NAVWEPS O&ROT-SO  AIRPLANE PERFORMANCE  specific fuel consumption is not adversely  affected as long as auto-lean or manual lean  power can be used at the cruise power setting.  One operating characteristic of the recipro-  cating engine is distinctly different from that  of the turbojet.  Water vapor in the air will  cause  a significant reduction in output power of  the reciprocating engine but a negligible loss  of thrust for the turbojet engine. This basic  difference exists because the reciprocating  engine operates  with a fixed displacement and  all air processed  is directly associated  with the  combustion process. If water vapor enters the  induction system of the reciprocating engine,  the amount of air available for combustion is  reduced and, since most carburetors do not  distinguish water vapor from air, an enrich-  ment of the fuel-air ratio takes place. The  maximum power output at takeoff requires  fuel-air ratios richer than that for maximum  -haezt re1m.e rn ,, C,I+P-  nnr:rLmm.c  . . ..I1 *-IF-  --A-“-\- “W . A....A c. b.IIA.cIIIIICIIL “1111 La&C  place with  subsequent loss of power. The  turbojet operates  with such great excess  of air  that the combustion process essentially is  unaffected and the reduction of air mass flow  is the principal consideration. As an example,  extreme conditions which would produce high  specific humidity may cause a 3 percent thrust  loss for a turbojet but a 12 percent loss of BHP  for a reciprocating engine.  Proper accounting  of the loss due to humidity is essential in the  operation of the reciprocating engine.  heavy detonation or preignition is common to  the high airflow at maximum power, the most  likely chance of detonation or preignition is at  takeoff. In order to suppress detonation or  allow greater power for takeoff, water injec-  tion is often used in the reciprocating engine.  At high power’settings, the injection of the  water-alcohol mixture can replace the excess  fuel required to suppress  detonation, and de-  richment provisions can reduce the fuel-air  ratio toward the value for maximum heat re-  lease. Thus, an increase  in power will be ob-  tained by the better fuel-air ratio.  In some  instances, a higher manifold pressure can be 1  utilized to produce additional power. The in-  jection fluid will require proportions of alcohol  and water quite different from the injection  fluid for jet engine thrust augmentation.  Since derichment of the fuel-air ratio is de-  sired, the anti-detonant injection (AOZ) will  rr
+l;n ,Ir,.Le, :.. -.....^*;*:- r-^--..-.*--.:J..-l  b”IICLIALI  PlC”ll”l111 yu ‘ a”c ’ l ’ L~  L” pC”LuL Ic>luual  fluid from fouling the plumbing.  When the fuel grades are altered during oper-  ation and the engine must be ’ operated on a  next lower fuel grade, proper account must be  made for the change in the operating limita-  tions.  This accounting must be made for the  maximum power for takeoff and the maximum  cruise power since both of these operating con-  ditions are near the detonation envelope. In  addition, when the higher grade of fuel again  becomes  available, the higher operating,limits  cannot be used until it is sure chat no contamina-  tion exists from the lower grade fuel remaining  in the tanks.  OPERATING  LIMITATIONS.  Recipro-  cating engines have achieved a great degree of  refinement and development and are one of the  most reliable of all types of aircraft power-  plants.  However, reliable operation of the re-  ciprocating engine is obtained only by strict  adherence  to the specific  operating limitations.  The most important operating limitations of  the reciprocating engine are those provided to  ensure that detonation and preignition do not  take place. The pilot must ensure that proper  fuel grades are used that limit MAP, BMEP,  RPM, CAT, etc., are not exceeded. Since  Spark plug fouling can provide certain high  as well as low limits of operating temperatures.  When excessively  low operating temperatures  are encountered, rapid carbon fouling of the  plugs will take place. On the other hand,  excessively high operating temperatures will  produce plug fouling from lead bromide de-  posits from the fuel additives.  Generally, the limited periods of time at  various high power settings are set to mini-  mize the accumulation of high rates of wear  144  Revised January 1965 