operating conditions. Remember that at cruising speeds, a  lean mixture is desirable for economy reasons, while at high  power output, the mixture must be rich to obtain maximum  power and to aid in cooling the engine cylinders. The power  enrichment system automatically brings about the necessary  change in the fuel/air ratio. Essentially, it is a valve that is  closed at cruising speeds and opened to supply extra fuel  to the mixture during high power operation. Although it  increases the fuel flow at high power, the power enrichment  system is actually a fuel saving device. Without this system,  it would be necessary to operate the engine on a rich mixture  over the complete power range. The mixture would then  be richer than necessary at cruising speed to ensure safe  operation at maximum power. The power enrichment system  is sometimes called an economizer or a power compensator. by fuel injection systems and have limited use on modern  aircraft engines. Fuel evaporation ice or refrigeration ice is formed because of  the decrease in air temperature resulting from the evaporation  of fuel after it is introduced into the airstream. As the fuel  evaporates, the temperature is lowered in the area where  the evaporation takes place. Any moisture in the incoming  air can form ice in this area. It frequently occurs in those  systems in which fuel is injected into the air upstream  from the carburetor throttle, as in the case of float-type  carburetors. It occurs less frequently in systems in which the  fuel is injected into the air downstream from the carburetor.  Refrigeration ice can be formed at carburetor air temperatures  as high as 100 °F over a wide range of atmospheric humidity  conditions, even at relative humidity well below 100 percent.  Generally, fuel evaporation ice tends to accumulate on the  fuel distribution nozzle in the carburetor. This type of ice can  lower manifold pressure, interfere with fuel flow, and affect  mixture distribution. Although the various systems have been discussed separately,  the carburetor functions as a unit. The fact that one system  is in operation does not necessarily prevent another from  functioning. At the same time that the main metering system  is discharging fuel in proportion to the airflow, the mixture  control system determines whether the resultant mixture  is rich or lean. If the throttle is suddenly opened wide, the  accelerating and power enrichment systems act to add fuel to  that already being discharged by the main metering system. The float-type carburetor, the most common of all carburetor  types, has several distinct disadvantages. The effect that  abrupt maneuvers have on the float action and the fact that its  fuel must be discharged at low pressure leads to incomplete  vaporization and difficulty in discharging fuel into some  types of supercharged systems. The chief disadvantage of the  float carburetor, however, is its icing tendency. Since the float  carburetor must discharge fuel at a point of low pressure, the  discharge nozzle must be located at the venturi throat, and  the throttle valve must be on the engine side of the discharge  nozzle. This means that the drop in temperature due to fuel  vaporization takes place within the venturi. As a result, ice  readily forms in the venturi and on the throttle valve. Throttle ice is formed on the rear side of the throttle, usually  when the throttle is in a partially “closed” position. The  rush of air across and around the throttle valve causes a low  pressure on the rear side; this sets up a pressure differential  across the throttle, which has a cooling effect on the fuel/ air charge. Moisture freezes in this low pressure area and  collects as ice on the low pressure side. Throttle ice tends  to accumulate in a restricted passage. The occurrence of a  small amount of ice may cause a relatively large reduction in  airflow and manifold pressure. A large accumulation of ice  may jam the throttles and cause them to become inoperable.  Throttle ice seldom occurs at temperatures above 38 °F. A pressure-type carburetor discharges fuel into the airstream  at a pressure well above atmospheric. This results in better  vaporization and permits the discharge of fuel into the  airstream on the engine side of the throttle valve. With the  discharge nozzle located at this point, the drop in temperature  due to fuel vaporization takes place after the air has passed  the throttle valve and at a point where engine heat tends  to offset it. Thus, the danger of fuel vaporization icing is  practically eliminated. The effects of rapid maneuvers and  rough air on the pressure-type carburetors are negligible  since its fuel chambers remain filled under all operating  conditions. Pressure carburetors have been replaced mostly  Impact ice is formed either from water present in the  atmosphere as snow, sleet, or from liquid water which  impinges on surfaces that are at temperatures below 32 °F.  Because of inertia effects, impact ice collects on or near a  surface that changes the direction of the airflow. This type of ice  may build up on the carburetor elbow, as well as the carburetor  screen and metering elements. The most dangerous impact ice  is that which collects on the carburetor screen and causes a  very rapid reduction of airflow and power. In general, danger  from impact ice normally exists only when ice forms on the  leading edges of the aircraft structure. Under some conditions,  ice may enter the carburetor in a comparatively dry state and  2-9 Carburetor Types Carburetor Icing There are three general classifications of carburetor icing: 1.  Fuel evaporation ice 2.  Throttle ice 3.  Impact ice