Most general aviation piston engines burn a mixture as lean as 16 parts air and one part fuel - 16:1 - and as rich as 8:1. An engine achieves maximum power at a ratio of approximately 12:1, and reaches its highest combustion temperature at approximately 15:1.
The correct mixture ratio is important for proper engine operation. An excessively lean mixture can cause backfiring through the induction system; a loss of power; and excessive cylinder head, piston, valve, and oil temperatures. An excessively rich mixture will result in a loss of power and a waste of fuel.
Most general aviation piston engines today are "air cooled," but, really, they are "fuel-air cooled." Compared to a water-cooled engine of the same power, more fuel flows through an air-cooled aircraft engine than it needs, and the "extra" fuel helps cool the cylinder heads from the inside. To provide this "extra" cooling fuel at higher power settings, an aircraft carburetor incorporates a device called an economizer system. When you hear the term "economizer" - think "save engine," not "save fuel."
As a piston engine climbs to high altitudes, the weight (density) of the surrounding air decreases, and the engine mixture becomes richer as a normal result of this decrease in air weight. To maintain the most desirable mixture ratio for both performance and fuel economy, pilots must lean the mixture as the aircraft climbs.
The mixture not only affects the engine's performance, it affects the exhaust gas temperature (EGT). Because EGT is a good measure of the mixture, many airplanes have an EGT gauge on the instrument panel. This gauge is wired to a temperature probe in the exhaust gas stream close to a cylinder's exhaust port.
As you slowly lean the mixture, the exhaust gas temperature will begin to rise - the fuel/air mixture ratio changes as less fuel is added to the same amount of air. At some point the EGT will reach a peak temperature. Any further leaning causes the EGT to decrease because the mixture lacks enough fuel to sustain normal combustion. At some point the engine will not continue to run.
The exhaust gas temperature system is straightforward and simple, and it helps pilots get the most out of their airplanes. However, a few caveats apply. The simplest EGT systems are based on a single probe in the exhaust pipe of one cylinder. That cylinder may run hotter or cooler than some other cylinders. Multi-cylinder EGT probes and gauges are much more accurate for leaning and monitoring engine temperatures. Also, because the probes live in an extremely hot, hostile environment, they wear out. If you know that the EGT probe or probes in the airplane you fly have not been replaced or serviced in some time, view the EGT gauge indications with skepticism.
In a carbureted engine, leaning to roughness and then increasing the mixture slightly is a reliable alternative to using the EGT gauge. An airplane's operating handbook will give the manufacturer's recommended leaning procedure for that airplane.
When leaning an engine using an EGT gauge, it's best to proceed slowly, so the EGT probes have time to react to the exhaust temperature change and indicate it accurately. If you lean too quickly, you may overshoot the desired leaning point, which might damage the engine before the EGT system indicates a problem. When you become familiar with a given aircraft and its leaning requirements, you learn to lean more quickly to an intermediate setting to reduce fuel consumption, then fine-tune the mixture to the desired setting.
Naturally, before a pilot touches the little red knob, he or she should be familiar with the manufacturer's recommended leaning procedure using an EGT gauge. Traditionally, most engines are leaned to some number of degrees on the rich side of peak. However, some engines introduced in recent years are designed and intended to be leaned to some number of degrees on the lean side of peak. It is important to know which type your engine is because leaning to the wrong side can lead to engine damage.
Pilots of multiengine aircraft should be aware that it's very unlikely for any two engines to indicate exactly the same EGT or the same fuel flow when they reach the desired leaning point. This happens because of numerous small differences, particularly in the fuel-injection and induction systems, between engines of the same make and model. Pilots should lean each engine individually, regardless of whether the EGT readings differ substantially, or if you see a difference in the way the engine reacts during the leaning procedure, you should record the readings and discuss the situation with an aircraft maintenance technician.
Most of the normally aspirated (non-turbocharged) engines that power general aviation aircraft use a full-rich mixture for takeoff, climb, and for cruising at low altitudes (typically, below 3,000 feet). However, in some conditions, such as high ambient temperatures or high runway elevations, leaning is definitely necessary for takeoff because an airplane uses less fuel to achieve the optimum fuel/air mixture at higher altitudes. I used to fly Cessna 421s into Aspen, Colorado, which is located at 7,000 feet MSL. At that altitude, the aircraft would hardly even start with a full-rich mixture, and a takeoff at that mixture setting would have been "interesting," to say the least, because the engines would be flooded with fuel and not be producing anything near optimum power. Aircraft flight manuals discuss proper leaning procedures, and give recommendations on the circumstances when leaning is both desirable and not desirable.
Besides helping pilots lean the mixture accurately, the EGT gauge can help them recognize problems. Many general aviation airplanes that have EGT gauges also have fuel flow gauges. The fuel flow gauges actually measure fuel pressure, even though their indicators read gallons per hour. On carbureted engines these gauges reliably show the actual fuel flow to the engine because all the fuel flows through one carburetor and disperses mechanically to the individual cylinders as a fuel/air mixture.
On fuel-injected engines, air and fuel reach the cylinders by separate routes. The induction system carries the air, and each cylinder has its own fuel-injector nozzle. The nozzles are calibrated and flow-checked to ensure they all match perfectly.
The fuel flow gauge gives a reliable reading if all the nozzles are injecting fuel, but if the system is restricted, often because of a blocked nozzle, the fuel flow gauge will give a false high reading. Remember, the fuel flow gauge measures fuel pressure, even though the indicator is marked for fuel flow.
When you see a high fuel-flow reading, your first reaction might be to reduce the fuel flow by leaning the mixture. If you do this, the mixture, which is already too lean in one cylinder, will become critically lean and can cause damage or even engine failure. But when you cross-check the fuel flow reading with the EGT indication, you'll know something is amiss because, generally speaking, a certain fuel flow should produce a fairly consistent EGT indication. Properly leaning the engine by the EGT gauge will prevent you from falling victim to a false fuel flow reading and perhaps avoid engine damage caused by a too-lean mixture.
Multiple-probe EGT gauges are useful for quickly spotting a problem in one of the cylinders. For example, an abnormally low or high reading in one cylinder might be an indication of a completely-blocked fuel injector nozzle - the cylinder not receiving any fuel. An unusually high reading could signal a partially blocked injector - the mixture too lean compared to the other cylinders.
A mixture control in an aircraft is there to be used, and an EGT gauge can give you valuable guidance about how to use it. You should adjust the fuel/air ratio to ensure the engine receives the proper mixture - not too much fuel for the amount of air - and not too little.
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