Pilots are famous for arguing the finer points of various procedures they employ when flying. In recent years, few topics have generated more “discussion” and disagreement than when and how to lean a gasoline-fueled piston-engines fuel mixture. A previous article (“The Red Knob,” April 2011) explored the benefits of modern multi-cylinder engine monitors, closely balanced fuel flow between all cylinders of an engine and the effects leaning from full rich have on parameters like exhaust gas temperature (EGT), internal cylinder pressure (ICP) and cylinder head temperature (CHT). With that earlier article in mind as a foundation, what we need to explore next is where the mixture control should be positioned for the various flight operations we might conduct. For example, there are times when we want the mixture rich of peak EGT (ROP); on other occasions we may want it lean of peak (LOP). And we may want it at peak EGT sometimes, too. Acknowledging that some piston-powered aircraft utterly lack a mixture control-and therefore some of this discussion doesnt apply to their pilots-lets explore those conditions and how we can use our basic knowledge of engine operation, plus the tools and basic instrumentation most piston-engine airplanes have, to make some educated decisions. Everythings Relative One definition of a rich mixture might be where some fuel will be left over after combustion is completed. Meanwhile, a lean mixture is one where is some air is left after combustion is completed. With a rich mixture, all the air is consumed before all of the fuel can be burned. With a lean mixture, all of the fuel will be burned before all of the air is used. Depending on the aircrafts instrumentation, theres another way to think of this. For example and considering an engine conforming to its design and equipped with a multi-cylinder monitor, as we lean the mixture from full rich, EGTs rise. If EGTs are rising as we lean the mixture, the engine is ROP. If we continue leaning the mixture, at some point EGTs will peak, then begin to decline. The point at which EGTs peak, of course, is the peak. Beyond that point-and when EGTs drop as we lean further-the mixture is said to be lean, or LOP. A critical point to understand is that both rich and lean mixtures can be used to slow the flame front, the rate at which the fuel-air combination in each cylinder burns. Generally, we want to avoid the fastest flame fronts as they generate the most heat and the highest internal cylinder pressures. Both are to be avoided if we want to minimize wear and tear on the cylinders and for the engine to remain healthy. One way to visualize the effects of leaning from ROP through peak EGT and into the LOP area is by understanding the so-called “landmarks” graph we published with Aprils article, another version of which is presented as Figure 1 on page 14. Another important point to keep in mind is that absolute EGT numbers, usually expressed in degrees Fahrenheit, are meaningless. On any given two flights, the maximum EGT values an engine might generate will differ, sometimes drastically, depending on variables like altitude and air density, plus how much power is being produced. The important thing to keep in mind is EGT is a relative value, and it along with other engine parameters can be manipulated with the mixture control. ROP Operating an engine rich of peak is easy: just leave the mixture control at its full-rich position. By doing so, you may be wasting fuel, at least when compared with leaning the mixture for operations at less than full power. Depending on where you are and what youre doing-e.g., attempting a full-power takeoff at a high density altitude-you also may be giving up some amount of engine power. In our view, the question isnt whether you should be running the engine ROP (or LOP, for that matter). Instead, the issue is whether youre running far enough ROP or LOP to avoid high ICPs and high CHTs. In other words, we all run our engine(s) ROP at one time or another-at engine start, for example, or during a full-power, sea-level takeoff-and theres nothing wrong with that. In fact, thats what the manufacturers recommend to extract the most performance. But some guidance-for example, leaning to 50 deg. F ROP for “best power” during cruise-can place the mixture squarely in whats come to be called the “red box,” which is presented in Figure 2 on page 14. Note how ICPs are peaking in the area where the mixture is 50 deg. F ROP. At relatively low power settings, the ICPs arent that high and running the engine at this mixture isnt likely to have any adverse effect on anything except fuel economy, and possibly a fouled spark plug or two. But at high power settings, ICPs will be near their peak, and this is an area to be avoided. LOP Just as ROP operations are desirable from time to time, the same can be said for LOP. As a rule, the engine will run cooler and cleaner, fuel economy is improved and spark plugs are lots less likely to foul. Not every engine equipped with a mixture control can be operated LOP, however (see the sidebar on page 15). But theres no free lunch. For example, operating LOP means well give up some horsepower-refer again to the graphs on page 14-and therefore be flying more slowly. Also, to reliably and confidently obtain the benefits of LOP operations, we need to install a multi-probe engine monitor and learn how to use it. Neither are inexpensive-except perhaps when compared to cylinder overhauls from running the engine incorrectly, or to the fuel bills we might be paying when running well ROP. Finally, your airplane/engine combination may not run well LOP, or run at all. Some big-bore Lycomings are particularly troublesome to coax into LOP operations, while some similar-displacement Continentals can run that way out of the box, without balanced fuel injectors. And carbureted engines are always somewhat finicky when it comes to LOP operations. See the sidebar on page 15 for some of the ways these engines can be “tamed” to run LOP. At Peak Operating the engine at its peak EGT also is an option. For many operators, such a setting can offer the best compromise between fuel economy and power. But, some caveats apply. The major caveat is no one recommends operating at peak EGT and high power settings, the latter of which usually are considered to be 65 percent of the engines rated horsepower or greater. At these higher power settings, we should be far enough ROP or LOP to avoid the so-called “red box,” highlighted in Figure 2 at left. But at power settings below 65 percent power, the red box effectively ceases to exist. How can we determine if were pulling less than 65 percent from our engine(s)? Thats easy, and there are two ways we recommend doing it when considering non-turbocharged engines. The first is by reference to the manufacturer-provided power-setting tables or charts in our POH/AFM. With that information, setting the engine controls for an rpm and manifold pressure (if so instrumented) resulting in less than 65-percent power is not only simple but its FAA- and manufacturer approved. The second way is by reference to the airplanes altitude. A normally aspirated (non-turbocharged) engine cannot produce more than 65 percent power above around 8000 feet msl. In other words, if we routinely cruise our non-turbod airplane at or above, say, 8500 feet, we pretty much can place the mixture control anywhere we want it. Instead of worrying so much about EGTs, ICPs, etc., we can lean the engine for smoothness, fuel economy, speed or some other operational consideration. On The Side Of Caution We recommend employing LOP techniques in operating piston aircraft engines when theyre appropriate. And theyre appropriate quite frequently in our experience. There are good reasons for it; the science has been proven since around WWII and modern instrumentation along with exhaustive tests performed by such organizations as General Aviation Modifications, Inc. (GAMI), provide detailed documentation of the benefits and considerations. But there are good reasons to run the engine at ROP settings, too. These obviously include operations like takeoffs and maximum-performance climbs when flying normally aspirated engines. They also can include when the airplane were flying either wont run well ROP for any number of reasons or when it lacks the instrumentation we need to reliably determine where the EGT is set. In the latter instance, running well ROP may be the safest and most reasonable thing we can do, fuel economy notwithstanding. In our book, the key isnt whether were ROP or LOP, but whether were enough ROP or LOP to avoid operating in the red box. Ideally, wed get some training from organizations like Advanced Pilot Seminars (www.advancedpilot.com), or from a flight instructor or trusted friend who has similar training and knows what theyre doing. It helps to have a well-maintained airplane with an engine conforming to its original design and a multi-probe engine monitor. Most personal aircraft delivered today are equipped with temperature probes on all cylinders; if they didnt come with such equipment, they should. Meanwhile, were not aware of an in-service piston-powered aircraft that cant be fitted with such instrumentation. Pilots need a clear understanding of the effects of mixture adjustments to take advantage of this equipment. If your aircraft is not equipped with probes on every cylinder, or you dont fully understand the effects of leaning the mixture on your engine, the safest thing probably is to ensure you operate it well on the rich side of peak EGT. Steven Gibb is a Maryland-based civilian- and Army-trained CFI/I/ME/Glider who has accumulated more than 3000 hours of flying time.
