The rich mixture requirements for an aircraft engine are  established by running a power curve to determine the fuel/ air mixture for obtaining maximum usable power. This curve  is plotted at 100 rpm intervals from idle speed to takeoff  speed.  [Figure 2-2]  Since it is necessary in the power range  to add fuel to the basic fuel/air mixture requirements to  keep cylinder-head temperatures in a safe range, the fuel  mixture must become gradually richer as powers above  cruise are used.  [Figure 2-1]  In the power range, the engine  runs on a much leaner mixture, as indicated in the curves.  However, on the leaner mixture, cylinder-head temperature  would exceed the maximum permissible temperatures and  detonation would occur. The best economy setting is established by running a series  of curves through the cruise range, as shown in the graph in  Figure 2-3,  the low point (auto-lean) in the curve being the  fuel/air mixture where the minimum fuel per horsepower is  used. In this range the engine operates normally on slightly  leaner mixtures and obviously operates on richer mixtures  than the low-point mixture. If a mixture leaner than that  specified for the engine is used, the leanest cylinder of the  engine is apt to backfire because the slower burning rate of  the lean mixture results in a continued burning in the cylinder  when the next intake stroke starts. absorb this heat energy and turn it into power by expansion.  The mixture proportion of fuel and air by weight is of extreme  importance to engine performance. The characteristics of a  given mixture can be measured in terms of flame speed and  combustion temperature. Gasoline and other liquid fuels do not burn at all unless they  are mixed with air. If the mixture is to burn properly within  the engine cylinder, the ratio of air to fuel must be kept  within a certain range. It would be more accurate to state that  the fuel is burned with the oxygen in the air. Seventy-eight  percent of air by volume is nitrogen, which is inert and does  not participate in the combustion process, and 21 percent is  oxygen. Heat is generated by burning the mixture of gasoline  and oxygen. Nitrogen and gaseous byproducts of combustion  The composition of the fuel/air mixture is described by the  mixture ratio. For example, a mixture with a ratio of 12 to 1  (12:1) is made up of 12 pounds of air and 1 pound of fuel. The  ratio is expressed in weight because the volume of air varies  greatly with temperature and pressure. The mixture ratio can  also be expressed as a decimal. Thus, a fuel/air ratio of 12:1  and a fuel/air ratio of 0.083 describe the same mixture ratio.  Mixtures of air and gasoline as rich as 8:1 and as lean as 16:1  will burn in an engine cylinder, but beyond these mixtures,  either lean or rich blow out could occur. The engine develops  maximum power with a mixture of approximately 12 parts  of air and 1 part of gasoline by weight. From a chemist’s point of view, the perfect mixture for  combustion of fuel and air would be 0.067 pounds of fuel  to 1 pound of air (mixture ratio of 15:1). The scientist calls  this chemically correct combination a stoichiometric mixture  (pronounced stoy-key-o-metric). With this mixture (given  sufficient time and turbulence), all the fuel and all the oxygen  in the air is completely used in the combustion process. The  stoichiometric mixture produces the highest combustion  temperatures because the proportion of heat released to a  mass of charge (fuel and air) is the greatest. If more fuel is  added to the same quantity of air charge than the amount  giving a chemically perfect mixture, changes of power  and temperature occur. The combustion gas temperature is  lowered as the mixture is enriched, and the power increases  until the fuel/air ratio is approximately 0.0725. For mixtures  from 0.0725 fuel/air ratio to 0.080 fuel/air ratio, the power  remains essentially constant even though the combustion  2-4 Figure 2-2.  Power versus fuel/air mixture curve. Figure 2-3.  Specific fuel consumption curve.  Fuel/Air Mixtures Fuel/air mixture Lean  Rich Specific fuel consumption High Low Auto lean Auto rich Fuel/air mixture Lean  Rich Power High Brake HP Low Idle Takeoff Auto lean Auto rich