Mixture Management, Getting It Right: Cruise Flight

“Just pull the red knob until the engine runs rough, then push it in a bit” … oh really. “Unscrew the mixture control until you find the peak EGT, then enrichen to get 25 degrees ROP” … “find peak EGT, then enrichen to get 75 to 100 degrees ROP” … “find the first cylinder to peak, then lean to get 25 degrees on the lean side of peak.” These are all the proper way to lean the mixture in our airplanes — depending on who you talk to. If the expanse of accepted methods tell us anything, it’s that leaning may be a hard action to screw up, but that doesn’t make doing it right any easier.

Almost all our airplanes’ engines are air-cooled. They depend on air flow around the cylinders to keep them from overheating. This works great at cruise speed, but at high power/low airspeed configurations (read: “takeoff and climb”), there’s not a whole lot of airflow available for cooling. Hence, almost all airplane engines are set up to run with very rich fuel mixtures. With the “red knob” all the way forward, more fuel than can combine with induction air is sent to the cylinders… and subsequently spit out of the exhaust.

Translation: At “full rich” mixture settings, we’re effectively “liquid-cooling” our cylinder heads with unburned fuel, to make up for the lack of air flow. So, at cruise power, a full rich mixture simply wastes fuel — reducing range and endurance, washing lubricating oil off the inside of cylinders, increasing wear, and making it more expensive to fly… in a variety of ways.

So we know we need to lean the mixture for cruise flight, but which technique is right?

Most Pilot’s Operating Handbooks (POHs) recommend leaning for all cruise power settings and altitudes (some old handbooks recommend leaning above 5000 feet, but it’s “correct” to lean at any altitude). How you do it is a matter of what you’re trying to achieve, and how the airplane is equipped. Take a look at the illustration, below.

Figure 1

The graph shows the relationship between mixture settings (see the scale along the bottom — leaner mixtures to the left, richer settings to the right) and operating temperatures (the vertical scale, with higher temperatures further up on the graph). The two upper lines represent the Cylinder Head Temperature (CHT) and Exhaust Gas Temperature (EGT) effect of making a mixture adjustment. Note: The actual numerical temperature achieved for a given mixture setting isn’t important here, and will vary by engine and the power setting used, but the relationship holds true for almost all engines.

As you lean the mixture, the EGT (uppermost line on the graph) will rise. It’s cooler when you operate Rich Of Peak (ROP), because there’s excess fuel flowing through the cylinders, cooling them down. Eventually you’ll get to the point where all the fuel efficiently burns, giving the highest (“peak”) EGT. Lean further, and the EGT will again drop — this is Lean Of Peak (LOP), and there’s too much air inside the cylinders to combine with too little fuel, the resultant combustion is less efficient, produces less power, and results in lower EGTs.

Now look at the second line from the top on the graph — the CHT. As you lean the mixture, the CHT will rise as well. It’s important to note, though, that the cylinder head temperatures are at their highest around 25 degrees Fahrenheit ROP. Most POHs commonly recommended a 25-degrees-Fahrenheit ROP cruise mixture setting, which may actually reduce cylinder life. In larger engines (Cessna 182/Piper Dakota-sized and bigger), you may need to add extra fuel for engine longevity. Many engine experts advise we avoid this POH recommended setting as being an area of “possible danger”– the CHTs run too hot. Common practice among engine-wise pilots is to fly with EGTs at 50- to 75-degrees Fahrenheit ROP, a good compromise between fuel burn and engine health.

Remember: Change the airflow and you’ll have to re-lean the mixture. This includes throttle changes, altitude changes, and the use of carburetor heat and/or alternate induction air.

Is Richer Better? To an extent, yes. Run at 100- to 120-degrees Fahrenheit ROP and you’ll be at a best power mixture setting — getting the most efficient work out of your engine, at the expense of a slightly higher fuel burn. Looking at the graph, you’ll see this gives you lower CHTs as well. If you run too rich, however (150F-200F or more ROP), you may again be displacing lubricating oil in the cylinders with unburned fuel, fouling up the spark plugs, and simply wasting fuel by pumping it overboard through the exhaust. So there’s an “upper limit” on ultra-rich mixture settings, designed for engine health and operating economy.

Is Leaner Better? Many engine operators now recommend LOP operations — under some circumstances. You’ll get lower operating temperatures and significantly lower fuel burns. Evidence suggests there are additional engine-longevity benefits of cruising LOP. But it’s the trade-off that makes that true — look at the third line from the top, the “percent of power” curve. You’ll see that power potential is highest at close to 100- to 125-degrees Fahrenheit ROP, hence a “best power” setting. The curve is fairly flat all the way to the peak EGT point, then drops precipitously as you lean further. In fact, that’s why most engines run so roughly when LOP — the power development of individual cylinders can vary greatly on the lean side of peak, since most engines run with cylinders hitting their individual peak at different fuel flows. (When you leaned a Skyhawk or a Cherokee until it “ran rough,” you were leaning until at least one cylinder passed the peak point.) “Push the red knob in a little” meant “get it back on the rich side of peak, so it’ll run smooth.”

Luckily, temperature control isn’t a terrible problem in low-power engines, but transition to a big-bore Continental or Lycoming, and such cavalier mixture management can cause damage.

Some companies, notably General Aviation Modifications, Inc. (www.gami.com) and Teledyne-Continental Motors (www.tcmlink.com) offer “balanced” fuel injectors for some engines, which allow smooth LOP operation and evidence of other long-term cylinder benefits. As the graph shows, LOP cruise will provide low temperatures and fuel flows — at the expense of power. In fact, a power setting at 100F ROP will give almost 10% more power than the same throttle/prop settings at 25F LOP.

NOTE: Pilots flying turbocharged airplanes can usually “make up” the lost power by adding back manifold pressure — getting phenomenal performance with high power, cool engine temperatures and low fuel flows.

Bottom Line?
It Depends. Mixture management technique depends on what you want, and how your airplane is equipped. Universal Truths: You need to lean for cruise to get better range, and for long-term engine health. If you’re looking for speed, then lean to peak EGT and enrichen about 100- to 125- degrees cooler. More economy minded? Find peak EGT, then enrichen to 50- to 75-degrees Fahrenheit ROP to stay away excessive cylinder heating. Will the engine run smoothly lean of peak, and are you willing to accept the airspeed loss that results from lower horsepower? Then you might consider cruising on the lean side of peak — find the peak point, and continue to lean to around 25 Fahrenheit LOP.

BOTTOM LINE: Look at the graph, look at your mission, and look at your goals — and you can make a better informed decision about cruise mixture management.

Editor’s Note: For more on leaning, please see also:

Jeff Pardo’s ‘Behind the Red Knob