temperature continues downward. Mixtures from 0.0725  fuel/air ratio to 0.080 fuel/air ratio are called best power  mixtures, since their use results in the greatest power for  a given airflow or manifold pressure. In this fuel/air ratio  range, there is no increase in the total heat released, but the  weight of nitrogen and combustion products is augmented  by the vapor formed with the excess fuel. Thus, the working  mass of the charge is increased. In addition, the extra fuel in  the charge (over the stoichiometric mixture) speeds up the  combustion process, which provides a favorable time factor  in converting fuel energy into power. cooling is obtained from unused fuel when fuel/air ratios  above 0.067 are used. The same function is performed by  excess air when fuel/air ratios below 0.067 are used. Varying the mixture strength of the charge produces  changes in the engine operating condition affecting power,  temperature, and spark-timing requirements. The best power  fuel/air ratio is desirable when the greatest power from a  given airflow is required. The best economy mixture results  from obtaining the given power output with the least fuel  flow. The fuel/air ratio which gives most efficient operation  varies with engine speed and power output. If the fuel/air ratio is enriched above 0.080, there is loss of  power and a reduction in temperature. The cooling effects  of excess fuel overtake the favorable factor of increased  mass. This reduced temperature and slower rate of burning  lead to an increasing loss of combustion efficiency. If, with  constant airflow, the mixture is leaned below 0.067, fuel/air  ratio power and temperature decrease together. This time, the  loss of power is not a liability but an asset. The purpose in  leaning is to save fuel. Air is free and available in limitless  quantities. The object is to obtain the required power with  the least fuel flow. A measure of the economical use of fuel  is called specific fuel consumption (SFC), which is the fuel  weight in pounds per hour per horsepower. In the graph showing this variation in fuel/air ratio, note that  the mixture is rich at both idling and high-speed operation  and is lean through the cruising range.  [Figure 2-1]  At idling  speed, some air or exhaust gas is drawn into the cylinder  through the exhaust port during valve overlap. The mixture  that enters the cylinder through the intake port must be  rich enough to compensate for this gas or additional air. At  cruising power, lean mixtures save fuel and increase the range  of the airplane. An engine running near full power requires  a rich mixture to prevent overheating and detonation. Since  the engine is operated at full power for only short periods, the  high fuel consumption is not a serious matter. If an engine  is operating on a mixture that is too lean, and adjustments  are made to increase the amount of fuel, the power output  of the engine increases rapidly at first, then gradually until  maximum power is reached. With a further increase in the  amount of fuel, the power output drops gradually at first, then  more rapidly as the mixture is further enriched. By using this ratio, the engine’s use of fuel at various power  settings can be compared. When leaning below 0.067 fuel/ air ratio with constant airflow, even though the power  diminishes, the cost in fuel to support each horsepower  hour (SFC) also is lowered. While the mixture charge is  becoming weaker, this loss of strength occurs at a rate  lower than that of the reduction of fuel flow. This favorable  tendency continues until a mixture strength known as best  economy is reached. With this fuel/air ratio, the required  hp is developed with the least fuel flow or, to put it another  way, the greatest power produced by a given fuel flow. The  best economy fuel/air ratio varies somewhat with rpm and  other conditions, but for cruise powers on most reciprocating  engines, it is sufficiently accurate to define this range of  operation as being from 0.060 to 0.065 fuel/air ratios on  aircraft where manual leaning is practiced. There are specific instructions concerning mixture ratios  for each type of engine under various operating conditions.  Failure to follow these instructions results in poor  performance and often in damage to the engine. Excessively  rich mixtures result in loss of power and waste of fuel. With  the engine operating near its maximum output, very lean  mixtures cause a loss of power and, under certain conditions,  serious overheating. When the engine is operated on a lean  mixture, the cylinder head temperature gauge should be  watched closely. If the mixture is excessively lean, the  engine may backfire through the induction system or stop  completely. Backfire results from slow burning of the lean  mixture. If the charge is still burning when the intake valve  opens, it ignites the fresh mixture and the flame travels back  through the combustible mixture in the induction system. Below the best economical mixture strength, power and  temperature continue to fall with constant airflow while  the SFC increases. As the fuel/air ratio is reduced further,  combustion becomes so cool and slow that power for a given  manifold pressure gets so low as to be uneconomical. The  cooling effect of rich or lean mixtures results from the excess  fuel or air over that needed for combustion. Internal cylinder  The carburetor must measure the airflow through the  induction system and use this measurement to regulate  the amount of fuel discharged into the airstream. The air  SFC =  pounds fuel/hour horsepower Carburetion Principles Venturi Principles 2-5