When one is operating a Cessna, Piper, or other small aircraft with a conventional aero gas engine, there will most likely be a mixture control element present in the cockpit. With this control, pilots have the ability to adjust the fuel-and-air ratio in the carburetor, as well as stop the engine when the flight concludes. While some aircraft will automatically adjust mixtures, others may provide manual control for the benefit of various flight operations and needs. In this blog, we will discuss the leaning of aircraft engines, that of which is when the amount of air in the mixture is higher than standard operating ratios.
During typical engine operations and conditions, a slightly rich mixture is relied on as it will deter the chance of detonation, exhaust valve overheating, and preignition. With a small amount of excess fuel, cylinder assemblies can be cooled through evaporation which benefits the burning of richer mixtures. Once the aircraft has reached a cruise altitude and speed, it can potentially take advantage of lean operations to prioritize optimal power or fuel-economy. Generally, manual leaning will be carried out while operating below the 75% threshold of maximum power.
When leaning, having precise control over adjustments is the most advantageous to ensure that fuel-and-air mixtures are exactly as needed. If an aircraft is fitted with gauges for exhaust gas temperature (EGT) and cylinder head temperature (CHT) values, then a pilot will have a much easier time precisely leaning the mixture in the engine. Despite this, all cylinders will exhibit varying temperatures when the engine features a carburetor, though this can be controlled with the use of a balanced fuel injected system. Another way to establish increased control is through the installation of a FADEC, such systems preventing any cylinder’s mixture from becoming too rich or too hot. With bolstered precision, pilots can carefully control the mixture of fuel-and-air while saving money on fuel.
Depending on the particular needs of a pilot, there are a few fuel-and-air ratios that one may take advantage of. For example, the best power is often reached when a ratio of 1:13 is maintained. Meanwhile, the best specific fuel consumption can be achieved with a leaner ratio of 1:17. For peak EGT, the ratio should be adjusted to 1:14, while peak CHT is achieved with a slightly richer ratio. To determine any ratio, the weight of fuel and air that enters into all cylinder assemblies should be controlled.
While a leaner mixture can be beneficial for certain needs, it is important to be careful when managing power settings as well. If one is operating with a lean mixture while high power settings are established, cylinder temperature can rise to the point where detonation, burnt exhaust valves, piston damage, and power failure are all a hazard. Nevertheless, too rich of a mixture can also be detrimental, potentially leading to rough running, power losses, spark plug fouling, lead deposits, and more. As all of these can lead to issues big and small, having ample control over mixture ratios is very important.
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