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The Fuel Air Mixture

Proper leaning benefits engine performance, longevity

By Ken Gardner

Proper operation of the piston or reciprocating aircraft engine requires considerably more attention and technical skill than does its automotive cousin.

One such area of technical skill is the proper selection and subsequent regulation of fuel-air mixtures, generally referred to as mixture leaning or enrichment. The process should really be termed mixture regulation, since the operator can control both lean and rich modes. A common misconception, especially to the student pilot, is the belief that reciprocating aircraft engines require mixture regulation but automotive engines do not. This is not correct, as one who has driven an automobile up the road to Pike's Peak can tell you.

Both automotive and aircraft engine carburetors contain devices that effect automatic mixture ratio changes. However, these devices function in relation to power ranges and are not sensitive to air density changes. Most automobiles never get to Pike's Peak and those based in high-altitude areas usually require a change in carburetor metering jets for satisfactory operation. In light of day-to-day operating conditions, a mixture control would be a nuisance in the stock automobile.

Although a great many light airplanes such as the Piper J-3 Cub, Taylorcraft B-12, etc., operated reasonably well without mixture controls, they would, and do, provide more satisfactory operation if fitted with them. Such a device is most desirable especially at levels in excess of 5,000 feet density altitude.

All internal combustion engines are air-breathing; consequently, they are quite sensitive to any change in pressure and quality of the air they breathe. Neither reciprocating nor gas turbine engines "suck" in the air they displace; rather, air is forced into the engine by atmospheric pressure. The same is true of the supercharger which simply displaces its volume at a much faster rate and supplies air to its engine at a pressure above atmospheric, up to a given differential.

Since the pressure of the atmosphere decreases with altitude, likewise will be the force available to push air into the engine. A decrease in atmospheric pressure also results in expansion of the air causing it to become less dense, so what air does go into the engine has less oxygen because of expansion.

Under such conditions the naturally aspirated (breathing solely by atmospheric pressure) engine's power output will be proportional to the atmospheric pressure present at any given altitude. Decreasing air density could further aggravate power loss if the fuel flow is not reduced to match the lesser amounts of oxygen associated with less dense air.

The gas turbine engine is equipped with a barometric fuel control that senses these changes and automatically adjusts fuel flow to match. Although a similar type of device (automatic mixture control or AMC) is employed on some reciprocating engines, most do not have them. Therefore, mixture regulation becomes necessary at flight levels above 5,000 feet density altitude (DA) for satisfactory engine operation. Notice we did not mention fuel economy, and for good reason. Fuel economy is secondary, and not the primary reason for mixture regulation.

No doubt you have heard much about the perfect mixture. Names such as stoichiometric and chemically correct are used to enhance descriptions of the perfect mixture which every good pilot should strive for. (There is no singular perfect mixture for the reciprocating aircraft engine or the stock automotive engine, either.) A stoichiometric mixture is one having a ratio of fuel and oxygen that will result in the absence of both upon completion of combustion — no fuel or oxygen remain in the spent gases. However such a mixture is not satisfactory for all modes of engine operation and it is more perfect in definition than in application. Chemically correct is even more confusing.

At full takeoff power the aircraft engine requires a full rich mixture. The term "full rich" in this particular application describes a mixture as rich as possible without a substantial loss of power. Such a mixture does result in the loss of some power; however, the loss is slight and the added fuel flow greatly aids internal engine cooling at a time when the engine needs it most. Thus the power/cooling tradeoff is a good one.

Such a mixture would indeed be the "perfect mixture" for these requirements. The takeoff mixture conditions just described occur with throttle full open, mixture control in full rich, and sea level ambient conditions. This same takeoff mixture would be too rich and virtually intolerable at Denver on a 90 degree Fahrenheit day.

On the other hand a full-power takeoff from sea level Atlantic City on a 0 degree F day would be a hair on the lean side even with the mixture control in full rich. Under these conditions the engine would actually be developing more than its normal full rated power due to the below-sea-level air density.

Mixture ratios most definitely do influence combustion characteristics. While you need not know the actual proportions of a mixture ratio such as 8 to 1, for example, you should be familiar with the effects in terms of engine behavior. Mixture ratios span a spectrum from a rich of 6 to 1 to a lean of 18 to 1 depending on combustion chamber design and operating conditions.

Widely accepted among most pilots is the belief that lean mixtures burn hotter than rich mixtures and therefore produce the most power. That belief is highly misleading and seldom, if ever, correct. Under ideal conditions the stoichiometric mixture would produce the hottest flame. However, this does not necessarily apply to the greatest power mixture. The amount of mixture inducted into the cylinder is more relative to power output than are minor ratio differences.

The expansive qualities of the mixture inducted into the cylinder play a major role in power development. For example, an increase in design compression ratio will result in a considerable increase in power, but with a lesser increase in combustion temperature. In fact, exhaust gas temperatures are actually lower on higher-compression engines because a greater amount of the heat released in combustion is converted into work. Thus, if the expansive forces of a slightly richer mixture result in an increase in power output, then combustion temperature is not the sole significant factor.

Furthermore, a lean mixture of 16 to 1 is not going to burn as hot as a rich mixture of 8 to 1. Both lean and rich mixtures produce flame temperatures less than stoichiometric. A lean mixture burns slower than a normal or rich mixture and in doing so exposes the engine longer to actual combustion temperatures. It is this factor more than any other that causes an engine to run hotter on lean mixtures.

Now let's apply these facts to some actual situations using a Cessna 182 Skylane. All takeoffs up to 5,000 feet DA should be made at full throttle with a full rich mixture. Our first example will be a takeoff at Kansas City. The DA is approximately sea level. We climb out to 400 feet above the runway and reduce power to 75 percent for climb. Upon attaining 5,000 feet DA we depart the altitudes where a full rich mixture was essential for takeoff and climb power.

Since all carburetors are not exactly alike in their metering characteristics, we will check the mixture at this point. Gently ease the mixture control from the full rich position towards lean. If the engine gets a little smoother it indicates that the mixture had been too rich. Return the mixture control to full rich and repeat the procedure. Cease leaning at the point where an increase in smooth operation occurred.

If, on the other hand, no such increase in smooth operation occurred and the engine actually began to roughen from continued leaning, return the mixture to full rich and leave it; it was lean enough.

The Cessna 182 has a normally aspirated (NA) engine and, unless otherwise stated by the engine manufacturer, 75-percent climb power on an NA engine should always be conducted with a rich mixture for added engine cooling. If the mixture gets too rich the engine will begin to get rough. During the climb you want to lean only enough to maintain smooth operation and still remain in the rich mode.

If our aircraft had a fixed-pitch propeller, such as the Cessna 172, you would use the same method, only you could observe the tachometer and watch for an increase in engine rpm. If the mixture is too rich a slight increase (25 to 50) in rpm should occur, as well as smoother operation. No increase in either would indicate an already satisfactory condition in full rich and will be reason to return the mixture to full rich. Make the same check each 2,000 feet of climb, each time ceasing the leaning procedure the moment smoother operation and/or a slight increase in rpm occurs. (The Cessna 182 has a constant-speed propeller and therefore will not show any increase in rpm.)

Once cruising altitude is attained, go about the necessary cockpit chores and allow the aircraft to attain maximum speed for the power setting you have established before you attempt cruise leaning. This allows sufficient time for engine temperature to reduce from climb, and the airspeed peak provides the cruising ram air for which you should lean. At this point there are two choices of lean mixtures available to you. If you prefer maximum performance, lean almost to roughness then gradually enrich the mixture while noting the airspeed. Maximum indicated airspeed (IAS) will occur when the mixture is at best power. This procedure will require some practice at first, but eventually you will become quite good at it.

The second choice is maximum economy and must never be used for cruise power settings above 75 percent — and never for climb power. Gradually lean from rich until the engine begins to roughen up. Now gradually enrich just enough to be out of the roughness. Maximum mixture smoothness occurs just rich of best power. Once you have leaned the mixture to your satisfaction no further changes should be necessary as long as engine power settings, altitude and ambient conditions remain unchanged.

By the way, the roughness associated with excessively lean or rich mixtures is the result of cylinder misfiring. Due to induction distribution irregularities, one cylinder will nearly always lead the others in becoming too lean or too rich, and will misfire, producing a momentary unevenness in engine rhythm, which we call roughness. Such roughness constitutes no immediate harm to the engine so long as it is not continued for any length of time. Often a pilot will lean for maximum economy, then shortly thereafter nudge the mixture control to a little richer position for fear of being too lean.

What actually is too lean? The following conditions apply to direct-drive, naturally aspirated engines and not necessarily to other types. A mixture less than normal full rich for takeoff and climb below 5,000 feet DA would be too lean. In these operating modes insufficient fuel flow could invite detonation and internal heating.

The climb above 5,000 feet DA to cruising level should be as rich as possible and still provide smooth engine operation. Actually this is the same mixture as takeoff and climb to 5,000 feet DA, only regulated to a leaner position to compensate for decreasing air density. The effects of inadequate fuel in the climb regions above 5,000 feet DA are the same as those at sea level up to 5,000 feet, only on a decreasing basis with increasing altitude.

Excessive leaning at cruise power above 75-percent power will invite engine damage from overheated valves and incur the possibility of detonation. The probability of damage from over-leaning decreases rapidly as cruise power is reduced from 75 percent. For example, there is considerably less, if even any, possibility of engine damage from over-lean mixtures at 50-percent power. However, an over-lean mixture can foul spark plugs and combustion chambers because of cylinder misfiring.

Many older engines are equipped with non-alloy steel exhaust valves. During normal cruise power (approximately 50 percent to 75 percent) the exhaust valve will become glowing hot during the period that it is open and exposed to the hot gases leaving the cylinder. If the mixture is lean enough to create an oxidizing atmosphere, the exhaust valves usually will be damaged. (An oxidizing atmosphere is always present in lean mixtures.)

The corrosion-resistant, alloy-steel valves employed in nearly, if not all, present engines are not too susceptible to the effects of oxidizing atmospheres. For those valves that are susceptible, the oxygen present in the spent gases tends to combine with the steel in the valve when it reaches incandescence. This action scales the exterior of the valve and destroys the finely ground face of the valve seat, causing the valve to begin leaking. Once the valve begins to leak its operating temperature elevates further, weakening it. Very high combustion temperatures will eventually torch a groove in the face of the valve, requiring its immediate replacement — an expensive price to pay for the miserly amount of fuel that may have been saved.

These situations still happen, simply because of improper leaning procedures. If you lean to roughness then back to the point where the roughness abates, such damage is not likely to occur, especially as cruise power descends from 75 percent maximum. Most leaning damage occurs from improper leaning above 75-percent power, most often during climb.

Another method of checking cruise power mixture after you have leaned is to switch to a single magneto; the engine is more critical of lean mixtures when operating this way. If the engine demonstrates only slight roughness and power loss the mixture is not excessively lean. Care must be taken not to switch the magnetos off. Should you inadvertently switch the ignition off completely, leave if off and retard the throttle to idle position. Then switch both magnetos on before bringing the power back in to prevent induction backfiring and exhaust afterfiring.

Both conditions are structurally hazardous to the systems involved. The single magneto-mixture check is limited to direct-drive, NA engines and should never be attempted on geared or mechanically aspirated (supercharged) engines.

Leaning during descent is another important phase of mixture regulation. Lean the mixture to match the power during descent, and if you are below 50-percent power the leanest condition satisfactory to smooth engine operation is best. Don't forget to enrich the mixture prior to increasing power when you level off from your descent. Many pilots do a masterful job of climb and cruise leaning, then push the mixture to full rich for a lower power descent from altitude — that really fouls the plugs and combustion chambers. A properly leaned descent adds greatly to keeping your engine and plugs "perky clean" at all times.

For high-altitude takeoffs (above 5,000 feet DA) the mixture should be leaned just as is done in the climb, just enough to avoid overrich roughness and subsequent power loss. This can be accomplished on the takeoff roll or by holding the aircraft with the brakes and leaning at full static run-up.

Taxiing and ground operation can be improved and fouling greatly reduced while visiting a high-altitude airport if the mixture is leaned for ground operation. (Some newer trainers, especially those with fuel-injected engines, should almost always be leaned while on the ground; check the pilot's operating handbook.) Operate the engine at 1,700 rpm, lean to roughness, then enrich just enough to restore smooth operation — then throttle back to idle speed. The engine should idle smoothly; it may be necessary to further lean or enrich to obtain best results. (A slightly richer position will be necessary for starting, especially in colder temperatures.)

The procedures given here are for direct-drive, carburetor-equipped, naturally aspirated engines. Fuel-injected and turbocharged engines are another ballgame. Proper leaning is essential to good engine operation and it pays off in more ways than just fuel economy. If you pay attention to the necessary details and perform mixture regulation properly you will be richer for it and your engine will operate longer and better.