Before we get into that, let's think about the carburetor's basic function. Simply put, it is to deliver air and fuel to the engine in the correct proportion-about 15 parts of air for every one part of fuel. This combination of fuel and air, called, not surprisingly, a mixture, is then pulled into the engine (a result of the piston moving down the cylinder while the intake valve is open), compressed, and fired off with the spark plug on the power or expansion stroke of the engine. If you add in the final expulsion of spent gases during the exhaust stroke, you have just described the basic Otto-cycle engine-yes, your common Lycoming or Continental.
The carburetor is a fairly basic device. It consists of a tube, through which the air flows to the engine. This tube, called the throat, is constricted, venturi-fashion, which helps to accelerate the airflow through the carburetor. Such acceleration is needed to help the fuel mix well with the incoming air; this fuel is then distributed by a small ring or tube located in the middle of the throat. Here is the elegant part of the carburetor: The fuel itself is carried into the airstream by siphon action proportional to the amount of air being admitted into the engine.
Naturally, you need some means of controlling the airflow; this is accomplished with a throttle plate (also called a butterfly valve), located past the point where the fuel is admitted, very close to where the carburetor is mounted to the engine. The butterfly valve is connected directly to the throttle control in the cockpit.
Another control in the cockpit is labeled "mixture." We don't need variable mixture controls on carbureted cars because they don't have to deal with the altitude extremes that airplanes do, and the electronic fuel-injection systems so common in today's cars automatically take care of such a task should it ever be needed. But the airplane must work in widely varying air densities, so the necessity of having a mixture control became clear early in engine development and remains so today.
The carburetor has a small holding tank of sorts-the float bowl. Gas from the fuel tanks and any external pumps (there are none in a Cessna 150 or 152) comes into the bowl, maintained at a preset level by a float and needle valve arrangement. As you might imagine, there is a port from the bowl to the discharge mechanism in the carburetor. It's a fairly simple matter, then, to restrict this port to control the ratio of fuel admitted to the amount of air flowing through the carburetor. This is accomplished with a tapered rod moving into and out of a fixed opening. Move the mixture control to full rich, and the rod retreats completely from the space, allowing maximum fuel flow. Select idle/cutoff on the mixture control, and the rod completely fills the hole, blocking fuel from reaching the engine.
Now that you understand how the mixture is controlled, you might want to know how and why you must control it. A big part of understanding mixture management is to think like an engine. Simply put, an engine wants a lot of fuel when it's making a lot of power. So it will also want less fuel-sometimes a lot less-when the throttle is pulled back, as it would be during descent.
Many instructors teach students to lean the mixture on the ground, during taxi. This procedure is more common on certain types of trainers and in warmer climates. The object of leaning on the ground is to keep excess fuel from leaving lead deposits on the spark plugs. A rough runup will usually disclose when one or more plugs has become fouled and therefore cannot fire as intended.
When operating out of high-elevation strips, there's an additional step-leaning for best power before attempting the takeoff. Sitting at the end of the runway and holding the brakes, apply full throttle and slowly lean the mixture until rpm peaks; use this setting for the takeoff unless unusually rough running or high engine temperatures occur, in which case you need to enrichen the mixture a bit.
Most POHs recommend leaving the mixture full rich until reaching 3,000 or 5,000 feet. Keep in mind that this refers to density altitude, so plug in those temperatures accordingly. You can and should lean in the climb once you've reached the recommended altitude. As during ground operations, slowly pull back the mixture until the rpm peaks, then enrichen it slightly. If you leave the mixture full rich during cruise, your carefully calculated fuel consumption figures will be way off the mark. Remember, the numbers in the POH reflect proper leaning.
And what about carb heat? Because the venturi restriction in the carburetor body both accelerates and cools the air, there is the chance for ice crystals to form on the throttle plate, downstream of the venturi. Left unchecked, the icing can completely close off the carb throat and cause engine stoppage. Applying carb heat simply directs air heated by proximity to the exhaust system (but not exhaust gases themselves) to the carburetor. Depending on airplane type, you will often use carb heat as a preventive measure at low power settings and before landing to prevent the formation of carb ice. In some airplanes, you will only pull on the carb heat if you suspect icing, which would manifest itself first as a drop in rpm and in the more advanced stages as a rough engine.
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