Since the use of carburetor heat tends to reduce the output  of the engine and to increase the operating temperature,  carburetor heat should not be used when full power is required  (as during takeoff) or during normal engine operation, except  to check for the presence of, or to remove, carburetor ice.  Some aircraft are equipped with a carburetor air temperature  gauge, which is useful in detecting potential icing conditions.  Usually, the face of the gauge is calibrated in degrees Celsius  with a yellow arc indicating the carburetor air temperatures  where icing may occur. This yellow arc typically ranges  between –15 °C and +5 °C (5 °F and 41 °F). If the air  temperature and moisture content of the air are such that  carburetor icing is improbable, the engine can be operated  with the indicator in the yellow range with no adverse effects.  If the atmospheric conditions are conducive to carburetor  icing, the indicator must be kept outside the yellow arc by  application of carburetor heat.  Certain carburetor air temperature gauges have a red radial  that indicates the maximum permissible carburetor inlet air  temperature recommended by the engine manufacturer. If  present, a green arc indicates the normal operating range.  Most aircraft are also equipped with an outside air  temperature (OAT) gauge calibrated in both degrees  Celsius and Fahrenheit. It provides the outside or ambient  air temperature for calculating true airspeed and is useful in  detecting potential icing conditions.  In a fuel injection system, the fuel is injected directly into  the cylinders, or just ahead of the intake valve. The air  intake for the fuel injection system is similar to that used  in a carburetor system, with an alternate air source located  within the engine cowling. This source is used if the external  air source is obstructed. The alternate air source is usually  operated automatically, with a backup manual system that  can be used if the automatic feature malfunctions.  A fuel injection system usually incorporates six basic  components: an engine-driven fuel pump, a fuel-air control  unit, a fuel manifold (fuel distributor), discharge nozzles,  an auxiliary fuel pump, and fuel pressure/flow indicators.  [Figure 7-13]  The auxiliary fuel pump provides fuel under pressure to the  fuel-air control unit for engine starting and/or emergency  use. After starting, the engine-driven fuel pump provides fuel  under pressure from the fuel tank to the fuel-air control unit.  This control unit, which essentially replaces the carburetor,  meters fuel based on the mixture control setting and sends it  to the fuel manifold valve at a rate controlled by the throttle.  7-11  Figure 7-13.  Fuel injection system.  Outside Air Temperature Gauge  Carburetor Air Temperature Gauge  Fuel Injection Systems  Fuel tank  Auxiliary fuel pump  Engine-driven fuel pump  Fuel-air control unit  Fuel manifold valve  Fuel lines 