this range. Not only are the pressures high, but the highest pressures (peak pressures) occur at a less-than-ideal moment in crankshaft rotation.
A more efficient crankshaft angle, lower combustion pressures, and the resulting cooler cylinder temperatures are the reasons for operating lean of peak. When increased fuel economy is added to these three pluses, GAMI's argument for lean-of-peak mixture settings is very persuasive. There's a catch, of course. Most fuel-injected general aviation engines will need a set of GAMI's balanced and finely tuned fuel-injection nozzles to successfully run lean of peak. In fairness, there are distinct advantages to the GAMI nozzles for pilots who are perfectly happy following more traditional leaning practices.
Last month's article (" Airframe & Powerplant: Fleet Fliers Run Rich — With Good Results," August Pilot) illustrated that running rich of peak — even full rich — doesn't automatically shorten engine life or damage a high-performance air-cooled airplane engine.
Leaning learning
The fuel metering systems installed on reciprocating general aviation engines, and this includes all types of systems, deliver much more fuel than is necessary for complete combustion. Complete combustion occurs when the fuel-to-air ratio is 100 pounds of air for 6.7 pounds of fuel (14.9-to-1). The scientific term for this chemically correct ratio of fuel-to-air is a stoichiometric mixture. This ratio also produces the hot.test temperatures within the cylinder combustion chamber. Since 1962, when Alcor Aviation, of San Antonio, introduced the Mixture Control Indicator, pilots have been able to accurately determine the point of peak exhaust gas temperature and have been able to accurately set mixtures both lean and rich of peak.
Waypoints on the mixture scale
The point in the fuel-air mixture that produces the maximum power for a given power setting (manifold pressure and propeller rpm) is termed best power. This best-power point is located at 80 degrees rich of peak; most pilots adjust for 100 degrees rich of peak and call that best power. It's close enough. Best power (100 pounds of air to 8 pounds of fuel, or 12.5-to-1) gives 100-percent engine power output for the power setting selected.
Under normal conditions, pilots flying high-performance turbocharged engines that are well maintained and baffled can keep the cylinder head temperatures (CHTs) under 400 degrees F.
When cylinder head temperatures rise, the normal course of action is to open the cowl flaps and reduce the power setting and/or pitch angle to increase airflow over the cylinders. The final and least desirable course of action is to richen the mixture. George Braly, the chief engineer at GAMI, says that the cylinders don't run cooler just because of the increased vaporization of the excess fuel being introduced into the mixture — they run cooler because the rich mixture slows the combustion rate, resulting in lower peak pressures that occur later in the combustion cycle. A college engineering text titled The Internal-Combustion Engine in Theory and Practice, by Charles Fayette Taylor, supports Braly's theory because it shows that mixtures that are both richer and leaner than the chemically correct mixture that occurs at peak EGT cause the speed of combustion to slow down.
Leaning signposts
Lycoming and Teledyne Continental publish service information that addresses leaning. TCM Service Bulletin M89-18 gives suggestions for fuel management using EGTs. Boiled down, this bulletin says that cruise mixtures should be limited to no leaner than 50 degrees F rich of peak at maximum cruise power settings (75 percent) and no leaner than peak EGTs at economy cruise power settings (65 percent and lower). This bulletin also cautions pilots to add 50 degrees to these settings if the engine is equipped with a single EGT probe (rather than a probe in each cylinder). Lycoming's general leaning information (Service Instruction No. 1094D) is similar except that leaning to peak is allowed at power settings of 75 percent and lower.
Based on GAMI's contention that operation within the plus-125-degree-and-minus-50-degree mixture range is less than ideal, you might conclude that both the TCM and Lycoming leaning guidelines are a compromise. There's no doubt about that — there are pluses and minuses that have to be considered in every machine. The engineers most likely decided that the higher temperatures and pressures that resulted from leaning to peak EGTs was a good trade-off — fuel consumption was decreased without a big compromise in utility or safety.
Even if Braly is right, there's a lot of evidence that following the manufacturers' suggestions doesn't measurably shorten engine life or stress cylinders.
These same manufacturers' leaning guidelines could be read to suggest that both manufacturers discourage running their engines with the mixture ratios set on the lean side of peak. But that's not the case. A number of Lycoming and TCM engine operating manuals give detailed instructions for lean-of-peak operations. In fact, one of the selling points of the TCM Aerosance full authority digital engine control (FADEC) system that will soon be certified for installation on TCM engines is the fuel savings that will result from the FADEC automatically leaning the mixture to lean-of-peak settings when cruise power is selected.
Lean-of-peak realities
One of the realities of operating lean of peak is that the power output drops off dramatically. When the mixture is leaned to 50 degrees lean of peak the power output is 8 percent lower than at best power. Counterbalancing the loss of power are cooler cylinder head temperatures and lower fuel consumption.
Cylinder head temperatures rise as the mixture is leaned until they top out with the mixture between 50 degrees rich of peak and peak. At 50 degrees F rich of peak the CHTs are down from the maximum by a few degrees; at 50 degrees F lean of peak CHTs are down by 35 to 45 degrees F. In exchange for a power drop-off (from peak EGT) of about 6 percent, the CHTs are reduced by more than 10 percent.
In an article in the October 1965 AOPA Pilot Al Hundere, an early light-airplane engine researcher and president of Alcor Aviation, wrote, "Few pilots realize how rapidly the structural strength of an engine can decrease with excessive temperatures." Hundere states that when the cylinder head temperature is raised from 300 to 450 degrees F, the yield strength of the aluminum head alloy drops by 76 percent. Since excess heat can also cause lubrication breakdown at the top of the ring travel and result in uneven cylinder cooling, lower temperatures should result in longer cylinder life.
The J.P. Instruments EDM-800 installed in the 2001 AOPA Sweepstakes Bonanza recorded CHTs in the 330-to-360-degree range during high-power lean-of-peak operations. This is well below the 380-degree cruise CHT limit suggested by Braly.
The Sweepstakes Bonanza
The AOPA staff operated the turbonormalized AOPA Sweepstakes Bonanza engine with the mixture set from 50 to 90 degrees F lean of peak for more than 80 flight hours (" Turbo On the Go," September 2001 Pilot) before it was passed on to the winning AOPA member. Although the idea of pulling the mixture control into lean-of-peak territory while flying AOPA's newly installed Superior Air Parts Certified Millennium engine required a leap of faith ("Go ahead, give it the big pull — it won't hurt the engine," Braly said into my headset), the engine performed as promised — it ran smoothly, fuel flows were down, and the CHTs were in the mid-300-degree range.
In spite of the evidence, lean-of-peak operations are controversial. A spokesman for one high-profile engine rebuild shop said, "We love those guys that fly lean of peak — that's one of the reasons business is so good." This shop, others contacted for this article, and an engineer at TCM all conceded that while they thought that lean-of-peak operations could be successful, they didn't recommend the practice for most pilots.
I, too, was skeptical so I set out to get another opinion. Noting that the turbonormalized engine in Braly's Bonanza was being overhauled, I called Monty Barrett of Barrett Performance Aircraft in Tulsa and asked him to give me his opinion of the condition of Braly's cylinders. The cylinders had been flown lean of peak for 900 hours before coming to Barrett for overhaul. Barrett replied that there was no more wear on Braly's cylinders than on any other set of regularly flown, well-maintained cylinders. At last year's American Bonanza Society convention I also spoke to a number of pilots who had experimented with lean-of-peak mixtures. None of them had elected to go back to the old way.
If hundreds of pilots have successfully used the lean-of-peak method, and most everyone concedes that it won't hurt a well-maintained engine, then why does the procedure create such controversy?
At the present time, the only drawbacks I've heard seem to revolve around pseudoscientific explanations such as "chemical reactions during the fuel burning that we don't fully understand," "the higher temperatures on the exhaust valve caused by the slower-burning lean mixture," and "the oxygen-rich atmosphere inside the cylinders causing rapid valve wear." No one has presented me with any credible scientific data that contradicts the lean-of-peak camp.
Keys for cylinder health
Every TCM fuel-injected-engine owner who wants trouble-free operation from his engine should insist that the fuel-injection system be adjusted in accordance with Service Information Directive 97-3 before the first flight after a new engine installation, at 50 hours after installation, and at each annual thereafter. That's always the first step. The second step is to ensure that the baffling and baffle seals are impeccable — that's how important these seemingly flimsy aluminum fences and floppy-looking rubbery parts are. A hole or leak as small as one square inch has been shown to reduce the efficiency of the engine cooling airflow by as much as 20 percent.
Pilots who treat their engines with respect by making throttle and mixture changes slowly and gradually, by waiting three or four minutes for temperatures to stabilize before gradually closing the cowl flaps, by warming up and cooling their engines gradually, and by accepting 65-percent power as their maximum cruise power setting have shown again and again that the leaning suggestions from the engine and airframe manufacturers will work. Then again, if the old methods work, why would anyone want to try the GAMI lean-of-peak operating theory?
Running outside the range of 125 degrees rich of peak and 50 degrees lean of peak has two important effects: Combustion peak pressures are reduced and the peaks occur later in the reciprocating engine cycle.
Here's why this is important for long engine life. Operating with the mixture set at best power (80 to 100 degrees rich of peak) results in maximum combustion pressures that average 10 percent higher than when the burn rate is slowed by setting the mixture outside of the critical range. Lower peak pressures also mean lower temperatures. At best-power mixtures the peak pressure occurs in the vicinity of 12 degrees of crankshaft rotation after the piston has passed top dead center (TDC) — according to engineering theory and real-time testing at GAMI, this is too early. The almost-direct alignment of the piston connecting rod and the crankshaft rod journal that occurs at only 12 degrees TDC transfers heavy loads to the crankshaft, and rod and main bearings. Running at 125 degrees rich or at 50 degrees lean of peak slows down the combustion so the pressure peak occurs at 18 to 20 degrees TDC. This gives a more favorable connecting rod-crankshaft angularity, resulting in less stress on the crankshaft and the bearings.
Lowering peak pressures and delaying the onset of the pressures also may help prevent rapid cylinder wall wear at the top of piston travel. Modern piston rings are designed so that the increasing combustion pressures push the ring out against the cylinder wall. This helps the ring seal against the oil film on the cylinder wall and increases efficiency. The lubricating oil film that protects the cylinder walls is better able to handle the lower pressures and later timing of peak pressures that occur under Braly's plan.
If you're foggy about lean-of-peak operations or have any doubts about your ability or desire to pay close attention to leaning, fuel flows, EGTs, and CHTs — or if you don't have an engine monitor installed — then you should leave lean-of-peak operations to others.
But if you want to fly efficiently, and like the challenge of sophisticated powerplant management techniques, bone up on the process, take the GAMI lean-find test to see if you need a set of matched nozzles, and go lean.
E-mail the author at [email protected].
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