The Red Knob

Ordinary auto maintenance shops use modern sophisticated computers to analyze engine status and performance. New cars and trucks use computer circuits to control ignition timing, fuel flow and mixture, among other parameters. Drivers do not need any specialized knowledge or skill to operate their automobile engines efficiently and safely.

Pilots are in a much different position. Aircraft maintenance shops use tools little different than they were a half-century ago. Aircraft piston engines have seen little or no improvement in decades. In fact, some argue aircraft engines are less reliable and durable than they were even a few years ago. Unless several physical laws are repealed, substantial improvement of aircraft piston engines does not appear to be imminent. In the foreseeable future, pilots of piston engine aircraft will need to understand safe and efficient use of the mixture control. Lets get started.

For most aircraft, the basic steps to start and operate the engine are: fuel on, mixture full rich, throttle cracked, master on, magneto switch to start and then to both once the engine is running. After starting, for most pilots, engine operation involves adjusting the power with throttle.

The vast majority of piston-engine aircraft also have a mixture control. Some POHs provide little information about using the mixture control, and pilots usually are given little formal training on using it. However, it is doubtful many pilots have a basic understanding of the effects of different mixture settings. Educated pilots should know how to establish correct mixture settings, how mixture affects engine temperatures and the dangers of incorrect settings.

The FAA does not emphasize mixture control in training or testing. This is true even when practical tests are taken in aircraft equipped with all the instrumentation needed for safe and efficient mixture adjustments. Similarly, engine manufacturers give very little guidance. The FAAs and engine manufacturers attitudes seem to be unknowledgeable pilots cannot get in trouble if they use the mixture control only for two settings: full rich and idle cutoff. The fact the mixture control is colored red is intended to indicate a danger. Certainly, incorrect mixture settings may be dangerous-depending on the engine, some settings can result in long-term damage-but knowledge of how to control fuel flow has real benefits in economics and engine longevity.

For many years the discussion of mixture control generally was limited to using a full-rich setting for takeoff and landing and the idle-cutoff position to stop the engine. If you were lucky and worked with an informed instructor or mentor, you learned about the need to lean the mixture to obtain full power at high-altitude airports. Although I had flown piston engines for hundreds of hours, I had a poor understanding of mixture control use before I attended an Advanced Pilot Seminar, www.advancedpilot.com, a few years ago.

Theres little reason many pilots have a poor understanding of the effects of mixture on cylinder pressures and temperatures because this knowledge is not recently discovered. Before the jet age, airlines and other big-equipment operators flew large piston engines many millions of hours with crews who knew how to adjust an engines mixture to reduce fuel consumption, increase range and lower engine temperatures. These engines were often flown several thousand hours between overhauls. Today, very few general aviation piston engines achieve even a fraction of those kinds of hours before some type of overhaul is required. Incorrect mixture adjustment is surely a factor in shortened engine life.

The effects of different fuel/air mixture ratios on temperatures, pressures and power are basically the same for all kinds of piston engines, from lawnmowers to very large radial aircraft engines. With proper fuel flow settings, appropriate instrumentation and knowledgeable pilot technique, its much easier to avoid plug fouling, greatly reduce fuel consumption, control peak cylinder pressures, keep cylinder head temperatures cool, minimize carbon deposits and prolong engine life. Meanwhile, misusing the mixture control can seriously shorten an engines life and even cause failures.

The basic reason mixture adjustments are made during aircraft engine operation is to control the transfer of heat generated by combustion events to the cylinder wall and piston. Too much heat is the enemy of long cylinder life: Metal in the cylinder, piston and rings becomes softer and expands, fatiguing the components and leading to premature failures. Gradually, or quickly in the case of very hot temperatures, excessive heat destroys the ideal fit between the moving parts. Most aircraft engines are designed to operate with cylinder head temperatures ranging between 250 and 400 degrees. Generally speaking, cooler is better.

Some piston aircraft may lack cylinder head temperature (CHT) or exhaust gas temperature (EGT) instrumentation. When operating the engine of such an aircraft, the pilot really only has the tachometer and his or her ears to manage it. In that situation, full rich can be the safest mixture setting, depending on the amount of power being produced.

Other aircraft may have a single-cylinder EGT gauge. While this type of gauge is better than nothing, it only allows determining peak EGT value for the instrumented cylinder. The pilot really needs to know what is happening in every cylinder to safely and efficiently make mixture control adjustments.

Manufacturers now install in almost all new aircraft probes for CHT and EGT on all cylinders, displaying these values on cockpit instruments, as well as other important information such as fuel flow. When combined into one standalone unit, these devices, known as engine monitors, can be installed on older airplanes.

In addition to displaying CHT and EGT for all cylinders, other engine and aircraft performance values can be tracked by an engine monitor, including turbine inlet temperature if turbocharged, electrical system voltage, oil temperature, fuel flow and more. Finally, these monitors also capture and store the monitored data in memory every few seconds. Later, a pilot or owner can download the information to a memory stick via a USB port and analyze it using furnished software. Monitors also allow operators to set alarms that will flash if any temperature value limits are exceeded. An example of a few seconds of engine flight data downloaded from an engine monitor is presented at the bottom of the previous page.

This type of engine monitor provides the pilot with all of the data needed to operate the engine safely and efficiently. Purchase and installation can cost four to six thousand dollars. However, a pilot can easily reduce fuel consumption by two or three gallons per hour with an engine monitor-and some training in how to use it-which can easily pay for the installation. In addition, skillful use of the mixture control could significantly prolong the life of your engine. Combined savings from reduced fuel consumption and longer engine life could pay for buying and installing an engine monitor in as little as 300 flying hours.

But perhaps the most important benefit of having this data-and understanding how to use it-is the pilot can know minute-by-minute what is happening in each cylinder during flight. This makes it possible to detect abnormal engine function well before catastrophic failure.

Very common advice given to pilots is to determine the peak EGT on a cylinder and then enrichen the mixture by 50 deg. F, or one or two twists of the mixture control. This is poor advice on several levels. Take a few minutes to study the meaning of the curves in the chart on the opposite page, one I refer to as the “Landmarks Chart,” supplied courtesy of Advanced Pilot Seminars.

The Landmarks Chart shows what happens to EGT and CHT as temperatures rise and fall when the pilot leans the mixture from rich on the left to lean on the right. Also depicted is the rise and fall of power and cylinder pressures.

The chart does not represent specific power settings or temperature values. For that matter, it doesnt represent a specific engine: All internal combustion engines will exhibit the same basic characteristics. The chart represents the relative changes in these variables for a single cylinder as the mixture is leaned.

The actual EGT, CHT and cylinder pressure values will be higher when using a high power setting and lower with a low power setting. At a specific throttle setting, as the red knob is moved to lean the mixture, the power produced increases until EGT is about 80 deg. F rich of peak (ROP) and then declines. Cylinder pressures and CHTs increase until about 40 deg. F ROP, then decline.

There are several takeaways from the Landmarks Chart. For example, the greatest cylinder pressure is directly related to peak CHT. This is important because higher pressure in the cylinder transfers more heat to the cylinder wall. At a particular setting, the amount of fuel supplied to each cylinder can vary substantially. Therefore, we need temperature probes on each cylinder to know if the values are actually rising and falling together on all cylinders.

Presume an engine has equal fuel distribution to all cylinders, but unequal airflow. What would happen if we established peak EGT on one cylinder and then use commonly given advice to enrichen the mixture 50 deg. F? Looking at the Landmarks Chart, we would expect CHTs to increase over the values obtained at peak EGT. Running the engine at this mixture setting and high power could seriously damage it.

The table above reproduces actual data I recorded to determine the fuel distribution to the six cylinders of the TCM TSIO-360 engine in a turbo Mooney. This data tells us a lot. As the mixture in leaned in .2 gph increments, the temperatures closely correspond to what we would expect based on the curves in the Landmarks Chart. Each cylinders peak CHT is highlighted in red; peak EGT is in yellow. Importantly, peak CHT occurs in every cylinder before it reaches peak EGT.

Fuel flow for this engine is set for a full-rich minimum of 24 gph during takeoff at 40 inches of MP. Full-rich cruise at 27 inches of MP would burn about 14 gph. This test showed fuel distribution was not well-balanced. The number one cylinders EGT peaked at 11.8 gph while number six peaked at 11 gph, a difference of almost one gallon. Note also the CHT range on the number five and six cylinders are significantly lower than the ranges on the number one and two cylinders. This is because these cylinders are located at the front of the engine cowling see much better cooling air flow.

After recording this test data, the fuel injector in the number one cylinder was adjusted to run richer and the injector on the number six cylinder was adjusted to run leaner. Another test was run and all cylinders achieved peak EGT within .2 gallons of fuel flow. Achieving equal fuel distribution allows the engine to be run lean of peak EGT (LOP), saving at least three gallons per hour for an equivalent power setting, and helps keep all cylinders relatively cool. The engine also runs with less vibration because the power generated by each cylinder is more equal to the others. Finally, the engine runs cleaner because the combustion event is more efficient-a greater proportion of the fuel entering each cylinder is burned.

Proper use of the mixture control, to maximize engine longevity and economy, requires understanding whats going on in each cylinder. An engine monitor, which employs temperature probes on all cylinders, is a basic requirement. For best results, fuel-injected engines may require balanced injectors, adjusted to compensate for unequal air distribution. Meanwhile, carbureted engines pose their own challenges; applying partial carb heat sometimes helps.

Without an engine monitor providing real-time data for all cylinders, the safest course is to be certain the mixture is at least 100 deg. F rich of peak EGT any time the engine is operating at more than 60 percent of its rated power. At lower settings, neither internal cylinder pressures nor CHT are likely to cause damage. Similarly, aggressive leaning is appropriate during low-power taxi and run-up operations, to avoid plug fouling. The mixture should be lean enough that throttle advancement to takeoff power would make it immediately obvious to the pilot the mixture has not been set properly for takeoff.

This article discussed how cylinder pressures and temperatures vary as the mixture is leaned. A subsequent article will explore why these cylinder pressures and temperatures rise and then fall as the mixture control is moved from full rich to leaner settings.