Relative humidity Outside air temperature 100% 50% 80% 60% 70% 90% 20 °F/–7 °C 32 °F/0 °C 70 °F/21 °C 100 °F/38 °C High carb uretor H i g h c a r b u r e t o r icing potential i c i n g p o t e n t i a l Carb uretor icing possib le Figure 4-14. Although carburetor ice is most likely to form when  temperature and humidity are in ranges indicated by this chart,  carburetor ice is also possible under conditions not depicted. 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 the fuel  and sends it to the fuel manifold valve at a rate controlled  by the throttle. After reaching the fuel manifold valve, the  fuel is distributed to the individual fuel discharge nozzles.  The discharge nozzles, which are located in each cylinder  head, inject the fuel/air mixture at the precise time for each  cylinder directly into each cylinder intake port. Some of the advantages of fuel injection are: dictate how susceptible the carburetor is to icing. Consult  the aircraft POH for the probability of carburetor ice for the  specific installation and for carburetor ice procedures.  In a fuel injection system, the fuel is injected either directly  into the cylinders or just ahead of the intake valve. A fuel  injection system usually incorporates these basic components:  engine-driven fuel pump, fuel/air control unit, fuel manifold  (fuel distributor), discharge nozzles, auxiliary fuel pump, and  fuel pressure/fl ow indicators.  [Figure 4-15] Disadvantages include: • No carburetor icing. • Better fuel fl ow. • Faster throttle response. • Precise control of mixture. • Better fuel distribution. • Easier cold weather starts. • Difficulty in starting a hot engine. • Vapor locks during ground operations on hot days. • Problems associated with restarting an engine that  quits because of fuel starvation. Fuel Injection Induction Systems Fuel tank Fuel lines to each cylinder Fuel manifold valve Electric fuel pump Fuel filter Throttle control Fuel/air control unit Fuel pressure regulator Figure 4-15.  Fuel injection system. 4-11