When the injector plunger is depressed, a fine spray   of   fuel   is   discharged   into   the   cylinder through  small  holes  in  the  nozzle.  The  smooth operation  of  the  engine  depends,  to  a  large extent, on the accuracy with which the plungers inject  the  same  amount  of  fuel  into  every cylinder. The amount of fuel injected into the cylinders on  each  stroke  is  controlled  by  rotating  the plungers  of  a  unit  injector.  The  throttle,  which regulates the speed of the engine, is connected to the  injectors  through  a  suitable  linkage.  A  change in  the  throttle  setting  rotates  the  plungers  and varies  the  amount  of  fuel  injected  into  the cylinders  on  each  stroke. LUBRICATION  SYSTEM The  lubrication  system  of  an  internal-combus- tion engine is very important. If the lubricating system should fail, not only will the engine stop, but many of the parts are likely to be damaged beyond  repair.  Therefore,  when  lubrication  failure occurs,  the  engine  can  seldom  be  run  again without  a  major  overhaul. The  lubricating  system  delivers  oil  to  the moving parts of the engine to reduce friction and to assist in keeping the parts cool. Most diesel and gasoline  engines  are  equipped  with  a  pressure lubricating  system  that  delivers  the  oil  under pressure  to  the  bearings  and  bushings  and  also lubricates  the  gears  and  cylinder  walls.  The  oil usually  reaches  the  bearings  through  passages drilled   in   the   framework   of   the   engine.   The lubricating  system  of  a  typical  diesel  engine  is shown  in  figure  7-10. All of the engine parts are lubricated with oil delivered  by  a  gear-type  oil  pump.  This  pump takes  suction  through  a  screen  from  an  oil  pan or sump. From the pump, the oil is forced through the oil filter and the oil cooler into the main oil gallery.  The  oil  is  fed  from  the  main  gallery, through individual passages, to the main crank- shaft  bearings  and  one  end  of  the  hollow camshaft.  All  the  other  moving  parts  and bearings are lubricated by oil drawn from these two sources. The cylinder walls and the teeth of many  of  the  gears  are  lubricated  by  oil  spray thrown off by the rotating crankshaft. After the oil has served its purpose, it drains back to the sump  to  be  used  again. The oil pressure in the line leading from the pump  to  the  engine  is  indicated  on  a  pressure gauge.  A  temperature  gauge  in  the  return  line provides   an   indirect   method   for   indicating variations  in  the  temperature  of  the  engine  parts. Any  abnormal  drop  in  pressure  or  rise  in temperature should be investigated at once. It is advisable to secure (shut down) the engine until the  trouble  has  been  located  and  corrected. Constant   oil   pressure,   throughout   a   wide range of engine speeds, is maintained by the oil pressure relief valve that allows the excess oil to flow back into the sump. All of the oil from the pump passes through the filter unless the oil is cold  and  heavy  or  if  the  filter  (or  oil  cooler)  is clogged.  In  such  cases,  the  bypass  valve  (filter bypass  valve  or  cooler  bypass  valve)  is  forced open;  and  the  oil  flows  directly  to  the  engine.  Part of  the  oil  fed  to  the  engine  is  returned  through the bypass filter, which removes flakes of metal, carbon  particles,  and  other  impurities. COOLING  SYSTEM Marine  engines  are  equipped  with  a  water- cooling  system  to  carry  away  the  excess  heat produced  in  the  engine  cylinders.  Fresh  water (coolant)  is  circulated  through  passages  in  the cylinder walls and in the cylinder head, where it becomes hot from absorbing engine heat. The hot coolant  then  passes  through  a  heat  exchanger, where  it  gives  up  its  heat  to  a  cooling  medium, becomes  cool,  and  returns  to  the  engine  to  remove more heat. The cooling medium may be either air or seawater. A  heat  exchanger  using  air  as  the  cooling medium  works  like  an  automobile  radiator.  A heat  exchanger  using  seawater  as  the  cooling medium  may  be  mounted  either  on  the  engine or  on  the  ship’s  hull.  Engine-mounted  heat exchangers  require  seawater  to  be  pumped  to and  from  them;  whereas,  hull-mounted  heat exchangers (keel coolers) are in constant contact with seawater and require the fresh water (coolant) to  be  pumped  through  the  cooler. STARTING  SYSTEMS There  are  three  types  of  starting  systems  used in   internal-combustion   engines—electric, hydraulic,  and  compressed  air. As  a  Fireman,  you  will  probably  have  more contact with the electric starting system than you will  with  the  other  two  types.  Lifeboats  aboard ships use an electric starter to start the engine. Electric  starting  systems  use  direct  current because electrical energy in this form can be stored in  batteries  and  drawn  upon  when  needed.  The battery’s  electrical  energy  can  be  restored  by 7-11