NAVWEPS 00-8OT-80  AIRPLANE PERFORMANCE  cruise power is the upper limit of power that  can be utilized for this operation. Higher air-  flows and higher power wirhout a change in  fuel-air ratio will intersect the knee of the  detonation envelope.  The primary factor relating the efficiency  of  operation of the reciprocating engine is the  brake specific fuel consumption, iWE%, or  simply c.  Brake suecific fuel consumution  The detonation properties of a fuel are de-  termined by the basic molecular structure of  the fuel and the various additives.  The fuel  detonation properties are generally specified  by the antidetonation or antiknock qualities of  an octane rating.  Since the antiknock proper-  ties of a high quality fuel may depend on the  mixture strength, provision must be made  in. the rating of fuels. Thus, a fuel grade of  IIS/  would relate a lean mixture antiknock  rating of 115 and a rich mixture antiknock  rating of 145. One of the most common opera-  tional causes  of detonation is fuel contamina-  tion.  An extremely small contamination of  high octane fuel with jet fuel can cause  a serious  ,decrease  in the antiknock rating.  Also, the  contamination of a high grade fuel with the  next lower grade will cause a noticeable loss of  antiknock quality.  Typical minimum values for c range from 0.4  to 0.6 lbs. per hr. per BHP and most aircraft  powerplaots average 0.5. The turbocompound  engine is generally the most efficient because  of the power recovery turbines and can ap-  proach values of c=O.38 to 0.42. It should be  noted that the minimum values of specific  fuel  consumption will be obtained only within the  range of cruise power operation, 30 to 60 per-  cent of the maximum power output.  Gen-  erally, the conditions of minimum specific  fuel  consumption are achieved with auto-lean or  manual lean scheduling of fuel-air ratios and  high BMEP and low RPM. The low RPM is  the usual requirement to minimize friction  horsepower and improve output efficiency.  The fuel metering requirements for an engine  are illustrated by the third graph of figure 2.16  which is a plot of fuel-air ratio versus engine  airflow.  The carburetor must provide specific  fuel-air ratios throughout the range of engine  airflow to accommodate  certain output power.  Most modern engines equipped with  auto-  matic mixture control provide a scheduling of  fuel-air ratio for automatic rich or automatic  lean operation. The auto-rich scheduling usu-  ally provides a fuel-air ratio at or near the  maximum heat release value for the middle  range of airflows. However, at high airflows  a power enrichment must be provided to sup-  press detonation. The auto-rich schedule gen-  erally will  provide an approximate fuel-air  ratio of 0.08 which increases  to 0.10 or 0.11 at  the airflow for takeoff power. In addition,  the low airflow and mixture dilution that oc-  curs in the idle power range requires enrich-  ment for satisfactory operation.  The effect of &it&c  is to reduce the engine  airflow and power output and supercharging  is necessary  to maintain high power output  at high altitude.  Since the basic engine is  able to process air only by the basic volume  displacement, the function of the supercharger  is to compress the inlet air and provide a  greater weight of air for the engine to process.  Of course, shaft power is necessary  to operate  the engine driven supercharger  and a tempera-  ture rise occurs through the supercharger  com-  pression. The effect of various forms of super-  charging on altitude performance  is illustrated  in figure 2.17.  The  unsupercharged-or  naturally  aspi-  rated-engine has no means of providing a  The schedule of fuel-air ratios with an auto-  matic lean fuel-air ratio will  automatically  provide maximum usable economy. If manual  leaning procedures  are applicable a lower fuel-  air ratio may be necessary  for maximum possi-  ble  efficiency. The  maximum  continuous  engine fuel flow  = brake horsepower  C=  lbs. per hr.  BHP  141 