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What's The Best Leaning Option For Your Engine?

Engine Primary Ken Hodge

There are lots of techniques to lean an engine - and most aircraft have two different leaning options for cruise: best power and best economy.

Every manufacturer has a different name for these settings - best power may be called "recommended lean mixture" in a Cessna, and best economy may be called "peak EGT." Regardless of the name, one's always a little bit rich and one's at peak EGT.

When planning your cross country on your check ride, you'll choose one. And, your examiner will probably ask why you chose what you did. Common answers are:

  • I need to get there fast, so I chose best power.
  • I'm not paying for the gas, so I chose best power.
  • I don't want rental costs to go up, so I chose best economy.
  • I want all of the range I can get, so I chose best economy.

In most aircraft, these answers really don't make sense. In fact, in the Cessna 172S, best economy (peak EGT) only increases your range by 4%, and it decreases your speed by only 3 knots! That's not much of a difference...

The Real Difference Between Best Power And Best Economy Is Engine Operating Temperature

For most training aircraft, the real difference between best power and best economy is operating temperature. At best economy, your engine runs hotter than at best power. Why?


rich-and-lean-cylindars 2

When you lean to peak EGT (best economy), you're burning 100% of the fuel that enters your cylinder. That's as efficient as you can get.

But, when you run the engine slightly rich of peak EGT (50 degrees Fahrenheit is common), you're ingesting more fuel into the cylinder than you can burn. That extra fuel evaporates, which absorbs heat. It's no different than the rain falling from a thunderstorm evaporating and cooling the air.

Best Power Or Best Economy - Which Is Better

This is an impossible question to answer with a hard and fast rule. You'll hear some owners and mechanics say that best economy can damage your cylinders and engine. And, you'll hear others say that best economy's just fine and best power could foul your spark plugs over time. You'll find countless articles online supporting both sides.

If you follow the operating rules in the POH, you won't damage the engine. And, use your knowledge as a pilot to decide what's best for a flight. If it's hot and you're running at a high power setting, I would suggest a rich mixture to keep the engine cool. If it's cold (I learned to fly in North Dakota, where it was -35 Fahrenheit during many winter days) and your power setting's lower, best economy is probably fine. (And in my case, I wanted all of the heat I could get. It's hard to get a Warrior's engine to heat the cabin at cruise on a -35 degree day...)

Most importantly, keep the engine within its temperature limits.

EGT, CHT and Oil Temp: Three Ways To Measure Your Engine

All aircraft have an oil temperature gauge, and most have an exhaust gas temperature (EGT) gauge. Some have cylinder head temperature (CHT) gauges. So, what does each measure?

temp-measurement

Oil Temperature

Your oil temperature gauge measures the temperature of... the oil. On a Cessna 172S with an Lycoming O-360 engine, the gauge measures the temperature of the oil at the rear of the engine, near the crankshaft. Oil temperature can tell you if your engine is overheating, but it's the slowest gauge to react. (It takes time to heat all of the oil up.) And, if the temperature is high, it may be caused by low oil.

Exhaust Gas Temperature (EGT)

The exhaust gas temperature (EGT) gauge really tells you nothing about the temperature of your engine. It simply tells you the temperature of your exhaust - which indicates whether you're running rich or lean of your most efficient mixture. It reacts almost immediately to changes in mixture, but it won't tell you if your engine's overheating.

Cylinder Head Temperature (CHT)

Some aircraft have cylinder head temperature (CHT) gauges. If you have them, they may measure one, some or all of your cylinders. These gauges tell you the temperature of the top of the cylinder, and they give you an almost immediate indication of engine operating temperature.

When using CHTs, you want to watch the temperature of your hottest cylinder. That's usually the cylinder closest to your firewall, because it receives the least amount of airflow and the air's already warm when it reaches the back of your engine.

rear-of-engine-is-hotter

CHT gauges can also tell you if your engine is cooling too quickly. If you're flying a high-performance airplane, you want to allow the engine to cool slowly, so that your cylinders don't crack when the metal cools and contracts. This is called "shock cooling."

What To Tell Your Examiner

What should you tell your examiner when he asks about your cruise mixture setting? Tell him that you picked a recommended leaning technique that balances efficiency and operating temperature. Explain how you considered the outside air temperature and power setting. If you do, your examiner will be impressed - and you'll know your stuff.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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