TM 1-1510-224-10 exhaust  plumbing.    These  valves  control  the  flow  of  hot exhaust gasses to the inlet air lip assemblies. (2) Fuel heater. An oil-to-fuel heat exchanger,    located    in    the    engine    accessory    case, operates  continuously  and  automatically  to  heat  the  fuel sufficiently   to   prevent   ice   from   collecting   in   the   fuel control unit. 2-21.-ENGINE FUEL CONTROL SYSTEM. a.  Description.   The  basic  fuel  system  for  each engine  consists  of  an  engine  driven  fuel  pump,  a  fuel control   unit,   a   fuel   flow   divider,   a   dual   fuel   manifold, fourteen  fuel  nozzles,  and  a  purge  system.    The  fuel purge  system  forces  residual  fuel  from  the  manifolds  to the combustion chamber where it is consumed. b.    Fuel  Control  Unit.    The  fuel  control  unit  is mounted on the accessory case of the engine.  The unit is  a  hydro-pneumatic  metering  device  which  determines the  proper  fuel  flow  schedule  required  for  the  engine  to produce  the  amount  of  power  requested  by  the  relative position of the associated  POWER  lever.    The  control  of developed  engine  power  is  accomplished  by  adjusting the   engine   gas   generator   (N₁)   speed.      N, speed   is controlled by varying the amount of fuel injected into the combustion  chamber  through  the  fuel  nozzles.    Engine shutdown   is   accomplished   by   moving   the   appropriate CONDITION lever to the full aft  FUEL  CUTOFF position, which shuts off the fuel supply. 2-22.  POWER LEVERS. CAUTION Moving the POWER lever below the flight idle gate  without  the  associated  engine  running may     result     in     damage     to     the     reverse mechanism linkage. The   two   POWER   levers   are   located   on   the control  pedestal  (fig.  2-14),  and  are  placarded  POWER. These  levers  regulate  power  in  the  reverse,  idle,  and forward   ranges,   operating   so   that   forward   movement increases engine power.  Power control is accomplished through  adjustment  of  the  N₁  speed  governor  in  the  fuel control   unit.      Power   is   increased   when   N₁    RPM    is increased.    The   POWER   levers   also   control   propeller reverse  pitch.    Distinct  movement  (pulling  up  and  then aft  on  the   POWER   lever)   by   the   pilot   is   required   for operation    in    the    ground    fine    and    reverse    ranges. Forward lever travel range is designated INCR (increase),  supplemented  by  an  arrow  pointing  forward. Lever   travel   range   is   marked   IDLE,   LIFT,   GROUND FINE,  LIFT, and  REVERSE.    A  placard  below  the  lever slots     reads:     CAUTION     -     REVERSE     ONLY     WITH ENGINES RUNNING. 2-23.  CONDITION LEVERS. The two   CONDITION  levers  are  located  on  the control  pedestal  (fig.  2-14).    Each  lever  starts  and  stops the   fuel   supply,   and   controls   the   idle   speed   for   its respective   engine.      The   levers   have   three   placarded positions:  FUEL CUTOFF,  LOW IDLE, and  HIGH IDLE. In the   FUEL  CUTOFF  position,  the    CONDITION  lever controls   the   cutoff   function   of   its   engine-mounted   fuel control  unit.    From    LOW   IDLE   to    HIGH   IDLE,   they control the governors of the fuel control units to establish minimum  fuel  flow  levels.    LOW  IDLE  position  sets  the fuel flow rate to attain 60 to 62% minimum N₁, and  HIGH IDLE position sets the rate to attain 71 to 73% minimum N₁   The POWER lever for the corresponding engine can select N₁ from the respective idle setting, up to maximum power.  An increase in low idle N, will be experienced at high field elevation. 2-24.  FRICTION LOCK KNOBS. Friction  drag  of  the  engine  and  propeller  control levers   is   adjusted,   as   applicable,   by   four   friction   lock knobs.      The   friction   lock   knobs,   placarded   FRICTION LOCK,  are  located  on  the  control  pedestal  (fig.  2-14). One knob is below the propeller levers, one is below the CONDITION   levers,   and   two   are   below   the   POWER levers.      When   a   knob   is   rotated   clockwise,   friction   is increased,  opposing  movement  of  the  affected  lever  as set  by  the  pilot.    Counterclockwise  rotation  of  the  knob will decrease friction, thus permitting free and easy lever movement. 2-25.  ENGINE FIRE DETECTION SYSTEM. a.  Description.  A fire detection system (fig. 2-16) is installed to provide an immediate warning in the  event  of  a  fire  or  overtemperature  in  each  engine compartment.      The   main   element   of   the   system   is  a temperature  sensing  tube,  which  is  routed  continuously throughout  each  engine  compartment  and  terminates  in a  responder  unit.    The  responder  unit  is  mounted  in  the accessory   area   on   the   upper   left   hand   engine   mount truss,  just  forward  of  the  engine  firewall.    The  responder unit   contains   two   sets   of   contacts:   a   set   of   integrity switch  contacts,  for  continuity  test  functions  of  the  fire detection  circuitry;  and  a  set  of  alarm  switch  contacts, which  complete  the  circuit  to  activate  the  fire  warning system   when   the   detector   (sensor   tubing)   senses   an overtemperature   condition   in   critical   areas   around   the engine.  The detector is dual functioning and responds to overall    "average"    temperature,    or    a    highly    localized "discrete"  temperature,  caused  by  flames  or  hot  gases. Both  the  average  and  discrete  temperatures  are  preset, and cannot be adjusted in the field. The sensor tubing consists of an outer tube that is  filled  with  an  inert  gas,  and  an  inner  core  that  is  filled with  an  active  gas.    The  gases  within  the  tube  form  a pressure barrier that keeps the contacts of the responder integrity 2-35