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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 

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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 

