Using The Red Knob To Manage Piston Engines

Saturday, June 03, 2023 at 08:46 PM ET

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< The chart at left can best be described as a graphical representation of what happens to engine-condition variables as we manipulate the mixture control. Changing the mixture by increasing or decreasing the amount of fuel flowing to the engine typically is referenced by how far rich of peak (ROP) or lean of peak (LOP) EGT the setting is, in Fahrenheit degrees. The idea is to place the mixture control so that the mixture is ROP or LOP enough that no damage occurs, that CHTs remain relatively cool and the engine produces the power we require. It’s important to remember that this chart ignores other variables, like rpm or manifold pressure, and focuses only on fuel flow as it relates to peak EGT. Figure 1

Key Takeaways:

Proper engine mixture management, guided by modern multi-cylinder EGT and CHT monitors, is essential for engine longevity, fuel economy, and preventing in-flight failure.
Pilots must understand and avoid the "Red Box"—a range of high power and mixture settings that create damaging engine stress (high CHT/ICP).
Exiting the Red Box can be achieved by operating well Rich of Peak (ROP) for high power needs (takeoff/climb) or, preferably for cruise, Lean of Peak (LOP) to reduce CHTs, save fuel, and extend engine life.
Maintaining CHTs below 400°F (ideally 360°F or cooler) through mixture adjustments and cowl flap management is a critical task for engine protection.

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While conducting flight reviews for two decades, I’ve noted a subset of informed pilots employing up-to-date concepts to properly manage the air/fuel mixture entering their piston engine. Other pilots—probably because they were initially trained that way—leaned to peak exhaust gas temperature (EGT), no matter the engine power setting, and then enrichened it by 50 degrees F. Others merely set cruise power and pulled back the mixture about an inch and left it there. And, of course, another subset refused to lean the mixture at all, sometimes stating it was “too complicated.”

Except for the first group of pilots, the rest ignore that properly managing the engine’s mixture can limit cylinder head temperatures (CHTs), has vital implications for fuel economy and engine longevity. The trick is understanding that a full-rich mixture may be most appropriate in some operations and lean of peak (LOP) EGT may be best in others.