or   ports,   permits   an   intake   of   air,   or   an intake   of   a   mixture   of   fuel   and   air,   into the  combustion  area  of  the  cylinder  (intake valve).   The   other   opening,   or   port,   permits the  burned  gases  to  escape  from  the  cylinder (exhaust  valve).  The  two  ports  have  valves in   them.   These   valves   close   off   either   one or  the  other  of  the  ports,  or  both  of  them, during  various  stages  of  engine  operation.  The camshaft   (a   shaft   with   a   number   of   cam lobes   along   its   length)   opens   the   valves   and holds   them   open   for   short   periods   during the  piston  stroke.  The  camshaft  is  driven  by the   crankshaft   through   timing   gears   or   by a  timing  chain. On   a   four-stroke   engine, the  camshaft  turns  at  one-half  crankshaft speed.  This  permits  each  valve  to  open  and close  once  for  every  two  turns  of  the  crank- shaft. The   following   sections   give   a   simplified explanation  of  the  action  that  takes  place within  the  engine  cylinder.  For  the  purpose  of explanation,  we  will  show  the  action  of  a  four- stroke  diesel  engine.  This  type  of  engine  is referred  to  as  a  four-stroke  engine  because  it requires four complete piston strokes to complete one cycle. These strokes are known as the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. In   a   four-stroke   engine,   each   piston   goes through four strokes, and the crankshaft makes two  revolutions  to  complete  one  cycle.  Each piston delivers power during one stroke in four, or each piston makes one power stroke for each two  revolutions  of  the  crankshaft. We will take one cylinder and trace its opera- tion  through  the  four  strokes  that  make  up  a cycle  (fig.  7-4).  The  engine  parts  shown  in  this figure  include  a  cylinder,  a  crankshaft,  a  piston connecting rod, and the intake and exhaust valves. The   valve-operating   mechanism   and   the   fuel system  have  been  omitted. During  the  intake  stroke  shown  in  view  A,  the intake  valve  is  open  and  the  exhaust  valve  is closed.  The  piston  is  moving  downward  and drawing a charge of air into the cylinder through the intake valve. When the crankshaft has rotated to the posi- tion shown in view B, the piston moves upward to almost the top of the cylinder. Both the intake and exhaust valves are closed during this stroke. The  air  that  entered  the  cylinder  during  the intake stroke is compressed into the small space above the piston. This is called the compression stroke. The  high  pressure,  which  results  from  the compression stroke, raises the temperature of the air far above the ignition point of the fuel. When the piston nears the top of the compression stroke, a  charge  of  fuel  is  forced  into  the  cylinder  through the injector, as shown in view C. The air, which has   been   heated   by   compression,   ignites   the fuel. NOTE: The injection portion of a cycle is not considered  a  stroke. During the power stroke (view D), the intake and exhaust valves are both closed. The increase in  temperature  resulting  from  the  burning  fuel greatly increases the pressure on top of the piston. This   increased   pressure   forces   the   piston downward and rotates the crankshaft. This is the only  stroke  in  which  power  is  furnished  to  the crankshaft. During  the  exhaust  stroke  (view  E),  the exhaust valve is open and the intake valve remains closed.  The  piston  moves  upward,  forcing  the burned  gases  out  of  the  combustion  chamber through  the  exhaust  valve.  This  stroke,  which completes  the  cycle,  is  followed  immediately  by the   intake   stroke   of   the   next   cycle,   and   the sequence  of  events  continues. The  four-stroke  gasoline  engine  operates  on the same mechanical, or operational, cycle as the diesel  engine.  In  the  gasoline  engine,  the  fuel  and air  are  mixed  in  the  intake  manifold;  and  the mixture is drawn into the cylinders through the intake valve. The fuel-air mixture is ignited near the  top  of  the  compression  stroke  by  an  electric spark  that  passes  between  the  terminals  of  the spark  plug. Two-stroke diesel engines are widely used by the  Navy.  Every  second  stroke  of  a  two-stroke cycle  engine  is  a  power  stroke.  The  strokes between are compression strokes. The intake and exhaust  functions  take  place  rapidly  near  the bottom of each power stroke. With this arrange- ment,   there   is   one   power   stroke   for   each 7-5