each cylinder of the engine must be provided with the proper  fuel/air mixture, usually metered by the carburetor. On some  fuel-injection engines, fuel is metered by the fuel injector  flow divider and fuel-injection nozzles. The relation between fuel/air ratio and power is illustrated in  Figure 10-44 . Note that, as the fuel mixture is varied from  lean to rich, the power output of the engine increases until  it reaches a maximum. Beyond this point, the power output  falls off as the mixture is further enriched. This is because the  fuel mixture is now too rich to provide perfect combustion.  Note that maximum engine power can be obtained by setting  the carburetor for one point on the curve. In establishing the carburetor settings for an aircraft engine,  the design engineers run a series of curves similar to the one  shown. A curve is run for each of several engine speeds. If,  for example, the idle speed is 600 rpm, the first curve might  be run at this speed. Another curve might be run at 700 rpm,  another at 800 rpm, and so on, in 100-rpm increments, up to  takeoff rpm. The points of maximum power on the curves  are then joined to obtain the best power curve of the engine  for all speeds. This best power curve establishes the rich  setting of the carburetor. In establishing the detailed engine requirements regarding  carburetor setting, the fact that the cylinder head temperature  varies with fuel/air ratio must be considered. This variation is  illustrated in the curve shown in  Figure 10-45 . Note that the  cylinder head temperature is lower with the auto-lean setting  than it is with the auto-rich mixture. This is exactly opposite common belief, but it is true. Furthermore, knowledge  of this fact can be used to advantage by flight crews. If,  during cruise, it becomes difficult to keep the cylinder head  temperature within limits, the fuel/air mixture may be leaned  out to get cooler operation. The desired cooling can then be  obtained without going to auto-rich with its costly waste of  fuel. The curve shows only the variation in cylinder head  temperature. For a given rpm, the power output of the engine  is less with the best-economy setting (auto-lean) than with  the best-power mixture. The decrease in cylinder head temperature with a leaner  mixture holds true only through the normal cruise range.  At higher power settings, cylinder temperatures are higher  with the leaner mixtures. The reason for this reversal hinges  on the cooling ability of the engine. As higher powers are  approached, a point is reached where the airflow around the  cylinders do not provide sufficient cooling. At this point,  a secondary cooling method must be used. This secondary  cooling is done by enriching the fuel/air mixture beyond the  best-power point. Although enriching the mixture to this  extent results in a power loss, both power and economy must  be sacrificed for engine cooling purposes. Many older, large, high-powered radial engines were  influenced by the cooling requirements on fuel/air mixture,  by effects of water injection.  Figure 10-46  shows a fuel/air  curve for a water-injection engine. The dotted portion of the  curve shows how the fuel-air mixture is leaned out during  water injection. This leaning is possible because water, rather  than extra fuel, is used as a cylinder coolant. These types of  systems are not used on modern aircraft. This permits leaning out to approximately best-power mixture  without danger of overheating or detonation. This leaning  out gives an increase in power. The water does not alter the  combustion characteristics of the mixture. Fuel added to the  auto-rich mixture in the power range during dry operation is  Fuel/air mixture Lean  Rich Power High Brake HP Low Auto lean Auto rich Fuel/air mixture Lean  Rich Power High Brake HP Low Auto lean Auto rich Figure 10-45.  Variation in head temperature with fuel/air mixture  (cruise power). Figure 10-44.  Power versus fuel/air mixture curve.  10-32