Two-Stroke-Cycle  Engine  Timing

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Start TDC with the beginning of the POWER STROKE. Compression is at its peak when fuel injection has been completed and combustion is taking place. Power is delivered to the crankshaft as the piston is driven downward by the expanding gases in the cylinder. Power delivery ends when the exhaust valve opens. After  the  exhaust  valve  opens,  the  piston continues downward to BDC and then upward in the EXHAUST STROKE. The exhaust gases are pushed out of the cylinder as the piston rises to TDC, and the exhaust valve closes a few degrees after   TDC   to   ensure   proper   scavenging.   The crankshaft  has  made  a  complete  revolution  during the  power  and  exhaust  strokes. The intake valve opens a few degrees before TDC near the end of the upward exhaust stroke to   aid   in   scavenging   the   cylinder.   As   the crankshaft  continues  to  rotate  past  TDC,  the INTAKE   STROKE   begins.   The   intake   stroke continues  for  the  whole  downward  stroke  and  part of  the  next  upward  stroke  to  take  advantage  of the  inertia  of  the  incoming  charge  of  fresh  air. The  rest  of  the  upward  stroke  is  the  COM- PRESSION  STROKE,  which  begins  at  the  instant of  intake  valve  closing  and  ends  at  TDC  FUEL INJECTION  may  begin  as  much  as  40°  before TDC  and  continue  to  TDC,  thus  completing  the power  cycle  and  the  second  complete  revolution of  the  engine. By showing an approximate ignition point in place  of  fuel  injection,  figure  3-6  could  easily represent a timing diagram for a typical gasoline engine. For  additional  information  on  diesel  fuel injection system tests that can be made both in the    shop    and    in    the    field,    refer    to    the manufacturer’s  service  manual. Two-Stroke-Cycle  Engine  Timing Figure 3-7 shows a timing diagram of a two- stroke-cycle diesel engine. This engine is typical of  the  General  Motors  series,  which  uses  a  blower to send fresh air into the cylinder and to clear out the  exhaust  gases.  The  movement  of  the  piston itself does practically none of the work of intake and  exhaust,  as  it  does  in  a  four-stroke-cycle engine. This fact is important to the mechanic in detecting  two-stroke-cycle  diesel  engine  power losses. Beginning at TDC (fig. 3-7), the fuel has been injected,  and  combustion  is  taking  place.  The piston is driven down, and the power is delivered to the crankshaft until the piston is just a little more  than  halfway  down.  The  exhaust  valves  (two in  each  cylinder)  open  92  1/2°  after  TDC.  The exhaust  gases  blow  out  through  the  manifold,  and the  cylinder  pressure  drops  off  rapidly. At  132°  after  TDC  (48°  before  BDC),  the intake  ports  are  uncovered  by  the  downward movement  of  the  piston.  Scavenging  air  under blower   pressure   swirls   upward   through   the cylinder and clears the cylinder of exhaust gases. This flow of cool air also helps to cool the cylinder and the exhaust valves. Scavenging continues until the  piston  reaches  44  1/2°  after  BDC.  At  this point, the exhaust valves are closed. The blower continues  to  send  fresh  air  into  the  cylinder  for just  a  short  time  (only  3  1/2°  of  rotation),  but it is sufficient to give a slight supercharging effect. The intake ports are closed at 48° after BDC, and  compression  takes  place  during  the  remainder of  the  upward  stroke  of  the  piston.  Injection begins  at  about  22  1/2°  before  TDC  and  ends about  5°  before  TDC,  depending  on  the  engine speed  and  load. The  whole  cycle  is  completed  in  one  revolution of  the  crankshaft,  and  the  piston  is  ready  to deliver  the  next  power  stroke. Multiple-Cylinder Engines Theoretically,  the  power  stroke  of  a  piston continues  for  180°  of  crankshaft  rotation  on  a four-stroke-cycle   engine.   Best   results   can   be obtained,   however,   if   the   exhaust   valves   are opened  when  the  power  stroke  has  completed about   four-fifths   of   its   travel.   Therefore,   the period  that  power  is  delivered  during  720°  of crankshaft rotation, or one four-stroke cycle, will be 145° multiplied by the number of cylinders in the  engine.  This  may  vary  slightly  according  to the manufacturers’ specifications. If an engine has two cylinders, power will be transmitted for 290° of the 720° necessary to complete the four events of  the  cycle.  The  momentum  of  the  flywheel rotates the crankshaft for the remaining 430° of travel. 3-6

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