In order to burn, the fuel must be vaporized and mixed with air at the correct ratio. This job can be performed by either a carburetor or a fuel injection system.
Carburetors
A carburetor contains an open pipe with a shaped restriction, or venturi, inside. The venturi momentarily accelerates the intake air, lowering its pressure and temperature. A fuel nozzle, placed at the venturi, allows the lowered air pressure to pull fuel through the nozzle and into the intake air. The sprayed fuel quickly vaporizes in the air.
Next, the fuel/air mixture encounters a throttle valve inside the carburetor. The pilot manipulates the position of the throttle valve to adjust the engine’s power. At idle power, the valve is nearly closed, greatly restricting the flow of fuel and air to the engine. At full power, the valve is completely open.
Carburetor Icing
Air is cooled inside the carburetor as a result of the reduced pressure and vaporization of fuel. The temperature can drop from relatively warm temperatures prior to the venturi to below freezing just after it. Ice can form inside the carburetor, restricting the flow of air to the engine reducing engine performance. If the ice isn't dealt with, it can choke off the engine completely, resulting in an engine failure. Favorable conditions for carburetor icing are temperatures below 70 degrees with high humidity. Although, carb icing is possible at temperatures as high as 100 degrees.
A pilot can counteract the formation of carburetor ice through the use of carburetor heat. The carburetor heat control in the cockpit moves a valve, which directs intake air through a small heat exchanger. The heat exchanger uses hot engine exhaust to warm the intake air. The use of carburetor heat will also cause an engine performance decrease. This is due to the air thinning as a result of being heated.
Because of this lower density hotter air going into the engine, the fuel/air mixture is enriched. At higher altitudes, this condition is aggravated because of the already reduced air density, where the mixture is already richer than at sea level.
For airplanes with a fixed-pitch propeller, the first indication of carb ice is often the loss of engine RPM. Check for carburetor icing by applying carb heat. Because application of carb heat enriches the fuel/air mixture, expect a 100-150 RPM drop with carb heat application. If there is no carb ice, the engine will continue to run smoothly, and the engine RPM will remain constant.
If carb ice is present, the engine RPM will usually drop more significantly and become rough. Additional engine roughness will result from the ingestion of melted carb ice into the engine. Then, the engine will return to running smoothly.
A common mistake pilots make is to become nervous by the lowered RPM and rougher running engine that occurs when carb ice is discovered. The nervous pilot makes the mistake of immediately turning the carb heat off to reacquire a smooth running engine. However, it has now been verified that carb ice exists. The engine has to be allowed to choke on the melting ice until it is gone, otherwise, it will soon be unable to breath.
Once you have found yourself to be in conditions conducive to the formation of carburetor icing, use the carburetor heat continuously. You already got carb ice once. You know for a fact if you don't use carb heat, carb ice will form again.
Since the use of carb heat enriches the fuel/air mixture, the pilot may use the mixture control to lean the mixture. By cutting some of the fuel to the engine, you will return the mixture to an optimum ratio and obtain best engine power.
Fuel Injection
A fuel injection system uses a fuel injection pump to supply fuel under pressure to a fuel/air control unit. This unit meters fuel and air to the engine according to the throttle and mixture control setting. The metered fuel then flows to a fuel flow divider, which splits the fuel equally among the cylinders. Each cylinder has its own fuel injector, which vaporizes the fuel just prior to the cylinder’s intake valve. Since there is no venturi in a fuel injected system, a fuel injection system is less susceptible to internal icing. The high temperature of the cylinder makes ice not likely to develop at the fuel injector, either.
Impact Icing
Both carbureted and fuel injected engines are susceptible to impact icing, which results when the airplane flies through visible moisture that forms ice on the engine’s air intake or air filter. This impact ice blocks the flow of air into the engine. To counteract impact icing, many engines include a manual or automatic valve, which provides an alternate intake air supply.